Houston Motor Rewinding, Preventing Unexpected Shutdowns and Extending Motor Life
Looking for Houston Motor Rewinding? 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.
Key terms
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).
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.
For additional info, please see our electrical mechanical application formulas. In need or have questions about maintenance or Houston motor rewinding? Call CMI now!
