Safe Locked Rotor Time How Safe is It

708

IEEE TRANSACTIONS ON INDUSTRY AND GENERAL APPLICATIONS, VOL.

Safe

Locked Rotor Time: RICHARD L. NAILEN,

Abstract-Stalling of an induction-motor, or its failure to accelerate upon start-up, produces both thermal and mechanical stress within the stator and rotor which can be damaging. Whether the stator winding or rotor cage reaches unsafe stress limits first depends upon individual motor design. "Safe locked time" is considered the maximum period a motor can be locked on the line without significant loss of motor life. The nature of these stresses imposed on motor components, how they vary with design, and why the nature of acceleration heating differs from that of locked rotor heating is explained. Equally important, the effect of the motor's "safe time-current characteristic" (which expresses internal stress limits in terns of line current) on the problem of protective relaying is described. Solution of this problem inust depend upon the system designer's understanding of this characteristic.

ALLING an induction motor by another name can create confusion. It is not good engineering to refer to a motor as a "fuse" which when locked on the line at rated voltage will "blow out" in some fixed number of seconds. It just does not work that way. Let us look at the concept of "safe locked rotor time" to see what it does mean, in terms of motor design characteristics, relay protection problems, and motor manufacturing variations. When a motor is stalled at full voltage, the windings (stator and rotor) heat up hundreds of times faster than when the motor is running at rated load and speed. Insulation, copper, and iron undergo rapid rates of differential thermal expansion, potentially damaging mechanically to 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 is difficult to protect from. Thermal devices are too slow in operation and other devices *- are often set too high to recognize a locked rotor condition" [1]. If relays take the motor off the line too quickly, a normal start may be impossible; if they do not act fast enough, damaging overloads can occur at running speed [2]. The usual recommendation is to supply thermal relays responsive to winding temperature plus long-time induction overcurrent 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 manufacturer sets for his product. The considerable variation in what is "allowable" by different manufacturers was brought out years ago [4] and is summarized in Table I. Paper 71 TP 69-IGA, approved by the Petroleum and Chemical Industry Committee of the IEEE IGA Group for presentation at the 1970 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.

IGA-7,

NO.

How Safe

6, NOVEMBER/DECEMBER 1971

Is It?

SENIOR MEMBER, IEEE

TABLE I ALLOWABLE TOTAL TEMPERATURES AT LOCKED ROTOR (NOT TEMPERATURE RISE) FOR 200-HP MOTORa

Stator WindingS Average value 156 Range of all replies 105-190

Rotor Bar Rotor End Ring 285 140 190-640 85-200

a Compiled from ten different replies to a survey of various manufacturers.

Common limits for larger rotor design, stated as rises above ambient, are, for normal acceleration, 45°C in the stator, 200°C in the rotor bars, and 40°C in the rotor end rings. Under "emergency" conditions, such as locked rotor on 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 to satisfy himself that the limits he uses will not get him into trouble. Differences in individual experience and application, even for what seem to be "similar" motors, are bound to cause variation in that judgment. Figs. 1 and 2 indicate the 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 to expand lengthwise in their slots without damaging other parts, need only be kept below the high temperatures at which loss of fatigue strength begins to be important. Bar ends in the usual squirrel-cage construction, as in Fig. 1(b), are bent back and forth as the end rings heat and cool; this leads to fatigue failure which is accelerated at high temperatures (see Fig. 3). To limit this bending, end-ring temperature is not allowed to go nearly as high. The average in the Table I is high probably because the motor on which the manufacturers were polled was rated only 200 hp; some versions of it were probably built with cast aluminum rotors in which ring expansion becomes a minor factor. Even the 350-hp motor tested in [4] was so constructed. Note, however, that at locked rotor the heating limit may be either in the squirrel-cage rotor or in the stator winding. Many large two-pole motors are "stator limited"; under locked or accelerating heating, the stator winding gets dangerously hot before the rotor reaches its unsafe temperature. 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 therefore of "safe operating time") beyond which failure is instantaneous or even imminent. Although seldom understood, this is a most important point. For example, some people in the industry speak of the motor as a "fuse," having a precise failure or "melting" point at a certain time-current

709

NAILEN: SAFE LOCKED ROTOR TIME

(a)

(b)

(c)

(e)

(d)

(a)

(f)

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

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).

