Application of (Motor Protection) Circuit Breakers in Combination with Variable Frequency Drives

Motor Protection Circuit Breakers (MPCB) are used in multi motor applications with Variable Frequency Drives (VFD). Devices positioned load side of a drive failed after only several months through thermal degradation. Investigations showed that MPCBs rated lower than 10 A suffer most. Similar designs of different brands were compared and showed in principle the same behaviour. Steep slopes of the DC voltage pulses in combination with the surge impedance of the motor and MPCB are responsible for excessive heat generation in the switchgear destroying the short circuit protection function. Similar effects are known in motors directly connected to drives with cables when exceeding a critical length above which reflected voltage waves occur. It is shown that chopping frequencies should not exceed 4 kHz and critical wire lengths of about 20 m should be respected to avoid damage.