Working principle of ZNB-S digital display intelligent motor protector control circuit

The functions and features of this protector are basically the same as those of ZNB-P. It has protections such as overload, phase failure, stall, and three-phase imbalance. It can be interchanged with thermal relays such as JR16 and JR36. Phase failure protection operates within 3s; stall protection operates within 0.3s. The working power supply voltage is 220V or 380V; output relay contact capacity:

250V, 7A; Reset method: When the motor trips due to a fault, the fault indicator lights up and the protector is in memory mode. Press the reset button on the panel to reset. The reset time is less than 2s.

(1) Brief description of working principle

The circuit is shown in the figure below. Because the rated current of the motor is relatively large, a current transformer is used to press into the main terminal of the protector. If the rated current of the motor is less than the rated current of the protector, the three-phase power line can be directly connected to the three terminals of the protector. This circuit protector uses a 380V power supply, and a 220V power supply can also be connected between the terminals ((1), (2)). The coil power supply of the contactor KMI intermediate relay is also 380V, and the coil voltage can also be 220V. Because the power of the motor is relatively large, the contactor used is also relatively large, so an intermediate relay is used to control the coil of the contactor KM to achieve the purpose of "controlling the large with the small".

When the start button SB2 is pressed, the coil of the intermediate relay KA is energized and its normally open auxiliary contact KA-1 is closed and self-locked, the normally open contact KA-2 is closed, the coil of the AC contactor KM is energized and its main contact KM-1 is closed, the three-phase power supply is connected, and the motor M is energized to run. When it is necessary to stop, press the stop button SBI to cut off the power supply of the contactor and the intermediate relay coil, and the motor M stops running.

When the motor M has a fault such as phase failure, overload, stall, or three-phase current imbalance, the protector detects the fault and trips. Its normally closed contacts ((3), (4)) separate, cutting off the power supply of the control circuit and stopping the motor, thus protecting the motor. Only after the fault is eliminated and the protector is manually reset can the motor be restarted.

(2) Electrical appliance selection　

The circuit breaker QF uses D220-400, 400A, three-pole; the AC contactor uses CJ20-400, coil voltage 380V; the intermediate relay uses J27-44, coil voltage 380V; the current transformer uses LM21-0.5, current ratio 400/SA; the protector uses ZNB-S type; the three-phase asynchronous motor uses YZR400LA, power 160kW, rated current 244A.

(3) Common troubleshooting

1) When the button SB2 is pressed, the contactor relay does not close and the motor does not run. The control circuit of this circuit is controlled by the relay KA, so it is closely related to the relay KA coil circuit. Possible reasons are: fuse FU is blown; the contact of the button SB1 or SB2 is in poor contact; the relay KA coil is open circuit; the protector contacts ((3), (4)) are damaged. a. If the fuse FU is blown, the multimeter measures the resistance at both ends to be infinite. You can replace it with a fuse of the same specification. b. If the contact of the button SB1 or SB2 is in poor contact and cannot conduct electricity to connect the coil circuit power supply, you can use a knife or sandpaper to rub the contacts to make the contact resistance less than 1n, and this fault can be eliminated. c: If the coil of the intermediate relay KA is open circuit, the multimeter measures the resistance to be infinite. You can replace it with an intermediate relay of the same model. d. If the fault cannot be eliminated after the above treatment, check the protector contacts ((3), (4)). If the contacts are damaged and cannot be reset, replace the protector.

2) Press SB2. The contactor KM can be closed, but the motor M does not run. This situation is often encountered. Because the motor power is large, the main circuit current is large, and when the main contact of the contactor is turned on and off, the starting current impact is large, which can easily cause the main contact KM1 to be burned by the arc, and the contact surface is uneven, causing poor contact and unable to connect to the Santong power supply. If the contact is severely burned, you have to replace the contactor.