relationship. But this is not true. What we mean by safe 00 operating time at locked rotor or any other overload condi- '-5 tion is that time at which thermal or mechanical strain on the motor becomes great enough that the manufacturer is * 60 no longer willing to warrant normal motor life. This is analogous to the life-load relationship of a ball bearing. At a given speed and load, the bearing manu- 4-4 facturer quotes a specific bearing life in terms of probability t~~~rass that most bearings of that kind will last as least as long as a 54 Copper stated minimum number of hours. If you increase the bearing load 25 percent, 50 percent, or any other such -;20 figure, you do not necessarily cause immediate bearing failure, unlike overloading a fuse, which blows instantly if 4-4t0, overloaded past a certain point. 0 C 500 The bearing may fail, of course, because there is only a 300 200 100 400 high statistical probability (not a certainty) that it would Temperature, Degrees C have lasted the quoted or catalog life even if not overloaded. 3. Loss of strength in rotor bar alloys is liable to be marked at But in general nothing will happen immediately. What will Fig.high temperatures. Limits set up by motor designers for locked rotor or acceleration are meant to keep rotor winding temperatures happen is that instead of lasting 40 000 h, the typical bearbelow 300°C maximum. 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 has Overheating of the stator due to locked rotor conditions doubtless been fostered by statements such as this [6]. can result in the following. m0 C,

.4

Overheating of the squirrel cage rotor can result in the following types of damage. 1. Cracking of bars and end rings 2. Melting of ... cast aluminum rotor. 3. Loosening of rotor core and separation of ... punchings due to oxidation and deterioration of punching enamel.

1. Rapid insulation deterioration and possible immediate 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 conditions for a definite period without incurring some damage of the type mentioned.

710

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

Actually, however, it is no more true that the safe locked rotor time represents the threshold of immediate failure than that insulation life of a stator coil at rated load becomes zero as soon as that load is exceeded by any amount. Life versus load is a continuous function without abrupt discontinuities 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 impossible. It should be noted that locked rotor current exists essentially undiminished during the motor acceleration period. For this reason, the accelerating time should be shorter than the permissible locked rotor time to permit a satisfactory setting of the induction disc relay. [7]

MOTOR

Ii 00 \ SAF*E t TIME A SEC7

N'

SAFE TIME us. AMP CURVE

r, jACCEL.

Ri00o%v. ACCEL.

AXT

I

1~~~~

.T/MP

6ooO -"' Fig. 4. Relation between acceleration time and safe locked time. Note that current does not remain constant at locked rotor value throughout acceleration and that reducing voltage does reduce current but may also dangerously lengthen acceleration time by lowering motor accelerating torque. 100 ZOO 300 400 500

1O0 SAFE TI ME, L--. SEC.

---I

Fig. 4 shows the unwarranted pessimism of that statement. Note that current actually does decrease significantly during the acceleration period. To assume that conditions are 100 200 300 400 500 600JQ%AMPS actually as shown in Fig. 5 may be conservative for the re5. Here we see how assumption that full locked rotor current lay application engineer, but it can impose a great burden Fig.persists throughout acceleration can create protection problem where, as Fig. 4. shows, there actually is no problem. on the motor designer who must perhaps commit unnecessary dollars to an oversize design capable of needlessly lengthening the safe locked rotor time. 2) Reduced voltage starting, to lower locked rotor and A accelerating current, will ease the protection problem in SAFE TIME, a]l cases. Fig. 4 shows why this may not be true. (See [8].) SEC. C Let us say more here about the so-called safe time versus 10 current or "thermal damage curve" illustrated in Figs. 4 and 5. Papers, textbooks, and articles have for years explained how to coordinate this curve with the correspond100 200 300 400 500 6( 0 %AMPS ing curve for thermal protective devices such as relays or Fig. 6. Typical safe time versus current curve. Current at point A represents possible running overload; current at point C cannot be fuses. What is overlooked generally, however, is that while physically realized without stalling motor, so that actual line this curve is a simple continuous function for the proteccurrent immediately goes to point B. tive device, it is much more complex for the motor. Consider the example in Fig. 6. Other properties of the 100A same motor appear in Fig. 7. Rated full load current can be sustained without damage for an indefinitely long time, 80 so at the value of current the curve indicates no limit on RPM6. safe operating time. But overload the motor to 200 perTORQUE 40cent of rated current and the rate of heating in the stator AMPS 20winding rises so fast that safe operating time drops greatly 1,W_

