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November 2003
Understanding Motor Temperature Rise Limits By Tom Bishop, P.E. EASA Technical Support Specialist
EASA MEMBER BENEFIT
How do we know if a motor is operating within its temperature rating? The simple answer, and a good one, is that the National Electrical Manufacturers Association (NEMA) has defined temperature rise for electric motors in Motors and Generators, NEMA standard MG 1-1998. In this article we will focus on temperature rise and temperature sensing of three-phase induction motors. We will begin by identifying some key terms. Temperature rise is the increase in temperature above ambient. Ambient temperature is the temperature of the air (or other cooling medium) in the area surrounding the motor, frequently termed “room temperature.” The sum of the ambient temperature and the temperature rise is the overall, or “hot,” temperature of a component. Insulation temperature classes are based on the overall temperature. For example, a Class B winding system is rated 130º C. The normal maximum ambient, per NEMA, is 40º C. The temperature rise limit for the Class B winding would be estimated at 90º C (130-40).
in MG1-12.43, and shown in Table 1. The temperature rise values are in degrees Celsius (º C) and are based on a maximum ambient of 40º C. In the most common speed ratings the NEMA designation of medium motors includes horsepower ratings from 1/2 horsepower (hp) (0.37 kW) up to 500 hp (370 kW) for 2 and 4 poles, and up to 350 hp (260 kW) for 6 poles. Temperature rise limits for large motors, those above medium motor ratings, differ based on the service factor. Table 2 is taken from MG1-20.8.1 and lists the temperature rise for motors with a 1.0 service factor (SF) and Table 3, taken from MG120.8.2, applies to motors with 1.15 SF.
Measuring Resistance The resistance method is useful for motors that do not have embedded detectors such as thermocouples or resistance temperature detectors (RTDs). Note that temperature rise limits (Table 1) for meOther Factors To Consider dium motors are based on resistance. The However, there are other factors that NEMA temperatures of large motors can be measured by uses in establishing temperature rise for motors. resistance or by embedded detectors. Resistance Therefore, our estimate is not precise and not applitesting is performed by measuring the lead-to-lead cable to motor windings. The other factors are resistance of line leads of the winding. An initial primarily allowances for hot spot temperatures. test is done with the motor “cold,” i.e., room (amThat is, a safety factor is built in to the rating to acbient) temperature. Verify that the motor is at room count for parts of the winding that may be hotter temperature by checking the winding temperature than the location at which temperature is measured. directly if possible. Temperature rise limits for medium motors are Alternative checks that are Table 1: Temperature rise by resistance method for medium not as precise would be checking the temperature of the induction motors. stator core or the external frame. The motor winding hot Medium Induction Motors Insulation Class and resistance is tested after the Temperature Rise º C winding temperature stabilizes with the motor operating at Motor Type A B F H rated load and the change in resistance is used to determine 1 Motors with 1.0 service factor (SF) the hot temperature. (Note: It other than those in 3 or 4. 60 80 105 125 may take as long as 8 hours at 2 All motors with 1.15 or higher SF 70 90 115 --rated load for the winding tem3 Totally-enclosed nonventilated perature to stabilize.) The motors with 1.0 SF 65 85 110 130 ambient temperature is subtracted from the hot winding 4 Motors with encapsulated windings temperature to determine the and with 1.0 SF, all enclosures 65 85 110 ---
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Understanding Motor Temperature . . . Continued From Page 1 Table 2: Temperature rise for large motors with 1.0 service factor. Large Motors with 1.0 Service Factor at Rated Load Motor Rating
Insulation Class and Temperature Rise º C
Method of determination
A
B
F
H
1 All horsepower (or kW) ratings
Resistance
60
80
105
125
2 1500 hp (1120 kW) and less
Embedded detector
70
90
115
140
3 Over 1500 hp (1120 kW) and 7000 volts or less
Embedded detector
65
85
110
135
4 Over 1500 hp (1120 kW) and over 7000 volts
Embedded detector
60
80
105
125
Table 3: Temperature rise for large motors with 1.15 service factor. Large Motors with a 1.15 Service Factor at Service Factor Load
Insulation Class and Temperature Rise º C
Motor Rating
Method of determination
A
B
F
H
1 All horsepower (or kW) ratings
