eric assignm

8/9/2019 Eric Assignm

1/27

INTRODUCTION TO ELECTRIC MOTORS ....................................................................................................... 3

I.1. Single phase induction Motor ........................................................................................................................3

I.2. Wound rotor induction Motor ........................................................................................................................3

I.3. Capacitor-start induction motor ....................................................................................................................3

I.4. Synchronous motor .................................................................................................................................... 3

I.4.1. Single phase Synchronous motor ........................................................................................................... 3I.4.2. Three-phases phase Synchronous motor ................................................................................................ 4

I.5. Special Motors ...............................................................................................................................................5

I.5.1. Stepper Motor ......................................................................................................................................... 5

I.5.2. Single phase series motor ...................................................................................................................... 6

I.5.3. Compensated series motor .....................................................................................................................6

I.5.4.Universal motor ...................................................................................................................................... 6

I.5.5. Repulsion motor ................................................................................................................................... 6

I.5.6. Repulsion start induction motor ............................................................................................................ 6I.5.7. Hysteresis motor ..................................................................................................................................... 7

I.5.8.Eddy current clutch ................................................................................................................................7

II. ELECTRIC MOTORS PROTECTION .............................................................................................................. 7

II.1. Overcurrent Protection ................................................................................................................................ 7

II.2. Overload Protection .....................................................................................................................................7

II.3. Differential Protection .................................................................................................................................8

II.4. Other Motor Protection Devices .................................................................................................................. 9

II.4.1. Low Voltage Protection ........................................................................................................................ 9II.4.2. Medium Voltage Motor Surge Protection ............................................................................................ 9

II.4.3. Overvoltage........................................................................................................................................................................ 10

II.4.4. Phase Failure Protection ...................................................................................................................... 10

II.4.5. Phase Reversal Protection ................................................................................................................... 11

II.4.6. Ground Fault Protection ......................................................................................................................11

II.4.7. Unbalance Protection .........................................................................................................................11

II.4.8. Short Circuit Protection .......................................................................................................................11

II.4.11.Other Motor Protection Devices ........................................................................................................ 12

II.4.12.Sizing Motor Overcurrent Protection .................................................................................................12

II.4.13.Types of Overcurrent Devices ............................................................................................................12

II.4.14.Standard Fuse Response ..................................................................................................................... 12

II.4.11.1. Standard (Non-Time Delay, Single Element) Fuses .................................................................. 12

II.4.11.2. Time-Delay (Dual Element) Fuses .............................................................................................13

II.5. Circuit Breakers ..........................................................................................................................................13Page 1 of27 Industrial Safety and Maintenance

By Eric MWIZERWA (Kigali Institute of Science and Technology(0250 788 55 3598)


2/27

II.5.1. Inverse CB Trip Curve ........................................................................................................................ 13

II.5.2. Instantaneous Trip Circuit Breakers ....................................................................................................14

II.6. Motor Overload Protection .........................................................................................................................15

II.6.1. Magnetic & Thermal Overloads ..........................................................................................................15

II.6.2. Magnetic Overload Relays .................................................................................................................. 15

II.6.3. Thermal Overload Relays ....................................................................................................................15II.6.3.1. Melting-Alloy Thermal Overload Relays .................................................................................... 15

II.6.3.2. Bimetallic Thermal Overload Relays ...........................................................................................16

II.6.3.3. Automatic Reset Devices .............................................................................................................16

II.6.4.Electronic Overloads ............................................................................................................................17

II.6.5. Fuses ....................................................................................................................................................17

II.6.6.Overload Trip Time ..............................................................................................................................17

II.6.7. Sizing Motor Overload Protection ......................................................................................................18

II.6.8. Ambient Temperature Compensation ................................................................................................. 18II.6.8.1. Ambient Compensated Heaters ....................................................................................................19

II.7. Sizing Motor Protection Systems ............................................................................................................... 19

III.ELECTRIC MOTORS INSTALLATION ........................................................................................................20

III.1. General Guidelines to electrical Motors Installations ...............................................................................20

III.2. Motor Start and Speed control .................................................................................................................. 21

III.3. Running 3-phase motors on 1-phase ......................................................................................................... 21

III.4. Self-starting static phase converter Run capacitor ...................................................................................22

III.5. More efficient static phase converter Start capacitor ................................................................................22III.6. Electronic variable speed drive .................................................................................................................22

III.7. Stepping a Permanent magnet stepper Motor ........................................................................................... 23

III.8. Samples on Installation of DC Motors ......................................................................................................23

III.8.1. Shunt wound Motors ..........................................................................................................................23

III.8.2. Series Wound Motors ........................................................................................................................ 24

III.8.3. Compound Wound Motors ................................................................................................................24

III.9. Sample on installation of Induction Motors ..............................................................................................25

III.9.1. Installation of Single phase Capacitor run-Capacitor start Motor ..................................................... 25

III.9.2. Star-delta starters with overload relay .............................................................................................. 25

III.10. Visual and Mechanical inspections ........................................................................................................26

III.11. Possible relevant standards .....................................................................................................................27

IV. CONCLUSION ................................................................................................................................................27

V. REFERENCES ..................................................................................................................................................27

Page 2 of27 Industrial Safety and MaintenanceBy Eric MWIZERWA (Kigali Institute of Science and Technology(0250 788 55 3598)


3/27

INTRODUCTION TO ELECTRIC MOTORS

Many different energy efficiency standards for IMs are currently in use (e.g. NEMA and EPAct in USA, CSAin Canada, CEMEP/EU in Europe, AS/NZ in Australia and New Zealand, JIS in Japan, and GB in China) withnew classes currently being developed in several countries around the world.