(see point A, Fig. 6). Such overloading normally can result only from increasing the horsepower (or torque) demanded of the motor. Fig. 7, however, shows that this cannot be done much past 200 percent of rating. Still greater load will simply cause the motor to stall, passing rapidly and uncontrollably down to zero speed with an attendant rise in current to the locked rotor value at point B, Fig. 6. Thus the region between "infinity" and point A is a "real" curve, determined solely by certain assumptions about stator heating. Some of these are that short-time winding hot-spot temperature rise may be 30°C above

O

00 100

300 100

0 0

%AMPa

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 the stalled condition) and is determined either by stator or rotor, as explained previously. Normal ventilation or cooling 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 motor normal insulation rating; winding cooling (heat loss) rate to run when the current is at some such value as point C. remains the same as at rated horsepower; all heat gen- This means that the entire curve is of necessity a composite erated in the winding, over and above the rated full load of stator and rotor limitations put together in the attempt loss, remains stored in the copper. to match characteristics of the protective devices. A high

711

NAILEN: SAFE LOCKED ROTOR TIME

degree of precision in such a curve is not to be expectedagain, 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 typical quotations. Availability of data on motor damage characteristics is rather limited and shows wide variations Some standardization would reduce the problem of developing and applying devices for motor protection. [1 ] In obtaining motor heating curves for use in applying thermal protection, the kind of information provided by different manufacturers has varied widely. It would be a great help if standards could be established We would expect relatively small differences if the curves were prepared on the same basis, since the motors were selected to have similar characteristics. [9] ...

...

...

An expression of the heating rate of the machine at relatively high overcurrents does not appear on the motor nameplate. The listing, in a standard fashion of this thermal constant, however complicated, would be a step in the right direction for the more successful application of certain motor protective devices. [101

They overlook several important conditions. In any event, it is wishful thinking to expect this kind of standardization, certainly until such time as power system operation, driven machine design, and petrochemical plant design are standardized. For one thing, safe locked rotor time (the "anchor point" of thermal damage curves) depends on power transferred across the motor air gap, which is in turn directly related to motor accelerating torque. A high torque design will tend to have lower safe locked time that one based on standard National Electrical Manufacturers Association (NEMA) torques. Here are some specific differences between various motor designs which affect heating rates and therefore the thermal damage characteristics: 1) use of high resistance copper alloy rotor bars to improve accelerating torque or to redistribute the generation 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 allowable bar thermal stresses and the distribution of heating

throughout the section; 4) use of expansion restraints on some designs, such as the "retention caps" shown in Fig. 1 (c); 5) different construction methods, as illustrated in Figs. 1 and 2. Not only do construction differences influence how locked rotor heat is generated throughout the motor and the temperature limits allowed for the various parts, but they also influence heat transfer from windings to the laminations and the outside air. Cast rotor bars, for example, are in intimate contact with the laminations along the sides of the slots; heat travels from bars to laminations much faster than in a fabricated bar rotor where the contact is relatively poor.

In the stator, both mechanical fatigue and thermal degradation of the insulation must be considered, which means a fairly low limit on short-time overheating. Use of different insulation systems and different methods and degrees of winding bracing will again mean variation between manufacturers. Users have complained not only about nonuniformity of thermal damage data, but about the difficulty of getting any data at all. Time-current curves for motors are rather difficult to obtain and are frequently not available when relay settings are made. In most cases, a single point corresponding to permissible locked rotor time is usually adequate for selecting and setting relays for pipeline pump motors. [7]

Speaking only for one motor manufacturer, we encounter very few requests for such curves-less than half of the motor orders above 1000 hp ever call for a safe time versus current curve, except for power plant auxiliary drives, where the fraction is appreciably higher. Frankly, industrial users often seem unaware of the real significance of the curve, leading us to believe they are unsure of the finer 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 than some performance curves (such as speed versus torque) which are requested far more often. We are more than willing to furnish safe time versus current data on any motor, because a customer who makes intelligent use of that data has a better chance of a successful motor application than one who does not. "Normal" range of safe locked rotor time for large induction motors is from 5 to 50 s, the higher values applying to totally enclosed ribbed frame machines with relatively poor ventilation and therefore oversize for the rating compared to open motors. Safe locked times as low as 2'/2 s are occasionally encountered. Values less than 5 s may pose a protection problem. If the motor cannot tolerate normal acceleration conditions without some loss of life, the user may decide such loss is quite tolerable rather than to risk tripping the motor off the line at every start. So he sets his relays high enough to avoid these nuisance trips. This is not an uncommon situation. Because of the imprecise nature of the motor time versus current curve in the region A to B in Fig. 6, as already explained, the problem is usually not too serious. But we are now just beginning to encounter another situation, one in which the refinery or chemical plant user wishes to use relays having a minimum operating time of 15 s at about 650 percent of rated motor current and not suffer any loss of protection. His reasons are economic. Explosion-proof equipment is required because arcing contacts are present; the 'motors themselves (250-800 hp) need not be explosion-proof and hence have fairly short safe locked times, usually 5 to 10 s. The 15-s relay, which can be oil-immersed for explosive atmospheres (Fig. 8), costs under $50'.' Conventional induction-type relays, entirely enclosed in' explosion-proof housings, would cost as much as $1000 per motor. This

