Schematic diagram of common control circuits for motors Analysis of common control circuits for motors

1. One-way start control circuit

1. Circuit working analysis

1) Close the power switch QS, press the start button SB2, the contactor coil KM is energized, the normally open main contact is closed, and the motor is connected to the power supply to start. At the same time, the auxiliary normally open contact of the contactor connected in parallel with the start button is closed. When SB2 is released, the contactor coil continues to be energized through the normally open auxiliary contact to ensure continuous operation of the motor.

2) A circuit that relies on the normally open auxiliary contacts of the contactor to keep the coil energized is called a self-locking or self-holding circuit. The auxiliary normally open contacts are called self-locking contacts.

3) When the motor needs to stop, press the stop button SB1 to cut off the contactor KM coil circuit, the KM normally open contact and the normally open auxiliary contact are disconnected, the motor power circuit and control circuit are cut off, and the motor stops running.

2. Circuit protection link

1) Short circuit protection

The fuses FU1 and FU2 are used to implement short-circuit protection for the main circuit and the control circuit respectively. To expand the protection range, the fuse should be installed close to the power supply side in the circuit, usually under the power switch.

2) Overload protection

Since the fuse has the characteristics of inverse time protection and dispersion, it is difficult to achieve long-term overload protection for the motor. Therefore, the thermal relay FR is used as the long-term overload protection for the motor. When the motor is overloaded for a long time, the bimetallic strip connected in series in the motor stator circuit overheats and deforms, causing the normally closed contact of the thermal relay FR connected in series in the control circuit to open, cutting off the KM coil circuit, and the motor stops running, achieving overload protection.

3) Undervoltage and loss of voltage protection

When the power supply voltage is seriously undervoltage or loses voltage for some reason, the electromagnetic attraction of the contactor drops sharply or disappears, the armature is released, the normally open contact and the self-locking contact are disconnected, and the motor stops running. When the power supply voltage returns to normal, the motor will not start running automatically to avoid accidents. Therefore, the control circuit with self-locking has undervoltage and loss of voltage protection functions.

2. Forward and reverse control circuit

The figure below shows a control circuit in which two buttons control two contactors respectively to change the phase sequence of the motor and implement reversible rotation of the motor.

1. As shown in Figure (a), the normally closed contacts of KM1 and KM2 are connected in series in the other coil circuit to form a mutually restrictive control, which is called interlocking or interlocking control. The interlocking of normally closed contacts of contactors or relays is called electrical interlocking. If this circuit wants to make the motor rotate from forward to reverse or from reverse to forward, the stop button must be pressed first, and then reverse start.

2. The motor that requires frequent forward and reverse rotation can be controlled by the control circuit shown in Figure (b). Figure (b) is based on Figure (a), and the normally closed contacts of the forward start button SB2 and the reverse start button SB3 are connected in series in the normally open contact circuit of the other party. The connection method that uses the mechanical interlocking of the normally open and normally closed contacts of the buttons to restrict each other in the circuit is called mechanical interlocking. This control circuit with electrical and mechanical dual interlocking is a commonly used and reliable motor reversible control circuit.

3. Y-△ start control circuit

1. Y-△ reduced voltage starting means that when the motor with △ connection is started in normal operation, the stator winding is connected into a Y shape. The starting voltage applied to each phase of the stator winding is only 1/√3 of that of the △ connection, the starting current is 1/3 of that of the △ connection, and the starting torque is also only 1/3 of that of the △ connection. After the motor is started, the stator winding is changed to a △ shape to make the motor run at full voltage. This reduced voltage starting method is only suitable for starting under light load or no load.

2. Time relay automatic switching Y-△ start control circuit

As shown in the figure above, first close the power switch QS, press the start button SB1, KM is energized and self-locked, KM is energized, and the motor is connected to the voltage reduction start. At the same time, the auxiliary normally closed contact of KM is disconnected, so that KM△ is in a power-off state when the motor is connected to the voltage reduction start. While pressing SB1, KT is energized, and after a delay, the normally closed contact of KT is disconnected, KM is de-energized and released, and the neutral point of the motor is disconnected. The other pair of KT normally open contacts is closed, KM△ is energized and self-locked, and the motor is connected to the voltage reduction start. At the same time, the normally closed contact of KM△ is disconnected, and KM and KT are in a power-off state when the motor is connected to the voltage reduction start, so that the circuit works more reliably. At this point, the motor voltage reduction start-up of the ...

4. Reverse Braking Control Circuit

1. Reverse braking is to reverse the power supply of the running motor to change the power supply phase sequence of the motor stator winding, so that the rotating magnetic field of the stator winding is in the opposite direction, generating a braking torque in the opposite direction to stop the rotor quickly. In order to prevent the motor from reversing after stopping, a speed relay is generally used in the reverse braking control circuit for automatic control to ensure that the power supply is quickly cut off when the motor speed is braked to near zero to prevent the motor from rotating in the opposite direction.

2. Schematic diagram of one-way reverse braking control

As shown in the figure below, the main circuit of the one-way reverse braking control is basically the same as the main circuit of the forward and reverse control, except that three current limiting resistors R are added. In the figure below, KM1 is the forward running contactor, KM2 is the reverse braking contactor, and the speed relay KA and the motor are connected by a dotted line to indicate coaxiality.

1) When starting, press SB1, KM1 is energized and self-locked, and the motor starts. When the speed increases to the set value, the normally open contact of the speed relay KA closes. Because the normally closed contact of KM1 is disconnected, the KM2 coil is not energized at this time, and the connection of KA is only for the preparation of reverse braking action.

2) When stopping, press the stop button SB2, the contactor KM1 is de-energized and released, the motor loses power and runs by inertia, and at the same time, the interlocking contact of KM1 is closed, the contactor KM2 is energized and attracted, and the motor is connected in series with a current-limiting resistor for reverse braking. When the speed drops to a set smaller value, the normally open contact of the speed relay KA is disconnected, the contactor KM2 is de-energized and released, the motor is disconnected from the power supply, and the braking process ends.

3) Due to the high relative speed between the rotating magnetic field and the rotor during reverse braking, the induced electromotive force is large, so the rotor current is larger than the current of direct starting. The reverse braking current is generally about 10 times the rated current of the motor. Therefore, a series resistor R is connected in the main circuit to limit the reverse braking current. When the motor capacity is small, the current limiting resistor can be omitted.