Because the starting stopping control of asynchronous motor with frequency converter is carried out through the control terminal command of frequency converter, rather than through electromagnetic contactor, so electromagnetic contactor DC contactor is needed in normal operation. However, in order to cut off the frequency converter the power supply when the frequency converter fails, it is necessary to set up an electromagnetic contactor. In addition, DC contactor should be set when braking resistor is used. If the circuit is cut off as soon as possible, the brake resistor with short time rating characteristics will be burnt out because of the continuous flow of large current in the braking resistor when the transistor of brake circuit fails. In this case, the signal of overload relay installed on the braking resistance can be used to release the DC contactor, so as to achieve the purpose of protecting the braking resistance. In the selection of DC contactor, attention should be paid to make its appearance meet: the rated current is greater than the input current value of frequency converter.

The common motor is designed on the premise of operating under the grid power supply, so it can operate continuously for a long time under the power supply of the grid. However, when such an ordinary motor is driven by a frequency converter for continuous operation, due to the influence of high-order harmonics the inverter output, even if the motor runs at a speed lower than the rated speed the current is below the rated current, it is difficult to meet the cooling needs by the fan alone. Especially when the load is a constant torque load, even if the speed of the motor is below the rated speed, the current of the motor is basically equal to the rated current. Compared with the grid power drive, the temperature rise of the motor becomes larger, even the motor may be burned. Therefore, when the motor works continuously in the low-speed area, the DC contactor selected for protection based on the rated current of the motor can provide protection for the motor, which should be paid attention to when determining the DC contactor.