I.1. Single phase induction Motor

The single coil of a single phase induction motor does not produce a rotating magnetic field, but a pulsatingfield reaching maximum intensity at 0o and 180o electrical.

A three phase motor may be run from a single phase power source. However, it will not self-start. It may behand started in either direction, coming up to speed in a few seconds. It will only develop 2/3 of the 3- powerrating because one winding is not used

I.2. Wound rotor induction Motor

A wound rotorinduction motor has a stator like the squirrel cage induction motor, but a rotor with insulatedwindings brought out via slip rings and brushes. However, no power is applied to the slip rings. Their solepurpose is to allow resistance to be placed in series with the rotor windings while starting

I.3. Capacitor-start induction motor

A larger capacitor may be used to start a single phase induction motor via the auxiliary winding if it is switchedout by a centrifugal switch once the motor is up to speed. Moreover, the auxiliary winding may be many moreturns of heavier wire than used in a resistance split-phase motor to mitigate excessive temperature rise. Theresult is that more starting torque is available for heavy loads like air conditioning compressors. This motor

configuration works so well that it is available in multi-horsepower (multi-kilowatt) sizes. It exists othermethods of placing the capacitor for different purposes like Cap. Run; Cap. Start-Cap. Run and Auxiliarywinding.

I.4. Synchronous motor

I.4.1. Single phase Synchronous motor

Single phase synchronous motors are available in small sizes for applications requiring precise timing such astime keeping, (clocks) and tape players. Though battery powered quartz regulated clocks are widely available,the AC line operated variety has better long term accuracy-- over a period of months. This is due to power plant

operators purposely maintaining the long term accuracy of the frequency of the AC distribution system. If itfalls behind by a few cycles, they will make up the lost cycles of AC so that clocks lose no time.

Above 10 Horsepower (10 kW) the higher efficiency and leading power factor make large synchronous motorsuseful in industry. Large synchronous motors are a few percent more efficient than the more common inductionmotors. Though, the synchronous motor is more complex.



4/27

Synchronous motor running in step with alternator

I.4.2. Three-phases phase Synchronous motor

A 3-phase synchronous motor generates an electrically rotating field in the stator. Such motors are not selfstarting if started from a fixed frequency power source such as 50 or 60 Hz as found in an industrial setting.Furthermore, the rotor is not a permanent magnet as shown below for the multi-horsepower (multi-kilowatt)motors used in industry, but an electromagnet.

Large industrial synchronous motors are more efficient than induction motors. They are used when constantspeed is required. Having a leading power factor, they can correct the AC line for a lagging power factor.

The number of poles is n. For rotor speed in rpm, multiply by 60.

S = f120/n

Where: S = rotor speed in rpm; f = AC line frequency and n = number of poles per phase

Large industrial synchronous motors are self started by embedded squirrel cage conductors in the armature,acting like an induction motor. The electromagnetic armature is only energized after the rotor is brought up tonear synchronous speed.

Block diagram of a synchronous motor or condenser supplying VAR to keep utility PF constant.



5/27

On the figure below the tachometer signal, a pulse train proportional to motor speed, is fed back to a phaselocked loop, which compares the tachometer frequency and phase to a stable reference frequency source such asa crystal oscillator.

Phase locked loop controls synchronous motor speed.

A motor driven by square waves of current, as provided by simple Hall Effect sensors, is known as a brushlessDC motor. This type of motor has higher ripple torque variation through a shaft revolution than a sine wavedriven motor. This is not a problem for many applications. Though, we are primarily interested in synchronous

motors in this section.

I.5. Special Motors

I.5.1. Stepper Motor

A stepper motor is a digital version of the electric motor. The rotor moves in discrete steps as commanded,rather than rotating continuously like a conventional motor. When stopped but energized, astepper(short forstepper motor) holds its load steady with a holding torque. Wide spread acceptance of the stepper motor withinthe last two decades was driven by the ascendancy of digital electronics. Modern solid state driver electronicswas a key to its success. And, microprocessors readily interface to stepper motor driver circuits. Application

wise, the predecessor of the stepper motor was the servo motor

Stepper motor vs. servo motor.

Stepper speed characteristics.



6/27

I.5.2. Single phase series motor

If a DC series motor equipped with a laminated field is connected to AC, the lagging reactance of the field coilwill considerably reduce the field current. While such a motor will rotate, operation is marginal. While starting,armature windings connected to commutator segments shorted by the brushes look like shorted transformerturns to the field. This results in considerable arcing and sparking at the brushes as the armature begins to turn.This is less of a problem as speed increases, which shares the arcing and sparking between commutatorsegments The lagging reactance and arcing brushes are only tolerable in very small uncompensated series ACmotors operated at high speed. Series AC motors smaller than hand drills and kitchen mixers may beuncompensated.