712

IEEE TRANSACTIONS ON

Fig. 8. Induction-type overload relay for oil-immersed application in hazardous atmosphere; minimum setting at about 650-percent rated motor current is 15 s. may be economic when horsepower is 1000 or more, but not for smaller ratings. We solve this problem by modifying our motor designs to provide extra thermal capacity, allowing an increase in safe locked rotor time. About half of our standard motor designs do not meet the 15-s limit without modification. Among the changes are higher rotor bar resistances, using alloys instead of copper, bigger end rings, larger stator slots to hold more copper, or even a larger frame size. Before going that far, however, it was necessary to establish with the user just what his operating conditions really were. From these discussions emerged the "cold start" and "infrequent emergency" concepts which justified substantial increase in temperature limits so that many designs could be used without change. The motor designer or application engineer usually does not know what user operating practices may lead to stalling or a failure to start. Some drives, such as rubber mills, pulverizers, or crushers, may be subjected to this abuse very frequently, depending on the operating cycle and the material being processed. Thus we compute the safe locked rotor time from the same allowable rotor and stator temperature limits as used for normal acceleration. The motor is assumed to be "hot" or up to full running temperature prior to the stall. However, in some applications, such as the refinery service calling for the 15-s minimum safe time, entirely

INDUSTRY AND GENERAL APPLICATIONS, NOVEMBER/DECEMBER 1971

different conditions apply. Here, the motor is never stalled while running. Instead, it may be unable to start after an idle period because of some abnormality such as "a 2 by 4 in the pipeline." This occurs "cold," with the motor only at ambient temperature. Furthermore, the condition is a true "emergency" in the sense of extremely infrequent occurrence, perhaps only a few times during the motor's entire lifetime. In that case, the allowable stator and rotor temperature rises may be increased 50 percent because of the infrequency and by about another 20 percent because of the low initial temperature. Again, we are not saying that failure is immediate or even imminent if the safe time so calculated is exceeded, meaning that the temperature limits so carefully figured are edged up even higher. All we are saying is that such abuse will shorten the life of the motor by an amount not exactly calculable, but enough that we are unwilling to warrant 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 no significant loss of motor life results if that time is not exceeded.

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 Electric Co., Newark, N. J., 1957. [4] W. J. Martiny, R. M. McCoy, and H. B. Margolis, "Thermal

[5]

[6] [7] [8] [9]

[101

relationships in an induction motor under normal and abnormal operations," presented at the AIEE Winter General Meeting, New York, N.Y., Jan. 31-Feb. 5, 1960, Paper 60-225. J. F. Heidbreder, "Induction motor temperature characteristics," AIEE Trans. (Power App. Syst.), vol. 77, pp. 801-804, Oct. 1958. V. J. Picozzi, "Factors influencing starting duty of large induction motors," AIEE Trans. (Power App. Syst.), vol. 78, pp. 401-407, June 1959. L. U. Eidson and A. A. Regotti, "Relaying requirements for pipe line pump motors," presented at the 1969 IEEE Petroleum and Chemical Industry Technical Conference, Los Angeles, Calif., Sept. 15-17, Paper PCI-69-35. R. L. Nailen, "Stop rotor troubles before they start," Plant Eng., pp. 156-160, Dec. 1966. J. M. Bisbee, "Problems in the application of thermal protection to motors," presented at the AIEE Fall General Meeting, Chicago, Ill., Oct. 3-7, 1955, Paper 55-767. R. E. Walters, "Characteristics of thermal relays that influence their selection when used as motor protective devices," presented 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 received 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 Electric Corporation, Sunnyvale, Calif., on electrical and mechanical design of machines through 19 000 kVA. In 1964 he became a Senior Engineer for the Louis Allis Company, Division of Litton 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 a Registered Professional Engineer in the State of Wisconsin.