SUMMARY OF APPLICATION RULES FOR AC DRIVES

SUMMARY OF APPLICATION RULES FOR AC DRIVES

1. Unbalanced ac input voltages to a diode input source cause a current unbalance up to 20 times the voltage unbalance.

2. The power factor of a diode source bridge without a dc link reactor can be as low as 50 to 60 percent.

3. Older transistors and GTOs are much slower (50 to 20 j.Ls) than IGBTs (100 to 150 ns).

4. a. Fast transient voltages can generate reflections that increase the voltage stress of the insulation system.

b. The voltage reflections increase with the length of the motor cable and the switching speed.

5. a. The reflections can be stopped by a motor terminal (matching) filter. However, this filter retains the switching speed.

b. The switching speeds are reduced by an inverter filter. This is the preferred solution.

c. The motor and cable insulation voltage should be increased if neither filter is used.

6. EMI radiation is generated from the motor and ac line cables of all PWM inverters, espe- cially the ones having IGBTs.

7. a. The motor connecting cables (including ground conductors) for PWM inverters having IGBTs must be symmetric.

b. The best cable for these applications has continuous aluminum sheath, three conduc- tors, and three grounds.

c. The bearing currents and radiated EMI are reduced by the cable symmetry and con- tinuous outer sheath.

8. The damage to the motor bearings is reduced by an inverter output filter or an electro- static shield.