Variable-frequency drive

Small variable frequency drive

A variable-frequency drive (VFD) is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive. Variable-frequency drives are also known as adjustable-frequency drives (AFD), variable-speed drives (VSD), AC drives, microdrives or inverter drives. Since the voltage is varied along with frequency, these are sometimes also called VvVF (variable voltage variable frequency) drives.

Variable-frequency drives are widely used. In ventilation systems for large buildings, variable-frequency motors on fans save energy by allowing the volume of air moved to match the system demand. They are also used on pumps, conveyor and machine tool drives.

VFD types

All VFDs use their output devices (IGBTs, transistors, thyristors) only as switches, turning them only on or off. Using a linear device such as a transistor in its linear mode is impractical, since the power dissipated in these devices would be about as much as the power delivered to the load.

Drives can be classified as:

Constant voltage Constant current Cycloconverter

In a constant voltage converter, the intermediate DC link voltage remains approximately constant during each output cycle. In constant current drives, a large inductor is placed between the input rectifier and the output bridge, so the current delivered is nearly constant. A cycloconverter has no input rectifier or DC link and instead connects each output terminal to the appropriate input phase.

The most common type of packaged VF drive is the constant-voltage type, using pulse width modulation to control both the frequency and effective voltage applied to the motor load.

VFD system description

VFD system

A variable frequency drive system generally consists of an AC motor, a controller and an operator interface.

VFD motor

The motor used in a VFD system is usually a three-phase induction motor. Some types of single-phase motors can be used, but three-phase motors are usually preferred. Various types of synchronous motors offer advantages in some situations, but induction motors are suitable for most purposes and are generally the most economical choice. Motors that are designed for fixed-speed operation are often used. Certain enhancements to the standard motor designs offer higher reliability and better VFD performance, such as MG-31 rated motors.

VFD controller

Variable frequency drive controllers are solid state electronic power conversion devices. The usual design first converts AC input power to DC intermediate power using a rectifier or converter bridge. The rectifier is usually a three-phase, full-wave-diode bridge. The DC intermediate power is then converted to quasi-sinusoidal AC power using an inverter switching circuit. The inverter circuit is probably the most important section of the VFD, changing DC energy into three channels of AC energy that can be used by an AC motor. These units provide improved power factor, less harmonic distortion, and low sensitivity to the incoming phase sequencing than older phase controlled converter VFD's. Since incoming power is converted to DC, many units will accept single-phase as well as three-phase input power (acting as a phase converter as well as a speed controller); however the unit must be derated when using single phase input as only part of the rectifier bridge is carrying the connected load.

As new types of semiconductor switches have been introduced, these have promptly been applied to inverter circuits at all voltage and current ratings for which suitable devices are available. Introduced in the 1980s, the insulated-gate bipolar transistor (IGBT) became the device used in most VFD inverter circuits in the first decade of the 21st century.

AC motor characteristics require the applied voltage to be proportionally adjusted whenever the frequency is changed in order to deliver the rated torque. For example, if a motor is designed to operate at 460 volts at 60 Hz, the applied voltage must be reduced to 230 volts when the frequency is reduced to 30 Hz. Thus the ratio of volts per hertz must be regulated to a constant value (460/60 = 7.67 V/Hz in this case). For optimum performance, some further voltage adjustment may be necessary especially at low speeds, but constant volts per hertz is the general rule. This ratio can be changed in order to change the torque delivered by the motor.

In addition to this simple volts per hertz control more advanced control methods such as vector control and direct torque control (DTC) exist. These methods adjust the motor voltage in such a way that the magnetic flux and mechanical torque of the motor can be precisely controlled.

The usual method used to achieve variable motor voltage is pulse-width modulation (PWM). With PWM voltage control, the inverter switches are used to construct a quasi-sinusoidal output waveform by a series of narrow voltage pulses with pseudosinusoidal varying pulse durations.

Operation of the motors above rated name plate speed (base speed) is possible, but is limited to conditions that do not require more power than nameplate rating of the motor. This is sometimes called "field weakening" and, for AC motors, means operating at less than rated volts/hertz and above rated name plate speed. Permanent magnet synchronous motors have quite limited field weakening speed range due to the constant magnet flux linkage. Wound rotor synchronous motors and induction motors have much wider speed range. For example, a 100 hp, 460 V, 60 Hz, 1775 RPM (4 pole) induction motor supplied with 460 V, 75 Hz (6.134 V/Hz), would be limited to 60/75 = 80% torque at 125% speed (2218.75 RPM) = 100% power. At higher speeds the induction motor torque has to be limited further due to the lowering of the breakaway torque of the motor. Thus rated power can be typically produced only up to 130...150 % of the rated name plate speed. Wound rotor synchronous motors can be run even higher speeds. In rolling mill drives often 200...300 % of the base speed is used. Naturally the mechanical strength of the rotor and lifetime of the bearings is also limiting the maximum speed of the motor. It is recommended to consult the motor manufacturer if more than 150 % speed is required by the application.

PWM VFD Output Voltage Waveform

PWM AC variable speed drive

An embedded microprocessor governs the overall operation of the VFD controller. The main microprocessor programming is in firmware that is inaccessible to the VFD user. However, some degree of configuration programming and parameter adjustment is usually provided so that the user can customize the VFD controller to suit specific motor and driven equipment requirements.

VFD operator interface

The operator interface, also commonly known as an Human Machine Interface (HMI), provides a means for an operator to start and stop the motor and adjust the operating speed. Additional operator control functions might include reversing and switching between manual speed adjustment and automatic control from an external process control signal. The operator interface often includes an alphanumeric display and/or indication lights and meters to provide information about the operation of the drive. An operator interface keypad and display unit is often provided on the front of the VFD controller as shown in the photograph above. The keypad display can often be cable-connected and mounted a short distance from the VFD controller. Most are also provided with input and output (I/O) terminals for connecting pushbuttons, switches and other operator interface devices or control signals. A serial communications port is also often available to allow the VFD to be configured, adjusted, monitored and controlled using a computer.

VFD Operation

When an induction motor is connected to a full voltage supply, it draws several times (up to about 6 times) its rated current. As the load accelerates, the available torque usually drops a little and then rises to a peak while the current remains very high until the motor approaches full speed.

By contrast, when a VFD starts a motor, it initially applies a low frequency and voltage to the motor. The starting frequency is typically 2 Hz or less. Thus starting at such a low frequency

avoids the high inrush current that occurs when a motor is started by simply applying the utility (mains) voltage by turning on a switch. After the start of the VFD, the applied frequency and voltage are increased at a controlled rate or ramped up to accelerate the load without drawing excessive current. This starting method typically allows a motor to develop 150% of its rated torque while the VFD is drawing less than 50% of its rated current from the mains in the low speed range. A VFD can be adjusted to produce a steady 150% starting torque from standstill right up to full speed. Note, however, that cooling of the motor is usually not good in the low speed range. Thus running at low speeds even with rated torque for long periods is not possible due to overheating of the motor. If continuous operation with high torque is required in low speeds an external fan is needed. Please consult the manufacturer of the motor and/or the VFD.

In principle, the current on the motor side is in direct proportion of the torque that is generated and the voltage on the motor is in direct proportion of the actual speed, while on the network side, the voltage is constant, thus the current on line side is in direct proportion of the power drawn by the motor, that is U.I or C.N where C is torque and N the speed of the motor (we shall consider losses as well, neglected in this explanation).

(1) n stands for network (grid) and m for motor(2) C stands for torque [Nm], U for voltage [V], I for current [A], and N for speed [rad/s]We neglect losses for the moment :Un.In = Um.Im (same power drawn from network and from motor)Um.Im = Cm.Nm (motor mechanical power = motor electrical power)Given Un is a constant (network voltage) we conclude : In = Cm.Nm/Un That is "line current (network) is in direct proportion of motor power".

With a VFD, the stopping sequence is just the opposite as the starting sequence. The frequency and voltage applied to the motor are ramped down at a controlled rate. When the frequency approaches zero, the motor is shut off. A small amount of braking torque is available to help decelerate the load a little faster than it would stop if the motor were simply switched off and allowed to coast. Additional braking torque can be obtained by adding a braking circuit (resistor controlled by a transistor) to dissipate the braking energy. With 4-quadrants recifiers (active-front-end), the VFD is able to brake the load by applying a reverse torque and reverting the energy back to the network.

Power line harmonics

While PWM allows for nearly sinusoidal currents to be applied to a motor load, the diode rectifier of the VFD takes roughly square-wave current pulses out of the AC grid, creating harmonic distortion in the power line voltage. When the VFD load size is small and the available utility power is large, the effects of VFD systems slicing small chunks out of AC grid generally go unnoticed. Further, in low voltage networks the harmonics caused by single phase equipment such as computers and TVs are such that they are partially cancelled by three-phase diode bridge harmonics.

However, when either a large number of low-amperage VFDs, or just a few very large-load VFDs are used, they can have a cumulative negative impact on the AC voltages available to other utility customers in the same grid.

