The electrical schematic diagram of the forward and reverse control of a three-phase asynchronous motor via contactor interlocking is shown in the figure. The circuit uses two contactors: KM1 for forward rotation and KM2 for reverse rotation, controlled by forward and reverse buttons SB2 and SB3, respectively. The main contacts of these two contactors have different power phase sequences; KM1 is wired in the L1-L2-L3 phase sequence, while KM2 has two phases reversed. There are two control circuits: one for forward rotation consisting of button SB2 and the KM1 coil, and another for reverse rotation consisting of button SB3 and the KM2 coil.
Control Principle: When the forward start button SB2 is pressed, the power supply phase is energized through the normally closed contact of the thermal relay FR, the normally closed contact of the stop button SB1, the normally open contact of the forward start button SB2, the normally closed auxiliary contact of the reverse AC contactor KM2, and the coil of the forward AC contactor KM1. This causes the forward contactor KM1 to operate, its main contacts closing to make the motor rotate in the forward direction, and it maintains its operation through the normally open auxiliary contact of contactor KM1. The reverse start process is similar, except that after contactor KM2 operates, the two power lines U and W phases are swapped (i.e., the power phase sequence is changed), thus achieving the reverse direction.
Interlocking Principle: The main contacts of contactors KM1 and KM2 must never close simultaneously, otherwise a two-phase power short circuit will occur. To ensure that when one contactor is energized, the other cannot be energized, thus preventing a phase-to-phase short circuit, the normally closed auxiliary contact of the reverse contactor KM2 is connected in series in the forward control circuit, and the normally closed auxiliary contact of the forward contactor KM1 is connected in series in the reverse control circuit. When contactor KM1 is energized, its normally closed contact in the reverse control circuit breaks, disconnecting the reverse control circuit and ensuring that when the main contacts of KM1 are closed, the main contacts of KM2 cannot close. Similarly, when contactor KM2 is energized, its normally closed contact breaks, disconnecting the forward control circuit and reliably preventing a two-phase power short circuit. This function, where the normally closed auxiliary contact of one contactor prevents the other contactor from being energized when the other is energized, is called interlocking. Normally closed contacts that achieve interlocking are called interlocking contacts (or mutual locking contacts).
Starting and running principle of a three-phase asynchronous motor
(1) When a three-phase asynchronous motor is connected to a three-phase AC power supply (each phase differs by 120 electrical degrees), the three-phase stator windings generate a three-phase magnetomotive force (stator rotating magnetomotive force) through the three-phase symmetrical current and generate a rotating magnetic field. This magnetic field rotates clockwise along the inner circle space of the stator and rotor at a synchronous speed n0.
(2) The rotating magnetic field has relative cutting motion with the rotor conductor. According to the principle of electromagnetic induction, the rotor conductor (the rotor winding is a closed circuit) generates an induced electromotive force and an induced current (the direction of the induced electromotive force is determined by the right-hand rule).
(3) According to the law of electromagnetic force, under the action of induced electromotive force, an induced current will be generated in the rotor conductor, which will be in a direction basically consistent with the induced electromotive force. The current-carrying rotor conductor is subjected to electromagnetic force in the magnetic field generated by the stator (the direction of the force is determined by the left-hand rule). The electromagnetic force forms an electromagnetic torque on the motor rotor shaft, driving the motor rotor to rotate along the direction of the rotating magnetic field. When the motor shaft is loaded with a mechanical load, it outputs mechanical energy. Since the magnetic flux in the part without the short-circuit ring leads the magnetic flux in the part with the short-circuit ring, the direction of motor rotation is the same as the direction of the rotating magnetic field.
Working principle of forward and reverse rotation of a three-phase asynchronous motor
For a rotating electric motor, when the stator windings are connected in the same way, simply changing the power supply phase sequence can change the direction of rotation. For a three-phase asynchronous motor, the stator windings are interlocked phase to phase, which also changes the direction of rotation.
The forward and reverse rotation of a three-phase asynchronous motor is achieved by switching one of the power phases. The switching is achieved using two AC contactors. One contactor connects the live wires together normally (see the diagram below); the other contactor swaps the power of two of the phases, thus achieving forward and reverse rotation.
This forward and reverse rotation control method is suitable for small-capacity motors.
As shown in the figure below, when the motor rotates forward, it will rotate in the opposite direction, and when the motor rotates in the reverse direction, this speed regulation method is the most commonly used.
Therefore, for a single-phase motor, only one contactor is needed to achieve forward and reverse rotation, which is to say, switching.
For a three-phase motor, a motor with only one contactor and a coil that has both forward and reverse rotation is usually connected in series in the main circuit of a single-phase motor.
If a three-phase motor has only one contactor coil, we can achieve forward and reverse rotation by shorting the two contactors together.
For example, how to control the forward and reverse rotation of a motor? By changing this connecting piece, you can achieve both forward and reverse rotation.