I.5.3. Compensated series motor

The arcing and sparking is mitigated by placing a compensating windingthe stator in series with the armaturepositioned so that its magnetomotive force (mmf) cancels out the armature AC mmf. A smaller motor air gapand fewer field turns reduce lagging reactance in series with the armature improving the power factor. All butvery small AC commutator motors employ compensating windings. Motors as large as those employed in a

kitchen mixer, or larger, use compensated stator windings.

I.5.4.Universal motor

It is possible to design small (under 300 watts) universal motors which run from either DC or AC. Very smalluniversal motors may be uncompensated. Larger higher speed universal motors use a compensating winding. Amotor will run slower on AC than DC due to the reactance encountered with AC. However, the peaks of thesine waves saturate the magnetic path reducing total flux below the DC value, increasing the speed of theseries motor. Thus, the offsetting effects result in a nearly constant speed from DC to 60 Hz. Small line

operated appliances, such as drills, vacuum cleaners, and mixers, requiring 3000 to 10,000 rpm use universalmotors.

I.5.5. Repulsion motor

A repulsion motorconsists of a field directly connected to the AC line voltage and a pair of shorted brushesoffset by 15oto 25o from the field axis. The field induces a current flow into the shorted armature whosemagnetic field opposes that of the field coils. Speed can be controlled by rotating the brushes with respect tothe field axis. This motor has superior commutation below synchronous speed, inferior commutation abovesynchronous speed. Low starting current produces high starting torque.

I.5.6. Repulsion start induction motor

When an induction motor drives a hard starting load like a compressor, the high starting torque of the repulsionmotor may be put to use. The induction motor rotor windings are brought out to commutator segments forstarting by a pair of shorted brushes. At near running speed a centrifugal switch shorts out all commutatorsegments giving the effect of a squirrel cage rotor. The brushes may also be lifted to prolong bush life. Startingtorque is 300% to 600% of the full speed value as compared to under 200% for a pure induction motor.



7/27

I.5.7. Hysteresis motor

If the low hysteresis Si-steel laminated rotor of an induction motor is replaced by a slotless windinglesscylinder of hardened magnet steel, hysteresis, or lagging behind of rotor magnetization, is greatly accentuated.The resulting low torque synchronous motor develops constant torque from stall to synchronous speed.Because of the low torque, the hysteresis motor is only available in very small sizes, and is only used forconstant speed applications like clock drives, and formerly, phonograph turntables.

I.5.8.Eddy current clutch

If the stator of an induction motor or a synchronous motor is mounted to rotate independently of the rotor aneddy current clutch results. The coils are excited with DC and attached to the mechanical load. The squirrelcage rotor is attached to the driving motor. The drive motor is started with no DC excitation to the clutch. TheDC excitation is adjusted from zero to the desired final value providing a continuously and smoothly variabletorque. The operation of the eddy current clutch is similar to an analog eddy current automotive speedometer.

II. ELECTRIC MOTORS PROTECTION

Motor protection safeguards the motor, the supply system and personnel from various operating conditions ofthe driven load, the supply system or the motor itself.

Motor protection categories include- Overcurrent Protection- Overload Protection- Other Types of Protection.The National Electrical Code requires that motors and their conductors be protected from both overcurrent andoverload conditions.

II.1. Overcurrent Protection

Overcurrent protection interrupts the electrical circuit to the motor upon excessive current demand on thesupply system from either short circuits or ground faults. Overcurrent protection is required to protect personnel, the motor branch circuit conductors,

control equipment, and motor from these high currents. Overcurrent protection is usually provided in the form of fuses or circuit breakers. Thesedevices operate when a short circuit, ground fault or an extremely heavy overload occurs.Most overcurrent sources produce extremely large currents very quickly.

II.2. Overload Protection

Overload protection is installed in the motor circuit and/or motor to protect the motor from damagefrom mechanical overload conditions when it is operating/running.The effect of an overload is an excessive rise in temperature in the motor windings due to current higher thanfull load current.



8/27

Properly sized overload protection disconnects the motor from the power supply when the heat generated in themotor circuit or windings approaches a damaging level for any reason. The larger the overload, the morequickly the temperature will increase to a point that is damaging to the insulation and lubrication of the motor.Unlike common instantaneous type fuses and breakers, overload devices are designed to allow high currents toflow briefly in the motor to allow for: Typical motor starting currents of 6 to 8 times normal running currentwhen starting

Short duration overloads such as a slug of product going through a system. If the motor inlets and outlets arecovered by a blanket of lint or if a bearing should begin to lock, excessive heating of the motorwindings will overload the motors insulation which could damage the motor.

The overcurrent device will not react to this low level overload. The motor overload device prevents this type ofproblem from severely damaging the motor and also provides protection for the circuit conductors since it israted for the same or less current as the conductors. Overload protection trips when an overload exists for morethan a short time. The time it takes for an overload to trip depends on the type of overload device, length of timethe overload exists, and the ambient temperature in which the overloads are located.

II.3. Differential Protection

This protection function is mostly used to protect induction and synchronous motors against phase-to-phasefaults. This function requires two sets of CTs, one at beginning of the motor feeder, and the other at the starpoint. Differential protection may be considered the first line of protection for internal phase to phase or phaseto ground faults. In the event of such faults, the quick response of the differential element may limit the damagethat may have otherwise occurred to the motor.



