In the case that the load is the motor, the current that is several times more than its rated current flows when the motor starts up. Also, the back electromotive force( E = -L x (di/dt) ) is generated when the motor stops.
1. Peak current by starting the motor
The peak current that is several times more than its rated current flows when the motor starts up. If this peak current is bigger than the rated current of the power supply, the output voltage of the power supply is decreased by activating the overcurrent protection . Then, because the voltage applying to the motor is decreased, the maximum torque can not be produced. Incidentally, this maximum torque is same as the torque using at the rated condition. If the maximum torque is needed when the motor starts up, the power supply in which the rated current is bigger than the motor's peak current needs to be used.
2. Back electromotive force by stopping the motor
If there is the circuit that is for the reversal of polarity, the reverse current flows by the back electromotive force generated by stopping the motor. And then, the output voltage of the power supply might be decreased. Please refer to Fig. 2.1 and Fig. 2.2.
Fig.2.1 The circuit for operating the motor
Fig.2.2 The reverse current from the motor to the power supply
The out put voltage of the power supply might be increased and be shut down by the overvoltage protection, for the same reason. Therefore, if the motor, the inductance and so on is used as the load, the diode for protection from the reverse current flow needs to be installed for not increasing the output voltage of the power supply.
Fig.2.3 The circuit for protection from the reverse current flow
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