When the utility voltage becomes misshapen and distorted the losses in other loads such as normal AC motors are increased. This may in the worst case lead to overheating and shorter operation life. Also substation transformers and compensation capacitors are affected, the latter especially if resonances are aroused by the harmonics.

In order to limit the voltage distortion the owner of the VFDs may be required to install filtering equipment to smooth out the irregular waveform. Alternately, the utility may choose to install filtering equipment of its own at substations affected by the large amount of VFD equipment being used. In high power installations decrease of the harmonics can be obtained by supplying the VSDs from transformers that have different phase shift.

Further, it is possible to use instead of the diode rectifier a similar transistor circuit that is used to control the motor. This kind of rectifier is called active infeed converter in IEC standards. However, manufacturers call it by several names such as active rectifier, ISU (IGBT Supply Unit), AFE (Active Front End) or four quadrant rectifier. With PWM control of the transistors and filter inductors in the supply lines the AC current can be made nearly sinusoidal. Even better attenuation of the harmonics can be obtained by using an LCL (inductor-capacitor-inductor) filter instead of single three-phase filter inductor.

Additional advantage of the active infeed converter over the diode bridge is its ability to feed back the energy from the DC side to the AC grid. Thus no braking resistor is needed and the efficiency of the drive is improved if the drive is frequently required to brake the motor.

Applications considerations

The output voltage of a PWM VFD consists of a train of pulses switched at the carrier frequency. Because of the rapid rise time of these pulses, transmission line effects of the cable between the drive and motor must be considered. Since the transmission-line impedance of the cable and motor are different, pulses tend to reflect back from the motor terminals into the cable. The resulting voltages can produce up to twice the rated line voltage for lone cable runs, putting high stress on the cable and motor winding and eventual insulation failure. Increasing the cable or motor size/type for long runs and 480v or 600v motors will help offset the stresses imposed upon the equipment due to the VFD (modern 230v single phase motors not effected). At 460 V, the maximum recommended cable distances between VFDs and motors can vary by a factor of 2.5:1. The longer cables distances are allowed at the lower Carrier Switching Frequencies (CSF) of 2.5 kHz. The lower CSF can produce audible noise at the motors. For applications requiring long motor cables VSD manufacturers usually offer du/dt filters that decrease the steepness of the pulses. For very long cables or old motors with insufficient winding insulation more efficient sinus filter is recommended. Expect the older motor's life to shorten. Purchase VFD rated motors for the application.

Further, the rapid rise time of the pulses may cause trouble with the motor bearings. The stray capacitance of the windings provide paths for high frequency currents that close through the bearings. If the voltage between the shaft and the shield of the motor exceeds few volts the stored charge is discharged as a small spark. Repeated sparking causes erosion in the bearing surface that can be seen as fluting pattern. In order to prevent sparking the motor cable should provide a low impedance return path from the motor frame back to the inverter. Thus it is essential to use a cable designed to be used with VSDs.

In big motors a slip ring with brush can be used to provide a bypass path for the bearing currents. Alternatively isolated bearings can be used.

The 2.5 kHz and 5 kHz CSFs cause less motor bearing problems than caused by CSFs at 20 kHz. Shorter cables are recommended at the higher CSF of 20 kHz. The minimum CSF for synchronize tracking of multiple conveyors is 8 kHz.

The high frequency current ripple in the motor cables may also cause interference with other cabling in the building. This is another reason to use a motor cable designed for VSDs that has a symmetrical three-phase structure and good shielding. Further, it is highly recommended to route the motor cables as far away from signal cables as possible.

Available VFD power ratings

Variable frequency drives are available with voltage and current ratings to match the majority of 3-phase motors that are manufactured for operation from utility (mains) power. VFD controllers designed to operate at 111 V to 690 V are often classified as low voltage units. Low voltage units are typically designed for use with motors rated to deliver 0.2 kW or 1/4 horsepower (hp) up to several megawatts. For example, the largest ABB ACS800 single drives are rated for 5.6 MW . Medium voltage VFD controllers are designed to operate at 2,400/4,162 V (60 Hz), 3,000 V (50 Hz) or up to 10 kV. In some applications a step up transformer is placed between a low voltage drive and a medium voltage load. Medium voltage units are typically designed for use with motors rated to deliver 375 kW or 500 hp and above. Medium voltage drives rated above 7 kV and 5,000 or 10,000 hp should probably be considered to be one-of-a-kind (one-off) designs.

Medium voltage drives are generally rated amongst the following voltages : 2,3 KV - 3,3 Kv - 4 Kv - 6 Kv - 11 KvThe in-between voltages are generally possible as well. The power of MV drives is generally in the range of 0,3 to 100 MW however involving a range a several different type of drives with different technologies.

Brushless DC motor drives

Much of the same logic contained in large, powerful VFDs is also embedded in small brushless DC motors such as those commonly used in computer fans. In this case, the chopper usually converts a low DC voltage (such as 12 volts) to the three-phase current used to drive the electromagnets that turn the permanent magnet rotor.

Ca principiu de functionare:

1. Tensiunea trifazica se redreseaza cu o punte de 6 diode, se filtreaza.

2. Se genereaza dintr-un microcontroller 6 pwm-uri necesare comenzii semipuntilor cu

tranzistori

3. Se aplica semnalul prin niste drivere specializate la semipunti

4. Si gata treaba.....

Nota:

La 1. Teoria redresarii trifazice - clasic.

La 2. Microcontrollerul trebuie sa genereze 3 semnale sinus decalate intre ele cu 120

grade (vezi pct 1 - forme de unda a semnalului trifazic).

Atentie, cele 3 semnale sunt de fapt 6 jumatati de sinus=semialternante, pentru a putea

comanda cei 6 tranzistori (in perechi de cate 2 conectati in semipunte H).

In cazul in care puterea nu depaseste 250 W poti sa mergi cu IRF-uri

Daca vrei >250W trebuie sa bagi ceva mai scump pe partea de putere IGBT-uri

(tranzistoare cu grila si emitor-colector, vezi BUP314).

Aici intervine si partea de monitorizare consum curent (de catre un motor de ex.) Pe

partea asta apar mari probleme daca este facuta electronica fara instrumente de masura

(osciloscop cu memorie, stand mecanic pentru simulat sarcini, etc).

La turatii mici (sub 10% din turatia normala a motorului pt. 50Hz) trebuie asigurat pe

langa limitare de curent si o monitorizare de tensiune pe cele 3 infashurari pentru a putea

mentinu un cuplu constant. Cei care au testat motoare pas cu pas stiu ca la o frecventa

(de rezonanta) motorul "pierde pasi" daca este pus la o sarcina apropiata de maximul dat

de catalog. Asa se intampla si cu trifazicul.

La frecvente mai mari de 75Hz motorul pirde mult din cuplu (asta in functie de

producatorul motorului - romanesc, siemens, etc)

Da, uite ca m-am luat cu alte prostii si nu mai termin...

La 3. Driverele sunt bune la ceva... (vezi IR2130) ataca direct tranzistorii si rezolva

problema celor 400V intre cele 2 grile a unei perechi de tranzistori (plus multe alte

protectii).

La 4. Poti sa termini treaba dupa ce poti sa gandesti o sursa "ce transforma" 220V / 50Hz

in 220V / 0Hz...100Hz (sinus). Vezi PIC sau ATMEL, IR2111, IRF740 (sau BUP314 + diode

IXIS pentru puteri mai mari).

P.S. Este o treaba care se poate rezolva, daca ai rabdare si staruintza.

Vezi MicroMaster420 de la Siemens, ce stie sa faca, ca sa sti ce sa faci.

Bafta si spor la treaba.

Circuit description

Top: Simple inverter circuit shown with anelectromechanical switch

and automatic equivalent

auto-switching device implemented with two transistors and split winding auto-transformer in place of the mechanical

switch.

Square waveform with fundamental sine wave component, 3rd harmonic and 5th harmonic

Basic designs

In one simple inverter circuit, DC power is connected to a transformer through the centre tap of the

primary winding. A switch is rapidly switched back and forth to allow current to flow back to the DC

source following two alternate paths through one end of the primary winding and then the other. The

alternation of the direction of current in the primary winding of the transformer produces alternating

current (AC) in the secondary circuit.

The electromechanical version of the switching device includes two stationary contacts and a spring

supported moving contact. The spring holds the movable contact against one of the stationary

contacts and an electromagnet pulls the movable contact to the opposite stationary contact. The

current in the electromagnet is interrupted by the action of the switch so that the switch continually

switches rapidly back and forth. This type of electromechanical inverter switch, called a vibratoror

buzzer, was once used in vacuum tube automobile radios. A similar mechanism has been used in

door bells, buzzers and tattoo guns.

As they became available with adequate power ratings, transistors and various other types

of semiconductor switches have been incorporated into inverter circuit designs. Certain ratings,

especially for large systems (many kilowatts) use thyristors (SCR). SCRs provide large power handling

capability in a semiconductor device, and can readily be controlled over a variable firing range.