9/27

The differential protection function can only be used if both sides of each stator phase are brought out of themotor for external connection such that the phase current going into and out of each phase can be measured.The differential element subtracts the current coming out of each phase from the current going into each phaseand compares the result or difference with the differential pickup level. If this difference is equal to or greaterthan the pickup level a trip will occur. GE Multilin motor protective relays support both three and six CTconfigurations. For three CT configuration both sides of each of the motors stator phases are being passedthrough a single CT. This is known as the core balance method and is the most desirable owing to its sensitivityand noise immunity.

II.4. Other Motor Protection Devices

II.4.1. Low Voltage Protection

Low Voltage Disconnects - Protection device operates to disconnect the motor when the supply voltage dropsbelow a preset value. The motor must be manually restarted upon resumption of normal supply voltage. LowVoltage Release - Protection device interrupts the circuit when the supply voltage drops below a preset valueand re-establishes the circuit when the supply voltage returns to normal.

II.4.2. Medium Voltage Motor Surge Protection

MSP (Motor Surge Protector) is designed to protect medium voltage motors and generators from voltage surgesdue to lightning and switching events. The MSP accomplishes this task better than any other product bydecreasing the slope and crest of impending voltageApplication of the MSP is guaranteed to reduce the likelihood of motor failures, resulting in less down-time andhigher productivity.- Reduced medium voltage motor and generator failures from voltage surges due to lightning, faults, andswitching events.- Units can be custom designed for direct mounting to generators, motor, and compressor housing.

- Units can be supplied with over-current and differential protection current transformers. Reduced downtimeand material waste from motor failure.



10/27

The Principle operation of the MSP is to decrease the crest voltage and rate of rise of the impending surges.High rates of rise damage end turns while high crest voltages damage winding to core insulation.

Motor Surge Protection Unit with Differential and Phase Overcurrent Current Transformers for DirectMounting on Equipment.

II.4.3. Overvoltage

When the motor is running in an overvoltage condition, slip will decrease as it is inversely proportional to thesquare of the voltage and efficiency will increase slightly. The power factor will decrease because the currentbeing drawn by the motor will decrease and temperature rise will decrease because the current has decreased(based on I2t). As most new motors are designed close to the saturation point , increasing the V/HZ ratio couldcause saturation of air gap flux causing heating.The overall result of an overvoltage condition is an increase incurrent and motor heating and a reduction in overall motor performance.

II.4.4. Phase Failure Protection

Interrupts the power in all phases of a three-phase circuit upon failure of any one phase.Normal fusing andoverload protection may not adequately protect a polyphase motor from damaging single phase operation.Without this protection, the motor will continue to operate if one phase is lost. Large currents can be developed



11/27

in the remaining stator circuits which eventually burn out. Phase failure protection is the only effective way toprotect a motor properly from single phasing.

II.4.5. Phase Reversal Protection

Used where running a motor backwards (opposite direction from normal) would cause operational or safetyproblems.Most three phase motors will run the opposite direction by switching the connections of any two of the threephases. The device interrupts the power to the motor upon detection of a phase reversal in the three phasesupply circuit. This type of protection is used in applications like elevators where it would be damaging ordangerous for the motor to inadvertently run in reverse.

II.4.6. Ground Fault Protection

Operates when one phase of a motor shorts to ground preventing high currents from damaging the statorwindings and the iron core.Damage to a phase conductors insulation and internal shorts due to moisture within the motor are common

causes of ground faults. A strategy that is typically used to limit the level of the ground fault current is toconnect an impedance between the neutral point of the motor and ground. This impedance can be in the form ofa resistor or grounding transformer sized to ensure that the maximum ground fault current is limited to a levelthat will reduce the chances of damage to the motor.

II.4.7. Unbalance Protection

Unbalanced load in the case of AC motors is mainly the result of an unbalance of the power supply voltages.The negative-sequence reactance of the three-phase motor is 5 to 7 times smaller than positive-sequencereactance, and even a small unbalance in the power supply will cause high negative sequence currents. Forexample for an induction motor with a staring current six times the full load current, a negative sequence

voltage component of 1% corresponds to a negative sequence current component of 6%. The negative-sequencecurrent induces a field in the rotor, which rotates in the opposite direction to the mechanical direction andcauses additional temperature rise. Main causes of current unbalance are: system voltage distortion andunbalance, stator turn-to-turn faults, blown fuses, loose connections, as well as faults.

II.4.8. Short Circuit Protection

The short circuit element provides protection for excessively high overcurrent faults. When a motor starts, thestarting current (which is typically 6 times the Full Load Current) has asymmetrical components. Theseasymmetrical currents may cause one phase to see as much as 1.7 times the RMS starting current. As a resultthe pickup of the short circuit element must be set higher than the maximum asymmetrical starting currents seenby the phase CTs to avoid nuisance tripping. The breaker or contactor that the relay is to control under suchconditions must have an interrupting capacity equal to or greater then the maximum available fault

II.4.9. Mechanical Jam

The mechanical jam element is designed to operate for running load jams due to worn motor bearings, loadmechanical breakage and driven load process failure. This element is used to disconnect the motor on abnormaloverload conditions before motor stalls. In terms of relay operation, the Mechanical Jam element prevents themotor from reaching 100% of its thermal capacity while a Mechanical Jam is detected. It helps to avoidmechanical breakage of the driven load and reduce start inhibit waiting time.