This operating method is exactly the same as the structure of a single-phase motor or a three-phase motor, except that the connecting piece is fixed at three ends and the included angle in the middle cannot be changed.
If a three-phase motor can only use the output terminals of a single-phase motor, we can use two contactors to achieve forward and reverse rotation.
1. The coil of this contactor is connected in the main circuit of the motor. You only need to connect the main contacts of the two contactors to the forward rotation control circuit. Connect three auxiliary contacts of one contactor and four contacts of the two contacts of the other contactor.
2. The forward and reverse terminals and wiring terminals are respectively set as two buttons, one for the top and one for the bottom, requiring a control circuit of four buttons and two contactors.
3. Rotate clockwise to the left. This method works on the same principle as manually rotating the schematic diagram. However, in actual operation, the mechanical schematic diagram does not distinguish between positive and negative poles.
4. Rotate clockwise to the left. This method is basically the same as the principle of a motor. That is, to achieve forward and reverse control of the motor in the motor control circuit, you only need to set the phase sequence of its power supply to the lower limit.
5. This method is different from the motor schematic diagram. In the motor control circuit, as long as the normally closed auxiliary contacts of one contactor are connected in series in the motor control circuit, the forward and reverse rotation of the motor can be achieved. This is what we call reverse braking.
The reverse circuit shown in the figure below is a small motor control circuit for PLC enthusiasts. Its principle is to use an energy-consuming braking resistor to start the button and a reverse braking resistor.
The forward and reverse rotation of electric motors is widely used and is one of the essential topics in low-voltage electrical engineering practical exams. Only by fully understanding its working principle and operation process can we flexibly apply it in actual work. Furthermore, when a fault occurs, we can quickly troubleshoot the problem through principle analysis.
Editing the forward and reverse working principle
To achieve forward and reverse rotation of a three-phase asynchronous motor, it's necessary to swap two phases of the three-phase power supply. There are many methods for phase swapping, such as using a changeover switch or a contactor. In practical applications, contactor phase swapping is generally used to achieve forward and reverse motor rotation.
Let's take a look at the circuit diagram for forward and reverse rotation. Divide the circuit diagram in half, the left side is the main circuit, and the right side is the control circuit.
Editing the main circuit principle
Let's first look at the main circuit. The three-phase power supply splits into two paths after passing through the fuse, each going to the main contacts of two contactors. At this point, the phase sequence of the incoming main contacts of the contactors corresponds one-to-one with the power supply. After the outgoing lines of the two main contacts are interchanged, they are connected in parallel, then connected to the thermal relay, and finally connected to the motor.
When the main contact of KM1 is closed, power supply L1 flows to the first phase of the three-phase motor, power supply L2 flows to the second phase of the three-phase motor, and power supply L3 flows to the third phase of the three-phase motor, causing the motor to rotate forward.
When the main contact of KM2 is closed, power supply L1 flows to the third phase of the three-phase motor, power supply L2 flows to the second phase of the three-phase motor, and power supply L3 flows to the first phase of the three-phase motor, causing the motor to reverse.
Therefore, we only need to control the opening and closing of the main contacts of contactor 1 and contactor 2 to achieve forward and reverse rotation of the motor; to control the main contacts of contactor 1 and 2, we only need to control their coils. In addition, the main contacts of contactor 1 and 2 cannot be closed simultaneously, otherwise a short circuit will occur in the power supply.
Editing control circuit principle
The single-phase 380V is converted to a safe 36V voltage by a transformer before supplying power to the control circuit. The 36V power supply first passes through the thermal relay and the stop switch SB3, and then to the forward button SB1, the reverse button SB2, and the normally open buttons KM1 and KM2, respectively.
If the forward rotation button SB1 is pressed, current will flow through SB1 and the normally closed KM2 circuit to the KM1 coil. At this time, the KM1 coil is energized, the KM1 main contacts are closed, and the motor rotates forward. Simultaneously, the normally open KM1 circuit locks the connection between the two ends of SB1, and the normally closed KM1 circuit opens to prevent a short circuit caused by accidentally pressing the reverse rotation button SB2.
If the stop button SB3 is pressed, the KM1 coil is de-energized, the KM1 main contacts open, and the motor stops running. At the same time, the normally open KM1 circuit breaks and loses its self-locking mechanism.
If the reverse button SB2 is pressed, current will flow through SB2 and the normally closed contact of KM1 to the KM2 coil. At this time, the KM2 coil is energized, the main contacts of KM2 are closed, and the motor reverses. Simultaneously, the normally open contact of KM2 connects and latches the two ends of SB2, while the normally closed contact of KM2 opens to prevent a short circuit caused by accidentally pressing the reverse button SB1.
If the stop button SB3 is pressed, the KM2 coil is de-energized, the KM2 main contacts open, and the motor stops running. At the same time, the normally open KM2 circuit breaks and loses its self-locking mechanism.
