Simple motor phase loss protection circuit

Specialized phase loss protection devices are mostly used in the starting and running circuits of large motors.

For most small and medium-sized motors, only short-circuit and overload protection are provided (as shown in the attached figure). When there is a short-circuit fault in the line, the air circuit breaker QF can trip. When there is an overload, the larger current can cause the thermal relay FR to operate. Thus, the contactor KM coil loses power, which plays a protective role. When the motor is running, if the phase loss occurs due to some reason, the motor will be burned out soon. The following introduces a simpler motor phase loss protection circuit, which is easy to connect to the original line and has a more reliable operation.

1. Working Principle
The phase loss protection circuit is shown in the dotted box in the figure.

Each phase leads out a wire through the capacitor and connects them at one point to form an artificial neutral point. When the line is normal, the neutral point voltage is zero. If one phase is open, the neutral point voltage rises, and the bridge rectifier circuit formed by the neutral point and the N line has a voltage output, which is filtered by capacitor C4 and causes the intermediate relay KA to close.

KA has a set of normally closed contacts connected in series in the original contactor coil circuit. After KA is energized, its normally closed contacts are disconnected, the contactor KM loses power, and the motor stops working, which plays a protective role. At the same time, the indicator light HL lights up, prompting the maintenance personnel that the phase loss protection is in action, which speeds up the maintenance personnel's inspection and troubleshooting.

2. Component selection
　　
Cl~C3: oil-immersed paper capacitors, 1.5μF/630V; VD1~VD5: rectifier diodes 1N4007:

C4: electrolytic capacitor 1201μF/50V;
KA: DC24V small intermediate relay:

HL: 24V indicator light:

QF: Air circuit breaker. Choose according to motor capacity:

KM: AC contactor. Select according to the motor capacity. Coil rated voltage AC220V; FR: Thermal relay. Select according to the motor capacity.

3. Notes

1. The rectifier part of this device uses the neutral line (N line) of the system, so the three-phase load of the system must be relatively balanced. If the neutral line voltage rises due to uneven three-phase load of the system, it will cause malfunction of the device.
2. The extraction point from the three-phase power supply in this circuit should be as close to the motor side as possible, or at least connected to the output side of the contactor. Because poor contact of the contactor contact is also a common phenomenon of phase failure.

3. Now many intermediate relays have freewheeling diodes on their coils (such as Omron relays), so diode VD5 can be omitted.