Operating temperature considerations and performance.pdf

7/30/2019 Operating temperature considerations and performance.pdf

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1120 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 37, NO. 4, JULY/AUGUST 2001

Operating Temperature Considerationsand Performance Characteristics

for IEEE 841 MotorsAustin H. Bonnett, Fellow, IEEE

AbstractThis paper reviews the operating temperature con-siderations and performance characteristics for totally enclosedfan-cooled motors as covered in the IEEE 841-2000 motor stan-dard. NEMA MG-1-1998 motor standards are also included sincethey are embodied in the IEEE standard. Although the scope ofproduct covered is for ac squirrel-cage induction motors through500 hp, the material presented has application with many othersizes and types of motor. The paper reviews this standard as it ap-plies to the motor operating temperature and various performancecharacteristics. The impact of temperature on the stator winding,rotor cage, bearings, lubrications, as well as the effects on motor

efficiency and other applicable life factors, are considered.

Index TermsAC induction motors, IEEE 841, insulation, rotortemperature, service factor, thermal life, winding temperature.

I. INTRODUCTION

THE effects of variations in voltage, load, speed, starting,

ambient temperature, service factor, and altitude will

each be reviewed. Examples of how these factors influence the

motor performance and life are provided. Reasonable operating

limits for these variations are provided and suggested rules of

thumb are presented that will enhance the motor reliability

and longevity. Although the IEEE 841-2000 motor standard

also covers vertical motors, only horizontal motors will beaddressed in this paper. Many of the principles apply to both

types of motors.

Fig. 1 shows a typical IEEE 841 TEFC horizontal motor in-

tended for use in the petroleum and chemical industry and other

severe duty totally enclosed fan-cooled (TEFC) applications up

to 500 hp.

II. TEMPERATURE CONSIDERATIONS

The motor has three main elements. They are the stator, rotor,

and bearing system. Each of these elements must be considered

individually and with regard to how they influence each other.

Each of these elements must be balanced against the motorcooling system which is made up of conduction (the frame,

Paper PID 0113, presented at the 2000 IEEE Petroleum and ChemicalIndustry Technical Conference, San Antonio, TX, September 1014, and ap-proved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONSby the Petroleum and Chemical Industry Committee of the IEEE IndustryApplications Society. Manuscript submitted for review September 15, 2000and released for publication May 1, 2001.

The author, retired, was with U.S. Electrical Motors, Emerson Motors, St.Louis, MO 63136 USA. He is now at 4546 Gatemont Drive, Chesterfield, MO63017 USA (e-mail: [email protected]).

Publisher Item Identifier S 0093-9994(01)05898-4.

Fig. 1. Typical IEEE 841 TEFC motor.

lamination, and shaft), radiation (end turns, rotor end rings,

and frame) and convection (the fan and cooling surfaces of the

motor).

The base sources of heat generation are the losses to the

rotor and stator, the core loss of the electrical laminations, the

stray load loss associated with the air gap and tooth surfaces of

the rotor and stator and the bearing system losses.

Combining these three elements, the motor is made up of its

heat source within the stator, rotor and bearing system, and the

cooling system which is made up of the conduction, convection,

and radiation elements.

All of these must be balanced so the heat generated bythe motor load and ambient can be satisfactorily disposed

of through the motor cooling system while maintaining an

equilibrium and without damaging the stator, rotor, or bearings.

Successful operation is measured against motor performance

criteria outlined in the proposed IEEE 841-2000, NEMA

MG-1-1998, and IEEE standards, practices, and recommended

guidelines.

Fig. 2 graphically illustrates the basic motor elements, heat

sources and cooling system for a typical IEEE 841-2000 TEFC

horizontal motor.

00939994/01$10.00 2001 IEEE


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BONNETT: PERFORMANCE CHARACTERISTICS FOR IEEE 841 MOTORS 1121

Fig. 2. Cooling circuit and heat sources for typical IEEE 841 TEFC.

TABLE ITHERMAL AGING PROCESS

III. THERMAL AGING PROCESS

The thermal aging process as shown in Table I is always

present and is occurring even when the motor is not running. At

this extreme, it is aging at the rate caused by the ambient tem-

perature which the winding is exposed to. The other extreme

is when the motor is operating under service factor conditions

which is limited to 155 C (Class F) average winding tempera-ture when the motor is running.

