hampson, j, hanssen, s 2013, electrical trade principles: section_7-3

PowerPoint to accompany:

Section 7Alternating current rotating machines

7.3 Starter operationsand applications

Copyright © 2013 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442545960/Hampson/Electrical Trade Principles /3e 2

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Copyright © 2013 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442545960/Hampson/Electrical Trade Principles /3e

Topic outcomes

3


Topic outcomes

•State the operating characteristics and applications for various types of motor starters

3


Topic outcomes

•State the operating characteristics and applications for various types of motor starters

•Interpret motor starter wiring diagrams

3


4

Motor starters


A motor starter is a device designed to supply a motor with sufficient current to establish the required starting torque and to accelerate the motor to rated-load speed.

4

Motor starters


A motor starter is a device designed to supply a motor with sufficient current to establish the required starting torque and to accelerate the motor to rated-load speed.

It is the starting current that poses difficulties for the energy circuits the motor is connected across, while the developed torque at starting can cause problems with the mechanical coupling devices (chain drives, gearboxes, conveyor belts) between the motor and the load.

4

Motor starters


5

Starting current


When a three-phase motor is started direct on line (DOL) the current drawn by the motor can be up to six times its full-load current rating. Current of this magnitude can cause a voltage drop across the supply conductors, resulting in a voltage dip. This voltage drop of the supply can affect the operation of other equipment and machines that are connected across the supply in the rest of the installation.

5

Starting current


When a three-phase motor is started direct on line (DOL) the current drawn by the motor can be up to six times its full-load current rating. Current of this magnitude can cause a voltage drop across the supply conductors, resulting in a voltage dip. This voltage drop of the supply can affect the operation of other equipment and machines that are connected across the supply in the rest of the installation.

If a motor is of a significant size the voltage disturbance at starting can affect other consumers of energy that are also connected across the supply company’s energy product. A sudden large increase in current required from a source (distribution transformer) will cause a larger voltage to be developed across the source’s impedance. This will result in a reduction in voltage as seen by the load.

5

Starting current


6

Torque


The torque developed by a motor at start-up can vary from approximately 110% to 280% of its full-load value depending upon the design parameters of the motor. The effects of full-voltage starting torque can damage equipment. This means that the motor characteristics must be matched to the starting and accelerating torque requirements of the driven load.

6

Torque


The torque developed by a motor at start-up can vary from approximately 110% to 280% of its full-load value depending upon the design parameters of the motor. The effects of full-voltage starting torque can damage equipment. This means that the motor characteristics must be matched to the starting and accelerating torque requirements of the driven load.

Once the motor has been chosen for the load requirements a starter must be selected. The starter chosen must enable the motor to develop the starting torque required to turn the rotor under load conditions. Most starters reduce the motor’s starting current drawn from the supply.

6

Torque


Motor starters

A motor starter is a device designed to supply a motor with sufficient current to establish the required starting torque and to accelerate the motor to rated full-load speed.

Page 379

7


8

Motor starter circuits


The most efficient and also the most economical method of starting a three-phase motor is by a DOL starter. However, there are restrictions placed by supply authorities on DOL starting. There are two types of DOL starters available—the manually operated and the contactor controlled.

8

Motor starter circuits


9

Motor starter circuits – simple DOL


The simple DOL starter uses a circuit breaker on/ off switch with a thermal overload trip protection. Closing the circuit breaker places the motor directly on line. The thermal (bimetallic) elements of the circuit breaker trip the breaker whenever the current in one of the phases exceeds the designed starting and full-load current (FLC) for a predetermined length of time.

9




This type of starting can be applied to motors up to 7.5 kW output which have intermittent starting requirements or a full-load current rating of less than 14 A and for which the mechanical equipment allows DOL starting.

9




This type of starting can be applied to motors up to 7.5 kW output which have intermittent starting requirements or a full-load current rating of less than 14 A and for which the mechanical equipment allows DOL starting.

All DOL starters should be capable of 15 starts per hour except where plugging duty is required. A recommended duty cycle for a DOL plugging starter is 40 starts per hour.

9




Direct-on-line (DOL) current can be up to 10 times the motor’s full-load current rating.

The effects of full-voltage starting torque can damage equipment.