12/27

II.4.10. Load Loss Detection

Undercurrent protection is useful for indicating the loss of suction in a pump application, or a broken belt inconveyor application. The second method of load loss detection is to use of the underpower protection element.

II.4.11.Other Motor Protection Devices

Bearing Temperature Monitors & Protection Winding Temperature Monitors & Protection Devices CurrentDifferential Relays (Phase Unbalance) Vibration Monitors & Protection

II.4.12.Sizing Motor Overcurrent Protection

Circuit overcurrent protection devices must be sized to protect the branch-circuit conductors and also allow themotor to start without the circuit opening due to the in-rush current of the motor.

II.4.13.Types of Overcurrent Devices

Selection of the size of the overcurrent protection device is made using NEC Table 430-152 which listsinformation for four types of devices:1) Standard (non-time delay) fuses2) Time-Delay (dual element) fuses3) Instantaneous Trip Circuit Breaker4) Inverse Time Circuit Breaker

II.4.14.Standard Fuse Response

II.4.11.1. Standard (Non-Time Delay, Single Element) Fuses

Standard fuses protect against short circuits and ground faults using thermal features to sense a heatbuildup in the circuit. Once blown standard fuses are no longer usable and must be replaced.. The NEC allows standard fuses as overcurrent protection devices sized up to a maximum of300% of the motors ratings to allow the motor to start.An exception allows the use of the next higher size fuse when the table value does not correspond to a standardsize device. An additional exception allows the use of the next size larger device until an adequate size is foundif the motor will not start without operating the device. Standard fuses will hold 500% of their current rating forapproximately one fourth of a second.



13/27

NOTE:

Some special standard fuses will hold 500% of their current rating for up to two seconds.In order for a standard fuse to used as motor overload protection, the motor would have to start and reach itsrunning speed in one-fourth of a second or less.

Standard fuses will not generally provide any overload protection for hard starting installations because they

must be sized well above 125% of a motors FLA to allow the motor to start.

II.4.11.2. Time-Delay (Dual Element) Fuses

These are generally dual element fuses with both thermal and instantaneous trip features that allow the motorstarting current to flow for a short time without blowing the fuse.

Time delay fuses can also be used to provide some degree of overload protection which standard fuses cannot.The NEC allows time delay fuses to be sized up to a maximum of 175% of a motors FLA for overcurrentprotection. Time-delay fuses will hold 500% of their amp rating for 10 seconds which will allow most motors tostart without opening the circuit. Under normal conditions, a 100-amp time-delay fuse will start any motor witha locked-rotor current rating of 500 amps or less.

II.5. Circuit Breakers

II.5.1. Inverse CB Trip Curve

Inverse time circuit breakers have both thermal and instantaneous trip features and are preset to tripat standardized levels. This is the most common type of circuit breaker used in the building trades forresidential, commercial, and heavy construction. The thermal action of this circuit breaker responds to heat.



14/27

If a motors ventilation inlets and outlets are not adequate to dissipate heat from the windings of the motor, theheat will be detected by the thermal action of the circuit breaker.

If a short should occur, the magnetic action of the circuit breaker will detect the instantaneous values of currentand trip the circuit breaker.

The National Electrical Code requires inverse time circuit breakers to be sized to a maximum of 250%of the motor FLA.The rating of an inverse time circuit breaker can be multiplied by 3 and this total amperage will start anymotor with less locked-rotor amperage. The time it takes to reach the 300% level varies with the amperage andvoltage ratings of the breaker as shown in the curve.

II.5.2. Instantaneous Trip Circuit Breakers

Instantaneous trip circuit breakers respond to immediate (almost instantaneous) values of current from a shortcircuit, ground fault, or locked rotor current. This type of circuit breaker will never trip from a slow heatbuildup due to motor windings overheating. A stuck bearing or a blanket of lint covering the inlets and outletsof the motors enclosure will cause the motor to overheat and damage the windings.

The National Electrical Code allows instantaneous trip circuit breakers to be sized to a maximum of 800% of amotors FLA value. They are used where time-delay fuses set at five times their ratings or circuit breakers at three times theirrating will not hold the starting current of a motor. Some instantaneous trip circuit breakers have adjustable tripsettings. The instantaneous trip ratings of an instantaneous trip circuit breaker can be adjusted above the locked-rotor current of a motor to allow the motor to start and come up to its running speed.



15/27

II.6. Motor Overload Protection

Motors larger than 1 horsepower must be provided separate motor overload protection devices.The most common devices typically used include:1) Magnetic or thermal overload devices2) Electronic overload relays3) Fuses

II.6.1. Magnetic & Thermal Overloads

Overload devices are usually located in the motors starter and connected in series with the motors electricalsupply circuit and can be operated by either magnetic or thermal action. The same amount of current passesthrough the overload relay and the motor. If the current or heat through the overload device is higher than thedevices rating, it trips and shuts down the electric power to the motor.

II.6.2. Magnetic Overload Relays

A magnetic overload relay is an electro-mechanical relay operated by the current flow in a circuit..

When the level of current in the circuit reaches a preset value, the increased magnetic field opens a set ofcontacts. Electromagnetic overload relays operate on the magnetic action of the load current flowing through acoilWhen the load current becomes too high, a plunger is pulled up into the coil interrupting the circuit. Thetripping current is adjusted by altering the initial position of the plunger with respect to the coil.

