PWM-2 CONSIDERATIONS
1. Motors must be derated due to the relatively high output harmonics; i.e., two-level PWM drive cannot be used with standard ac induction motors.
2. Due to the fast switching speed of the IGBTs (100 to 200 ns), high motor bearing cur- rents are generated. They flow through the bearings from the rotor to the stator and frame.
3. The fast voltage transients and the capacitance between the stator windings and the rotor create motor bearing current. This current flows through the motor bearings from the rotor to the stator (frame). A special motor construction must be used to minimize the capacitance between the stator windings and the rotor.
4. The fast switching transients create voltage reflection problems at the motor due to the difference in impedance between the motor connection cables and the motor windings.
These reflections result in an increase in the voltage transient levels on the terminals up to twice the nominal level. This problem is normally dealt with by using special motor insulation to withstand the higher voltages or by using a motor or an inverter filter.