Motors are very important in anyindustrial process, since they are usedas prime movers in productionprocesses. Effectiveness, profitability,accuracy and continuity of theprocesses are decided byuninterrupted running motors.

Motors are to be protected frominternal failures as well s externaladversities. Winding burnouts,insulation failures, and short circuitsare considered as internal failures.Bearing failure, coupling shaft failure,overloads due to processes, andundervoltage are all external causes.

Motor failures can cause major losses.Either such failures should beidentifies quickly and isolated or theyshould be prevented. In order to arrestthe losses, timely interruption of powersupply to the motor is essential. Thisis achieved by the protection relays.

Industry has been using fuse-backed-up starters for motor protection as wellas control. This combination offerscoarse but cost effective protection forsmall motors.

However, for larger motors, suchcoarse protection is not adequate.

Comprehensive motor protection relayshave been in use for quite some timenow. Advancement in microprocessortechnology and introduction ofIntelligent Electronic Devices (IEDs) forprotection and control have given riseto integrated motor protectioncontrollers. Right from simple, basicthermal overload protection, theserelays offer exhaustive list ofprotections such as unbalance,undervoltage, undercurrent and loadincrease. With the extensive list, it islikely that the user's maintenance andoperation engineer starts worryingabout how to set such a relayprecisely and how to offer the bestprotection to the motor.

Deciding applicable protections is notenough. The motor data is often notavailable. How to set the relay withoutsuch data? What to do with a rewoundmotor? Often, these questions areraised and do not get answered clearly.These have been the cases in the

past where non-availability of motordata resulted in motor protection relaynot being available for years after themotor was commissioned. Operationand maintenance engineers used toprovide make shift protection to run theprocesses.

New generation of relays do notrequire such elaborate motor data. Itis possible to set a motor protectionrelay with more precision even withminimum data. In this issue ofL & T Current Trends, we bringinformation on comprehensive motorprotection and an example on how toset motor protection relay withminimum available data. In fact, withMotorvision relays, it is possible togenerate motor data even for arewound motor.

Certain features of the relays are tobe used for protective checks so thatfailures can be prevented. How can itbe achieved? The lead articledescribes this aspect too.

Prospect / Retrospect

July - Sept 2001

Setting Motor Protection Relay

Visit us at www.LNTEBG.com

Motor protection has always been achallenging as well as interestingsubject. Motors are the prime moversof any industry. Failure of a motor canhave a severe impact on the processand overall cost effectiveness of theplant. Hence, irrespective of the motorsize, careful attention must be paid toits protection, specifically if it is a partof an important process.

Statistical data of motor failures/faultsover past 5 years indicate :

about 60% failures/faults areelectrical(in motor)

about 15% failures/faults arerelated to starters

about 25% failures/faults aremechanical

Out of the 60% electrical faults, most(85%) are related to stator. Theanalysis shows:

about 65% failures are due tostator winding burnout

about 15% failures are due to

stator winding short circuits. About 5% failures are due to

stator winding open circuit.

Balance 15% failures are of motorwinding or insulation deterioration.This analysis clearly indicates thatwhile considering the topic of motorprotection, maximum precaution mustbe taken against stator windingfailures.

Motor protection in stator usingbimetal relays:Normally, in a starter circuit, switch,fuse, contactor and bimetallic overloadrelay are provided. The overload relayoffers protection against statorwindings getting overheated due tooverload and the fuse offers protectionagainst short circuits and earth faults.Present generation of bimetal relaysoffer a coarse protection againstsingle phasing also.

Fuses isolate the fault current

themselves but a contactor isactivated (opened) by the overloadrelay for fault isolation. Hence, thecurrent breaking capacity of thecontactor has to be co-ordinated whileselecting overload relay range in astarter feeder. This point is taken intoaccount while deciding the startercomponents, if the selection confirmsto type 2 co-ordination as per IS13947.

When the motor starting time ishigher, the overload relaycharacteristic is cut by the motorstarting curve. This means, the motorwill trip during starting. In such acase, a flatter overload relaycharacteristic is chosen by usingsaturable CT operated overload relay.In some extreme cases, the overloadrelay circuit is even bypassed duringstarting.

Care must be taken in all cases toensure that the cold withstand timeand the hot withstand time of themotor fall above the respectiveoverload relay curves, as indicated inFig.1.

