7/27/2019 electrical-engineering-portal.com-Stator_Overheating_Protection.pdf

1/5

Stator Overheating Protection (on photo bottom of the Motor Stato r Winding; by electrical-f orensics.com)

http://electrical-engineering-portal.com/stator-overheating-protection December 17, 2012

Stator Overheating Protection

Overheating

All mot ors need prot ect ion against overheating resulting from overload, stalled rotor, orunbalanced stator currents.

For complete protection, three-phase motors should have an overload element in eachphase. This is because an open circuit in the supply to the power t ransformer feeding a mot orwill cause twice as much current t o f low in one phase of the mot or as in either of the other twophases, as shown in Figure 1.

Consequently, to be sure that there will be an overload element in the most heavily loadedphase no matter which power-t ransformer phase is open-circuited, one should provideoverload elements in all three phases.

In spite of the desirability of overload elements in all three phases, motors rated about 1500 hpand below are generally provided with elements in only two phases, on the assumption that theopen-phase condition will be detected and corrected before any motor can overheat.

http://electrical-engineering-portal.com/basics-of-3-phase-induction-motor-4http://electrical-engineering-portal.com/basics-of-3-phase-induction-motor-3http://www.printfriendly.com/print/?url=http://electrical-engineering-portal.com/stator-overheating-protection

7/27/2019 electrical-engineering-portal.com-Stator_Overheating_Protection.pdf

2/5

Figure 1 - Illust rating the need f or overcurrentprotection in each phase

Single-phase motors require an overloadelement in only one of the twoconductors.

Motors Other than Essential

Service

Except for some essent ial-servicemotors, whose protection will bediscussed later, it is the practice fo rmotors rated less than about 1500 hp toprovide either replica-type thermal-overload relays or long-t ime inverse-t ime-overcurrent relays or direct-acting trippingdevices to disconnect a motor f rom itssource of supply in the event of overload.

Which type of relay to use is largely amatt er ofpersonal preference.

Other things being equal, the replica t ypewill generally provide the bestprotection because, as shown in Figure2, its t ime-current characteristic more nearly matches the heating characterist ic of a motor overthe full range of overcurrent; also, it may take into account the heating effect of the load onthe mot or before the overload condition occurred.

http://electrical-engineering-portal.com/thermal-overload-motor-relay-protection

7/27/2019 electrical-engineering-portal.com-Stator_Overheating_Protection.pdf

3/5

Figure 2 - Typical motor- heating and protective-relay characteristics. A,motor; B, replica relay; C, inverse-time relay.

Inverse-Time Overcurrent type e ICM

21 - ABB

The inverse-time-overcurrent relaywilltend to overprotect atlow currents and tounder protect at highcurrents, as shown inFigure 2.

However, theovercurrent relay is veryeasy to adjust and test ,and it is self-reset. Forcontinuous-rated motorswithout service facto r orshort-t ime overloadratings, the protectiverelays or devices shouldbe adjusted to t rip at not

more than about 115%of rated mot or current.

For motors with 115%service factor, trippingshould occur at notmore than about 125%of rated motor current.For mot ors with specialshort-t ime overloadrat ings, or with other

service factors, themotor characterist ic will determine the required t rippingcharacterist ic, but t he tripping current should notexceed about 140% of rated mot or current.

The manufacturers recommendations should beobtained in each case.

The overload relays will also provide protection in theevent of phase-to-phase short circuits, and in practiceone set of such relays serves fo r both purposes

wherever possible.

A survey of the practice of a number of powercompanies45 showed that a single set oflongtime inverse-t ime-overcurrent relays, adjusted topick up at 125% to 150% of rated motor current, isused for combined short-circuit and overload protectionof non-essential auxiliary mot ors; they aresupplemented by instantaneous overcurrent relaysadjusted as already described.

Such inverse-t ime overload relays must withstandshort-circuit currents without damage for as long as ittakes to trip the breaker. Also the minimumrequirements as to the number of relays or devices for either funct ion must be fulfilled.

7/27/2019 electrical-engineering-portal.com-Stator_Overheating_Protection.pdf

4/5

Motors rated higher than about 1500 hp are generally provided with resistance temperaturedetectors embedded in the stator slots between the windings. If such temperature detectorsare provided, a single relay operat ing from these detectors is used instead of the replica-typeor inverse-time-overcurrent relays.

Also, current-balance relays capable of operat ing on about 25% or less unbalance betweenthe phase currents should be supplied. If the motor does not have resistance temperaturedetectors, but is provided with current-balance relays, a single replica-t ype thermal overload

relay may be substitutedfor the resistance-temperature-detector relay.

Specially cooled or ventilated motors may require other types of protective equipmentthan those recommended here. For such motors, the manufacturers recommendationsshould be obtained.

Reference 50 gives more useful information on the subject o findustrial-motor protection.

Essential-Service Motors

The protection recommended for some essential-service motors is based on minimizing thepossibility of unnecessarily tripping the motor, even though such practice may sometimesendanger the motor. In other words, long-t ime inverse-t ime overcurrent relays are provided forall motor ratings, but they merely cont rol an alarm and leave tripping in the cont rol of anoperator.

Then, for motors that can suffer locked rotor, supplementary instantaneous overcurrent relays,adjusted to pick up at about 200% to 300% of rated motor current are used, and their contactsare connected in series with the contacts of the inverse time-overcurrent relays to trip themotor breaker automatically.

The instantaneous relays should be of the high-reset t ype to be sure that they will reset whenthe current returns to normal after the starting inrush has subsided. The protection provided bythis type of equipment is illustrated in Figure 3.

http://electrical-engineering-portal.com/motor-protection-depending-on-size-and-voltage-level

7/27/2019 electrical-engineering-portal.com-Stator_Overheating_Protection.pdf

5/5

Figure 3 - Protection characterist ic for essential-service motors . A,motor; B, inverse-t ime relay; C, instanteneous relay.

For essent ial-servicemotors for whichautomatic trippingisdesired in addition tothe alarm for overloadsbetween about 115% ofrated currentand the

pickup of theinstantaneousovercurrent relays,thermal relays of eitherthe replica type or theresistance temperature-detector type should beused, depending on thesize of the motor.

Such relays permit

operation for overloadsas far as possible beyondthe point where the alarmwill be sounded, butwithout damaging themotor to the extent t hatit must be repaired beforeit can be used again.

Resource: The ART &SCIENCE of protective

relaying C. RussellMason