The production of three-phase asynchronous motor phase loss protection circuit

During the normal operation of the motor, if one phase fuse is blown or one phase is disconnected for some reason, the power supply changes from three phases to single phase, and the stator magnetic field changes from three-phase rotating magnetic field to single-phase pulsating magnetic field. This single-phase pulsating magnetic field can be divided into two rotating magnetic fields in opposite directions. The forward rotating magnetic field will generate a forward torque to make the motor rotor continue to rotate, but this torque is much lower than the original electromagnetic torque. The reverse rotating magnetic field generates a reverse braking torque, which offsets part of the forward torque, and reduces the electromagnetic torque that has already been reduced. Therefore, the output torque of the motor is greatly reduced, and the motor will not be able to start. The rotor swings left and right, and there is a strong "buzzing" sound. If the power supply of the motor is missing one phase during operation, although the motor can continue to rotate, the output torque will be greatly reduced, the motor current will increase, and the motor will burn out. Therefore, when the power supply of the motor is disconnected during operation, the protection device should be used to stop the motor from running. In rural electricity use, especially the control circuit of submersible pumps, the circuit is simple, and some control circuit boards are mobile. It is very easy for users to have phase failure accidents when using them, causing the water pump to burn out and affecting agricultural production. Common phase loss protectors are voltage type and current type. Although the voltage type has a simple structure, it cannot protect the motor when the terminal and the internal phase are missing. The current type has a complex circuit structure, high cost and instability. Based on the above shortcomings, the author has designed a new current type three-phase asynchronous motor phase loss protector. This protector is not only simple in structure, reliable in action, easy to use, and low in price, but also maintenance-free, long life and small in size. The circuit is shown in the figure below. The protector consists of diodes, relays, three-phase transformers and other components.

After the components are connected, they are placed in a box and output 8 wires: al, a2, b1, b2, c1, c2, d1, d2. Connect them to the circuit as shown in the figure below and they will work normally.

1. Working Principle

When the circuit is normal, close the power switch QS, press the SB1 button, the coil KM1 is energized, the main contact of KM1 is closed, the self-locking contact is closed and self-locked, and the motor rotates forward continuously. At this time, the three-phase current of the three-phase motor passes through the primary winding of the transformer, and the three-phase secondary winding of the transformer T generates an induced voltage. The wiring of the secondary winding is connected as shown in the figure below. The sum of the three-phase electromotive force of the closed loop is zero, so no circulating current is generated, the voltage across the relay KM2 is zero, the relay does not work, the normally closed (d1, d2) of KM2 does not act, and the motor works normally. When one phase of the motor is powered off, the secondary side of transformer T lacks one-phase induced voltage, the sum of the three-phase electromotive force of the closed circuit is not zero, and a voltage is generated between AB. The voltage is the vector sum of the two-phase induced voltage. This voltage is rectified by the diode and a DC voltage is generated at both ends of the relay (adjust the turns ratio of the transformer so that the voltage is 24V). Relay KM2 is activated, and its normally closed contacts (d1, d2) are activated to disconnect the control circuit, the KM1 coil loses power, the main contacts of KM1 are disconnected, and the motor stops.

3. Component Selection

The protector consists of a three-phase transformer T. The low-voltage winding can be wound around the core 1 to 5 times according to the size of the motor current. The winding is connected in series in the main circuit. The secondary is a high-voltage winding. The number of turns can be selected according to the actual motor current to make the induced voltage about 12V. Diodes D1 and D2 are ordinary diodes 1N4007, and KM2 is a 24V, 3A DC relay.