IEEE TRANSACTIONS ON INDUSTRY AND GENERAL APPLICATIONS, VOL. IGA-7, NO. 6, NOVEMBER/DECEMBER 1971

Safe Locked Rotor Time: How Safe Is It?RICHARD L. NAILEN, SENIOR MEMBER, IEEE

Abstract-Stalling of an induction-motor, or its failure to accelerateupon start-up, produces both thermal and mechanical stress withinthe stator and rotor which can be damaging. Whether the statorwinding or rotor cage reaches unsafe stress limits first depends uponindividual motor design. "Safe locked time" is considered the maxi-mum period a motor can be locked on the line without significant lossof motor life. The nature of these stresses imposed on motor com-ponents, how they vary with design, and why the nature of accelera-tion heating differs from that of locked rotor heating is explained.Equally important, the effect of the motor's "safe time-current char-acteristic" (which expresses internal stress limits in terns of linecurrent) on the problem of protective relaying is described. Solutionof this problem inust depend upon the system designer's understand-ing of this characteristic.

ALLING an induction motor by another name can

create confusion. It is not good engineering to refer toa motor as a "fuse" which when locked on the line at ratedvoltage will "blow out" in some fixed number of seconds.It just does not work that way. Let us look at the conceptof "safe locked rotor time" to see what it does mean, in

terms of motor design characteristics, relay protectionproblems, and motor manufacturing variations.When a motor is stalled at full voltage, the windings

(stator and rotor) heat up hundreds of times faster thanwhen the motor is running at rated load and speed. In-sulation, copper, and iron undergo rapid rates of differen-tial thermal expansion, potentially damaging mechanicallyto the windings. Furthermore, the high temperature can

destroy insulation thermally if allowed to persist too long.Protective relays are intended to prevent this. But

"locked rotor is an extreme overload condition which isdifficult to protect from. Thermal devices are too slow inoperation and other devices *- are often set too high torecognize a locked rotor condition" [1].

If relays take the motor off the line too quickly, a normalstart may be impossible; if they do not act fast enough,damaging overloads can occur at running speed [2]. Theusual recommendation is to supply thermal relays respon-sive to winding temperature plus long-time induction over-

current relays [3].The terms "slow" or "fast" in this conitext refer to time

periods longer or shorter than some allowable limit or

"safe locked rotor time" which the motor manufacturersets for his product. The considerable variation in what is

"allowable" by different manufacturers was brought outyears ago [4] and is summarized in Table I.

Paper 71 TP 69-IGA, approved by the Petroleum and ChemicalIndustry Committee of the IEEE IGA Group for presentation at the1970 IEEE Petroleum and Chemical Industry Technical Conference,Tulsa, Okla., September 14-16. Manuscript received August 26,1971.The author is with the Louis Allis Company, Division of Litton

Industries, Milwaukee, Wis. 53201.

TABLE IALLOWABLE TOTAL TEMPERATURES AT LOCKED ROTOR

(NOT TEMPERATURE RISE) FOR 200-HP MOTORa

Stator WindingS Rotor Bar Rotor End Ring

Average value 156 285 140Range of all replies 105-190 190-640 85-200

a Compiled from ten different replies to a survey of various manu-facturers.

Common limits for larger rotor design, stated as risesabove ambient, are, for normal acceleration, 45°C in thestator, 200°C in the rotor bars, and 40°C in the rotor endrings. Under "emergency" conditions, such as locked rotoron a very infrequent basis, these become, respectively, 75,300, and8O0C [5].Some of these values were based on actual tests. Whether

by test or otherwise, however, each manufacturer has tosatisfy himself that the limits he uses will not get him intotrouble. Differences in individual experience and applica-tion, even for what seem to be "similar" motors, are boundto cause variation in that judgment. Figs. 1 and 2 indicatethe variations to be expected in rotor construction.The relative magnitudes of the average figures in Table I

can be explained this way: rotor bars, relatively free toexpand lengthwise in their slots without damaging otherparts, need only be kept below the high temperatures atwhich loss of fatigue strength begins to be important. Barends in the usual squirrel-cage construction, as in Fig. 1(b),are bent back and forth as the end rings heat and cool; thisleads to fatigue failure which is accelerated at high tempera-tures (see Fig. 3).To limit this bending, end-ring temperature is not al-

lowed to go nearly as high. The average in the Table I ishigh probably because the motor on which the manu-facturers were polled was rated only 200 hp; some versionsof it were probably built with cast aluminum rotors inwhich ring expansion becomes a minor factor. Even the350-hp motor tested in [4] was so constructed.