Page 379

10


11

Motor starter circuits – contactor controlled DOL


In addition to the main circuit breaker, this type of starter includes a contactor and a thermal overload relay to provide motor protection. The main element of the starter is the control contactor, which is a set of silver cadmium oxide contacts operated by an electromagnetic coil.

11



In addition to the main circuit breaker, this type of starter includes a contactor and a thermal overload relay to provide motor protection. The main element of the starter is the control contactor, which is a set of silver cadmium oxide contacts operated by an electromagnetic coil.

The inclusion of the contactor allows for remote pushbutton starting and emergency stop capability. Contactors are rated for various operating voltages and are sized according to the kW rating of the motor and the load conditions.

11



Motor starter circuits – contactor DOL

This type of starter includes a contactor and a thermal overload relay to provide motor protection.

The main element of the starter is the control contactor, which is a set of silver cadmium oxide contacts operated by an electromagnetic coil.

Page 380

12


13

Motor starter circuits – star–delta


Star–delta starters belong to the group of reduced-voltage starters. This means that the voltage applied across the motor is reduced only during the starting period. This starter is limited to motors that have each end (six ends in total) of the three-pole phase groups of stator coils available at the motor terminals for connection to the starter.

13




The reduced voltage to the motor at starting is achieved by the starter connecting the motor leads in star configuration and placing the motor on line. Upon reaching approximately 80% to 90% of its full-load speed the motor leads are then connected in delta configuration for full-load motor running conditions.

13




The reduced voltage to the motor at starting is achieved by the starter connecting the motor leads in star configuration and placing the motor on line. Upon reaching approximately 80% to 90% of its full-load speed the motor leads are then connected in delta configuration for full-load motor running conditions.

If the rotor speed is less than 80% the connection of the delta sequence introduces a large phase displacement between the stator and rotor fluxes.

13




Star–delta and autotransformer starters belong to the group of reduced-voltage starters.

The voltage applied across the motor by reduced voltage starters is reduced only during the starting period.

Page 381

14


15

Motor starter circuits – autotransformer


This type of starter uses transformer tappings to reduce the voltage, current and torque of the motor at starting. The autotransformer energises the motor with stepped voltage control until full operating voltage is placed across the motor terminals.

15




These starters can be utilised with either star-or delta-connected motors as only three leads from the motor are required. They find application in situations where a star–delta starter cannot deliver the necessary starting torque required by the load.

15




These starters can be utilised with either star-or delta-connected motors as only three leads from the motor are required. They find application in situations where a star–delta starter cannot deliver the necessary starting torque required by the load.

Several tappings are provided on the three autotransformers (primary and secondary windings are common) that are connected in star configuration. However, some autotransformers utilise two autotransformers connected in an open delta or ‘V-configuration’.

15





Page 382

16


17

Motor starter circuits – primary resistance


Primary resistance starters are classified as closed transition starters as the motor is not disconnected from the supply during the starting sequence. These starters incorporate a resistor bank that is connected in series with the stator windings. The resistor bank limits the initial current surge drawn by the motor at starting. Once the motor draws current a voltage drop occurs across the resistor bank resulting in a reduced voltage across the motor terminals.

17



Primary resistance starters are classified as closed transition starters as the motor is not disconnected from the supply during the starting sequence. These starters incorporate a resistor bank that is connected in series with the stator windings. The resistor bank limits the initial current surge drawn by the motor at starting. Once the motor draws current a voltage drop occurs across the resistor bank resulting in a reduced voltage across the motor terminals.

As motor speed increases, the back emf induced in the stator windings opposes the applied voltage and further reduces the starting current. While the voltage across the resistor bank decreases, the voltage across the motor terminals increases. This increase in voltage causes the motor torque also to increase.

17




Primary resistance starters are classified as closed transition starters as the motor is not disconnected from the supply during the starting sequence.

Page 383

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19

Motor starter circuits – electronic


Electronic starters are often referred to as stepless, reduced-voltage starters. Solid-state technology is employed to provide a method of starting without the initial current, transitional and torque surges that are a consequence of electromechanical starters. This fact translates into less electrical and mechanical stress on the complete motor system with corresponding longer service life obtained.

