Special phase loss protection devices are often used in the start-up circuits of large motors.
And for most small and medium-sized motors. Only equipped with short circuit and overload protection (as shown in the drawing). When the line has a short circuit fault, the air circuit breaker QF can trip. When the load is overloaded, a large current can cause the thermal relay FR to operate. Thereby the contactor KM coil is de-energized. Play a protective role. And when the motor is running. When the phase loss occurs for some reason, the motor will burn out quickly. The following describes a relatively simple motor phase loss protection circuit, which is convenient to connect with the original line and has relatively reliable operation.
First, the working principleThe phase loss protection circuit is shown in the dotted line in the figure. From the three-phase line.
Each phase leads a line through the capacitor. Connected at one point. Form an artificial neutral point when the line is normal. The neutral point voltage is zero. If a phase is open. Then, the neutral point voltage rises, and the bridge rectifier circuit composed of the N line has a voltage output, and is filtered by the capacitor C4. The intermediate relay KA is brought into close contact.
The KA has a set of normally closed contacts connected in series to the original contactor coil circuit. After KA pulls in, its normally closed contact is disconnected, the contactor KM loses power, and the motor stops working, which plays a protective role. At the same time, the indicator light HL is illuminated. It is suggested that the maintenance personnel are lack of phase protection action, speeding up the inspection and troubleshooting of maintenance personnel.
Second, the component selectionCl～C3: oil-impregnated paper capacitor, 1.5μF/630V; VDl～VD5: rectifier diode 1N4007:
C4: electrolytic capacitor 1201μF / 50V;
KA: DC24V small intermediate relay:
HL: 24V refers to the permanent light:
QF: Air circuit breaker. According to the size of the motor capacity:
KM: AC contactor. Select according to the size of the motor. Coil rated voltage AC220V; FR: thermal relay. Select according to the size of the motor.
1. The rectification part of the device is used to the neutral line (N line) of the system, so it is required to balance the three-phase load of the system. If the neutral line voltage rises due to the uneven three-phase load of the system, the device will be caused. Malfunction.
2. The removal point from the three-phase power supply in this circuit should be as close as possible to the motor side, and at the very least should be connected to the outlet side of the contactor. Poor contact of the contactor contacts is also a common phenomenon of phase failure.
3. Now many intermediate relays have a freewheeling diode (such as an OMRON relay) on the coil itself, which eliminates the need for diode VD5.
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