Note, however, that at locked rotor the heating limitmay be either in the squirrel-cage rotor or in the statorwinding. Many large two-pole motors are "stator limited";under locked or accelerating heating, the stator windinggets dangerously hot before the rotor reaches its unsafetemperature. Most other machines are "rotor limited."The word "limit" in all this is unfortunate. We do not

mean to imply some value of temperature (and thereforeof "safe operating time") beyond which failure is instan-taneous or even imminent. Although seldom understood,this is a most important point. For example, some peoplein the industry speak of the motor as a "fuse," having aprecise failure or "melting" point at a certain time-current

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NAILEN: SAFE LOCKED ROTOR TIME

(b)

(c) (d) (a)

(e) (f)

Fig. 1. Some of many currently used variations in rotor constructionfound in various types and sizes of motors above 200 hp, bothstandard and special purpose. (a) Die cast aluminum rotor, some-times used on ratings as large as 2000 hp. (b) Cap-type copper alloyend ring, brazed to bars. (c) Similar to (b) but with rolled copperend ring and steel retention cap to control end-ring thermal expan-sion. (d) Aluminum bars welded to cast aluminum end ring. (e), (f)Two versions of double-cage high-torque design.

relationship. But this is not true. What we mean by safeoperating time at locked rotor or any other overload condi-tion is that time at which thermal or mechanical strain onthe motor becomes great enough that the manufacturer isno longer willing to warrant normal motor life.

This is analogous to the life-load relationship of a ballbearing. At a given speed and load, the bearing manu-facturer quotes a specific bearing life in terms of probabilitythat most bearings of that kind will last as least as long as astated minimum number of hours. If you increase thebearing load 25 percent, 50 percent, or any other suchfigure, you do not necessarily cause immediate bearingfailure, unlike overloading a fuse, which blows instantly ifoverloaded past a certain point.The bearing may fail, of course, because there is only a

high statistical probability (not a certainty) that it wouldhave lasted the quoted or catalog life even if not overloaded.But in general nothing will happen immediately. What willhappen is that instead of lasting 40 000 h, the typical bear-ing will fail in 30 000 or 20 000 h.Thus the fuse concept of motor behavior under locked

rotor conditions must be discarded as misleading. It hasdoubtless been fostered by statements such as this [6].

Overheating of the squirrel cage rotor can result in thefollowing types of damage.

1. Cracking of bars and end rings2. Melting of ... cast aluminum rotor.3. Loosening of rotor core and separation of ... punch-

ings due to oxidation and deterioration of punchingenamel.

kV)Fig. 2. Rotor construction showing considerable differences which

affect locked rotor and acceleration heating. (a) High-speed rotor,resembles Fig. 1 (c). (b) Low-speed rotor, resembles Fig. 1 (e).

00m0

'-5C,

.4

* 60

4-4

54

-;204-4t0,

0C 100

t~~~rassCopper

200 300 400 500

Temperature, Degrees C

Fig. 3. Loss of strength in rotor bar alloys is liable to be marked athigh temperatures. Limits set up by motor designers for lockedrotor or acceleration are meant to keep rotor winding temperaturesbelow 300°C maximum.

Overheating of the stator due to locked rotor conditionscan result in the following.

1. Rapid insulation deterioration and possible im-mediate failure of windings.

2. Melting of soldered connections.3. Burning of stator iron due to winding failure.

An induction motor can only withstand locked rotor condi-tions for a definite period without incurring some damage ofthe type mentioned.

(a)

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IEEE TRANSACTIONS ON INDUSTRY AND GENERAL APPLICATIONS, NOVEMBER/DECEMBER 1971

Actually, however, it is no more true that the safe lockedrotor time represents the threshold of immediate failurethan that insulation life of a stator coil at rated load be-comes zero as soon as that load is exceeded by any amount.Life versus load is a continuous function without abruptdiscontinuities over quite a wide range.Some other fallacies to be found throughout the folklore

of motor application are the following.1) Safe locked rotor time must always be greater than

drive acceleration time or proper relay protection is im-possible.

It should be noted that locked rotor current exists essen-tially undiminished during the motor acceleration period.For this reason, the accelerating time should be shorterthan the permissible locked rotor time to permit a satis-factory setting of the induction disc relay. [7]

Fig. 4 shows the unwarranted pessimism of that statement.Note that current actually does decrease significantly dur-ing the acceleration period. To assume that conditions areactually as shown in Fig. 5 may be conservative for the re-lay application engineer, but it can impose a great burdenon the motor designer who must perhaps commit unneces-sary dollars to an oversize design capable of needlesslylengthening the safe locked rotor time.