19



Electronic starters are often referred to as stepless, reduced-voltage starters. Solid-state technology is employed to provide a method of starting without the initial current, transitional and torque surges that are a consequence of electromechanical starters. This fact translates into less electrical and mechanical stress on the complete motor system with corresponding longer service life obtained.

Power semiconductors such as the thyristor silicon-controlled rectifier (SCR) controlled by electronics provide the means by which the voltage and current drawn by the motor can be controlled, thereby providing a very smooth stress-free start (often called ‘ramp-up’).

19




Electronic starters are often referred to as stepless, reduced-voltage starters.

Page 384

20


21

Motor starter circuits – secondary resistance


The name ‘secondary resistance starter’ refers to the fact that resistance is added to the rotor or secondary circuit of the motor. Only slip-ring or wound-rotor induction motors have the rotor design necessary for this type of motor starting. Slip-ring or wound-rotor induction motors have very different performance characteristics to that of squirrel-cage motors even though the operating principle is the same.

21



The name ‘secondary resistance starter’ refers to the fact that resistance is added to the rotor or secondary circuit of the motor. Only slip-ring or wound-rotor induction motors have the rotor design necessary for this type of motor starting. Slip-ring or wound-rotor induction motors have very different performance characteristics to that of squirrel-cage motors even though the operating principle is the same.

The maximum torque characteristic of squirrel-cage motors cannot be altered as it depends upon rotor design and usually occurs at about 80% of rotor speed. The wound-rotor motor can have its rotor characteristic altered by inserting variable external resistance (metal or liquid) via slip rings in series with the rotor winding. This allows the maximum developed torque to be realised at any point between zero and 80% of full-load speed.

21




Secondary resistance starters add resistance to the rotor or secondary circuit.

Only slip-ring or wound-rotor induction motors have the rotor design necessary for secondary resistance motor starting.

Page 385

22


Most three-phase motor starters reduce the:

rotor inductance

starting current

stator resistance

supply voltage

Pop quiz


24

Torque-speed relationship


Torque is proportional to the square of the motorterminal voltage.

24




An induction motor will only increase the speed of the applied load when it produces more torque than the load can contain.

24





Full-load torque is calculated by:

24



= 60P 2πn 60P 60P 2πn 2πn 60P 2πn Torque is proportional to the square of the motorterminal voltage.



24






T = torque in newton-metres (Nm)

24







P = power output in watts (W)

24







P = power output in watts (W)

n = rotational speed in revolutions per minute (rpm)

24


Page 386


25

Torque – speed relationships


Torque/Speed Curve – Squirrel-Cage Rotors.

25




25


Speed (rpm)

Torq

ue (

%)

0

100

Ratedspeed

Ns

Rated Torque



25


Speed (rpm)

Torq

ue (

%)

0

100

Ratedspeed

Ns

Rated Torque

Slip{



25


Speed (rpm)

Torq

ue (

%)

0

100

Ratedspeed

Ns

Rated Torque

Breakdowntorque

Slip{



25


Speed (rpm)

Torq

ue (

%)

0

100

Ratedspeed

Ns

Rated Torque

Locked rotor torque

Breakdowntorque

Slip{



26


Speed (rpm)To

rque (

%)

0

100

Ratedspeed

Ns

Rated Torque

Locked rotor torque

Breakdowntorque

Slip

{



26


For small values of slip, torque is assumed to be proportional to the slip.

Speed (rpm)To

rque (

%)

0

100

Ratedspeed

Ns

Rated Torque

Locked rotor torque

Breakdowntorque

Slip

{



26



As load increases, torque increases and speed decreases until torque reaches breakdown torque (max value).

Speed (rpm)To

rque (

%)

0

100

Ratedspeed

Ns

Rated Torque

Locked rotor torque

Breakdowntorque

Slip

{



26




If the motor is loaded beyond this point, torque and speed decrease to stand still point.

Speed (rpm)To

rque (

%)

0

100

Ratedspeed

Ns

Rated Torque

Locked rotor torque

Breakdowntorque

Slip

{



26





Starting torque is typically 1.5 times rated torque.

Speed (rpm)To

rque (

%)

0

100

Ratedspeed

Ns

Rated Torque

Locked rotor torque

Breakdowntorque

Slip

{



26





Starting torque is typically 1.5 times rated torque.