II.6.3. Thermal Overload Relays

A thermal overload relay is an electro-mechanical relay that is operated by heat developed in therelay. When the level of current in a circuit reaches a preset value, the increased temperature opens a set ofcontacts. The increased temperature opens the contacts through a bimetallic strip or by melting an alloy thatactivates a mechanism that opens the contacts. Two types include melting alloy and the bi-metallic strip.

II.6.3.1. Melting-Alloy Thermal Overload Relays

These are probably the most popular type of overload protection.



16/27

The motor current passes through a small heater winding and under overload conditions, the heat causes aspecial solder to melt allowing a ratchet wheel to spin thus opening the control circuit contacts.- Must be reset by hand operation-Heater coil and solder pot in one unit non-tamperable

II.6.3.2. Bimetallic Thermal Overload Relays

This design uses a bimetal strip associated with a current-carrying heater coil. When an overload occurs, theheat causes the bimetal to deflect and actuate a tripping mechanism which opens a set of contacts in the controlcircuit interrupting power to the coil and opening the power contacts.

Most relays are adjustable over a range from 85% to 115% of their value. They are available with ambientcompensation. An ambient compensated devices trip point is not affected by ambient temperature and performsconsistently at the same value of current.

II.6.3.3. Automatic Reset Devices

Automatic reset is an advantage where the starter is inaccessible and the motor is provided threewire control from a magnetic starter.This control doesnt allow the motor to restart until the start push button is manually pushed. This permits theoverload condition to be removed before the motor restarts.



17/27

II.6.4.Electronic Overloads

Electronic overloads sense the load current and the heating effect on the motor is computed. If anoverload condition exists, the sensing circuit interrupts the power circuit. The tripping current can be adjusted tosuit the particular application. Electronic overloads often perform additional protective functions such as ground

fault and phase loss protection.

II.6.5. Fuses

Fuses have limited application as the primary means of overload protection for motors but can be effectivelyused to provide back up overload protection.Single-element fuses are not designed to provide overload protection. Their basic function is to protect againstshort circuits and ground faults. If sized to provide overload protection, they would blow when the motor startsdue to high motor inrush current. Dual-element fuses can provide motor overload protection, but they have to bereplaced when they blow which can be a disadvantage.There is a risk of single-phasing damage to the motor when only one fuse blows unless Single-phase protectionis provided.

II.6.6.Overload Trip Time

The time it takes an overload to trip depends on the length of time the overload current exists.A Heater Trip Characteristics chart shows the relationship between the time an overload takes to trip and thecurrent flowing in the circuit based on the standard 40EC ambient temperature installation.



18/27

The larger the overload (horizontal axis), the shorter the time required to trip the overload (vertical axis).Any change from ambient temperature affects the tripping time of an overload.For temperatures higher than 40EC, the overloads trip at a current rating less than the value of the overload.

II.6.7. Sizing Motor Overload Protection

There are several types of devices that can be used to provide overload protection and the sizingprocedure can vary depending on the type of device used.It is important to keep differences in the procedures separate and understood well so as not to install overloadsthat do not provide adequate protection to the motor. The simplest and most straightforward sizing proceduresfor motor overload protection are applied when sizing overload relays using the cover of the motor starter,control center, or manufacturers catalog.The National Electrical Code specifies methods to calculate the maximum size motor overload protection forspecific motors if a manufacturers chart is not available. Installations relying on fuses and circuit breakers asback-up overload protection must be calculated using the NEC method.For motors rated 40EC with a Service Factor of 1.15 or greater, 125% of the motors FLA is used to calculatethe maximum size device for overload protection.The size overloads required to protect the windings of a motor can be determined by taking the motors full-load current rating and selecting the size overloads from the cover of a magnetic starter,a motor control center, or the manufacturers catalog. The following things should be kept in mind when usingmanufacturers charts. When the overload size is selected from the cover of a magnetic starter or controller, thenameplate full-load running current of the motor is used. The full-load running current is

II.6.8. Ambient Temperature Compensation

The ambient temperature in which a starter and motor is located must be considered when selecting overloadsbecause a high ambient temperature reduces overload trip time.

Reduced overload trip time can lead to nuisance tripping if a motor is located in a cooler ambient temperaturethan the starter and lead to motor burnout when the motor is located in a hotter ambient temperature than thestarter. Most thermal overload devices are rated for use at a maximum temperature of 40 degrees C which isabout 104 degrees F.The overload device trips at less than 100 percent rated current when the ambient temperature exceeds 104degrees F which can result in nuisance tripping.

If the temperature is significantly below 104 degrees F, the overload device allows significantly more currentthrough than it is rated for resulting in potential motor overload and failure without the overload tripping themotor off.



19/27


20/27

STEP 3: Determine the branch circuit overcurrent device size.

The maximum branch circuit overcurrent device size is calculated based on the type of protectivedevice selected (standard fuse, time-delay fuse, instantaneous breaker, inverse time breaker) and percentagemultiplier from according to the standards.

Multiply the motors design FLA by the appropriate percentage in NEC Table 430-152.

1. When the value found does not match a standard fuse/breaker size the NEC permits thenext higher STANDARD size for a branch circuit overcurrent device.