Output waveforms

The switch in the simple inverter described above, when not coupled to an output transformer,

produces a square voltage waveform due to its simple off and on nature as opposed to

the sinusoidal waveform that is the usual waveform of an AC power supply. Using Fourier

analysis, periodic waveforms are represented as the sum of an infinite series of sine waves. The sine

wave that has the same frequency as the original waveform is called the fundamental component. The

other sine waves, called harmonics, that are included in the series have frequencies that are integral

multiples of the fundamental frequency.

The quality of the inverter output waveform can be expressed by using the Fourier analysis data to

calculate the total harmonic distortion (THD). The total harmonic distortion (THD) is the square root of

the sum of the squares of the harmonic voltages divided by the fundamental voltage:

The quality of output waveform that is needed from an inverter depends on the characteristics of the

connected load. Some loads need a nearly perfect sine wave voltage supply to work properly. Other

loads may work quite well with a square wave voltage.

Advanced designs

H-bridge inverter circuit with transistor switches and antiparallel diodes

There are many different power circuit topologies and control strategies used in inverter designs.

Different design approaches address various issues that may be more or less important depending on

the way that the inverter is intended to be used.

The issue of waveform quality can be addressed in many ways. Capacitors and inductors can be used

to filter the waveform. If the design includes a transformer, filtering can be applied to the primary or the

secondary side of the transformer or to both sides. Low-pass filters are applied to allow the

fundamental component of the waveform to pass to the output while limiting the passage of the

harmonic components. If the inverter is designed to provide power at a fixed frequency,

aresonant filter can be used. For an adjustable frequency inverter, the filter must be tuned to a

frequency that is above the maximum fundamental frequency.

Since most loads contain inductance, feedback rectifiers or antiparallel diodes are often connected

across each semiconductor switch to provide a path for the peak inductive load current when the

switch is turned off. The antiparallel diodes are somewhat similar to the freewheeling diodes used in

AC/DC converter circuits.

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Fourier analysis reveals that a waveform, like a square wave, that is anti-symmetrical about the 180

degree point contains only odd harmonics, the 3rd, 5th, 7th, etc. Waveforms that have steps of certain

widths and heights can attenuate certain lower harmonics at the expense of amplifying higher

harmonics. For example, by inserting a zero-voltage step between the positive and negative sections

of the square-wave, all of the harmonics that are divisible by three (3rd and 9th, etc.) can be

eliminated. That leaves only the 5th, 7th, 11th, 13th etc. The required width of the steps is one third of

the period for each of the positive and negative steps and one sixth of the period for each of the zero-

voltage steps.

Changing the square wave as described above is an example of pulse-width modulation (PWM).

Modulating, or regulating the width of a square-wave pulse is often used as a method of regulating or

adjusting an inverter's output voltage. When voltage control is not required, a fixed pulse width can be

selected to reduce or eliminate selected harmonics. Harmonic elimination techniques are generally

applied to the lowest harmonics because filtering is much more practical at high frequencies, where

the filter components can be much smaller and less expensive. Multiple pulse-width or carrier

basedPWM control schemes produce waveforms that are composed of many narrow pulses. The

frequency represented by the number of narrow pulses per second is called the switching

frequency or carrier frequency. These control schemes are often used in variable-frequency motor

control inverters because they allow a wide range of output voltage and frequency adjustment while

also improving the quality of the waveform.

Multilevel inverters provide another approach to harmonic cancellation. Multilevel inverters provide an

output waveform that exhibits multiple steps at several voltage levels. For example, it is possible to

produce a more sinusoidal wave by having split-rail direct current inputs at two voltages, or positive

and negative inputs with a central ground. By connecting the inverter output terminals in sequence

between the positive rail and ground, the positive rail and the negative rail, the ground rail and the

negative rail, then both to the ground rail, a stepped waveform is generated at the inverter output. This

is an example of a three level inverter: the two voltages and ground.[7]

Three phase inverters

3-phase inverter with wye connected load

Three-phase inverters are used for variable-frequency drive applications and for high power

applications such as HVDCpower transmission. A basic three-phase inverter consists of three single-

phase inverter switches each connected to one of the three load terminals. For the most basic control

scheme, the operation of the three switches is coordinated so that one switch operates at each 60

degree point of the fundamental output waveform. This creates a line-to-line output waveform that has

six steps. The six-step waveform has a zero-voltage step between the positive and negative sections

of the square-wave such that the harmonics that are multiples of three are eliminated as described

above. When carrier-based PWM techniques are applied to six-step waveforms, the basic overall

shape, or envelope, of the waveform is retained so that the 3rd harmonic and its multiples are

cancelled.

3-phase inverter switching circuit showing 6-step switching sequence and waveform of voltage between terminals A and

C (23-2 states)

To construct inverters with higher power ratings, two six-step three-phase inverters can be connected

in parallel for a higher current rating or in series for a higher voltage rating. In either case, the output

waveforms are phase shifted to obtain a 12-step waveform. If additional inverters are combined, an

18-step inverter is obtained with three inverters etc. Although inverters are usually combined for the

purpose of achieving increased voltage or current ratings, the quality of the waveform is improved as

well.

Early inverters

From the late nineteenth century through the middle of the twentieth century, DC-to-AC power

conversion was accomplished using rotary converters or motor-generator sets (M-G sets). In the early

twentieth century, vacuum tubes and gas filled tubes began to be used as switches in inverter circuits.

The most widely used type of tube was the thyratron.

The origins of electromechanical inverters explain the source of the term inverter. Early AC-to-DC

converters used an induction or synchronous AC motor direct-connected to a generator (dynamo) so

that the generator's commutator reversed its connections at exactly the right moments to produce DC.

A later development is the synchronous converter, in which the motor and generator windings are

combined into one armature, with slip rings at one end and a commutator at the other and only one

field frame. The result with either is AC-in, DC-out. With an M-G set, the DC can be considered to be

separately generated from the AC; with a synchronous converter, in a certain sense it can be

considered to be "mechanically rectified AC". Given the right auxiliary and control equipment, an M-G

set or rotary converter can be "run backwards", converting DC to AC. Hence an inverter is an inverted

converter.

Controlled rectifier inverters

Since early transistors were not available with sufficient voltage and current ratings for most inverter

applications, it was the 1957 introduction of the thyristor or silicon-controlled rectifier (SCR) that

initiated the transition to solid state inverter circuits.

12-pulse line-commutated inverter circuit

The commutation requirements of SCRs are a key consideration in SCR circuit designs. SCRs do not

turn off or commutate automatically when the gate control signal is shut off. They only turn off when

the forward current is reduced to below the minimum holding current, which varies with each kind of

SCR, through some external process. For SCRs connected to an AC power source, commutation

occurs naturally every time the polarity of the source voltage reverses. SCRs connected to a DC

power source usually require a means of forced commutation that forces the current to zero when

commutation is required. The least complicated SCR circuits employ natural commutation rather than

forced commutation. With the addition of forced commutation circuits, SCRs have been used in the

types of inverter circuits described above.

In applications where inverters transfer power from a DC power source to an AC power source, it is

possible to use AC-to-DC controlled rectifier circuits operating in the inversion mode. In the inversion

mode, a controlled rectifier circuit operates as a line commutated inverter. This type of operation can

be used in HVDC power transmission systems and in regenerative braking operation of motor control

systems.

Another type of SCR inverter circuit is the current source input (CSI) inverter. A CSI inverter is

the dual of a six-step voltage source inverter. With a current source inverter, the DC power supply is

configured as a current source rather than a voltage source. The inverter SCRs are switched in a six-

step sequence to direct the current to a three-phase AC load as a stepped current waveform. CSI

inverter commutation methods include load commutation and parallel capacitor commutation. With

both methods, the input current regulation assists the commutation. With load commutation, the load is

a synchronous motor operated at a leading power factor.

As they have become available in higher voltage and current ratings, semiconductors such as

transistors or IGBTs that can be turned off by means of control signals have become the preferred

switching components for use in inverter circuits.

Rectifier and inverter pulse numbers

Rectifier circuits are often classified by the number of current pulses that flow to the DC side of the

rectifier per cycle of AC input voltage. A single-phase half-wave rectifier is a one-pulse circuit and

a single-phase full-wave rectifier is a two-pulse circuit. A three-phase half-wave rectifier is a three-

pulse circuit and a three-phase full-wave rectifier is a six-pulse circuit.

With three-phase rectifiers, two or more rectifiers are sometimes connected in series or parallel to

obtain higher voltage or current ratings. The rectifier inputs are supplied from special transformers that

provide phase shifted outputs. This has the effect of phase multiplication. Six phases are obtained

from two transformers, twelve phases from three transformers and so on. The associated rectifier

circuits are 12-pulse rectifiers, 18-pulse rectifiers and so on...

When controlled rectifier circuits are operated in the inversion mode, they would be classified by pulse

number also. Rectifier circuits that have a higher pulse number have reduced harmonic content in the

AC input current and reduced ripple in the DC output voltage. In the inversion mode, circuits that have

a higher pulse number have lower harmonic content in the AC output voltage waveform.