Breakdown torque is typically 2 times rated torque.

Speed (rpm)To

rque (

%)

0

100

Ratedspeed

Ns

Rated Torque

Locked rotor torque

Breakdowntorque

Slip

{



27


Speed (rpm)To

rque (

%)

0

100

Ratedspeed

Ns

Rated Torque

Locked rotor torque

Breakdowntorque

Slip

{



27


The resistance of the rotor conductors remains constant.

Speed (rpm)To

rque (

%)

0

100

Ratedspeed

Ns

Rated Torque

Locked rotor torque

Breakdowntorque

Slip

{



27



Inductive reactance XL decreases as the rotor speed increases.

Speed (rpm)To

rque (

%)

0

100

Ratedspeed

Ns

Rated Torque

Locked rotor torque

Breakdowntorque

Slip

{



27




Torque reaches maximum when the rotor resistance in ohms is equal to the rotor reactance in ohms.

Speed (rpm)To

rque (

%)

0

100

Ratedspeed

Ns

Rated Torque

Locked rotor torque

Breakdowntorque

Slip

{



27




Torque reaches maximum when the rotor resistance in ohms is equal to the rotor reactance in ohms.

Since resistance is fixed, the breakdown torque can only be altered by altering the inductance of the rotor. Speed (rpm)

Torq

ue (

%)

0

100

Ratedspeed

Ns

Rated Torque

Locked rotor torque

Breakdowntorque

Slip

{


28

Torque/speed curve – squirrel-cage rotors


For low slip, torque is assumed to be proportional to slip.

28





28





If motor is loaded beyond this point, motor will stall.

28






Starting torque – typically 1.5 times rated torque.

28







Breakdown torque – typically 2 times rated torque.

28








Resistance of rotor conductors remains constant.

28









Inductive reactance (XL) decreases as rotor speed increases.

28










Maximum torque when rotor resistance equals rotor reactance.

28










Maximum torque when rotor resistance equals rotor reactance.

Since resistance is fixed, breakdown torque can only be altered by altering rotor inductance.

28


Page 386


29

Torque – speed relationships for DOL


DOL starters are at full voltage across the line starter. This means that the torque developed at the instant of starting will be very high due to the fact that torque is directly proportional to the square of the voltage. The starting torque of motors using this method of starting for a standard resistance rotor is 150% of full-load torque.

29

Torque – speed relationships for DOL


30

Torque – speed relationships for star-delta


A guide that is used for the maximum starting time of a star–delta starter is the rating of the motor in kilowatts divided by three, plus nine in seconds.

30



A guide that is used for the maximum starting time of a star–delta starter is the rating of the motor in kilowatts divided by three, plus nine in seconds.

All contactor-type star–delta starters use a timing relay for switching from star to delta and the timer needs to be correctly set for each individual application. This type of starter provides very low torque (33% of DOL torque = 50% FLT according to the graph in Figure 7.49) at the instant of starting. It is therefore suitable for no-load or very light load starting applications (lathes, drilling machines).

30




Torque is directly proportional to the square of the voltage.

DOL starters are full voltage starters producing very high torque, which is about 150% of full-load torque for a standard resistance rotor.

Star–delta starter provides very low torque (33% of DOL) at the instant of starting making it suitable for no-load or very light load starting applications (lathes, drilling machines).

Page 386

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32

Torque – speed relationships for autotransformer


Three starting torques are available with the standard tappings of 80%, 65% and 50% of line voltage depending upon design requirements with respect to connected load. The tappings allow for easy adjustment of the starting torque and starting current to suit the load parameters. Starting torque is proportional to the square of the voltage so that the starting torque on these taps will be 64%, 42% and 25% respectively of the maximum DOL starting torque.

32



Three starting torques are available with the standard tappings of 80%, 65% and 50% of line voltage depending upon design requirements with respect to connected load. The tappings allow for easy adjustment of the starting torque and starting current to suit the load parameters. Starting torque is proportional to the square of the voltage so that the starting torque on these taps will be 64%, 42% and 25% respectively of the maximum DOL starting torque.

Autotransformer starters are suitable for heavy loads requiring a high starting torque. Applications include centrifugal pumps, conveyors and air-compressors.