2) Reduced voltage starting, to lower locked rotor andaccelerating current, will ease the protection problem ina]l cases. Fig. 4 shows why this may not be true. (See [8].)

Let us say more here about the so-called safe time versuscurrent or "thermal damage curve" illustrated in Figs. 4and 5. Papers, textbooks, and articles have for years ex-plained how to coordinate this curve with the correspond-ing curve for thermal protective devices such as relays orfuses. What is overlooked generally, however, is that whilethis curve is a simple continuous function for the protec-tive device, it is much more complex for the motor.

Consider the example in Fig. 6. Other properties of thesame motor appear in Fig. 7. Rated full load current canbe sustained without damage for an indefinitely long time,so at the value of current the curve indicates no limit onsafe operating time. But overload the motor to 200 per-cent of rated current and the rate of heating in the statorwinding rises so fast that safe operating time drops greatly(see point A, Fig. 6).Such overloading normally can result only from increas-

ing the horsepower (or torque) demanded of the motor.Fig. 7, however, shows that this cannot be done much past200 percent of rating. Still greater load will simply causethe motor to stall, passing rapidly and uncontrollably downto zero speed with an attendant rise in current to the lockedrotor value at point B, Fig. 6.Thus the region between "infinity" and point A is a

"real" curve, determined solely by certain assumptionsabout stator heating. Some of these are that short-timewinding hot-spot temperature rise may be 30°C abovenormal insulation rating; winding cooling (heat loss) rateremains the same as at rated horsepower; all heat gen-erated in the winding, over and above the rated full loadloss, remains stored in the copper.

IiSAFTIMESEC

00 \ MOTOR SAFE TIME*E t N' us. AMP CURVE

7 Ar,jACCEL.Ri00o%v. AXT

ACCEL. I1~~~~ .T/MP

100 ZOO 300 400 500 6ooO -"'

Fig. 4. Relation between acceleration time and safe locked time.Note that current does not remain constant at locked rotor valuethroughout acceleration and that reducing voltage does reducecurrent but may also dangerously lengthen acceleration time bylowering motor accelerating torque.

1O0SAFETI ME,SEC.

100 200 300 400 500 600JQ%AMPS

Fig. 5. Here we see how assumption that full locked rotor currentpersists throughout acceleration can create protection problemwhere, as Fig. 4. shows, there actually is no problem.

1,W_

SAFETIME,SEC.

10

100

A

C

200 300 400 500 6( 0 %AMPSFig. 6. Typical safe time versus current curve. Current at point A

represents possible running overload; current at point C cannot bephysically realized without stalling motor, so that actual linecurrent immediately goes to point B.

100A

80

RPM6.

40- TORQUEAMPS

20-

00100 300 0 0 %AMPaO 100 2w %TORQUE

Fig. 7. Speed-torque and speed-current curves for same motor.Note that point C is below breakdown point on speed-torque curve.

Likewise, point B is a "real" point (representing thestalled condition) and is determined either by stator orrotor, as explained previously. Normal ventilation or cool-ing does not exist here, because the motor is not running.The region between points A and B is not a real curve

for the motor, because it is not possible to cause the motorto run when the current is at some such value as point C.This means that the entire curve is of necessity a compositeof stator and rotor limitations put together in the attemptto match characteristics of the protective devices. A high

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NAILEN: SAFE LOCKED ROTOR TIME

degree of precision in such a curve is not to be expected-again, a contrast to the fuse concept.Over the years, many authors have written wistfully of a

utopian era in which these safe time versus current curves

would all look pretty much alike. Here are some typicalquotations.

Availability of data on motor damage characteristics israther limited and shows wide variations ... Some stan-dardization would reduce the problem of developing andapplying devices for motor protection. [1 ]

In obtaining motor heating curves for use in applyingthermal protection, the kind of information provided bydifferent manufacturers has varied widely. It would be a

great help ... if standards could be established ... Wewould expect relatively small differences if the curves were

prepared on the same basis, since the motors were selectedto have similar characteristics. [9]

An expression of the heating rate of the machine at rela-tively high overcurrents does not appear on the motornameplate. The listing, in a standard fashion of this thermalconstant, however complicated, would be a step in the rightdirection for the more successful application of certainmotor protective devices. [101

They overlook several important conditions. In any

event, it is wishful thinking to expect this kind of standard-ization, certainly until such time as power system opera-

tion, driven machine design, and petrochemical plantdesign are standardized. For one thing, safe locked rotortime (the "anchor point" of thermal damage curves) de-pends on power transferred across the motor air gap, whichis in turn directly related to motor accelerating torque.A high torque design will tend to have lower safe lockedtime that one based on standard National Electrical Manu-facturers Association (NEMA) torques.