Other stresses present while the motor is running include di-

electric, mechanical, and environmental stresses (which may

also be present when the motor is not running). At some point,

the thermal aging renders the winding insulation vulnerable to

these stresses and the system begins to short out between

turns, or to ground at which time the insulation system has failed

by definition.

Fig. 3 provides temperature life curves for the standard motor

insulation systems that are available today. These curves assume

that the insulation life doubles for a 10 C decrease in temper-

ature.

Fig. 3. Temperature versuslife curves forinsulationsystem (per IEEE 117 and101).

IV. NEMA/IEEE INSULATION CLASSIFICATIONS AND

TEMPERATURE RISE

The following section comes directly from the newly pro-

posed IEEE 841-2000 Standard.

5.4 Insulation System and Temperature Rise:

a) Insulation system

1) The motor shall have a nonhygroscopic, chemical

and humidity-resistant insulation system. The


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TABLE IITHERMAL RISE NEMA MG-1-12.44

thermal rating of the insulation system shall be a

minimum of Class F as defined in section 1.66 of

NEMA MG 1-1998. A lead wire having a temper-

ature rating that is more than 5 C (9 F) less than

the temperature rating of the insulation system in

which it is connected shall be compatible and shall

be separated from the windings by a barrier orenvelope of a material compatible with the system.

The temperature rating of the lead wire shall not

be less than 125 C.

b) Temperature riseWhen operated at rated voltage, fre-

quency, and horsepower, the average temperature rise of

any phase of the stator winding shall not exceed 80 C by

winding resistance.

NEMA Temperature Limits: Note that, in Table II, NEMA

allows for a Class F system with a 1.15 service factor to have

a temperature rise of 115 C, whereas, the IEEE 841 standard

limits the temperature rise to 80 C. The result is a theoreticalincrease in the thermal life of the winding from 20 000 hours to

more than 1 000 000 hours when operated at full load.

Motor Service Factor: There is still much confusion as to

the proper definition of the motor service factor and how it is to

be applied. The IEEE 841 standard provides for a 1.15 service

factor that is not intended to be used as part of the base load, but

instead is intended to cover the following conditions as outlined

below.

Definition (NEMA MG-1-12.52): A multiplier, when

applied to the rated horsepower, indicates a permissible horse-

power loading, which may be carried under the conditions

specified for the service factor.

TABLE IIITEFC IEEE 841 FOUR-POLE 460-V 40 C AMBIENT MOTORS

Purposes:

1) To accommodate inaccuracy in predicting system peak

horsepower.

2) Lengthen insulation life by lowering winding temperature

at rated load.

3) Handle intermittent or occasional overloads.

4) Compensate for low or unbalanced supply voltages.

5) Enhance motor torque for intermittent heavier starting re-

quirements.

6) Allow use of nonsinusoidal power supply.

Cautions:1) Operation at service factor load will usually reduce the

motor speed, life, and efficiency.

2) Do not rely on the service factor capability to carry the

load on a continuous basis.

3) There are situations where the winding insulation is ca-

pable of the increased loading (i.e., Class H insulation,

Class B rise) but the bearing lubricant life may be dras-

tically reduced.

Table III provides a comparison of full load to service factor

load (1.15) versus efficiency and winding temperature at various

altitudes.

V. SPECIAL AMBIENT CONSIDERATIONS

Most industrial motors are designed to operate in a 40 C

ambient. Several key points to consider are the following.

1) Do not select average ambients to confirm that a 40 C

limit is acceptable. One hot month with a 50 C ambient

could damage the lubricant for the bearings even though

the Class F winding would still function satisfactorily.

2) Most of the time, the ambient consists of the heat gen-

erated by the heating or cooling system surrounding the

motor (this would include the sun or lack thereof). How-

ever, there are times when there are other heat sources in

close proximity to the motor that will have a significant


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Fig. 4. Winding thermal load versus ambient (class F system).

influence on the surrounding ambient. The bearing and

lubrication system is affected by these conditions.3) If the motor is located in a confined space, a condition re-

ferred to as recirculation may exist that could damage

the winding or the bearings. Recirculation occurs when

the hot exhaust air is drawn back into the motors intakes,

where the cooling air enters the motor. In this case, the

ambient plus the rise over ambient of the exhaust air com-

bine to create a total ambient higher than intended.

