Monitor and Reduce Motor Operating Temperature to Increase Reliability
Understanding and monitoring your motor’s recommended operating temperature can drastically lengthen its lifespan. Here we tell you how to do both, and so avoid early and unnecessary failure.
It is a striking fact that operating a three-phase induction motor at just 10°C above its rated temperature can shorten its life by half. Whether your facility has thousands of motors or just a few, regularly checking the operating temperature of critical motors will help extend their life and prevent costly, unexpected shutdowns. Here is how to go about it.
First, determine the motor’s insulation class (A, B, F or H) from its original nameplate or the ratings for three-phase induction motors in the National Electrical Manufacturers Association (NEMA) standard Motors and Generators, MG 1-2021 (hereafter MG1).
The insulation class indicates the maximum temperature that the motor’s winding can withstand without degrading (see Table 1) motor operation (called temperature rise), which is load-dependent. The rest is attributable to the ambient (room) temperature. Identifying both components of the winding temperature makes it possible to protect the motor winding under different operating conditions (e.g., a lower ambient temperature may permit a higher temperature rise). NEMA also incorporates a safety factor, but more on that later.
As with insulation class, every motor built to NEMA standards will have an ambient temperature rating (normally 40°C for three-phase motors). This is the maximum temperature for the air (or other cooling medium) surrounding the motor. Like the insulation class rating, you can find this rating on the motor nameplate or in MG1.
Determine the “Hot” temperature
The next step is to measure the overall (“hot”) temperature of the winding with the motor operating at full load–either directly using embedded sensors or an infrared temperature detector, or indirectly using the resistance method explained below. The difference between the winding temperature and the ambient temperature is the temperature rise. Put another way, the sum of the ambient temperature and the temperature rise equals the overall (or “hot”) temperature of the motor winding or a component.
Ambient temp. + Temp. rise = Hot temp.
To avoid degrading the motor’s insulation system, the hot temperature must not exceed the motor’s insulation class temperature rating.
Given that MG1’s maximum ambient temperature is normally 40°C, you would expect the temperature rise limit for a Class B 130°C insulation system to be 90°C (130° - 40°C), not 80°C as shown in Tables 2 and 3. But as mentioned earlier, MG1 also includes a safety factor, primarily to account for parts of the motor winding that may be hotter than where the temperature is measured, or that may not be reflected in the “average” temperature obtained by the resistance method.
Table 2 shows the temperature rise limits for MG1 medium electric motors, based on a maximum ambient of 40°C. In the most common speed ratings, the MG1 designation of medium motors includes ratings of 1.5 to 500 hp for 2- and 4-pole machines, and up to 350 hp for 6-pole machines.
Temperature rise limits for large motors–i.e., those above medium motor ratings–differ based on the service factor (SF). Table 3 lists the temperature rise for motors with a 1.0 SF; Table 4 applies to motors with 1.15 SF.
Resistance method of determining temperature rise
The resistance method is useful for determining the temperature rise of motors that do not have embedded detectors–e.g., thermocouples or resistance temperature detectors (RTDs). Note that temperature rise limits for medium motors in Table 2 are based on resistance. The temperature rise of large motors can be measured by the resistance method or by detectors embedded in the windings as shown in Tables 3 and 4.
To find the temperature rise using the resistance method, first measure and record the lead-to-lead resistance of the line leads with the motor “cold”–i.e., at ambient temperature. To ensure accuracy, use a milli-ohmmeter for resistance values of less than 5 ohms, and be sure to record the ambient temperature. Operate the motor at rated load until the temperature stabilizes (possibly up to 8 hours) and then de-energize it. After safely locking out the motor, measure the “hot” lead-to-lead resistance as described above.
Figure 2. It may be possible to determine the approximate temperature of the winding with a thermocouple.
Find the hot temperature by inserting the cold and hot resistance measurements into Equation 1. Then subtract the ambient temperature from the hot temperature to obtain the temperature rise.