Here are some specific differences between various motordesigns which affect heating rates and therefore the thermaldamage characteristics:

1) use of high resistance copper alloy rotor bars toimprove accelerating torque or to redistribute the gen-

eration of heat in case of a severe acceleration;2) use of random windings in some large low-voltage

machines, versus formed coil construction in others;3) rotor bar cross-sectional shape, affecting both allow-

able bar thermal stresses and the distribution of heatingthroughout the section;

4) use of expansion restraints on some designs, suchas the "retention caps" shown in Fig. 1 (c);

5) different construction methods, as illustrated inFigs. 1 and 2.Not only do construction differences influence how

locked rotor heat is generated throughout the motor andthe temperature limits allowed for the various parts, butthey also influence heat transfer from windings to thelaminations and the outside air. Cast rotor bars, for ex-

ample, are in intimate contact with the laminations alongthe sides of the slots; heat travels from bars to laminationsmuch faster than in a fabricated bar rotor where the con-

tact is relatively poor.

In the stator, both mechanical fatigue and thermaldegradation of the insulation must be considered, whichmeans a fairly low limit on short-time overheating.Use of different insulation systems and different methodsand degrees of winding bracing will again mean variationbetween manufacturers.

Users have complained not only about nonuniformityof thermal damage data, but about the difficulty of gettingany data at all.

Time-current curves for motors are rather difficult to ob-tain and are frequently not available when relay settingsare made. In most cases, a single point corresponding topermissible locked rotor time is usually adequate for select-ing and setting relays for pipeline pump motors. [7]

Speaking only for one motor manufacturer, we en-counter very few requests for such curves-less than half ofthe motor orders above 1000 hp ever call for a safe timeversus current curve, except for power plant auxiliarydrives, where the fraction is appreciably higher. Frankly,industrial users often seem unaware of the real significanceof the curve, leading us to believe they are unsure of thefiner points of motor protection with thermal relays.Once a curve is requested, however, we can supply it as

promptly as any other. It is actually simpler to plot thansome performance curves (such as speed versus torque)which are requested far more often. We are more than will-ing to furnish safe time versus current data on any motor,because a customer who makes intelligent use of that datahas a better chance of a successful motor application thanone who does not."Normal" range of safe locked rotor time for large in-

duction motors is from 5 to 50 s, the higher values applyingto totally enclosed ribbed frame machines with relativelypoor ventilation and therefore oversize for the rating com-pared to open motors. Safe locked times as low as 2'/2 s areoccasionally encountered.

Values less than 5 s may pose a protection problem. Ifthe motor cannot tolerate normal acceleration conditionswithout some loss of life, the user may decide such loss isquite tolerable rather than to risk tripping the motor off theline at every start. So he sets his relays high enough toavoid these nuisance trips.

This is not an uncommon situation. Because of the im-precise nature of the motor time versus current curvein the region A to B in Fig. 6, as already explained, theproblem is usually not too serious. But we are now just be-ginning to encounter another situation, one in which therefinery or chemical plant user wishes to use relays havinga minimum operating time of 15 s at about 650 percent ofrated motor current and not suffer any loss of protection.

His reasons are economic. Explosion-proof equipment isrequired because arcing contacts are present; the 'motorsthemselves (250-800 hp) need not be explosion-proof andhence have fairly short safe locked times, usually 5 to 10 s.The 15-s relay, which can be oil-immersed for explosiveatmospheres (Fig. 8), costs under $50'.' Conventional in-duction-type relays, entirely enclosed in' explosion-proofhousings, would cost as much as $1000 per motor. This

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IEEE TRANSACTIONS ON INDUSTRY AND GENERAL APPLICATIONS, NOVEMBER/DECEMBER 1971

Fig. 8. Induction-type overload relay for oil-immersed applicationin hazardous atmosphere; minimum setting at about 650-percentrated motor current is 15 s.

may be economic when horsepower is 1000 or more, butnot for smaller ratings.We solve this problem by modifying our motor designs

to provide extra thermal capacity, allowing an increase insafe locked rotor time. About half of our standard motordesigns do not meet the 15-s limit without modification.Among the changes are higher rotor bar resistances, usingalloys instead of copper, bigger end rings, larger statorslots to hold more copper, or even a larger frame size.