Items that could contribute to the higher than normal

ambient include the coupling or belting losses, the driven

equipment, the process, piping, or plumbing, and other

machines in close proximity. Typical belting systems are

in the 95% efficient range which means that their losses

could be as high as those of the motor.

It is best to think of the ambient temperature as the sum

total of all heat sources including recirculation, that are

influencing the motor intake cooling air.

Ambient Changes: Figs. 4 and 5 illustrate the allowed

temperature rise of the stator winding and bearing systems for

changes in the total ambient to which the motor is exposed.

Note that both conditions must be considered.

VI. ALTITUDE

NEMA MG-1-14.4 offers the following information for mo-

tors operated at altitudes over 3300 ft. Table III provides a com-

parison of the motor performance at 3000, 7000, and 9000 ft.

The temperature rises given for machines in MG-1 are based

upon operation at altitudes of 3300 ft (1000 m) or less and a

maximum ambient temperature of 40 C. It is also recognized

as good practice to use machines at altitudes greater than 3300

ft (1000 m) as indicated in the following paragraphs.

Machines having Class A, B, F, or H insulation systems and

temperature rises in accordance with MG-1 will operate satis-

factorily at altitudes above 3300 ft (1000 m) in those locations

Fig. 5. Bearing temperature versus ambient.

where the decreases in ambient temperature compensates for theincrease in temperature rise, as follows:

Ambient Temperature, Maximum Altitude,

Degrees C Feet (Meters)

Motors having a service factor of 1.15 or higher will operate

satisfactorily at unity service factor at an ambient temperatureof 40 C at altitudes above 3300 ft (1000 m) up to 9000 ft (2740

m, regardless of insulation class).

Motors which are intended for use at altitudes above 3300

ft (1000 m) at an ambient temperature of 40 C should have

temperature rises at sea level not exceeding thevalues calculated

from the following formula:When altitude is in feet,

When altitude is in meters,

where

test temperature rise in C at sea level;

temperature rise in C from the appropriate table in

MG-1;

altitude above sea level in feet (meters) at which ma-

chine is to be operated.

Caution: When considering the impact of altitude, care must

be taken to also take into account the ambient conditions. The

motor may be located in a building with a controlled ambient

of 20 C or sitting in the open where large temperature swings

exist.


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Fig. 6. Winding temperature allocations.

TABLE IVUNUSUAL SERVICE CONDITIONS [1]

Fig. 6 illustrates therelationship between thebasic motor load

temperature rise and the altitude factor, service factor and am-

bient. If the motor is operating at an altitude of 3300 ft and ex-

periences an additional temperature rise of 20 C, then the al-

lowable winding temperature of 80 C, plus a 40 C ambient

only allow for an increase in load that is equal to a 15 C rise

in winding temperature as shown in Fig. 6. The sum of these

components should not exceed 155 C.

VII. UNUSUAL SERVICE CONDITIONS [2]

Section 3.2 of the IEEE 841 standard cautions the user to ad-

vise those responsible for the design, manufacture, application

and operation of the motor of any unusual service conditions

that may affect the motor performance or life. Table IV is taken

from section 3.2 of the standard. Note that many of these con-

ditions affect the operating temperature of the motor, or are af-

fected by the operating temperature of the motor.

Table V is a more expanded version which may be helpful. In

all cases where the service conditions are different from those

specified as usual, the manufacturer should be consulted. Typ-

ical unusual service conditions are shown in Table V.

TABLE VUNUSUAL SERVICE CONDITIONS [3]

VIII. EFFECTS OF VOLTAGE UNBALANCE

NEMA 14.36.1 offers the following explanation of the im-

pact of voltage unbalance on motor performance and life. The

amount of unbalance is calculated as follows.

