Q&A(FAQ)
What is influence of back electromotive force from motor on PSU?

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.
Aluminum electrolytic capacitor is the most important parts in terms of reliabiliety, and life expectancy of a power supply can be estimated by calculating the estimation of life expectancy of aluminum electrolytic capacitor.

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.

The out put voltage of the power supply might be increased and be shut down by the overvoltage protection, by the same token. Therefore, if the motor, the inductance and so on is used as the load, the diode for protection from the reverse current needs to be assembled for not increasing the output voltage of the power supply.

Fig. 2.3 The circuit for protection from the reverse current

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