Variable-frequency drive

Small variable-frequency drive

Chassis of above VFD (cover removed)

A variable-frequency drive (VFD) (also termed adjustable-frequency drive, variable-speed drive, AC

drive, micro drive or inverter drive) is a type ofadjustable-speed drive used in electro-mechanical drive

systems to control AC motor speed and torque by varying motor input frequency andvoltage.

VFDs are used in applications ranging from small appliances to the largest of mine mill drives and

compressors. However, about a third of the world's electrical energy is consumed by electric motors in

fixed-speed centrifugal pump, fan and compressor applications and VFDs' global market penetration for all

applications is still relatively small. This highlights especially significant energy efficiency improvement

opportunities for retrofitted and new VFD installations.

Over the last four decades, power electronics technology has reduced VFD cost and size and improved

performance through advances in semiconductor switching devices, drive topologies, simulation and control

techniques, and control hardware and software.

VFDs are available in a number of different low and medium voltage AC-AC and DC-AC topologies.

System description and operation

VFD system

A variable frequency drive is a device used in a drive system consisting of the following three main sub-

systems: AC motor, main drive controller assembly, and drive operator interface.[5][4]

AC Motor

The AC electric motor used in a VFD system is usually a three-phase induction motor. Some types

of single-phase motors can be used, but three-phase motors are usually preferred. Various types

of synchronous motors offer advantages in some situations, but three phase induction motors are suitable

for most purposes and are generally the most economical choice. Motors that are designed for fixed-speed

operation are often used. Elevated voltage stresses imposed on induction motors that are supplied by VFDs

require that such motors be designed for definite-purpose inverter-fed duty in accordance to such

requirements as Part 31 of NEMA Standard MG-1.

Controller

The variable frequency drive controller is a solid state power electronics conversion system consisting of

three distinct sub-systems: a rectifier bridge converter, a direct current (DC) link, and an inverter. Voltage-

source inverter (VSI) drives (see 'Generic topologies' sub-section below) are by far the most common type

of drives. Most drives are AC-AC drives in that they convert AC line input to AC inverter output. However, in

some applications such as common DC bus or solar applications, drives are configured as DC-AC drives.

The most basic rectifier converter for the VSI drive is configured as a three-phase, six-pulse, full-wave diode

bridge. In a VSI drive, the DC link consists of a capacitor which smooths out the converter's DC

output ripple and provides a stiff input to the inverter. This filtered DC voltage is converted to quasi-

sinusoidal AC voltage output using the inverter's active switching elements. VSI drives provide higher power

factor and lower harmonic distortion than phase-controlled current-source inverter (CSI) and load-

commutated inverter (LCI) drives (see 'Generic topologies' sub-section below). The drive controller can also

be configured as a phase converter having single-phase converter input and three-phase inverter output.[7]

Controller advances have exploited dramatic increases in the voltage and current ratings and switching

frequency of solid state power devices over the past six decades. Introduced in 1983,[8] the insulated-gate

bipolar transistor (IGBT) has in the past two decades come to dominate VFDs as an inverter switching

device.

In variable-torque applications suited for Volts per Hertz (V/Hz) drive control, AC motor characteristics

require that the voltage magnitude of the inverter's output to the motor be adjusted to match the required

load torque in a linear V/Hz relationship. For example, for 460 volt, 60 Hz motors this linear V/Hz

relationship is 460/60 = 7.67 V/Hz. While suitable in wide ranging applications, V/Hz control is sub-optimal

in high performance applications involving low speed or demanding, dynamic speed regulation, positioning

and reversing load requirements. Some V/Hz control drives can also operate in quadratic V/Hz mode or can

even be programmed to suit special multi-point V/Hz paths.

The two other drive control platforms, vector control and direct torque control (DTC), adjust the motor

voltage magnitude, angle from reference and frequency[14] such as to precisely control the motor's magnetic

flux and mechanical torque.

Although space vector pulse-width modulation (SVPWM) is becoming increasingly popular, sinusoidal PWM

(SPWM) is the most straightforward method used to vary drives' motor voltage (or current) and frequency.

With SPWM control (see Fig. 1), quasi-sinusoidal, variable-pulse-width output is constructed from

intersections of a saw-toothed carrier frequency signal with a modulating sinusoidal signal which is variable

in operating frequency as well as in voltage (or current).

Operation of the motors above rated nameplate speed (base speed) is possible, but is limited to conditions

that do not require more power than the nameplate rating of the motor. This is sometimes called "field

weakening" and, for AC motors, means operating at less than rated V/Hz and above rated nameplate

speed. Permanent magnet synchronous motors have quite limited field weakening speed range due to the

constant magnet flux linkage. Wound rotor synchronous motors and induction motors have much wider

speed range. For example, a 100 hp, 460 V, 60 Hz, 1775 RPM (4 pole) induction motor supplied with 460

V, 75 Hz (6.134 V/Hz), would be limited to 60/75 = 80% torque at 125% speed (2218.75 RPM) = 100%

power. At higher speeds the induction motor torque has to be limited further due to the lowering of the

breakaway torque of the motor. Thus rated power can be typically produced only up to 130...150% of the

rated nameplate speed. Wound rotor synchronous motors can be run at even higher speeds. In rolling mill

drives often 200...300% of the base speed is used. The mechanical strength of the rotor limits the maximum

speed of the motor.

Fig. 1: SPWM carrier-sine input & 2-level PWM output

An embedded microprocessor governs the overall operation of the VFD controller. Basic programming of

the microprocessor is provided as user inaccessiblefirmware. User programming of display, variable and

function block parameters is provided to control, protect and monitor the VFD, motor and driven equipment.

[9][19]

The basic drive controller can be configured to selectively include such optional power components and

accessories as follows:

Connected upstream of converter - circuit breaker or fuses, isolation contactor, EMC filter, line reactor,

passive filter

Connected to DC link - braking chopper, braking resistor

Connected downstream of inverter - output reactor, sine wave filter, dV/dt filter.[b][21]

Operator interface

The operator interface provides a means for an operator to start and stop the motor and adjust the

operating speed. Additional operator control functions might include reversing, and switching between

manual speed adjustment and automatic control from an external process control signal. The operator

interface often includes analphanumeric display and/or indication lights and meters to provide information

about the operation of the drive. An operator interface keypad and display unit is often provided on the front

of the VFD controller as shown in the photograph above. The keypad display can often be cable-connected

and mounted a short distance from the VFD controller. Most are also provided with input and output (I/O)

terminals for connecting pushbuttons, switches and other operator interface devices or control signals.

A serial communications port is also often available to allow the VFD to be configured, adjusted, monitored

and controlled using a computer.[9][22][23]

Drive operation

Electric motor speed-torque chart

Referring to the accompanying chart, drive applications can be categorized as single-quadrant, two-

quadrant or four-quadrant; the chart's four quadrants are defined as follows:

Quadrant I - Driving or motoring, forward accelerating quadrant with positive speed and torque

Quadrant II - Generating or braking, forward braking-decelerating quadrant with positive speed and

negative torque

Quadrant III - Driving or motoring, reverse accelerating quadrant with negative speed and torque

Quadrant IV - Generating or braking, reverse braking-decelerating quadrant with negative speed and

positive torque.

Most applications involve single-quadrant loads operating in quadrant I, such as in variable-torque (e.g.

centrifugal pumps or fans) and certain constant-torque (e.g. extruders) loads.

Certain applications involve two-quadrant loads operating in quadrant I and II where the speed is positive

but the torque changes polarity as in case of a fan decelerating faster than natural mechanical losses.

Some sources define two-quadrant drives as loads operating in quadrants I and III where the speed and

torque is same (positive or negative) polarity in both directions.

Certain high-performance applications involve four-quadrant loads (Quadrants I to IV) where the speed and

torque can be in any direction such as in hoists, elevators and hilly conveyors. Regeneration can only occur

in the drive's DC link bus when inverter voltage is smaller in magnitude than the motor back-EMFand

inverter voltage and back-EMF are the same polarity.

In starting a motor, a VFD initially applies a low frequency and voltage, thus avoiding high inrush current

associated with direct on line starting. After the start of the VFD, the applied frequency and voltage are

increased at a controlled rate or ramped up to accelerate the load. This starting method typically allows a

motor to develop 150% of its rated torque while the VFD is drawing less than 50% of its rated current from

the mains in the low speed range. A VFD can be adjusted to produce a steady 150% starting torque from

standstill right up to full speed. However, motor cooling deteriorates and can result in overheating as speed

decreases such that prolonged low speed motor operation with significant torque is not usually possible

without separately-motorized fan ventilation.

With a VFD, the stopping sequence is just the opposite as the starting sequence. The frequency and

voltage applied to the motor are ramped down at a controlled rate. When the frequency approaches zero,

the motor is shut off. A small amount of braking torque is available to help decelerate the load a little faster

than it would stop if the motor were simply switched off and allowed to coast. Additional braking torque can

be obtained by adding a braking circuit (resistor controlled by a transistor) to dissipate the braking energy.

