PLC control motor forward and reverse circuit explanation

There are many ways to implement the PLC control of the motor forward and reverse circuit. Two circuits are listed below for your selection.

PLC Control Motor Forward and Reverse Circuit (I)

Figure 3-1(a) is a wiring diagram of a PLC-controlled motor forward and reverse circuit. Figures 3-1(b) and (c) show the ladder diagram and instruction statement table of the circuit. The input/output address allocation is shown in Table 3-1.

Figure 3-1 PLC controlled motor forward and reverse circuit (I)

Figure 3-1 PLC controls the forward and reverse rotation circuit of the motor (continued)

Table 3-1 PLC input/output address allocation table for the circuit shown in Figure 3-1(a)

The PLC control motor forward and reverse circuit consists of two parts: the main circuit and the control circuit. This is consistent with the composition of the relay-contactor motor control circuit.

When the motor is given a forward control signal, the make contact of the input relay X0 is closed, the break contact disconnects the coil of the reverse output relay Y1, the coil of the AC contactor KM2 loses power, and the motor stops reversing. At the same time, the break contact of Y1 is closed, the coil of the forward output relay Y0 is energized, the coil of the AC contactor KM1 is energized, and the motor rotates forward.

When the motor is given a reverse control signal, the make contact of the input relay X1 is closed and the break contact is disconnected. The break contact disconnects the coil of the forward output relay Y0, the coil of the AC contactor KM1 loses power, and the motor stops rotating forward. At the same time, the break contact of Y0 is closed, the coil of the reverse output relay Y1 is energized, the coil of the AC contactor KM2 is energized, and the motor rotates forward.

When a forward signal is given, the motor runs forward; when a reverse signal is given, the motor runs reversely; when a stop signal is given, the motor stops regardless of whether it is running forward or reverse. In other words, the motor can be controlled to achieve forward, reverse and stop.

In the main circuit, KM1 is a forward AC contactor, and KM2 is a reverse AC contactor. The main contacts of the two contactors KM1 and KM2 are not allowed to be closed at the same time, and it must be ensured that the main contact of one contactor can be closed only after the main contact of the other contactor is disconnected. For this reason, in the output circuit of the PLC, electrical interlocking measures are adopted between the coils of KM1 and KM2, mainly to avoid short circuit in the main circuit when the main contacts of the AC contactor are welded together and cannot be disconnected.

In order to ensure that the motor can switch directly from forward to reverse, a program similar to the mechanical interlock of a button must be added to the ladder diagram to achieve interlocking. In the coil circuit of the output relay Y0, add the break contact of the reverse control relay X1; in the coil circuit of the output relay Y1, add the break contact of the forward control relay X0, so that the motor can be directly switched from forward to reverse.

Thermal relay FR1 is used as an overload protection device for the motor. When the motor is overloaded, its normally closed contacts close and the overload signal is added to the PLC through input relay X2, interrupting the program operation, causing output relays Y0 and Y1 to lose power at the same time, the coils of AC contactors KM1 and KM2 to be de-energized, and the motor stops running.

hint

Since the coils of output relays Y0 and Y1 cannot be energized at the same time, program interlocking must be added in the ladder diagram. In the coil circuit of output relay Y0, add the break contact of Y1; in the coil circuit of output relay Y1, add the break contact of Y0. When the coil of Y0 is energized, the coil of Y1 cannot be energized because the break contact of Y0 is disconnected; in the same way, when the coil of Y1 is energized, the coil of Y0 cannot be energized because the break contact of Y1 is disconnected.

PLC Control Motor Forward and Reverse Circuit (Part 2)

Figure 3-2(a) shows another PLC-controlled motor forward and reverse circuit wiring diagram (the main circuit is the same as Figure 3-1 and is not drawn). Figures 3-2(b) and (c) show the ladder diagram and instruction statement table of the circuit, and its input/output address allocation is shown in Table 3-2.

Figure 3-2 PLC controlled motor forward and reverse circuit (II)

The working principle of this circuit is basically the same as the circuit shown in Figure 3-1. Readers are requested to analyze it by themselves.

Table 3-2 PLC input/output address allocation table for the circuit shown in Figure 3-2(a)

hint

In this circuit, since the protection contact of the thermal relay adopts the break contact input, the program X3 (FR break) in the ladder diagram adopts the make contact.

Programming Method of PLC Controlling the Forward and Reverse Rotation of Motor

There are many programming methods for PLC to control the forward and reverse circuit of the motor. Figure 3-3 shows the program that uses the set and reset instructions to control the forward and reverse operation of the motor, and Figure 3-4 shows the program that connects the break contact of the stop button X2 and the make contact of the thermal protection relay X3 in series to the Y0 and Y1 control circuits respectively.

Figure 3-3