When it comes to electric motor testing and maintenance, one of the chief concerns is motor temperature. In an effort to keep motors from overheating, special attention is typically paid to two values usually found on a motor nameplate: service factor and temperature rise. These two values should be considered together to ensure that motors are not allowed to run at loads that will cause them to suffer heat damage to the winding insulation.
Temperature rise for an electric motor is often arrived at using what is called the resistance method.There is a formula that applies, but simply stated it is the difference in electrical resistance of the motor windings when hot versus when they are cold. The formula calculates the allowable temperature rise for a particular motor based on that resistance difference. On a motor nameplate, the temperature rise is given as a single figure, for example, 50°C. That means that the motor’s allowable operating temperature is 50°C above the ambient operating temperature for the motor. It is important to remember too that temperature rise is always expressed for the maximum allowable load.
Service factor is defined by NEMA as “… a multiplier which, when applied to the rated horsepower, indicates a permissible horsepower loading which may be carried under the conditions specified for the service factor…” Simply put, service factor is an allowable level of overload for an electric motor. For example, if a motor has a service factor of 1.15 that means that it can operate at 115% of its rated load for intermittent overloading (under certain conditions not listed in this tip) without being driven to overheat, provided that the proper voltage and frequency are delivered to the motor. Selecting a higher service rating for a motor can be particularly helpful if the operating environment of the motor is not well defined. It provides some allowance for variance in operating temperature without risking insulation damage.
These two factors work together because temperature rise again is expressed for the maximum allowable load for the motor. So, if you have two motors at the same horsepower rating and temperature rise but one has a higher service factor, the one with the higher service factor will be able to operate at a higher temperature without risking thermal damage. Since heat is the enemy of electric motors, it is imperative that these factors be applied for maximum motor health.