32



33

Torque – speed relationships - primary resistance


The resistors in primary resistance starters are designed to allow approximately 60% of line voltage to be connected across the motor terminals at starting. Starting torque is proportional to the square of the voltage so that the starting torque that could be obtained with DOL starting developed at this voltage would be 36% DOL.

33



The resistors in primary resistance starters are designed to allow approximately 60% of line voltage to be connected across the motor terminals at starting. Starting torque is proportional to the square of the voltage so that the starting torque that could be obtained with DOL starting developed at this voltage would be 36% DOL.

These starters are for small kW rated motors with light starting requirements. The starter resistors are easily adjustable to suit the load and give smooth acceleration with a constantly increasing torque.

33




Autotransformer starters provide starting torques of 64%, 42% and 25% respectively of the maximum DOL starting torque with the standard tappings of 80%, 65% and 50%.

Autotransformer starters are suitable for heavy loads requiring a high starting torque. Applications include centrifugal pumps, conveyors and air-compressors.

The resistors in primary resistance starters are designed to allow approximately 60% of line voltage to be connected across the motor terminals at starting producing a starting torque of 36% DOL.

Primary resistance are for small kW rated motors with light starting requirements.

Page 387

34


35

Torque – speed relationships for electronic


This starter can be programmed to provide excellent stepless control of torque from zero to maximum speed depending upon the load requirements.

35

Torque – speed relationships for electronic


36

Torque – speed relationships - secondary resistor


The wound-rotor motor is able to develop a very high starting torque (up to 250% DOL value) from zero speed to full speed at a relatively low starting current. The starter has the capability of total control over the start characteristics of the wound-rotor motor by the selection of the rotor resistors.

36

Torque – speed relationships - secondary resistor



Electronic starters can be programmed to provide excellent stepless control of torque from zero to maximum speed depending upon the load requirements.

The wound-rotor motor is able to develop a very high starting torque (up to 250% DOL value) from zero speed to full speed at a relatively low starting current.

Page 388

37


Which of the following torque parameters designatesthe maximum torque the rotor is able to generate at rated voltage and frequency?

breakdown torque

pull-out torque

locked-motor torque

rated-load torque

Pop quiz


39

Speed control


The relative motion between the rotating field and the rotating rotor is called the slip and is given by:

39

Speed control


= 120 f P 120 f 120 f P P 120 f P The relative motion between the rotating field and the rotating rotor is called the slip and is given by:

39

Speed control




39

Speed control




f = line frequency in hertz (Hz)

39

Speed control




f = line frequency in hertz (Hz)

P = number of magnetic pole pairs

39

Speed control


40

Speed control


Since the rotor current is proportional to the relative motion between the rotating field and the rotor speed, the rotor current and hence the torque are both directly proportional to the slip.

40

Speed control



The rotor current is proportional to the rotor resistance. Increasing the rotor resistance will reduce the current and increase the slip; hence a form of speed and torque control is possible with wound-rotor motors.

40

Speed control



The rotor current is proportional to the rotor resistance. Increasing the rotor resistance will reduce the current and increase the slip; hence a form of speed and torque control is possible with wound-rotor motors.

This speed control technique is only useful over a range of 50% to 100% of full speed.

40

Speed control


41

Speed control – pole changing


With a squirrel-cage induction motor, the speed is dependent upon the number of poles. With a one-winding stator only one speed is possible.

41




However, by using a tapped winding the three-phase induction motor can have its speed varied by changing the stator winding pole connections to create a different pole configuration.

41




However, by using a tapped winding the three-phase induction motor can have its speed varied by changing the stator winding pole connections to create a different pole configuration.

Alternatively, another stator winding can be provided. Several speeds can be obtained by altering the number of poles.

41



42

Speed control – variable-speed drives


The speed regulation of a three-phase induction motor can also be achieved by varying the supply frequency. Four major variable-speed-drive designs are commonly used today: pulse width modulation (PWM), current source inverter (CSI) and voltage source inverter (VSI) and the flux vector drive (FVD).

42




These drives take the 50 Hz ac input voltage and frequency, covert it to dc using rectifiers, then convert it back to ac in an inverter which then pulses the output voltage for varying lengths of time to mimic an alternating current at the frequency desired.