The voltage unbalance in percent may be defined as follows:

Voltage

Unbalance

Maximum Voltage Deviation

from Average Voltage

Average Voltage

Example: If , , and , 467, and 450 V, respec-

tively, the maximum deviation from average is 9, and the percent

unbalance is

percent

Effects of Unbalanced Voltages on the Performance of

Polyphase Induction Motors (NEMA 14:36): When the line

voltages applied in a polyphase induction motor are not equal,

unbalanced currents in the stator winding will result. A small

percentage voltage unbalance will result in a much larger

percentage current unbalance. Consequently, the temperature

rise of the motor operating at a particular load and percentage

voltage unbalance will be greater than for the motor operating

under the same conditions with balanced voltages.
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Fig. 7. Derating factor.

Voltages preferably should be evenly balanced as closely as

can be read on a voltmeter. Should voltages be unbalanced, the

rated horsepower of the motor should be multiplied by the factor

shown in Fig. 7 (NEMA Figure 14-1) to reduce the possibility of

damage to the motor. Operation of the motor above a 5% voltage

unbalance condition1 is not recommended.

When the derating curve of Fig. 7 (NEMA Figure 14-1) is

applied for operating on unbalanced voltages, the selection and

setting of the overload device should take into account the com-

bination of the derating factor applied to the motor and increase

in current resulting from the unbalanced voltages. This is a com-

plex problem involving the variation in motor current as a func-

tion of load and voltages unbalanced in addition to the charac-teristics of the overload device relative to or .

In the absence of specific information, it is recommended that

overload devices be selected or adjusted, or both, at the min-

imum value that does not result in tripping for thederating factor

and voltage unbalance that applies. When unbalanced voltages

are anticipated, it is recommended that the overload devices be

selected so as to be responsive to in preference to

overload devices responsive to .

IX. MEDIUM MOTOR DERATING FACTOR DUE TO

UNBALANCED VOLTAGE

NEMA 14.35 provides the recommended derating factorshown in Fig. 7 to apply to the motor load when it exceeds 1%.

1The 5% voltage unbalance is too high except for very short periods of time.Frequently theoperatordoes not know what theactualload is,nor canhe controlit. Fig. 6 shows that a 3% unbalance will result in at least a 10 C increase inthe winding temperature which will reduce the thermal life to 1/2 of its originalvalue. A goodrule of thumb is: The percent increase in temperature rise is abouttwice the square of the percent voltage unbalance.The impact of increased heating in the rotor by the negative-sequenced voltagemay also affect the bearing and lubrication life.It is recommend that the voltage unbalance be held to no more than 1%.

TABLE VITYPICAL TEMPERATURE ( C) AND EFFICIENCY (%) VALUES FOR TEFC

FOUR-POLE IEEE 841 MOTORS

X. EFFECTS OF VOLTAGE VARIATION

Although MG1-12.45 states that a motor should operate suc-

cessfully under rated load with a voltage variation of plus or

minus 10%, it should be understood that the motor performance

may change. Table VI compares winding temperature and ef-

ficiency for 414, 460, and 506 V. Other performance such as

power factor and slip are also affected.

XI. THERMAL CONSIDERATIONS FOR CONSTANT-SPEED

MOTORS USED ON A SINUSOIDAL BUS WITH HARMONIC

CONTENT AND GENERAL PURPOSE MOTORS USED

WITH VARIABLE-VOLTAGE OR VARIABLE-FREQUENCY

CONTROLS OR BOTH

NEMA MG1-12:45 states that a motor should operate suc-

cessfully with a plus or minus 5% variation in frequency. How-

ever, the motor performance may be affected.

When motors are exposed to a power supply with harmonic

content or energized by variable voltage/frequency drives, it is

necessary to consider the impact on the motor heating and tem-perature rise.

NEMA Part 30, Figure 30-1, provides derating curves

and a methodology for calculating the harmonic content on

a sinusoidal bus. These curves cannot be applied to vari-

able-voltage/variable-frequency drives. For these applications,

it is necessary to take into consideration the impact of reduced

cooling air at slower speeds. NEMA Figures 30.2 and 30.3

provide examples of torque derating curves. NEMA MG-1

offers the following advice.

