Increase motor reliability by monitoring and reducing operating temperature
Although summer heat waves provide poignant reminders that “heat kills,” high temperatures can harm electric motors as well as people. In fact, 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 pay huge dividends—by preventing unexpected shutdowns and extending motor life. Here’s how to go about it.
First, determine the motor’s temperature rating from its original nameplate or the ratings for three-phase induction motors in the National Electrical Manufacturers Association (NEMA) standard Motors and Generators, MG 1-2011. Once you know the rating, you can measure the temperature rise directly using sensors or an infrared temperature detector, or indirectly using the resistance method explained below.
Brief definitions of a few commonly used terms will make it easier to follow the various procedures.
Ambient temperature (often referred to as “room temperature”) is the temperature of the air (or other cooling medium) that surrounds the motor. The difference between the ambient temperature and that of a motor operating under load is called 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 or a component.
Ambient temperature + Temperature rise = Hot temperature
NEMA rates winding insulation by its ability to withstand overall temperature. A Class B insulation system, for example, is rated 130 C, while a Class F system is rated 155 C. Since NEMA’s maximum ambient temperature is normally 40 C, you would expect the temperature rise limit for a Class B system to be 90 C (130 to 40 C). But NEMA also includes a safety factor, primarily to account for parts of the motor winding that may be hotter than where the temperature is measured.
Table 1 shows the temperature rise limits for NEMA medium electric motors, based on a maximum ambient of 40 C. In the most common speed ratings, the NEMA 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 2 lists the temperature rise for motors with a 1.0 SF; Table 3 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 1 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 Table 3.
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 (room) temperature. To ensure accuracy, use a milliohmmeter 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.
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.
Example: To calculate the hot winding temperature for an un-encapsulated, open dripproof 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:
The temperature rise equals the hot winding temperature minus the ambient temperature, or in this case:
Temperature rise = 125 C – 20 C = 105 C
As Table 1 shows the calculated temperature rise of 105 C in this example equals the limit for a Class F insulation system (105 C). 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 center 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.
What if you want to determine the operating temperature of a motor winding that does not have embedded detectors? For motors rated 600 V 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’s 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 NEMA’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% to 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
Another factor to deal with in some motor applications is the ambient temperature. If it exceeds the usual NEMA 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 1 is 105 C, decrease the temperature rise limit by 8 C (48 to 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.
Thomas Bishop is a senior technical support specialist at the Electrical Apparatus Service Association (EASA), St. Louis, Mo.