Before going that far, however, it was necessary to estab-lish with the user just what his operating conditions reallywere. From these discussions emerged the "cold start" and"infrequent emergency" concepts which justified sub-stantial increase in temperature limits so that manydesigns could be used without change.The motor designer or application engineer usually does

not know what user operating practices may lead to stall-ing or a failure to start. Some drives, such as rubber mills,pulverizers, or crushers, may be subjected to this abusevery frequently, depending on the operating cycle and thematerial being processed. Thus we compute the safe lockedrotor time from the same allowable rotor and statortemperature limits as used for normal acceleration. Themotor is assumed to be "hot" or up to full running tem-perature prior to the stall.

However, in some applications, such as the refineryservice calling for the 15-s minimum safe time, entirely

different conditions apply. Here, the motor is never stalledwhile running. Instead, it may be unable to start after anidle period because of some abnormality such as "a 2 by 4in the pipeline." This occurs "cold," with the motoronly at ambient temperature. Furthermore, the conditionis a true "emergency" in the sense of extremely infrequentoccurrence, perhaps only a few times during the motor'sentire lifetime. In that case, the allowable stator and rotortemperature rises may be increased 50 percent because ofthe infrequency and by about another 20 percent becauseof the low initial temperature.

Again, we are not saying that failure is immediate oreven imminent if the safe time so calculated is exceeded,meaning that the temperature limits so carefully figuredare edged up even higher. All we are saying is that suchabuse will shorten the life of the motor by an amount notexactly calculable, but enough that we are unwilling towarrant the application for normal service life.Thus the answer to the question "safe locked rotor time:

how safe is it?" becomes simply: safe enough so that nosignificant loss of motor life results if that time is notexceeded.

REFERENCES[1] 0. A. Lentz and T. Niessink, "Problems in medium size motor

protection," presented at the AIEE Fall General Meeting,Chicago, Ill., Oct. 3-5, 1955, Paper 55-696.

[2] D. Beaman, Ed., Industrial Power Systems Handbook, 1st ed.New York: McGraw-Hill, 1955.

[3] Westinghouse Applied Protective Relaying, Westinghouse ElectricCo., Newark, N. J., 1957.

[4] W. J. Martiny, R. M. McCoy, and H. B. Margolis, "Thermalrelationships in an induction motor under normal and abnormaloperations," presented at the AIEE Winter General Meeting,New York, N.Y., Jan. 31-Feb. 5, 1960, Paper 60-225.

[5] J. F. Heidbreder, "Induction motor temperature character-istics," AIEE Trans. (Power App. Syst.), vol. 77, pp. 801-804,Oct. 1958.

[6] V. J. Picozzi, "Factors influencing starting duty of large induc-tion motors," AIEE Trans. (Power App. Syst.), vol. 78, pp.401-407, June 1959.

[7] L. U. Eidson and A. A. Regotti, "Relaying requirements for pipeline pump motors," presented at the 1969 IEEE Petroleum andChemical Industry Technical Conference, Los Angeles, Calif.,Sept. 15-17, Paper PCI-69-35.

[8] R. L. Nailen, "Stop rotor troubles before they start," PlantEng., pp. 156-160, Dec. 1966.

[9] J. M. Bisbee, "Problems in the application of thermal protec-tion to motors," presented at the AIEE Fall General Meeting,Chicago, Ill., Oct. 3-7, 1955, Paper 55-767.

[101 R. E. Walters, "Characteristics of thermal relays that influencetheir selection when used as motor protective devices," pre-sented at the AIEE Midwest General Meeting, Oct. 1948.

Richard L. Nailen (M'51-SM'68) was born in San Jose, Calif., on January 2, 1928. He re-ceived the B.E.E. degree from the University of Santa Clara, Santa Clara, Calif., in 1950.From 1953 to 1964 he was employed in the Motor Engineering Section, Westinghouse Elec-

tric Corporation, Sunnyvale, Calif., on electrical and mechanical design of machines through19 000 kVA. In 1964 he became a Senior Engineer for the Louis Allis Company, Division ofLitton Industries, Milwaukee, Wis., where he is now Chief Electrical Engineer, Large Motors.He is the author of a number of technical papers on motor applications.

Mr. Nailen is a member of Tau Beta Pi, the National Fire Protection Association, and is aRegistered Professional Engineer in the State of Wisconsin.

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