Torque Derating Based on Reduction in Cooling: To account

for the effect of the reduction in cooling resulting from any re-

duction in operating speed, the motor should be derated in ac-

cordance with Fig. 8. The proposed derating may require selec-tion of an oversize motor. The curves are applicable only to the

NEMA frame sizes and Design types as indicated, and as noted,

additional derating for harmonics may be required. For larger

NEMA frames or other Design types, consult the motor manu-

facturer.

The curves in Fig. 8 represent the thermal capability of De-

sign A and B motors under the conditions noted, and are based

on noninjurious heating which may exceed rated temperature

rise for the class of insulation. This is analogous to operation

of a 1.15 service factor motor at service factor load (with rated

voltage and frequency applied) as evidenced by the 115% point

at 60 Hz for a 1.15 service factor motor.


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Fig. 8. NEMA Figure 30-2.

Torque Derating During Inverter Operation: In the case of

inverter operation, the available torque from the motor is usu-ally lower than on a sinusoidal voltage source. The reduction re-

sults from the additional temperature rise due to harmonic losses

and also from the voltagefrequency characteristics of some in-

verters.

The temperature rise at any load-speed point depends on the

individual motor design, the type of cooling, the effects of the

reduction in speed on the cooling, the voltage applied to the

motor, and the characteristics of the inverter. When determining

the derating factor, the thermal reserve of the particular motor is

important. Taking all of these matters into account, the derating

factor at rated frequency ranges from 0% to 20%.

It is not possible to produce a curve which applies to all cases.

Other motors with different thermal reserve, different methodsof cooling (self-circulation cooling or independent cooling), and

used with other types of inverters will have different derating

curves.

There is not an established calculation method for deter-

mining the derating curve for a particular motor used with a

particular inverter that can be used by anyone not familiar with

all of the details of the motor and inverter characteristics. The

preferred method for determining the derating curve for a class

of motors is to test representative samples of the motor design

under load while operating from a representative sample of

the inverter design and measure the temperature rise of the

winding.

XII. BEARING OPERATING TEMPERATURE RANGE

The safe bearing operating temperature range is based upon

the following factors:

1) winding temperature;

2) lubricant temperature;

3) motor thermal circuit (cooling path and method);

4) oil and grease viscosity;

5) bearing seals, shields, and lubricant type;

6) amount of grease in the bearing and cavity;

7) radial internal clearance;

8) ambient conditions, including contamination;

TABLE VIIRECOMMENDED BEARING TEMPERATURE LIMITS

9) loading and speed;

10) bearing type and size.

As a general guide, temperature limits for bearings are shown

in Table VII.

For most applications, actual temperatures will usually be

lower than those above. For maximum protection, the user

should determine the normal bearing operating temperature

for the application and adjust the alarm setpoint 10 C15 C

higher than the normal operating temperature.

XIII. BEARING AND WINDING TEMPERATURE RELATIONSHIPS

Because proper care of the motor bearings is enhanced by an

understanding of the effect the motor has as a heat source to the

bearings, this section has been included as a reference for those

dealing with this issue. Figs. 911 should help to visualize the

relationship between the various heat sources and the bearings.

Maru and Zotos [5] conducted an excellent study [4] on

the relationship between bearing and winding temperatures

for TEFC NEMA size horizontal motors. They reached the

following conclusions.

1) The temperature differential between the bearing outerrace and the end bell (bracket) is not significant as long as

the influence of the air over the motor is avoided. Hence, a

measurement of the end bell temperature (protected from

the air stream) will approximate the bearing temperature.

2) The bearing regreasing frequency should be doubled for

every 15 C increase in the motor operating temperature.

3) Shielded bearings will operate approximately 5 C hotter

than open bearings.

4) An enclosed fan-cooled motor (i.e., IEEE 841) will have

a bearing temperature rise that will not exceed 45 C for

up to 1800 r/min and 50 C for 3600 r/min if the average

winding temperature does not exceed 90 C rise.

A review of the MaruZotos data confirms that for TEFC mo-tors the Pulley End (PE) bearing will normally have a higher

temperature rise than the Short End (SE) bearing at full load

and that the temperatures will converge as the load is decreased

due to the external cooling fan. This fan and the fan cover guard

make it difficult to obtain temperature readings from this area

once the motor is put into service. When considering the am-

bient, be sure to include the effects of the driven equipment

which can be a significant source of heat to the PE bearing.