Resistance
70
90
115
135
2 1500 hp (1120 kW) and less
Embedded detector
80
100
125
150
3 Over 1500 hp (1120 kW) and 7000 volts or less
Embedded detector
75
95
120
145
4 Over 1500 hp (1120 kW) and over 7000 volts
Embedded detector
70
temperature rise. An example will help illustrate how this is done. An un-encapsulated, open dripproof medium motor with a Class F winding and a 1.0 SF has a lead to lead resistance of 1.02 ohms at an ambient temperature of 25º C, and a hot resistance of 1.43 ohms. The formula to determine temperature is: Th = [ (Rh/Rc) x (K + Tc) ] – K The meanings of the terms are: Th hot temperature Tc cold temperature Rh hot resistance Rc cold resistance K 234.5 (a constant for copper) The hot winding temperature for the example is calculated as follows: Th = [ (1.43/1.02) x (234.5 + 25) ] – 234.5 = 129.3º C The temperature rise is the hot winding tem-
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perature minus the ambient. The temperature rise for the example is 129º C (rounded value) hot temperature minus the 25º C ambient, or 104º C (129 – 25) rise. The temperature rise limit for Class F in Table 1 is 105º C. The example value is only one degree below the temperature rise limit. That is acceptable, but it also indicates that any increase in load would result in excessive temperature rise and consequent thermal degradation. Further, if the ambient at the motor installation were to go above 40º C, the motor load would have to be reduced so as not to exceed the total temperature (hot winding) capability.
Embedded Detectors Monitor Temperature Motors equipped with temperature detectors embedded in the windings are monitored by directly reading the output of the detectors with appropriate instrumentation. Typically, the motor control center has panel meters indicating the temperatures sensed by the detectors. If the detectors are in the windings but not connected to the controls, a hand held temperature meter can sense the output of the detector leads. The output temperature displayed is the hot winding temperature at the location of the sensor. If the detector read 129º C as in the example above, the same temperature concerns would apply. From a practical perspective, it is easier to measure a detector output rather than the winding lead-to-lead resistance. Further, the detector resistance can be measured when the motor is operating. That can’t be done with the lead-to-lead resistance test. What if we want to determine the winding 90
115
135
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November 2003
Understanding Motor Temperature . . . Continued From Page 2 temperature of a motor that does not have embedded detectors and that can’t be shut down? If the motor voltage is rated 600 volts or less it may be possible to (following all applicable safety rules) open the terminal box and access the back of the stator core iron laminations. The stator lamination temperature will not be the same as the winding temperature, but it will be closer to it than any other readily accessible part of the motor. If the lamination temperature minus the ambient exceeds the temperature rise for the motor, it can be assumed that the winding is operating beyond its rating. For example, if the stator core temperature measured 136º C and the motor was the same one as in the example above, the stator core rise would be 136 – 25, or 111º C rise. That is above the 105º C limit for the winding, and the winding can be expected to be hotter than the laminations. Hot Spot Temperature Is Key The critical limitation on winding temperature
is the hot or overall temperature. That is the sum of ambient plus rise. In large part, the load determines the rise. The other key factor that must be dealt with is ambient. The NEMA standard limit for ambient temperature is 40º C. Operation above that temperature may require de-rating of the motor so as not to exceed the hot winding limit. The limit for the temperature rise must be reduced by the number of degrees the ambient exceeds 40º C. If the ambient were 50º C, and the motor had a temperature rise limit 105º C from one of the tables above, the temperature rise limit would have to be reduced 10º C (50º – 40º C ambient difference) to 95º C. The total temperature in both cases is limited to the same amount. That is, 105 plus 40 equals 145º C, and 95 plus 50 also equals 145º C. Regardless of the method used to sense winding temperature, it is the total, or hot spot, temperature that is the real limitation. Further, winding life can be halved if the temperature increases 10º C, so the lower the hot winding temperature, the better.
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