42




These drives take the 50 Hz ac input voltage and frequency, covert it to dc using rectifiers, then convert it back to ac in an inverter which then pulses the output voltage for varying lengths of time to mimic an alternating current at the frequency desired.

By varying frequency, the speed can be adjusted over a wide range or to vary the speed precisely using exact changes in the electrical frequency input to the motor.

42



Speed control

The speed of a three-phase induction motor is determined by the number of poles.

The speed regulation of a three-phase induction motor can also be achieved by varying the supply frequency.

When wound-rotor motors are used for speed control they are used with resistance in the rotor circuit.

Page 389

43


With a squirrel-cage induction motor, the speed is dependent upon:

the supply voltage

stator reactance

the number of poles

power rating of the motor

Pop quiz


45

Braking


There are many types of braking systems that can be used with three-phase induction motors. Each of these types can be placed into one of the following categories:

45

Braking



• Internal braking

45

Braking



• Internal braking

• Exterior braking

45

Braking


46

Braking


Internal braking systems generate torque by converting the electric motor into a braking device. Internal brakes use electrical switch gear and electronic circuitry to perform the braking. Internal brakes can only be used for stopping; they are incapable of providing a holding function. The three methods of internal braking are:

46

Braking



• Plug braking

46

Braking



• Plug braking

• Regenerative braking

46

Braking



• Plug braking

• Regenerative braking

• Dynamic braking

46

Braking


47

Braking


Exterior braking requires the addition of a braking device connected to the shaft of the three-phase induction motor. Exterior braking devices generate a braking and/or holding torque external to the motor. External braking devices include:

47

Braking



• Friction brakes

47

Braking



• Friction brakes

• Eddy-current brakes

47

Braking



• Friction brakes


• Hysteresis brakes

47

Braking



• Friction brakes



• Magnetic-particle brakes

47

Braking



• Friction brakes



• Magnetic-particle brakes

Braking is essentially the removal of stored kinetic energy in the form of motion from a motor– mechanical load.

47

Braking


48

Braking – electromechanical


Electromechanical actuated adhesion brakes convert the kinetic energy into heat energy by pushing friction material via solenoid action against rotating or moving parts to stop the rotor. Therefore, without friction-generating components such as the drum, disc and shoe-lining material the rotor cannot be stopped.

48




The armature of the electromagnet opposes the force of a spring when voltage is applied to the solenoid. This releases the brake and lets the rotor shaft freely rotate. When power is removed, the spring energises the brake and holds the load.

48




The armature of the electromagnet opposes the force of a spring when voltage is applied to the solenoid. This releases the brake and lets the rotor shaft freely rotate. When power is removed, the spring energises the brake and holds the load.

Many hoisting systems rely solely upon electromechanical brakes to stop the rotor movement when an emergency condition such as loss of power occurs. All emergency brakes are electromechanical.

48



49

Braking – dynamic


Dynamic braking, also called dc injection braking, utilises the ability of the alternating current drive motor to act as a generator. Disconnecting the ac feed to the stator and injecting dc across any pair of stator terminals obtains this effect.

49

Braking – dynamic



The injection of dc into the stator windings creates a stationary dc magnetic field within the stator, and the kinetic energy of the revolving rotor is dissipated into the rotor cage by generator action. When the rotor, via its motion, cuts the magnetic field of the stator an emf is induced across the rotor bars. This voltage develops circulating current in the rotor cage.

49

Braking – dynamic



The injection of dc into the stator windings creates a stationary dc magnetic field within the stator, and the kinetic energy of the revolving rotor is dissipated into the rotor cage by generator action. When the rotor, via its motion, cuts the magnetic field of the stator an emf is induced across the rotor bars. This voltage develops circulating current in the rotor cage.

Dynamic braking cannot provide the holding torque necessary to keep the motor at standstill and an electromechanical brake is required to complete the stopping of the motor.

49

Braking – dynamic


50

Braking – plug


Plug braking is the fast de-acceleration of a motor by reversing any two line feeds to the stator terminals while it is running at full speed. Although this draws a high line current, even exceeding the locked-rotor current value of the motor, plugging is the simplest and quickest braking method.