STEP 4: Determine the required size for the motor running overload protection.

1. Use the nameplate FLA directly to find the appropriate overload device heater on the motorstarter cover or from manufacturers tables.

2. Use the nameplate FLA and NEC Section 430-32 to calculate the maximum sizefor the motor overload protection in amps.

III.ELECTRIC MOTORS INSTALLATION

III.1. General Guidelines to electrical Motors Installations

A strong working knowledge of installation techniques is vital to the effective operation and maintenance of

motors.

Today's modern motors require your consideration of all aspects of selection, application, and maintenance aswell as details of assembly, hardware, and the interrelationship of components and materials. As a result,installation of these motors is more important than ever before.

Proper motor installation is essential in obtaining top-quality operation, efficient performance, and maximum

reliability. Whether you're an installer, engineer, or maintainer, this work demands close coordination, planning,

and teamwork with other disciplines. Rolling-element bearings used in electric motors potentially have many



21/27

failure modes if an incorrect strategy is implemented. These modes include incorrect lubricant selection,

contamination, loss of lubricant and overgreasing. This article will discuss several effective strategies to

minimize the likelihood of these failure modes.

Most electric motors are designed with grease-lubricated, anti-friction, rolling-element bearings. Grease is thelifeblood of these bearings, providing an oil film that prevents harsh metal-to-metal contact between the rotatingelement and races.

III.2. Motor Start and Speed control

Most AC motors are induction motors. Induction motors are favored due to their ruggedness and simplicity. Infact, 90% of industrial motors are induction motors.

Autotransformer induction motor starter

Nikola Tesla conceived the basic principals of the polyphase induction motor in 1883, and had a halfhorsepower (400 watts) model by 1888. Tesla sold the manufacturing rights to George Westinghouse for$65,000.

Most large (> 1 hp or 1 kW) industrial motors arepoly-phase induction motors. By poly-phase, we mean thatthe stator contains multiple distinct windings per motor pole, driven by corresponding time shifted sine waves.In practice, this is two or three phases. Large industrial motors are 3-phase. While we include numerousillustrations of two-phase motors for simplicity, we must emphasize that nearly all poly-phase motors are three-phase. By induction motor, we mean that the stator windings induce a current flow in the rotor conductors, likea transformer, unlike a brushed DC commutator motor.

III.3. Running 3-phase motors on 1-phase

Three-phase motors will run on single phase as readily as single phase motors. The only problem for either

motor is starting. Sometimes 3-phase motors are purchased for use on single phase if three-phase provisioningis anticipated. The power rating needs to be 50% larger than for a comparable single phase motor to make upfor one unused winding. Single phase is applied to a pair of windings simultaneous with a start capacitor inseries with the third winding. The start switch is opened in Figurebelow upon motor start. Sometimes a smallercapacitor than the start capacitor is retained while running.

Three phase power may be tapped off from the three stator windings for powering other 3-phase equipment.The capacitor supplies asynthetic phase approximately midway 90o between the 180o single phase powersource terminals for



22/27

starting. While running, the motor generates approximately standard 3-, as shown in Figure above.

III.4. Self-starting static phase converter Run capacitor

Since a static phase converter has no torque load, it may be started with a capacitor considerably smaller than anormal start capacitor. If it is small enough, it may be left in circuit as a run-capacitor. See Figure above.However, smaller run-capacitors result in better 3-phase power output as in Figurebelow. Moreover,

adjustment of these capacitors to equalize the currents as measured in the three phases results in the mostefficient machine.

III.5. More efficient static phase converter Start capacitor

However, a large start capacitor is required for about a second to quickly start the converter The above figshows construction details.

III.6. Electronic variable speed drive

Modern solid state electronics increase the options for speed control. By changing the 50 or 60 Hz linefrequency to higher or lower values, the synchronous speed of the motor may be changed. However, decreasingthe frequency of the current fed to the motor also decreases reactance X L which increases the stator current.This may cause the stator magnetic circuit to saturate with disastrous results. In practice, the voltage to themotor needs to be decreased when frequency is decreased.



23/27

Conversely, the drive frequency may be increased to increase the synchronous speed of the motor. However,the voltage needs to be increased to overcome increasing reactance to keep current up to a normal value andmaintain torque. The inverter (Figure) approximates sinewaves to the motor with pulse width modulationoutputs. This is a chopped waveform which is either on or off, high or low; the percentage of on timecorresponds to the instantaneous sine wave voltage.

III.7. Stepping a Permanent magnet stepper Motor

Permanent magnet stepper motors require phased alternating currents applied to the two (or more) windings. Inpractice, this is almost always square waves generated from DC by solid state electronics.Bipolardrive is

square waves alternating between (+) and (-) polarities, say, +2.5 V to -2.5 V. Unipolardrive supplies a (+) and(-) alternating magnetic flux to the coils developed from a pair of positive square waves applied to oppositeends of a center tapped coil. The timing of the bipolar or unipolar wave is wave drive, full step, or half stepWave drive. The wave drive waveforms below show that only one coil is energized at a time. While simple, thisdoes not produce as much torque as other drive techniques.

Waveforms: bipolar wave drive.