With a four-quadrant rectifier (active-front-end), the VFD is able to brake the load by applying a reverse

torque and injecting the energy back to the AC line.

Benefits

Energy savings

Many fixed-speed motor load applications that are supplied direct from AC line power can save energy

when they are operated at variable-speed, by means of VFD. Such energy cost savings are especially

pronounced in variable-torque centrifugal fan and pump applications, where the loads' torque and power

vary with the square and cube, respectively, of the speed. This change gives a large power reduction

compared to fixed-speed operation for a relatively small reduction in speed. For example, at 63% speed a

motor load consumes only 25% of its full speed power. This is in accordance with affinity laws that define

the relationship between various centrifugal load variables.

In the United States, an estimated 60-65% of electrical energy is used to supply motors, 75% of which are

variable torque fan, pump and compressor loads.[30] Eighteen percent of the energy used in the 40 million

motors in the U.S. could be saved by efficient energy improvement technologies such as VFDs.[31][32]

Only about 3% of the total installed base of AC motors are provided with AC drives.[33] However, it is

estimated that drive technology is adopted in as many as 30-40% of all newly installed motors.[34]

An energy consumption breakdown of the global population of AC motor installations is as shown in the

following table:

Global population of motors, 2009[35]

Small General Purpose - Medium-Size Large

Power10W to 750W

750W to 375kW 375kW to 100MW

Phase, voltage 1-ph., <240V 3-ph., 200V to 1kV 3-ph., 1kV to 20kV

% total motor energy 9% 68% 23%

Total stock 2 billion 230 million 0.6 million

Control performance

AC drives are used to bring about process and quality improvements in industrial and commercial

applications' acceleration, flow, monitoring, pressure, speed, temperature, tension and torque.[36]

Fixed-speed operated loads subject the motor to a high starting torque and to current surges that are up to

eight times the full-load current. AC drives instead gradually ramp the motor up to operating speed to lessen

mechanical and electrical stress, reducing maintenance and repair costs, and extending the life of the motor

and the driven equipment.

Variable speed drives can also run a motor in specialized patterns to further minimize mechanical and

electrical stress. For example, an S-curve pattern can be applied to a conveyor application for smoother

deceleration and acceleration control, which reduces the backlash that can occur when a conveyor is

accelerating or decelerating.

Performance factors tending to favor use of DC, over AC, drives include such requirements as continuous

operation at low speed, four-quadrant operation with regeneration, frequent acceleration and deceleration

routines, and need for motor to be protected for hazardous area.[37] The following table compares AC and

DC drives according to certain key parameters:[38][39][40]

Drive type DC AC VFD AC VFD AC VFD AC VFD

Control platform Brush type DCV/Hz

controlVector control

Vector control

Vector control

Control criteria Closed-loopOpen-loop

Open-loopClosed-

loopOpen-loop w.

HFI^

Motor DC IM IM IM Interior PM

Typical speed regulation (%) 0.01 1 0.5 0.01 0.02

Typical speed range at constant torque (%))

0-100 10-100 3-100 1-1500 1-100

Min. speed at 100% torque (% of base)

Standstill 8% 2% StandstillStandstill (200%)

Multiple-motor operation recommended

No Yes No No No

Fault protection (Fused only or inherent to drive)

Fused only Inherent Inherent Inherent Inherent

Maintenance (Brushes) Low Low Low Low

Feedback device Tachometer or encoder N/A N/A Encoder N/A

^ High frequency injection

VFD types and ratings

Generic topologies

Topology of VSI drive

Topology of CSI drive

Six-step drive waveforms

Topology of direct matrix converter

AC drives can be classified according to the following generic topologies:

Voltage-source inverter (VSI) drive topologies (see image): In a VSI drive, the DC output of

the diode-bridge converter stores energy in the capacitor bus to supply stiff voltage input to the inverter.

The vast majority of drives are VSI type with PWM voltage output.[d]

Current-source inverter (CSI) drive topologies (see image): In a CSI drive, the DC output of

the SCR-bridge converter stores energy in series-reactorconnection to supply stiff current input to the

inverter. CSI drives can be operated with either PWM or six-step waveform output.

Six-step[e] inverter drive topologies (see image):[43] Now largely obsolete, six-step drives can be

either VSI or CSI type and are also referred to as variable-voltage inverter drives, pulse-amplitude

modulation (PAM) drives,[44] square-wave drives or D.C. chopper inverter drives.[45] In a six-step drive,

the DC output of the SCR-bridge converter is smoothed via capacitor bus and series-reactor

connection to supply via Darlington Pair or IGBT inverter quasi-sinusoidal, six-step voltage or current

input to an induction motor.[46]

Load commutated inverter (LCI) drive topologies: In a LCI drive, a special CSI case, the DC output

of the SCR-bridge converter stores energy via DC link inductor circuit to supply stiff quasi-sinusoidal

six-step current output of a second SCR-bridge's inverter and an over-excited synchronous machine.

Cycloconverter  or matrix converter (MC) topologies (see image): Cycloconverters and MCs

are AC-AC converters that have no intermediate DC link for energy storage. A cycloconverter operates

as a three-phase current source via three anti-parallel connected SCR-bridges in six-pulse

configuration, each cycloconverter phase acting selectively to convert fixed line frequency AC voltage

to an alternating voltage at a variable load frequency. MC drives are IGBT-based.

Doubly fed  slip[f] recovery system topologies: A doubly fed slip recovery system feeds rectified slip

power to a smoothing reactor to supply power to the AC supply network via an inverter, the speed of

the motor being controlled by adjusting the DC current.

Control platforms

See also: Dqo transformation and Alpha–beta transformation

Most drives use one or more of the following control platforms:[41][47]

PWM V/Hz scalar control

PWM field-oriented control (FOC) or vector control

Direct torque control (DTC).

Load torque and power characteristics

Variable frequency drives are also categorized by the following load torque and power characteristics:

Variable torque, such as in centrifugal fan, pump and blower applications

Constant torque, such as in conveyor and displacement pump applications

Constant power, such as in machine tool and traction applications.

Available power ratings

VFDs are available with voltage and current ratings covering a wide range of single-phase and multi-phase

AC motors. Low voltage (LV) drives are designed to operate at output voltages equal to or less than 690 V.

While motor-application LV drives are available in ratings of up to the order of 5 or 6 MW,[48] economic

considerations typically favor medium voltage (MV) drives with much lower power ratings. Different MV

drive topologies (see Table 2) are configured in accordance with the voltage/current-combination ratings

used in different drive controllers' switching devices[49] such that any given voltage rating is greater than or

equal to one to the following standard nominal motor voltage ratings: generally either 2.3/4.16 kV (60 Hz) or

3.3/6.6 kV (50 Hz), with one thyristor manufacturer rated for up to 12 kV switching. In some applications a

step up transformer is placed between a LV drive and a MV motor load. MV drives are typically rated for

motor applications greater than between about 375 kW (500 hp) and 750 kW (1000 hp). MV drives have

historically required considerably more application design effort than required for LV drive applications.[50]

[51] The power rating of MV drives can reach 100 MW, a range of different drive topologies being involved for

different rating, performance, power quality and reliability requirements.[52][53][54]

Drives by machines & detailed topologies

It is lastly useful to relate VFDs in terms of the following two classifications:

In terms of various AC machines as shown in Table 1 below[55][56]

In terms of various detailed AC-AC converter topologies shown in Tables 2 and 3 below.[57][54][53][42][41][58][59]

[60][61]

Application considerations

AC line harmonics

Note of clarification:.[g]

While harmonics in the PWM output can easily be filtered by carrier frequency related filter inductance to

supply near-sinusoidal currents to the motor load,[62] the VFD's diode-bridge rectifier converts AC line

voltage to DC voltage output by super-imposing non-linear half-phase current pulses thus creating harmonic

current distortion, and hence voltage distortion, of the AC line input. When the VFD loads are relatively

small in comparison to the large, stiff power system available from the electric power company, the effects

of VFD harmonic distortion of the AC grid can often be within acceptable limits. Furthermore, in low voltage

networks, harmonics caused by single phase equipment such as computers and TVs are partially cancelled

by three-phase diode bridge harmonics because their 5th and 7th harmonics are in counterphase.

[63] However, when the proportion of VFD and other non-linear load compared to total load or of non-linear

load compared to the stiffness at the AC power supply, or both, is relatively large enough, the load can have

a negative impact on the AC power waveform available to other power company customers in the same

grid.

When the power company's voltage becomes distorted due to harmonics, losses in other loads such as

normal fixed-speed AC motors are increased. This may lead to overheating and shorter operating life.