Because of the excellent heat transfer through the PE bracket,

it is not unreasonable to assume that at full load the temper-

ature rise at point A and B of Fig. 9 are close enough to es-

timate the actual bearing temperature. For most TEFC motors
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Fig. 9. Typical IEEE 841 TEFC product.

Fig. 10. Temperature rise versus motor load, 3600-r/min motor.

Fig. 11. Temperature rise versus motor load, 1800-r/min motor. Windingtemperature average of seven ratings per speed (calculated). Bearingtemperature average of 11 ratings (actual test data of bracket to winding). SEbearing temperature range 10 C30 C cooler than PE bearing temperature. premium efficient IEEE 841 product line.

the SE bearing may be as much as 15 C cooler. For purposes

of bench marking and monitoring bearing temperatures, collec-

tion of data over time at point A is adequate.It is clear from Figs. 10 and 11 that, at reduced load, the motor

heating has very little impact on the bearing temperatures. It can

also be assumed that most of the bearing heating is not caused

by friction nor the mechanical load on the bearing, but is caused

by the heat generated in the stator, rotor and ambient, which

includes the coupling and driven equipment.

XIV. TEMPERATURE OF CRITICAL PARTS

There are at least three parts of the motor that are addressed in

the IEEE 841 standard (assuming Division II application). They

are the stator, rotor, and bearings.

TABLE VIIITEMPERATURE RISE VERSUS PERCENT LOADING FOR STATOR,

ROTOR, AND T.O.E. BEARING

The motor surfaces, including the rotor must be below the

auto-ignition-temperature (AIT) limit. The bearing temperature

as measured on the bracket housing must be below 40 C rise for

four-pole and slower motors and 50 C for two-pole motors and

the average winding temperature not to exceed 80 C rise. All of

these temperatures are at full load and rated voltage conditions.

In Table VIII are estimates of what these temperature rises

would be for typical IEEE 841 motors. It is assumed that the

motor surface temperature and bracket surface temperatures

would be 10 C cooler than the maximum values. Since it ispossible for the motor to be overloaded beyond the service

factor for brief periods of time, especially if the increase load

is combined with a reduced voltage condition, the 125% load

point is also shown.

It is interesting to note that these motors have a considerable

margin for safe operation under most conditions, which is what

the IEEE 841 standard was developed to achieve.

It is important to point out that in some applications bearing

temperature protection may be required to keep the bearing tem-

perature below the AIT in case of a catostropic bearing failure.

XV. LOAD ESTIMATING

Over the years, a number of methods have been devised to es-

timate the load based on temperature, speed, or current change.

The speed change method is the least accurate due to difficulties

in estimating rotor temperature or the impact of voltage varia-

tion. Figs. 12 and 13 show the relationship of temperature and

current as a function of load. Since accurate current measure-

ments are usually the easiest to obtain, it is the preferred method

for estimating the load on a motor in the field.

Ratioing current change between two load points to deter-

mine the load is much more accurate than ratioing temperature

change. The higher up the load curve, the more accurate. The

closer together the two load points are, the more accuracy.


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Fig. 12. IEEE 841 TEFC four-pole 460-V load versus current.

Fig. 13. IEEE 841 TEFC four-pole 460-V load versus temperature rise.

For example, using the straight lines projections in Fig. 12

For the 100-hp motor

hp

A Ahp

which is the actual load

hp

A Ahp

whereas the actual load is 50 hp.

From full load to 3/4 load,

hp

A Ahp

which is very accurate.

If the nameplate current is used as one of the two points, the

accuracy will depend upon the nominal voltage and the percent

Fig. 14. Typical aluminum die-cast skewed rotor.

TABLE IXLOCK ROTOR/STALL TEMPERATURE DISTRIBUTION VERSUS TIME

unbalance. All of the above calculations are at rated voltage withno unbalance or harmonic distortion.