Static overload relays:Electronics found its way inswitchgear through static protectionrelays and timers. Static relays offerprecise setting as against the bimetalrelays. However, the protectionoffered by bimetal relays is against thewinding burnout in motor and alsoburnout of the cables. Such burnoutsare due to the heat generated, and theprotection needs to take residual heatinto account. Also, for properprotection against heating, RMS valueof the current needs to be sensedand not the instantaneous value. Boththese requirements are not met bystatic relays and hence, staticoverload relays have not found to getgood acceptance in the industry.

FEATURE

Comprehensive Motor Protection -Mr. R. S. Mahajan

Electrical Systems & Equipment

Comprehensive Motor ProtectionRelays:About 80-85% of the motor populationis less than 37 kW (50 HP) motors.These motors are generally protectedby a bimetal overload relays and fusecombination, for cost effectiveness.However, the balance 15-20% higherrated motors are expensive and theyare normally protected bycomprehensive motor protection relays.In this article, we shall cover differentaspects of motor protection and howthey can be taken care of in acomprehensive motor protection relay.Comprehensive motor protectionshould cover at least the followingprotections:

Thermal overload Short circuit (not to be provided on

contactor controlled starters) Earth fault/Earth leakage Unbalance Locked rotor (stalling) Single phasing

In addition to these, for large motors,especially for ACB controlled motors,time delayed undervoltage protection isessential. For ACBs, normally thecontrol supply is DC and hence it isessential to open the ACBs controllinglarge motors on undervoltage.Whereas, for contactor controlledstarters, it is assumed that the controlsupply for a contactor coil is derivedfrom power supply and hence loss ofvoltage on power bus will automaticallyresult in dropping out of contactor coil.

L&T offers supervision seriesMotorvision relays for comprehensivemotor protection. In our April-June1999 issue of L&T Current Trends, wehave covered various protectionsoffered by Motorvision relay. To recap,the protections available are:Pre-start condition:

Undervoltage Lockout Process Inhibit (by digital inputs) Adequate thermal capacity to

permit successful start of themotor

Starting condition: Maximum Start Time (for stalling

during starting or inability to reachsynchronous speed due to excessload)

Thermal Overload Short Circuit Earth Fault Single Phasing

Running condition:With the current sampling at 0.7 msec,the Motorvision relay distinguishes"start" condition from "run" condition.Thereby, a separate set of protectionsis available in running condition of themotor. They are:

Thermal overload Short circuit Earth fault Locked rotor (low set overcurrent/

stalling) Unbalance Single phasing Undercurrent Undervoltage with time delay Load increase Thermistor / RTD protection

Protections such as unbalance andundervoltage cause nuisance trippingduring starting and hence are madeapplicable during "run" condition only.A low set overcurrent protection in"run" condition facilitates setting oflocked rotor protection at very lowvalues (say 150% for 2 sec. or so)thereby preventing mechanicaldamage/breakdown.

Relay settings:Availability of a comprehensive relaydemands proper selection of thesettings so that all protections offeredare fully exploited. Let us look at how tochoose the relay settings.

One of the highlights of Motorvisionrelay is that it simulates the thermalcapacity of the motor by means of athermal register. The heating effectinside of the motor is related to thelargest of the three line currents. 100%thermal capacity means the motortemperature has reached themaximum allowed and is the level atwhich an overload trip will occur.

When a motor is stopped for a longtime, the thermal capacity used is zero.This is known as "cold condition"and the motor has 100% of its thermalcapacity available for heating before atrip will occur.

When a motor starts, it draws highercurrent (approx. 6 * FLC) and henceconsumes higher thermal capacityduring starting (till it reaches ratedspeed). It has to be ensured thatadequate thermal capacity is availableon the motor for a successful start.Starting thermal capacity is measuredand recorded by the Motorvision relayand it monitors whether adequatethermal capacity is available on themotor at the time of the next start. Ifnot, on start command, the relay gives"thermal lockout" alarm.

When running at normal FLC for someperiod, the motor reaches hotcondition and a lower value of thermalcapacity is available on such motor.Hot/cold ratio determines the amount ofthermal capacity available on amotor in hot condition. Please refer toTable 1.

A motor with lower hot/cold ratio willhave lesser thermal capacity availablein running condition. For example, in amotor with hot/cold ratio of 70%, atsteady state, in hot condition, themotor has 74% thermal capacityavailability for overloads.

If the motor running current exceedsoverload setting, the thermal capacitywill eventually reach 100% and trip themotor. Time taken to trip depends onthe present value of thermal capacityused and the t6x time setting (the timeset to trip a motor in cold conditionwhen current is 6*FLC).