Also substation transformers and compensation capacitors are affected negatively. In particular, capacitors

can cause resonance conditions that can unacceptably magnify harmonic levels. In order to limit the voltage

distortion, owners of VFD load may be required to install filtering equipment to reduce harmonic distortion

below acceptable limits. Alternatively, the utility may adopt a solution by installing filtering equipment of its

own at substations affected by the large amount of VFD equipment being used. In high power installations

harmonic distortion can be reduced by supplying multi-pulse rectifier-bridge VFDs from transformers with

multiple phase-shifted windings.[64]

It is also possible to replace the standard diode-bridge rectifier with a bi-directional IGBT switching device

bridge mirroring the standard inverter which uses IGBT switching device output to the motor. Such rectifiers

are referred to by various designations including active infeed converter (AIC), active rectifier, IGBT supply

unit (ISU), active front end (AFE) or four-quadrant operation. With PWM control and suitable input reactor,

AFE's AC line current waveform can be nearly sinusoidal. AFE inherently regenerates energy in four-

quadrant mode from the DC side to the AC grid. Thus no braking resistor is needed and the efficiency of the

drive is improved if the drive is frequently required to brake the motor.

Two other harmonics mitigation techniques exploit use of passive or active filters connected to a common

bus with at least one VFD branch load on the bus. Passive filters involve the design of one or more low-

pass LC filter traps, each trap being tuned as required to a harmonic frequency (5th, 7th, 11th, 13th, . . .

kq+/-1, where k=integer, q=pulse number of converter).[65]

It is very common practice for power companies or their customers to impose harmonic distortion limits

based on IEC or IEEE standards. For example, IEEE Standard 519 limits at the customer's connection point

call for the maximum individual frequency voltage harmonic to be no more than 3% of the fundamental and

call for the voltage total harmonic distortion (THD) to be no more than 5% for a general AC power supply

system.[66]

Long lead effects

The carrier frequency pulsed output voltage of a PWM VFD causes rapid rise times in these pulses, the

transmission line effects of which must be considered. Since the transmission-line impedance of the cable

and motor are different, pulses tend to reflect back from the motor terminals into the cable. The resulting

voltages can produce overvoltages equal to twice the DC bus voltage or up to 3.1 times the rated line

voltage for long cable runs, putting high stress on the cable and motor windings and eventual insulation

failure. Note that standards for three-phase motors rated 230 V or less adequately protect against such long

lead overvoltages. On 460 or 575 V systems and inverters with 3rd generation 0.1 microsecond rise time

IGBTs, the maximum recommended cable distance between VFD and motor is about 50 m or 150 feet.

Solutions to overvoltages caused by long lead lengths include minimizing cable distance, lowering carrier

frequency, installing dV/dt filters, using inverter duty rated motors (that are rated 600 V to withstand pulse

trains with rise time less than or equal to 0.1 microsecond, of 1,600 V peak magnitude), and installing LCR

low-pass sine wave filters. Regarding lowering of carrier frequency, note that audible noise is noticeably

increased for carrier frequencies less than about 6 kHz and is most noticeable at about 3 kHz. Note also

that selection of optimum PWM carrier frequency for AC drives involves balancing noise, heat, motor

insulation stress, common mode voltage induced motor bearing current damage, smooth motor operation,

and other factors. Further harmonics attenuation can be obtained by using an LCR low-pass sine wave filter

or dV/dt filter.

[edit]Motor bearing currents

Main article: Shaft voltage

PWM drives are inherently associated with high frequency common mode voltages and currents which may

cause trouble with motor bearings.[74] When these high frequency voltages find a path to earth through a

bearing, transfer of metal or electrical discharge machining (EDM) sparking occurs between the bearing's

ball and the bearing's race. Over time EDM-based sparking causes erosion in the bearing race that can be

seen as a fluting pattern. In large motors, the stray capacitance of the windings provides paths for high

frequency currents that pass through the motor shaft ends, leading to a circulating type of bearing current.

Poor grounding of motor stators can lead to shaft ground bearing currents. Small motors with poorly

grounded driven equipment are susceptible to high frequency bearing currents.

Prevention of high frequency bearing current damage uses three approaches: good cabling and grounding

practices, interruption of bearing currents, and filtering or damping of common mode currents. Good cabling

and grounding practices can include use of shielded, symmetrical-geometry power cable to supply the

motor, installation of shaft grounding brushes, and conductive bearing grease. Bearing currents can be

interrupted by installation of insulated bearings and specially designed electrostatic shielded induction

motors. Filtering and damping high frequency bearing, or, instead of using standard 2-level inverter drives,

using either 3-level inverter drives or matrix converters.

Since inverter-fed motor cables' high frequency current spikes can interfere with other cabling in facilities,

such inverter-fed motor cables should not only be of shielded, symmetrical-geometry design but should also

be routed at least 50 cm away from signal cables.

Dynamic braking

See also: Dynamic braking and Regenerative braking

Torque generated by the drive causes the induction motor to run at synchronous speed less the slip. If load

inertia energy is greater that the energy delivered to the motor shaft, motor speed decreases as negative

torque is developed in the motor and the motor acts as a generator, converting output shaft mechanical

power back to electrical energy. This power is returned to the drive's DC link element (capacitor or reactor).

A DC-link-connected electronic power switch or braking DC chopper (either built-in or external to the drive)

transfers this energy to external resistors to dissipate the energy as heat. Cooling fans may be used to

prevent resistor overheating. Dynamic braking wastes braking energy by transforming it to heat. By

contrast, regenerative drives recover braking energy by injecting this energy on the AC line. The capital cost

of regenerative drives is however relatively high.

Regenerative drives

Line regenerative variable frequency drives, showing capacitors (top cylinders) and inductors attached, which filter the

regenerated power.

Simplified Drive Schematic for a Popular EHV[80]

Regenerative AC drives have the capacity to recover the braking energy of a load moving faster than the

designated motor speed (an overhauling load) and return it to the power system.

Cycloconverter, Scherbius, matrix, CSI and LCI drives inherently allow return of energy from the load to the

line, while voltage-source inverters require an additional converter to return energy to the supply.[81][82]

Regeneration is only useful in variable-frequency drives where the value of the recovered energy is large

compared to the extra cost of a regenerative system,[81] and if the system requires frequent braking and

starting. Regenerative variable-frequency drives are widely used where speed control of overhauling loads

is required.

Some examples:

Conveyor belt drives for manufacturing, which stop every few minutes. While stopped, parts are

assembled correctly; once that is done, the belt moves on.

A crane, where the hoist motor stops and reverses frequently, and braking is required to slow the load

during lowering.

Plug-in and hybrid electric vehicles of all types (see image and Hybrid Synergy Drive).

Motor electric

Motorul de inducţie trifazat este cel mai răspândit motor electric

Un motor electric (sau electromotor) este un dispozitiv ce transformă energia electrică în energie mecanică. Transformarea inversă, a energiei mecanice în energie electrică, este realizată de un generator electric. Nu există diferenţe de principiu semnificative între cele două tipuri de maşini electrice, acelaşi dispozitiv putând îndeplini ambele roluri în situaţii diferite.

Principiul de funcţionare

Majoritatea motoarelor electrice funcţionează pe baza forţelor electromagnetice ce acţionează asupra unui conductor parcurs de curent electric aflat în câmp magnetic. Există însă şi motoare electrostatice construite pe baza forţei Coulomb şi motoare piezoelectrice.

Utilizare

Fiind construite într-o gamă extinsă de puteri, motoarele electrice sunt folosite la foarte multe aplicaţii: de la motoare pentru componente electronice (hard disc, imprimantă) până la acţionări electrice de puteri foarte mari (pompe, locomotive, macarale).

Clasificare

Motoarele electrice pot fi clasificate după tipul curentului electric ce le parcurge: motoare de curent continuu şi motoare de curent alternativ. În funcţie de numărul fazelor în care funcţionează motoarele electrice pot fi motoare monofazate sau motoare polifazate.

Motoare de curent continuu

În funcţie de tipul de excitaţie se împart în trei categorii:

Motoare derivaţie

Motoare serie

Motoare mixte

Motoare de curent alternativ

Motoare asincrone

Motoare cu inele de contact ( rotorul bobinat)

Motoare cu rotorul în scurtcircuit

Motoare de tipuri speciale

Motoare cu bare înalte

Motoare cu dublă colivie Dolivo-Dobrovolski

Motoare sincrone

Elemente constructive

Indiferent de tipul motorului, acesta este construit din două părţi componente: stator şi rotor. Statorul este partea fixă a motorului, în general exterioară, ce include carcasa, bornele de alimentare, armătura feromagnetică statorică şi înfăşurarea statorică. Rotorul este partea mobilă a motorului, plasată de obicei în interior. Este format dintr-un ax şi o armătură rotorică ce susţine înfăşurarea rotorică. Între stator şi rotor există o porţiune de aer numită întrefier ce permite mişcarea rotorului faţă de stator. Grosimea întrefierului este un indicator important al performanţelor motorului.

Motorul de curent continuu

Motorul de curent continuu a fost inventat în 1873 de Zénobe Gramme prin conectarea unui generator de curent continuu la un generator asemănător. Astfel, a putut observa că maşina se roteşte, realizând conversia energiei electrice absorbite de la generator.