From these curves it is obvious that it is not always accurate toratio the temperature rise to determine the load especially over awide range.Notethatthe100-and 200-hpmotorstrack closely upto 1.25%of full load, butthenthe twocurves diverge. The100-hpmotorhas superiorheatdissipationcapabilityfor thegivenload.

There are a variety of other methods to predict winding tem-perature with respect to change in load. However, most of themare not very accurate for significant changes in load or at the ex-tremes. The mathematics can be quite complicated and is usu-ally not practical for field use. For a more detailed study of theissue, see [4, pp. 301307].

XVI. ROTOR TEMPERATURE CONSIDERATIONS

ThetypicalIEEE841rotorisdiecastwithaluminumandhasno

air ducts. The rotor fan blades are normally castas partof the end

rings, as shown in Fig. 14.For these size motors, most will have a

skewed rotorto improveaccelerationand noise characteristics.A

study of the thermal stability of the rotor involves these elements:

therotorlaminationsurface,therotorbars,andtherotorendrings.

Table IX shows the relationship of the elements for starting and
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Fig. 15. Typical aluminum die-cast rotor squirrel cage.

Fig. 16. Temperature distribution for two typical rotors after 60-s stall time.

stall conditions. Usually, underrunning and overload conditions,

the stator winding will be the limiting factor and not the rotor.

Under stall conditions, it maybe therotor, especially if themotor

is a two-pole or four-pole design. During repetitive starts or high

inertia applications it could be either the rotor or stator, or both

dependingontheparticulardesign.

Fig. 15 is a typical rotor cage after the laminations have been

etched away. Fig. 16 shows how the temperature at stall varies

in the various motor components. The message is that there is

significant variation in motor hot spots (weak points) depending

upon the design.

XVII. CONCLUSION

In conclusion, a basic understanding of the impact of motor

temperature on the stator, rotor and bearings can help achieve

Fig. 17. Horsepower versus percent load temperature ratio.

satisfactory motor life and performance. This paper has pre-

sented a number of simple rules and guidelines to assist in this

effort. Remember that most of these examples are focused on

TEFC motors that are normally used in the petro-chemical in-

dustry and may not necessarily apply to other types of motors.

A key point of the paper is that not only is the stator subject to

the harmful effects of temperature, but so are the rotor, bearings,

and lubricant. Lower operating temperatures normally translate

into higher efficiencies and longer life.

For readers who want a more detailed discussion on this very

broad topic, the author suggests consulting [1][6].

APPENDIX

WINDING TEMPERATURE UNDER LOAD

There are at least four methods for measuring the winding

temperature:

1) thermometers;

2) embedded detectors;

3) local temperature detectors;4) sensing a change in winding resistance.

Theembeddedtemperaturedetectorisquitesimpleandreason-

ablyaccurate,butmostmotorsarenotfittedwiththesesensors.

The next protection method is to measure the winding resis-

tance hot and cold and then calculate the temperature as shown

in the following equation.

Because of the difficulties involved in these methods, it may

be desirable to only estimate the winding temperature. Again,

there are a variety of methods which usually involve ratioing or

extrapolation from known data.

The following is one simple method of measuring the frame

temperature in the outlet box and then ratioing the winding tem-
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perature. This method may be used for TEFC motors. However,

one should remember that all of these methods are only approxi-

mations and may vary with size, speed, and enclosure. As shown

in Fig. 17, this method will only provide an approximation of the

average winding temperature.Approximation Rule for This Family of Motors: From mea-

sured temperatures on the frame in the outlet box areas, out

of the air stream, the average winding could be approximately1.52.2 higher when running under load between 50% to ser-vice factor 1.15%. Note from Fig. 17 that the ratio increaseswith the increase in motor size. For this to be true, the outletbox has to be in the center of the motor because the hot spot ofthe motor is along the centerline ( ). Keep in mind that this isa benchmark type of test that can be used to indicate a signifi-cant change from the norm.

In order to accurately determine the average winding temper-

ature, it is necessary to use another method. Although a thermo-

couple device will give improved accuracy, the best method is to

measure the winding resistances hot and cold and calculate the

average winding temperature rise using the following equation

and IEEE 112-8.3.3 procedure:

where

total temperature of winding when was measured, in

C;

resistance measured during test, in ;

reference value of resistance previously measured at

known temperature in ;

temperature of winding when reference value of resis-

tance was measured, in C;

234.5 for 100% International Annealed Copper Stan-

dard (IACS) conductivity copper;225 for aluminum, based on a volume conductivity of

62%.