III.8. Samples on Installation of DC Motors

III.8.1. Shunt wound Motors

are the most widely used as they have a linear characteristic of Voltage & Torque. Shunt motor has moreconstant and controllable speed over various loads.This type of motor runs practically constant speed, regardless of the load. It is the type generally used in

commercial practice and is usually recommended where starting conditions are not usually severs



24/27

III.8.2. Series Wound Motors

Series Wound Motor is used for High starting Torque Applications. Series motor has greater torque capabilities.This type of motor speed varies automatically with the load, increasing as the load decreases. Use of seriesmotor is generally limited to case where a heavy power demand is necessary to bring the machine up to speed,as in the case of certain elevator and hoist installations, for steelcars, etc. Series-wound motors should never beused where the motor can be started without load, since they will race to a dangerous degree.

Common uses of the series motor include crane hoists, where large heavy loads will be raised and lowered andbridge and trolley drives on large overhead cranes.

III.8.3. Compound Wound Motors

Compound Wound Motorsare used for Mixed Load Applications. The above two desirable characteristics canbe found in the same motor by placing both a Series field and Shunt field winding on the same pole. Thus wehave, the Compound motor. The Compound motor responds better to heavy load changes than a Shunt motorbecause of the increased current through the series field coils. This boosts the field strength, providing addedtorque and speed.A combination of the shunt wound and series wound types combines the characteristics of both. Characteristicsmay be varied by varying the combination of the two windings



25/27

III.9. Sample on installation of Induction Motors

III.9.1. Installation of Single phase Capacitor run-Capacitor start Motor

Above circuit is the simplest of all and acts just like an electro-mechanical reversing switch. The followingcircuits show the internal configuration and capacitor/split-phase connections

III.9.2. Star-delta starters with overload relay

Arrangement in the motor line

In a standard circuit configuration, the star-delta starter withoverload relay, including a thermally delayed overcurrent relay are

situated in the cables leading to the motor terminals U1, V1, W1 orV2, W2, U2. The overload relay can also be operated in a starcircuit as it is usually connected in series with the motor windingand the relay current flowing through it = rated motor current 0.58.

Arrangement in the mains supply line



26/27

Instead of the arrangement in the motor line, the overload relay canbe placed in the mains supply line. For drives where the F2 relaytrips out when the motor is starting in the star circuit, the F2 relayrated for the rated motor current can be switched in the mainsline. The tripping delay is thus increased by approximately four tosix times. In the star circuit the current also flows through the relaybut here the relay does not offer full protection since its limit

current is increased to 1.73 times the phase current. It does,however, offer protection against non-starting.

Configuration in the delta circuit

Instead of the arrangement in the motor line or mains supply line,the overload relay can be placed in the delta circuit. When heavy,long-starting procedures are involved (e.g. for centrifuges) the F2relay, rated for relay current = rated motor current 0.58, can alsobe connected in the connecting lines between delta contactor Q15

and star contactor Q13. In the star circuit no current then flowsthrough relay F2. This circuit is used wherever exceptionallyheavy and long starting procedures are involved and whensaturable core current transformer-operated relays react too quickly

III.10. Visual and Mechanical inspections

Accepted good practice is to keep the difference in voltage between the point at which power is and the motorterminals below 3 percent when the motor is delivering full load delivered from the utility.An important aspect of large machine maintenance is the visual and mechanical inspection.

1. Inspect the machine's physical and mechanical condition.

Check for signs of oil or water leakage. Check the water and oil supply piping.

Verify that air inlets are not plugged and abnormal sound or smells

Check the water and oil supply piping.

Check the drain piping and the surroundings for any environmental issues that may affect performance.2. Inspect anchorage, alignment and grounding of the motor, driven equipment and base.3. Verify the application of appropriate lubrication and lubrication systems.4. Verify the absence of unusual mechanical or electrical noise or signs of overheating.5. Perform special tests such as air-gap spacing and machine alignment, if applicable



27/27

III.11. Possible relevant standards

Standard n Title What it covers

NEMA MG 1-2.02 Motors and Generators Assists users in the proper selection andapplication of motors and generators.

NEMA MG 1-2.05 Energy Management Guidefor Selection and Use offixed Frequency Medium ACSquirrel-Cage PolyphaseInduction Motors.

Provides practical information concerningproper selection and application of polyphaseinduction and synchronous motors, includinginstallation, operation and maintenance.

ANSI/NETA MTS-2007

Standard for MaintenanceTesting Specifications forElectrical DistributionEquipment and Systems.

IV. CONCLUSION

Selecting the appropriate protection and full requirements for electric motors installation based on Standards,optimize the system efficiency and performance also allow the users to be safe and bring to the long life of themachine, as intent of a good maintenance program is to extend the service life of your motors.

V. REFERENCES

1. T.Cox Lloyd, Electric Motors and their Applivation, 1969

2. Electric Motor Control and Maintenance, www.abb.com/powergeneration

3. Blahut, R. E., Principles and Practice of Electrical installations, Addison-Wesley, 1987.

4. Cover, T. M., and J. A. Thomas, Elements of Electrical Machines, Wiley, 1991.5. Gallagher, R., Information Theory and Reliable Electric protection, Wiley, 1968.

6. Widup, R., Large Motor Maintenance: Basics for machine reliability. 2008.
http://www.abb.com/powergenerationhttp://www.abb.com/powergeneration

eric assignm

Documents