Motorul de curent continuu are pe stator polii magnetici şi bobinele polare concentrate care creează câmpul magnetic de excitaţie. Pe axul motorului este situat un colector ce schimbă sensul curentului prin înfăşurarea rotorică astfel încât câmpul magnetic de excitaţie să exercite în permanenţă o forţă faţă de rotor.

În funcţie de modul de conectare a înfăşurării de excitaţie motoarele de curent continuu pot fi clasificate în:

motor cu excitaţie independentă - unde înfăşurarea statorică şi înfăşurarea rotorică sunt conectate la două surse separate de tensiune

motor cu excitaţie paralelă - unde înfăşurarea statorică şi înfăşurarea rotorică sunt legate în paralel la aceaşi sursă de tensiune

motor cu excitaţie serie - unde înfăşurarea statorică şi înfăşurarea rotorică sunt legate în serie

motor cu excitaţie mixtă - unde înfăşurarea statorică este divizată în două înfăşurări, una conectată în paralel şi una conectată în serie.

Înfăşurarea rotorică parcursă de curent va avea una sau mai multe perechi de poli magnetici echivalenţi. Rotorul se deplasează în câmpul magnetic de excitaţie până când polii rotorici se aliniază în dreptul polilor statorici opuşi. În acelaşi moment, colectorul schimbă sensul curenţilor rotorici astfel încât polaritatea rotorului se inversează şi rotorul va continua deplasarea până la următoarea aliniere a polilor magnetici.

Pentru acţionări electrice de puteri mici şi medii, sau pentru acţionări ce nu necesită câmp magnetic de excitaţie variabil, în locul înfăşurărilor statorice se folosesc magneţi permanenţi.

Turaţia motorului este proporţională cu tensiunea aplicată înfăşurării rotorice şi invers proporţională cu câmpul magnetic de excitaţie. Turaţia se reglează prin varierea tensiunii aplicată motorului până la valoarea nominală a tensiunii, iar turaţii mai mari se obţin prin slăbirea câmpului de excitaţie. Ambele metode vizează o tensiune variabilă ce poate fi obţinută folosind un generator de curent continuu (grup Ward-Leonard), prin înserierea unor rezistoare în circuit sau cu ajutorul electronicii de putere (redresoare comandate, choppere).

Motor universal folosit la râşniţele de cafea

Cuplul dezvoltat de motor este direct proporţional cu curentul electric prin rotor şi cu câmpul magnetic de excitaţie. Reglarea turaţiei prin slăbire de câmp se face, aşadar, cu diminuare a cuplului dezvoltat de motor. La motoarele serie acelaşi curent străbate înfăşurarea de excitaţie şi înfăşurarea rotorică. Din această consideraţie se pot deduce două caracteristici ale motoarelor serie: pentru încărcări reduse ale motorului, cuplul acestuia depinde de pătratul curentului electric absorbit; motorul nu trebuie lăsat să funcţioneze în gol pentru că în acest caz valoarea intensităţii curentului electric absorbit este foarte redusă şi implicit câmpul de excitaţie este redus, ceea ce duce la ambalarea maşinii până la autodistrugere. Motoarele de curent continuu cu excitaţie serie se folosesc în tracţiunea electrică urbană şi feroviară (tramvaie, locomotive).

Schimbarea sensului de rotaţie se face fie prin schimbarea polarităţii tensiunii de alimentare, fie prin schimbarea sensului câmpului magnetic de excitaţie. La motorul serie, prin schimbarea polarităţii tensiunii de alimentare se realizează schimbarea sensului ambelor mărimi şi sensul de rotaţie rămâne neschimbat. Aşadar, motorul serie poate fi folosit şi la

tensiune alternativă, unde polaritatea tensiunii se inversează o dată în decursul unei perioade. Un astfel de motor se numeşte motor universal şi se foloseşte în aplicaţii casnice de puteri mici şi viteze mari de rotaţie (aspirator, mixer).

Motorul de curent alternativ

Motoarele de curent alternativ funcţionează pe baza principiului câmpului magnetic învârtitor. Acest principiu a fost identificat de Nikola Tesla în 1882. În anul următor a proiectat un motor de inducţie bifazat, punând bazele maşinilor electrice ce funcţionează pe baza câmpului magnetic învârtitor. Ulterior, sisteme de transmisie prin curent alternativ au fost folosite la generarea şi transmisia eficientă la distanţă a energiei electrice, marcând cea de-a doua Revoluţie industrială. Un alt punct important în istoria motorului de curent alternativ a fost inventarea de către Michael von Dolivo-Dobrowlsky în anul 1890 a rotorului în colivie de veveriţă.

Motorul de inducţie trifazat

Motorul de inducţie trifazat (sau motorul asincron trifazat) este cel mai folosit motor electric în acţionările electrice de puteri medii şi mari. Statorul motorului de inducţie este format din armătura feromagnetică statorică pe care este plasată înfăşurarea trifazată statorică necesară producerii câmpului magnetic învârtitor. Rotorul este format din armătura feromagnetică rotorică în care este plasată înfăşurarea rotorică. După tipul înfăşurării rotorice, rotoarele pot fi de tipul:

rotor în colivie de veveriţă (în scurtcircuit) - înfăşurarea rotorică este realizată din bare de aluminiu sau -mai rar- cupru scurtcircuitate la capete de două inele transversale.

rotor bobinat - capetele înfăşurării trifazate plasate în rotor sunt conectate prin interiorul axului la 3 inele. Accesul la inele dinspre cutia cu borne se face prin intermediul a 3 perii.

Prin intermediul inducţiei electromagnetice câmpul magnetic învârtitor va induce în înfăşurarea rotorică o tensiune. Această tensiune creează un curent electric prin înfăşurare şi asupra acestei înfăşurări acţionează o forţă electromagnetică ce pune rotorul în mişcare în sensul câmpului magnetic învârtitor. Motorul se numeşte asincron pentru că turaţia rotorului este întotdeauna mai mică decât turaţia câmpului magnetic învârtitor, denumită şi turaţie de sincronism. Dacă turaţia rotorului ar fi egală cu turaţia de sincronism atunci nu ar mai avea loc fenomenul de inducţie electromagnetică, nu s-ar mai induce curenţi în rotor şi motorul nu ar mai dezvolta cuplu.

Turaţia motorului se calculează în funcţie alunecarea rotorului faţă de turaţia de sincronism, care este cunoscută, fiind determinată de sistemul trifazat de curenţi.

Alunecarea este egală cu: , unde

n1 este turaţia de sincronism şin2 este turaţia rotorului.

, undef este frecvenţa tensiunii de alimentare şip este numărul de perechi de poli ai înfăşurării statorice.

Turaţia maşinii, în funcţie de turaţia câmpului magnetic învârtitor şi în funcţie de alunecare

este: .

Se observă că alunecarea este aproape nulă la mers în gol (când turaţia motorului este aproape egală cu turaţia câmpului magnetic învârtitor) şi este egală cu 1 la pornire, sau când rotorul este blocat. Cu cât alunecarea este mai mare cu atât curenţii induşi în rotor sunt mai intenşi. Curentul absorbit la pornirea prin conectare directă a unui motor de inducţie de putere medie sau mare poate avea o valoare comparabilă cu curentul de avarie al sistemelor de protecţie, în acest caz sistemul de protecţie deconectează motorul de la reţea. Limitarea curentului de pornire al motorului se face prin creşterea rezistenţei înfăşurării rotorice sau prin diminuarea tensiunii aplicate motorului. Creşterea rezitenţei rotorului se face prin montarea unui reostat la bornele rotorului (doar pentru motoarele cu rotor bobinat). Reducerea tensiunii aplicate se face folosind un autotransformator, folosind un variator de tensiune alternativă (pornirea lină) sau conectând iniţial înfăşurarea statorică în conexiune stea (pornirea stea-triungi - se foloseşte doar pentru motoarele destinate să funcţioneze în conexiune triunghi) sau prin înserierea de rezistoare la înfăşurarea statorică. La reducerea tensiunii de alimentare trebuie avut în vedere că cuplul motorului este proporţional cu pătratul tensiunii, deci pentru valori prea mici ale tensiunii de alimentare maşina nu poate porni.

Turaţia maşinii de inducţie se modifică prin modificarea alunecării sale sau prin modificarea turaţiei câmpului magnetic învârtitor. Alunecarea se poate modifica din tensiunea de alimentare şi din rezistenţa înfăşurării rotorice astfel: se creşte rezistenţa rotorică (prin folosirea unui reostat la bornele rotorice - doar la motoarele cu rotor bobinat) şi se variază tensiunea de alimentare (folosind autotransformatoare, variatoare de tensiune alternativă, cicloconvertoare) sau se menţine tensiunea de alimentare şi se variază rezistenţa din rotor (printr-un reostat variabil). Odată cu creşterea rezistenţei rotorice cresc şi pierderile din rotor şi implicit scade randamentul motorului. O metodă interesantă de reglare a turaţiei sunt cascadele de recuperare a puterii de alunecare. La bornele rotorice este conectat un redresor, iar la bornele acestuia este conectat un motor de curent continuu aflat pe acelaşi ax cu motorul de inducţie (cascadă Krämmer cu recuperare puterii de alunecare pe cale mecanică). Tensiunea indusă în rotor este astfel redresată şi aplicată motorului de curent continuu astfel încât cuplul dezvoltat de motorul de curent continuu se însumează cuplului dezvoltat de motorul de inducţie. Reglarea turaţiei motorului de inducţie se face prin reglarea curentului prin înfăşurarea de excitaţie. În locul motorului de curent continuu se poate folosi un invertor cu tiristoare şi un transformator de adaptare (cascadă Krämmer cu recuperare puterii de alunecare pe cale electrică). Tensiunea indusă în rotor este astfel redresată şi prin intermediul invertorului şi a transformatorului este reintrodusă în reţea. Reglarea vitezei se face din unghiul de aprindere al tiristoarelor.

