Coordination of MCCB and motor

Protection of motor

If the motor is overloaded, an overload current will flow, and the motor will burn. Thus, the circuit must be opened quickly. The branch circuit can be configured with the method shown in Table 4. 11.

Coordination with motor starting current

One problem for MCCB in a motor circuit is the unnecessary operation of MCCB caused by the motor starting current when the motor starts up. This is caused by the following two points.

q. The starting time is longer than MCCB thermal tripping characteristics.

w. Instantaneous tripping operation MCCB caused by starting rush current.

The size of the motor’s starting current is unique to the motor and differs according to the maker, model, capacity and number of poles. Normally it is 500% to 700% (in high cases 800%) of the total load current. The time that this starting current flows depends on the load GD2, and is usually within 15s. Exceeding 30s is said to be hazardous for the standard motor.

What must be cautioned in addition to this start time is the starting rush current mentioned in point w). This will be explained in detail in the next section. Fig. 4. 26 shows an illustration, which ignores accuracy to give an easy-to- understand explanation of points q and w above.

Coordination with motor starting rush current

    The motor’s starting rush current is generated during startup, during star-delta changeover, during instantaneous restart and during reverse breaking. Although it is short and only several cycles, this current is much larger than the starting current. The starting rush current is caused by the following points.

q.  Superimposition of transient direct current element caused by lower power factor of starting current as shown in Fig. 4. 27, a transient rush current flows because of the effect of the direct current element even when the alternating current’s amplitude is constant. If the starting current’s power factor is approx. 0.3, the rush current (peak value) will be approximately double the starting current (active value).

w.  Rush current during instantaneous reset caused by effect of residual voltage

When cutting off the motor from the power and connecting it again, there will be a residual voltage if the motor has not stopped yet. The residual voltage is not generated by only the residual magnetism, but because the iron core is excited by the residual current in the secondary coil.

This residual voltage does not cause a problem if the power voltage when reconnecting matches the phase. If not, the state will be the same as direct-ON starting with an overvoltage and will generate a large rush current. In other words, compared to starting from stopped state 

e.  Effect of magnetic saturation

This starting rush current is short being only several cycles long. however, MCCB instantaneous tripping operation will react even if the time is 1/2 cycle. Thus, MCCB instantaneous tripping current value must be larger than this starting rush-current. The following caution is required depending on the starting method.