Abstract: When 690 V voltage systems are driven by variable frequency drives, the unit shutdown caused by insulation breakdown of cables, electromagnetic interference and low frequency resonance of motors are analyzed. The insulation breakdown of cables can be solved by controlling system voltage or improving cable insulation. Low frequency resonance can be eliminated by changing the natural frequency of pumps or utilizing variable frequency motors. Electromagnetic interference can be prevented by using shielded cables with the shielding layer grounded at both ends. The short circuit breaking capacity of switches decreases in the 690 V voltage systems, and the decline in that of molded case switches is greater, while the fuses have extremely high short circuit breaking capacity. When the variable frequency drives match with the motor capacity, maximum load current is determined and the rated power of motors can be used for reference. Self-ventilated air cooling should be selected for the variable frequency motor of blowers and pumps.

Key words: variable frequency drives, insulation breakdown, short circuit breaking capacity, low frequency resonance, shield, electromagnetic interference, capacity matching, air cooling