Turaţia câmpului magnetic învârtitor se poate modifica din frecvenţa tensiunii de alimentare şi din numărul de perechi de poli ai maşinii. Numărul de perechi de poli se modifică folosind o înfăşurare specială (înfăşurarea Dahlander) şi unul sau mai multe contactoare. Frecvenţa de alimentare se modifică folosind invertoare. Pentru frecvenţe mai mici decât frecvenţa nominală a motorului (50 Hz pentru Europa, 60 Hz pentru America de Nord) odată cu

modificarea frecvenţei se modifică şi tensiunea de alimentare păstrând raportul U/f constant. Pentru frecvenţe mai mari decât frecvenţa nominală la creşterea frecvenţei tensiunea de alimentare rămâne constantă şi reglarea vitezei se face cu slăbire de câmp (ca la motorul de curent continuu).

Sensul de rotaţie al motorului de inducţie se inversează schimbând sensul de rotaţie al câmpului învârtitor. Aceasta se realizează schimbând două faze între ele.

Motorul de inducţie cu rotorul în colivie este mai ieftin şi mai fiabil decât motorul de inducţie cu rotorul bobinat pentru că periile acestuia se uzează şi necesită întreţinere. De asemenea, motorul de inducţie cu rotorul in colivie nu are colector şi toate dezavantajele care vin cu acesta: zgomot, scântei, poluare electromagnetică, fiabilitate redusă şi implicit întreţinere costisitoare. Motoarele de curent continuu au fost folosite de-a lungul timpului în acţionările electrice de viteză variabilă, deoarece turaţia motorului se poate modifica foarte uşor modificând tensiunea de alimentare însă, odată cu dezvoltarea electronicii de putere şi în special cu dezvoltarea surselor de tensiune cu frecvenţă variabilă, tendinţa este de înlocuire a motoarelor de curent continuu cu motoare de inducţie cu rotor în colivie.

Motorul de inducţie monofazat

În cazul în care sistemul trifazat de tensiuni nu este accesibil, cum este în aplicaţiile casnice, se poate folosi un motor de inducţie monofazat. Curentul electric monofazat nu poate produce câmp magnetic învârtitor ci produce câmp magnetic pulsatoriu (fix în spaţiu şi variabil în timp). Câmpul magnetic pulsatoriu nu poate porni rotorul, însă dacă acesta se roteşte într-un sens, atunci asupra lui va acţiona un cuplu în sensul său de rotaţie. Problema principală o constituie deci, obţinerea unui câmp magnetic învârtitor la pornirea motorului şi aceasta se realizează în mai multe moduri.

Prin ataşarea pe statorul maşinii la un unghi de 90° a unei faze auxiliare înseriată cu un condensator se poate obţine un sistem bifazat de curenţi ce produce un câmp magnetic învârtitor. După pornirea motorului se deconectează faza auxiliară printr-un întrerupător centrifugal. Sensul de rotaţie al motorului se poate schimba prin mutarea condensatorului din faza auxiliară în faza principală.

În locul fazei auxiliare se poate folosi o spiră în scurtcircuit plasată pe o parte din polul statoric pentru obţinerea câmpului învârtitor. Curentul electric indus în spiră se va opune schimbării fluxului magnetic din înfăşurare, astfel încât amplitudinea câmpului magnetic se deplasează pe suprafaţa polului creând câmpul magnetic învârtitor.

Servomotorul asincron monofazat

Servomotorul asincron monofazat este o maşină de inducţie cu două înfăşurări: o înfăşurare de comandă şi o înfăşurare de excitaţie. Cele două înfăşurări sunt aşezate la un unghi de 90° una faţă de cealaltă pentru a crea un câmp magnetic învârtitor. Rezistenţa rotorului este foarte mare pentru a realiza autofrânarea motorului la anularea tensiunii de pe înfăşurarea de comandă. Datorită rezistenţei rotorice mari, randamentul motorului este scăzut şi motorul se foloseşte în acţionări electrice de puteri mici şi foarte mici.

Motorul sincron trifazat

Motorul sincron trifazat este o maşină electrică la care turaţia rotorului este egală cu turaţia câmpului magnetic învârtitor indiferent de încărcarea motorului. Motoarele sincrone se folosesc la acţionări electrice de puteri mari şi foarte mari de până la zeci de MW.

Statorul motorului sincron este asemănător cu statorul motorului de inducţie (este format dintr-o armătură feromagnetică statorică şi o înfăşurare trifazată statorică). Rotorul motorului sincron este format dintr-o armătură feromagnetică rotorică şi o înfăşurare rotorică de curent continuu. Pot exista două tipuri constructive de rotoare: cu poli înecaţi şi cu poli aparenţi. Rotorul cu poli înecaţi are armătura feromagnetică crestată spre exterior şi în crestătură este plasată înfăşurarea rotorică. Acest tip de motor are uzual o pereche de poli şi funcţionează la turaţii mari (3000 rpm la 50 Hz). Rotorul cu poli aparenţi are armătura feromagentică sub forma unui butuc poligonal pe care sunt plasate miezurile polilor rotorici şi bobine polare concentrate. În unele situaţii în locul bobinelor polare concentrate se pot folosi magneţi permanenţi. Motorul sincron cu poli aparenţi are un număr mare de poli şi funcţionează la turaţii mai reduse. Accesul la înfăşurarea rotorică se face printr-un sistem inel-perie asemănător motorului de inducţie. Motoarele sincrone cu poli aparenţi pot avea cuplu chiar şi în lipsa curentului de excitaţie, motorul reactiv fiind cel ce funcţionează pe baza acestui cuplu, fără înfăşurare de excitaţie şi fără magneţi permanenţi.

Înfăşurarea rotorică (de excitaţie) a motorului parcursă de curent continuu creează un câmp magnetic fix faţă de rotor. Acest câmp „se lipeşte” de câmpul magnetic învârtitor statoric şi rotorul se roteşte sincron cu acesta. Datorită inerţiei, câmpul magnetic rotoric nu are timp să se lipească de câmpul magnetic învârtitor şi motorul sincron nu poate porni prin conectare directă la reţea. Există trei metode principale de pornire a motoarelor sincrone:

pornirea în asincron - pe tălpile polare rotorice este prevăzută o colivie asemănătoare coliviei motorului de inducţie şi motorul porneşte pe acelaşi principiu ca al motorului de inducţie.

pornirea la frecvenţă variabilă - este posibilă doar atunci când este disponibilă o sursă de tensiune cu frecvenţă variabilă sau un convertor cu frecvenţă variabilă. Creşterea frecvenţei se face lent, astfel încât câmpul învârtitor să aibă viteze suficient de mici la început pentru a putea permite rotorului să se „lipească” de câmpul magnetic învârtitor.

pornirea cu motor auxiliar - necesită un motor auxiliar ce antrenează motorul sincron conectat la reţea. Când motorul ajunge la o turaţie apropiată de turaţia de sincronism motorul auxiliar este decuplat, motorul sincron se mai accelerează puţin până ajunge la turaţia de sincronism şi continuă să se rotească sincron cu câmpul magnetic învârtitor.

Motorul sincron monofazat

Este realizat uzual ca motor sincron reactiv cu sau fără magneţi permanenţi pe rotor. Asemănător motoarelor de inducţie monofazate, motoarele sincrone monofazate necesită un câmp magnetic învârtitor ce poate fi obţinut fie folosind o fază auxiliară şi condensator fie folosind spiră în scurtcircuit pe polii statorici. Se folosesc în general în acţionări electrice de puteri mici precum sistemele de înregistrare şi redare a sunetului şi imaginii.

Motorul pas cu pas

Motorul pas cu pas este un tip de motor sincron cu poli aparenţi pe ambele armături. La apariţia unui semnal de comandă pe unul din polii statorici rotorul se va deplasa până când

polii săi se vor alinia în dreptul polilor opuşi statorici. Rotirea acestui tip de rotor se va face practic din pol în pol, de unde şi denumirea sa de motor pas cu pas. Comanda motorului se face electronic şi se pot obţine deplasări ale motorului bine cunoscute în funcţie de programul de comandă. Motoarele pas cu pas se folosesc acolo unde este necesară precizie ridicată (hard disc, copiatoare).