Protection of DC Motor

The protection of the DC motor is to ensure the normal operation of the motor, prevent damage to the motor or mechanical equipment, and protect personal safety. Therefore, the protection of the DC motor is an indispensable part of the electrical control system. These protection links include short circuit protection, overvoltage and loss of voltage protection, overload protection, speed limit protection, excitation protection, etc. Some protection links are exactly the same as those of AC asynchronous motors. This section mainly introduces overload protection and excitation protection.

1. Overload protection of DC motor

DC motor in the starting, braking and short-term overload, the current will be very large, the current should be limited to allow the range of overload. Overload protection of DC motors is generally realized by overcurrent relays. The protection circuit is shown in the figure. In Figure 9.9, the armature circuit is connected in series with overcurrent relay KA2.

The working principle of the circuit is: when the motor load is normal, the armature current passing through the overcurrent relay is normal, KA2 does not operate, its movable contact remains closed, and the control circuit can work normally. Once an overload situation occurs, the current of the armature circuit will increase. When its value exceeds the set value of KA2, the overcurrent relay KA2 will act, and its movable contact will be disconnected, cut off the control circuit, and the DC motor will be separated from the power supply. Play the role of overload protection.

2. Excitation protection of DC motor

Under the normal operation of the DC motor, if the voltage of the excitation circuit drops more or the power is suddenly cut off, the speed of the motor will rise sharply and the phenomenon of "flying" will appear. Once the "flying car" phenomenon occurs, it will seriously damage the motor or mechanical equipment. The protection of DC motor to prevent loss of excitation or weakening of excitation is realized by using under-current relay. The excitation circuit is connected in series with the undercurrent relay KA1. When the excitation current is appropriate, the undercurrent relay is closed, its moving contact is closed, and the control circuit can work normally. When the excitation current is reduced or zero, the under-current relay is released due to the low current, and its moving contacts return to the open state, cut off the control circuit, and disconnect the motor from the power supply, which plays the role of excitation protection.