How to Realize Motor Forward and Reverse Control with Mitsubishi PLC

Electric motors are the main control objects of drag control systems. In industrial control, the controlled objects have many operating modes, such as jog and continuous control. Under some working conditions, only a small capacity electric motor needs to operate continuously in one direction to meet the requirements, such as small ventilators, water pumps, and conveyors. Circuit diagram of relay contactor for motor start stop control.

However, production equipment often requires upward and downward, left and right, forward and backward, and forward and backward movements, such as the forward and backward movement of the machine tool workbench, the forward and reverse rotation of the machine tool spindle, and the upward and downward movement of the elevator. These all require the electric motor to achieve forward and reverse control. The implementation method of forward and reverse rotation of the motor is to change the phase sequence of the three-phase power supply connected to the stator winding of the motor, that is, to switch any two of the three-phase power supply incoming lines connected to the motor, and the motor will change from forward to reverse.

To ensure the reliable operation of the forward and reverse control of the motor, the dynamic breaking contacts of the KM1 and KM2 forward and reverse contactors are connected in series in the coil circuit of the other party in the control circuit, forming a mutually restrictive control. This mutual restrictive relationship is called interlocking control. Interlock control is used in places where "when the first contactor is required to work, the second contactor cannot work, while the second contactor cannot work".

In addition, an interlock for the start button has been added to the circuit, forming a control circuit with electrical and button interlocking (also known as mechanical interlocking). The advantage of this circuit is that the forward and reverse directions can be directly switched, without the need to press the stop button again, making operation more convenient.

Press the forward rotation button SB1 to make the state of the input relay X1 connected to it "1". In the ladder diagram, the dynamic closing contact of X1 is closed. This closed contact, along with the subsequent dynamic breaking contacts such as X0 and X3, drives the state of output relay Y1 to "1". At the same time, the dynamic closing contact of Y1 is closed to form self-locking, that is, the KM1 coil is energized and self-locking, and the positive sequence power supply is connected, causing the motor to rotate forward.

Press the reverse start button SB2 to make the state of the input relay X2 connected to it "1". In the ladder diagram, the dynamic closing contact of X2 is closed, which drives the state of output relay Y2 to "1" along with the dynamic breaking contacts of X0 and X3. At the same time, the dynamic closing contact of Y2 is closed to form self-locking, that is, the KM2 coil is energized and self-locking, and the reverse sequence power supply is connected, causing the motor to reverse.

During the forward rotation of the motor, SB1 is not only the start button for the forward rotation of the motor, but also the button to stop the reverse rotation of the motor due to the series connection of X1 and Y1 dynamic break contacts in the reverse circuit; Similarly, it can be seen that SB2 is the start button for reversing the motor and also the button to stop the forward rotation of the motor. This design is called interlocking (soft interlocking), which can ensure that the motor only has one of the forward and reverse sequence power sources at the same time, protecting the motor from being burned out.

When the button SBO connected to X0 is pressed, the status of the input relay X0 connected to it is set to "1". The dynamic break contact of X0 in the ladder diagram is disconnected, causing all the forward and reverse circuits of the motor to be disconnected, acting as a stop button.

Although there are already interlocking contacts for soft relays in the ladder diagram, it is also necessary to use the dynamic break contacts of KM1 and KM2 for hard interlocking in external hardware output circuits. Because the interlocking of internal soft relays in PLC only differs by one scanning cycle, while the opening time of external hardware contactor contacts is often greater than one scanning cycle, and there is no time to respond.

For example, although Y1 is disconnected, the contact of KM1 may not be disconnected yet. Without external hardware interlocking, the contact of KM2 may be connected, causing a short circuit in the main circuit. Therefore, it is necessary to adopt dual software and hardware interlocking