50

Braking – plug


Plug braking is the fast de-acceleration of a motor by reversing any two line feeds to the stator terminals while it is running at full speed. Although this draws a high line current, even exceeding the locked-rotor current value of the motor, plugging is the simplest and quickest braking method.

Plug braking develops a strong counter-torque (retarding force) on the motor and places considerable stress on the stator and rotor windings and motor bearings. The generated heat losses in both stator and rotor windings can be up to three times that experienced during normal acceleration.

50

Braking – plug


51

Braking – regenerative


Regenerative braking is an extension of dynamic braking with the generated energy available to do work instead of being dissipated as heat. Regenerative braking takes advantage of the fact that when a motor stops being powered and just coasts it acts as a generator and can feed energy back into the alternating current supply system. This energy is now available to power other devices.

51



Regenerative braking is an extension of dynamic braking with the generated energy available to do work instead of being dissipated as heat. Regenerative braking takes advantage of the fact that when a motor stops being powered and just coasts it acts as a generator and can feed energy back into the alternating current supply system. This energy is now available to power other devices.

This form of braking finds application with overhauling-type loads. Examples of overhauling loads include the slowing of a flywheel, the slowing of trains and trams, the control of an elevator car under the force of gravity as it descends, and drilling and sawing operations where a sudden drop in torque occurs when these machines complete their designated operation.

51



52

Braking – eddy current


Eddy-current brakes are primarily used with variable-speed three-phase induction motors. They consist principally of a stationary field coil and metal assembly and a large metal rotor.

52




A small air gap exists between the smooth-surface rotor and the stationary field assembly.

52





The metal rotor is connected to a shaft which is coupled to the shaft of the motor. In order to stop the motor the field coil must be energised. When dc is applied to the field coil a magnetic flux is established in the metal assembly and links with the turning rotor.

52





The metal rotor is connected to a shaft which is coupled to the shaft of the motor. In order to stop the motor the field coil must be energised. When dc is applied to the field coil a magnetic flux is established in the metal assembly and links with the turning rotor.

As the rotor moves through the stationary magnetic field eddy currents are induced in it. These eddy currents react with the magnetic field in the field assembly and produce a torque that opposes motion of the rotor.

52



53

Braking – magnetic particle


Magnetic-particle brakes consist of a stationary field coil, stainless steel magnetic particles and a rotor. The rotor turns freely inside a housing containing the inactive magnetic particles.

53




The rotor shaft can be coupled to an induction motor shaft. When the coil is energised the magnetic particles begin to bind together forming chains along the magnetic flux lines linking the rotor to the housing.

53




The rotor shaft can be coupled to an induction motor shaft. When the coil is energised the magnetic particles begin to bind together forming chains along the magnetic flux lines linking the rotor to the housing.

As the rotor turns, the chains continuously shear, generating torque and heat. The increasing binding effect of the magnetic particles begins to slow down and eventually stops the rotor. Removal of the current flowing to the stationary coil allows the magnetic particles to return to their inactive state and the rotor is free to move.

53



54



Magnetic-particle brakes are used where light tensioning, positioning and continuous changes of speed are required.

54



Magnetic-particle brakes are used where light tensioning, positioning and continuous changes of speed are required.

Applications include tensioning rolls of labels to be put on packages and tensioning spools for unwinding wire and fabric.

54



Braking

Braking is essentially the removal of stored kinetic energy in the form of motion from a motor–mechanical load.

Braking methods convert kinetic energy into heat.

All emergency brakes are electromechanical.

Dynamic braking utilises the ability of the alternating current drive motor to act as a generator.

Plug braking is the fast de-acceleration of a motor by reversing any two line feeds to the stator terminals while it is running at full speed.

Regenerative braking is an extension of dynamic braking with the generated energy available to do work instead of being dissipatedas heat.