To trip a motor faster, sometimesspeed switch input is given to theMotorvision relay. It decreases thethermal trip time to 50% of the set hotand cold operating times, when themotor is stalled.

Flexibility to choose hot/cold ratio, t6xtime and max. start time independentlyprovide complete protection for motorswhich have not withstand time lesserthan the starting time.

When a motor is overload for morethan 20%, in place of standardoverload characteristics, fastertrippings can be achieved by using"load increase" protection. In thiscase, the tripping are with a definitetime delay of few seconds. Thisprotection saves the motor fromcarrying overloads for longer durations.Settings for t6x, Hot/Cold ratio or max.start time are based on the datanormally supplied by the motor

manufacturer. In case such data isnot available (which can be a case ofrewound motor), typical settings areto be used. Alternatively, the time-current graph recorded by theMotorvision relay can be used forworking out these settings.

For the clarity on this aspect, letus take a sample data and work on thesettings of Motorvision relay forsuch a motor.

1. Manufacture2. Type and fra3. Application4. Rated outpu5. Rated voltag6. No. of phase7. CTR8. Starting curr9. Starting time10. Locked rotor

(Hot conditio11. Locked rotor

(Cold conditi12. Maximum pe starting time13. No load curre14. Minimum vol % of rated vo15. Starting time starting volta16. Locked rotor min. starting 17. Max. permiss at F.L. at 75%Inputs from proc Normal load

FLC =

Hot / cold ratio =

Printed4703 EEstate,contencustom

Electrica

r : M/s. B.H.E.L., Bhopalme size : Sq. Cage, 1LA7 634-4

: PA FANt (kW) : 650e (Volts) : 6600s and frequency : 3 Ph. 50 Hz.

: 100 A / 1 Aent in % of FLC : 600 %

: 22 sec. withstand time : 23 sec.n) withstand time : 33 sec.on)rmissible : 39 sec.

nt : 30 Atage for starting in : 85 %ltage at minimum : 33 sec.ge withstand time at : 32 sec. (Hot)voltage 45 sec (Cold)ible running time : 5 min. rated voltageess

: 500 kW

SAMPLE MOTOR DATA SHEET

1. Maximum s2. Hot / cold ra3. T6x 4. Under curre

Time delay5. Load Increa

Time delay6. Overcurren

Time delay7. Single phas8. Unbalance

Time delay9. Under volta

Time delay10. Under volta11. Earth fault

Time delay12. Number of

start inhibit 13. Overvoltage

Time delay14. Short circui

650 / √3 * 6.6 (V) * 0.85 (PF) = 67A t6x = Time s(Norma

Hot withstand / Cold withstand 33 * 0.(23/33 * 100)% = 69.6969%~70%

Cold withstand tHence

CALCULATION

by Printania Offset Pvt. Ltd., D 20/21, Shalimar Industrial Estate, Matunga (eamail: pds6@bol.net.in Edited by Dr. Arun C. Vakil for Larsen & Toubro Lim Mumbai 400 001. The views expressed in this magazine are not necessarily thts of this magazine should not be reproduced without the written permission ofers. Editors: Sharmila Banerjee and Lawrence Mohan. Editorial Coordinators: R

For further details on this subject, please contact:l Systems & Equipment Division, Larsen & Toubro Limited, Saki-Vihar Road, P.

Fax : 022-8581024 & e-mail : kotnisnd@lntebg.com

9

tart time : 24 sec.tio : 70%

: 26 sec.nt : 50% of ARC

(ARC=52 A based on normal load): 10 sec.

se : 130%: 10 sec.

t(Trip) : 300% (During running): 5 sec.

e : Enabledcurrent : 15%

: 30 sec.ge : 80 %

: 3 sec.ge lockout : 85% (Refer 14 in data)

: 15%: 1 sec.

starts / hour : 10time : 15 min

: 110%: 10 sec.

t : 10 In

MOTORVISION RELAY SETTINGS

et to trip the relay at 6 *FLClly 80% of cold withstand time)

8 = 26.4 sec. ~ 26 sec.

ime > t6x > Max. start time > Starting time, Max. start time = 24 sec.

st), Mumbai 400 019. Tel: 407 7996/8866/4540 Fax: 402ited, from L & T House, Narottam Morarji Marg, Ballardose of the management of larsen & Toubro Limited. The the Editor. Not for sale - only for Circulation among the.S.Mahajan and Neelam D. Kotnis, EBG-ESE, Powai.

O. Box 8901, Powai, Mumbai 400 072