Note: For other winding materials, a suitable value of (in-

ferred temperature for zero resistance) must be used.

Since a small error in measuring resistance will make a

comparatively large error in determining temperature, the

winding resistance should be measured by a double bridge, or

other means of equivalent accuracy, and checked by a second

instrument, if possible. When using the above equation to

calculate temperature, both the reference resistance and the test

resistance should be measured using the same equipment.

This procedure includes the following steps.

1) Measure the winding line-to-line resistance cold andrecord the ambient temperature (the motor is de-ener-

gized). This assumes easy access to the motor leads.

2) Run the motor under load until the temperature is stable.

This maytakeseveral hours.Shutthe motordown, read the

line-to-lineresistancewith anappropriatebridgeobtaining

46pointsasshowninFig.18,andrecordthehotambient.

3) Extrapolate back to Time 0.

Now there is enough information to calculate the temperature

under load using the following equation:

Fig. 18. Temperature versus resistance.

Fig. 19. Placement of thermocouple.

Required data are as follows:

ambient cold and hot;

resistance cold and hot;

for copper.

If it is not practical to expose the motor connections to access

the leads to apply the resistance bridge, an alternate approach

would be to attach a thermocouple to the stator lamination at

the center to the stator core as shown in Fig. 19.

For totally enclosed motors, the lamination temperature at the

center line (A in Fig. 18) is usually 510% less than the average

winding temperature. If the depth of the frame is known, a hole


12/12


TABLE XSUMMARY OF ACTUAL TEST DATA

can be drilled down to the lamination (B in Fig. 19). Thismethod

would be more than adequate for field measurements.

The use of thermography to obtain the lamination tempera-

ture is another alternative.

Table X is a summary of actual test data sampled to compare

lamination and winding temperatures. This too is a benchmark

type test, but with a much higher degree of certainty.

REFERENCES

[1] IEEE Standard for Petroleum and Chemical IndustrySevere DutyTotally Enclosed FanCooled (TEFC) Squirrel Cage Induction

MotorsUp to and Including 500 HP, IEEE 841-2000.

[2] NEMA Motor-Generator Standard, MG-1-1998.[3] R. W. Smeaton, Motor Application and Maintenance Handbook. New

York: McGraw-Hill, 1969.[4] R. L. Nailen, The Plant Engineers Guide to Industrial Electric Mo-

tors. Chicago, IL: Barks Publications, 1985.[5] B. Maru andP.Zotos, Anti-friction bearing temperature rise forNEMA

frame motors, in Proc. IEEE PCIC88, Dallas, TX, 1988, p. 205.[6] IEEE Standard Test, Procedure for Polyphase Induction Motors and

Generators, IEEE Standard 112-1996.

Austin H. Bonnett (M68SM90F92) was borninLos Angeles, CA, in 1936. He received the B.S. de-gree in electrical engineering from California StateUniversity, Los Angeles, and the Masters Degree inbusiness from the University of Phoenix, Phoenix,AZ.

He served in the U.S. Navy from 1955 to 1958as an Electrician aboard the Icebreaker, Burton Is-land. He joined U.S. Electrical Motors, a Division ofEmerson Electric Company, in 1963 and has held po-sitions in the Service, Manufacturing, Quality Con-

trol, and Engineering Departments. He was the Plant Manager of the Prescott,AZ,facility forfive years prior to holding thepositionof Vice Presidentof Engi-neering, directing all U.S. Electrical Motors engineering functions for ten years.Presently, he holds the position of Vice President-Technology Emeritus and hasretired.

Mr. Bonnett serves on NEMA, IEEE, EPRI, EASA and DOE Committeesand received the 1994 IEEE Meritorious Award. In 1996, he was selected forthe IEEE Industry Applications Society Outstanding Achievement Award. Hereceived the 1999 NEMA Kite and Key Award for outstanding service to theelectrical industry.

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