Page 391

55


All emergency brakes are:

electromechanical

dynamic

regenerative

plug type

Pop quiz


57

Reversing direction of rotation


57


0 60 120 180 240 300 360

N

S

B CA


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N

SS

A

B C

0 60 120 180 240 300 360

N

S

B CA


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N

SS

N

SN

A

B C

0 60 120 180 240 300 360

N

S

B CA


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N

SS

N

SN

S

SN

A

B C

0 60 120 180 240 300 360

N

S

B CA


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N

SS

N

SN

S

NN

S

SN

A

B C

0 60 120 180 240 300 360

N

S

B CA


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N

SS

N

SN

S

NN

S

SN

S

NSA

B C

0 60 120 180 240 300 360

N

S

B CA


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N

SS

N

SN

S

NNN

NS

S

SN

S

NSA

B C

0 60 120 180 240 300 360

N

S

B CA


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N

SS

N

SN

S

NNN

NS

S

SN

S

NSN

SS

A

B C

0 60 120 180 240 300 360

N

S

B CA


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N

SS

N

SN

S

NNN

NS

S

SN

S

NSN

SS

A

B C

Result = Anticlockwise Rotation

0 60 120 180 240 300 360

N

S

B CA


58


0 60 120 180 240 300 360

N

S

B CA


58


Now, Lets swap phases B and C!

0 60 120 180 240 300 360

N

S

B CA


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N

SS

A

C B

0 60 120 180 240 300 360

N

S

B CA


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N

SS

N

NS

A

C B

0 60 120 180 240 300 360

N

S

B CA


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N

SS

N

NS

S

NS

A

C B

0 60 120 180 240 300 360

N

S

B CA


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N

SS

N

NS

S

NN

S

NS

A

C B

0 60 120 180 240 300 360

N

S

B CA


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N

SS

N

NS

S

NN

S

NS

S

SNA

C B

0 60 120 180 240 300 360

N

S

B CA


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N

SS

N

NS

S

NNN

SN

S

NS

S

SNA

C B

0 60 120 180 240 300 360

N

S

B CA


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N

SS

N

NS

S

NNN

SN

S

NS

S

SNN

SS

A

C B

0 60 120 180 240 300 360

N

S

B CA


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N

SS

N

NS

S

NNN

SN

S

NS

S

SNN

SS

A

C B

Result = Clockwise Rotation

0 60 120 180 240 300 360

N

S

B CA


59


Now, Lets swap original phases A and B!

0 60 120 180 240 300 360

N

S

B CA


59


S

SN

B

A C

0 60 120 180 240 300 360

N

S

B CA


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S

SN

N

SN

B

A C

0 60 120 180 240 300 360

N

S

B CA


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S

SN

N

SN

N

SS

B

A C

0 60 120 180 240 300 360

N

S

B CA


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S

SN

N

SN

N

NS

N

SS

B

A C

0 60 120 180 240 300 360

N

S

B CA


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S

SN

N

SN

N

NS

N

SS

S

NSB

A C

0 60 120 180 240 300 360

N

S

B CA


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S

SN

N

SN

N

NSS

NN

N

SS

S

NSB

A C

0 60 120 180 240 300 360

N

S

B CA


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S

SN

N

SN

N

NSS

NN

N

SS

S

NSS

SN

B

A C

0 60 120 180 240 300 360

N

S

B CA


59


S

SN

N

SN

N

NSS

NN

N

SS

S

NSS

SN

B

A C

Result = Clockwise Rotation

0 60 120 180 240 300 360

N

S

B CA


60



Interchanging the connections of any two of the three power conductors on a three-phase motor reverses the direction of the electromagnetic stator field thus reversing the direction of shaft rotation. The regular number of starts per day over a period of months or years influences the life of a motor and its power and control system.

60



Interchanging the connections of any two of the three power conductors on a three-phase motor reverses the direction of the electromagnetic stator field thus reversing the direction of shaft rotation. The regular number of starts per day over a period of months or years influences the life of a motor and its power and control system.

Excessive cycling affects the life of control components such as starters, sensors and relays. The number of starts and reverses that a motor sees (on and off) can also cause motor shaft damage (twisting stress), bearing damage, stressed insulation and motor overheating (for every 10 °C rise the motor insulation life is reduced by half). These conditions can reduce motor life.

60

Page 393



How can the direction of rotation of a three-phase motor be reversed?

reversing the rotor

turning the motor frame around

by connecting in delta rather than in star

interchanging the connections of any two of the three power conductors

Pop quiz

hampson, j, hanssen, s 2013, electrical trade principles: section_7-3

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