- Starting current:
- high current(I =6…7 × motor rated current).
- low power factor(cosϕ≈0,1…0,2).
- infrequent operation.
- due to the higher interrupted current the choppingcurrent can higher values.
- the arcing time for some switching devices may be longer than when the motor is running.
- overvoltages when breaking the starting current.
are usually higher than when breaking no-load or load-current
- No-load current:
- the no-load current of a small HV motor may be lower than the chopping current level of the circuit breaker.
- the power-frequency recovery-voltage is very small compared to the case of breaking the starting current.
- re-ignitions are not likely as the recovery voltage is low.
- low powerfactor(cosϕ≈0,1).
- Load current:
- high power factor(cosϕ=0,7 – 0,9).
- power-frequency recovery voltage is low.
- low overvoltages, no re-ignitions.
With regard to breaking capability and overvoltage generation, it is clear that the breaking of starting current is the most severe case.
Motor switching is analogous to the non-effectively earthed shunt reactor switching case.
The motor switching equivalent circuit is shown in Figure.