Equation 1. Hot winding temperature
Th = [ (Rh/Rc) x (K + Tc) ] - K
Where:
Th = hot temperature
Tc = cold temperature
Rh = hot resistance
Rc = cold resistance
K = 234.5 (a constant for copper)
Example. To calculate the hot winding temperature for an un-encapsulated, open drip-proof medium motor with a Class F winding, 1.0 SF, lead-to-lead resistance of 1.21 ohms at an ambient temperature of 20°C, and hot resistance of 1.71 ohms, proceed as follows:
Th = [ (1.71/1.21) x (234.5 + 20) ] - 234.5 = 125.2°C (round to 125°C)
The temperature rise equals the hot winding temperature minus the ambient temperature, or in this case:
Temp. rise = 125°C - 20°C = 105°C
As Table 2 shows, the calculated temperature rise of 105°C in this example equals the limit for a Class F insulation system.
Although that is acceptable, it is important to note that any increase in load would result in above-rated temperature rise and seriously degrade the motor’s insulation system. Further, if the ambient temperature at the motor installation were to rise above 20°C, the motor load would have to be reduced to avoid exceeding the machine’s total temperature (hot winding) capability.
Determining temperature rise using detectors
Motors with temperature detectors embedded in the windings are usually monitored directly with appropriate instrumentation. Typically, the motor control centre has panel meters that indicate the hot winding temperature at the sensor. If the panel meters were to read 125°C as in the example above, the same concerns about the overall temperature would apply.
Don’t let excessive heat kill your motors before their time.
What if you want to directly measure the operating temperature of a motor winding that does not have embedded detectors? For motors rated 600 volts or less, it may be possible to open the terminal box (following all applicable safety rules) with the motor de-energized and access the outside diameter of the stator core iron laminations with a thermocouple (see Figure 2). The stator lamination temperature will not be the same as winding temperature, but it will be nearer to it than the temperature of any other readily accessible part of the motor.
If the stator lamination temperature minus the ambient exceeds the rated temperature rise, it is reasonable to assume the winding is also operating beyond its rated temperature. For instance, had the stator core temperature in the above example measured 132°C, the temperature rise for the stator would have been (132°C - 20°C), or 112°C. That significantly exceeds MG1's limit of 105°C for the winding, which can be expected to be hotter than the laminations.
The critical limit for the winding is the overall or hot temperature. Again, that is the sum of ambient temperature plus the temperature rise. The load largely determines the temperature rise because the winding current increases with load. A large percentage of motor losses and heating (typically 35 - 40%) is due to the winding I2R losses. The “I” in I2R is winding current (amps), and the “R” is winding resistance (ohms). Thus, the winding losses increase at a rate that varies as the square of the winding current.
Adjusting for ambient
If the ambient temperature exceeds the usual MG1 limit of 40°C, you must derate the motor to keep its total temperature within the overall or hot winding limit. To do so, reduce the temperature rise limit by the amount that the ambient exceeds 40°C.
For instance, if the ambient is 48°C and the temperature rise limit in Table 2 is 105°C, decrease the temperature rise limit by 8°C (48°C - 40°C ambient difference) to 97°C. This limits the total temperature to the same amount in both cases: 105°C plus 40°C equals 145°C, as does 97°C plus 48°C.
Regardless of the method used to detect winding temperature, the total, or hot spot, temperature is the real limit; and the lower it is, the better. Each 10°C increase in operating temperature shortens motor life by about half, so check your motors under load regularly. Don’t let excessive heat kill your motors before their time.
Text: Thomas H. Bishop, P.E. EASA SENIOR TECHNICAL SUPPORT SPECIALIST
Photos: easa and Shutterstock
About the author
Thomas Bishop is a senior technical support specialist at EASA.
EASA is an international trade association of over 1800 electromechanical sales, service and repair firms in nearly 80 countries.
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