Tom Bishop, P.E.
EASA Technical Support Specialist
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 130ºC. The normal maximum ambient, per NEMA, is 40ºC. The temperature rise limit for the Class B winding would be estimated at 90ºC (130-40).
Other factors to consider
However, there are other factors that NEMA uses in establishing temperature rise for motors. Therefore, our estimate is not precise and not applicable to motor windings. The other factors are primarily allowances for hot spot temperatures. That is, a safety factor is built in to the rating to account for parts of the winding that may be hotter than the location at which temperature is measured.
Temperature rise limits for medium motors are 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 medium motors are based on resistance. The temperatures of large motors can be measured by resistance or by embedded detectors. Resistance testing is performed by measuring the lead-to-lead resistance of line leads of the winding. An initial test is done with the motor “cold,” i.e., room (ambient) temperature. Verify that the motor is at room temperature by checking the winding temperature directly if possible.
Alternative checks that are not as precise would be checking the temperature of the stator core or the external frame. The motor winding hot resistance is tested after the winding temperature stabilizes with the motor operating at rated load and the change in resistance is used to determine the hot temperature. (Note: It may take as long as 8 hours at rated load for the winding temperature to stabilize.) The ambient temperature is subtracted from the hot winding temperature to determine the temperature rise. An example will help illustrate how this is done. An un-encapsulated, open drip-proof 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
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 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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