When the motor is in normal operation, if the fuse is blown by a phase fuse or for some reason, the power supply becomes a single phase from three phases. The stator magnetic field changes from a three-phase rotating magnetic field to a single-phase pulsating magnetic field. The single-phase pulsating magnetic field can be divided into two mutually rotating magnetic fields. Where the forward rotation of the magnetic field will produce a positive torque to continue the rotation of the motor rotor, but this torque is much lower than the original electromagnetic torque, the reverse rotation of the magnetic field generated reverse braking torque, which offset a part Forward torque, so that the already reduced electromagnetic torque and reduced a lot. So the motor output torque greatly reduced, the motor will not start. The rotor swings left and right, with a strong "buzzing" sound. If the motor is running in the power phase is missing, although the motor can continue to rotate, but the output torque will be greatly reduced, the motor current will increase, resulting in burning the motor. Therefore, when the motor in the operation of the power phase failure phase, the use of protective devices should be used to make the motor out of the operation.
In the rural electricity, especially the submersible pump control circuit, the line is simple, and some control circuit board or mobile, the user in the use of prone to phase failure accident, causing the pump burned, affecting agricultural production. Common phase-failure protector voltage and current type, the voltage type of the structure is simple, but the motor terminal and the internal phase, the lack of protection, and current type, the circuit structure is complex, high cost, unstable The According to the above design of a new type of current-type Three Phase Electric Motor phase protection, the protection of the structure is not only simple, reliable, easy to use, low prices, and maintenance-free, long life, small size. Circuit as shown below, the protector by the diode, relay, three-phase transformer and other components. The new Three Phase Electric Motor phase protection circuit is as follows:
After connecting the components in a box, the output of eight lines: al, a2, b1, b2, c1, c2, d1, d2, as shown below connected to the circuit, you can work properly.
First, the 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 the motor is running continuously. At this point the Three Phase Electric Motor current transformer through a winding, transformer T three-phase secondary winding to produce the induced voltage, the secondary winding wiring connected as shown below, closed loop three-phase electromotive force and zero, So no circulation, relay KM2 at both ends of the voltage is zero, the relay does not work, KM2 normally closed (d1, d2) does not move, the motor work properly. When the motor phase power phase, the transformer T secondary side of the lack of a phase of the induced voltage, closed loop of the sum of the three-phase electromotive force is not zero, AB between a voltage, the voltage size of the two-phase induced voltage vector sum, the voltage (D1, d2) action disconnect the control circuit, KM1 coil loss of power, the relay is turned off at both ends of the relay, The main contact of KM1 is disconnected and the motor stops.
Third, the component selection
The protector consists of three-phase transformer T, low-voltage winding according to the size of the motor current can be on the core around 1 to 5 laps, the winding in series in the main circuit, the secondary high-voltage winding, the number of turns can be selected according to the actual motor current The induced voltage is about 12V. Diode D1, D2 for the ordinary diode 1N4007, KM2 for the 24V, 3A DC relay.