WILL YOUR MOTOR INSULATION SURVIVE A NEW ADJUSTABLE-FREQUENCY DRIVE

The latest family of insulated-gate-bipolar-transistor (IGBT)-type adj ustable-frequency drives (AFD's) produce voltage wavefronts that are e xtremely fast as a result of increasing carrier frequency. The frequen cy may be as high as 20 kHz. Motors which are designed for low-voltage sinusoidal power run more efficiently and quieter under this conditio n. However, the insulation in small-and medium-size motors may experie nce undesirable stress, resulting in accelerated aging. This aging usu ally leads to premature failure of motors, sometimes days or months af ter startup. Even new motors make this an issue. Harsh environments ma y also add to the problem. This paper will present a practical approac h to the motor insulation concerns of applying AFD power to less than 1000-V three-phase older motors and give specification details on new motors for AFD service.