Working principle of ZNB-P digital display intelligent motor protection starting circuit

The intelligent motor protector has protection functions such as overload, phase failure, stall, and three-phase imbalance. It can be used interchangeably with thermal relays such as JR16 and JR36. When the motor power is off, it will act within 3S: in the running state, when the motor current is greater than 6 times the set value, the protector will operate within Is; the reset time is less than 2s. Reset method: When the motor fails and the brake is turned off, the fault indicator light is on and the protector is in the memory state. Press the reset button on the panel to reset it. The protector consists of seven parts: current sensor start current delay circuit, overload protection, phase failure protection, stall speed protection, three-phase current imbalance protection, and fault display memory. The signal is taken out by the current sensor, and after rectification, filtering, detection, identification and other links, it is sent to the corresponding part for comparison and processing, and the power circuit is driven to make the relay act. When the motor current increases due to overload of the driving part, the voltage signal obtained from the current sensor will increase. When this current value is greater than the set value of the protector, the overload protection circuit will act. After the RC delay circuit delays, the drive relay acts, causing the contactor to cut off the main power supply and display the fault memory.

(1) Brief description of working principle

The three-phase power supply is connected to the main input terminal of ZNB-P through QF and KM2, and directly connected to the three-phase power line of the motor through three output terminals after passing through the internal current transformer and four protection circuits. The circuit is shown in the figure below. Its Al and A2 are connected to AC220V power supply (380V can also be connected, but the terminals are different).

When the button SB2 is pressed, the AC contactor KM line is energized and attracted, and its auxiliary normally open contact KM-1 is closed and self-locked, and its main contact KM-2 is closed, the three-phase power supply is connected, and the motor M is powered to run. The power supply path of the coil KM is: power supply L3→circuit breaker QF→fuse FU→stop button SBI→start button SB2/KM1 closed and self-locked→KM coil→protector normally closed ((95), (96)→neutral line N, forming a power supply circuit. When it is necessary to stop, press the stop button SB1. The motor stops running.

When the motor M fails due to a phase failure, overload, stall, or three-phase current imbalance, the protector detects the failure and operates. Its normally closed contacts ((95) and (96)) separate, disconnecting the power supply to KM and causing the motor M to stop running. The motor can only be started again after the failure is eliminated and the protector is reset.

(2) Electrical appliance selection

The circuit breaker QF is D247-63, the three-pole rated current is 10A, the AC contactor KM is CDC10-20, the coil voltage is 220V, the protector is ZNB05-P, the three-pole asynchronous motor M is rated power 1.1kW, rated current 3.2A, speed 910r/min, model Y9-6.

(3) Common troubleshooting

1) Press SB2. Contactor KM does not close.

This kind of fault is usually caused by: fuse FU is blown; button S'Bl or SB2 contacts are in poor contact; contactor KM coil is open circuit; protector normally closed contacts ((95), (96)) are damaged. a. If fuse FU is blown, and the multimeter measures the resistance at both ends to be infinite, the fuse is burnt out and can be replaced with a fuse of the same specification. b. If the button SB1 or SB2 contacts cannot be connected and the power supply circuit of coil KM is turned on, the contact surface can be rubbed with a knife or sandpaper to make it smooth and the contact resistance is less than 1n. The fault can be eliminated. c. If the fault cannot be eliminated after the above treatment, check the protector contacts ((95) and (96)). If the multimeter detects that the contact resistance is too large or infinite, and the protector is not in action, the protector needs to be replaced.

2) During the operation of the motor, there is an abnormal sound, the protector is activated, and the fault indicator HLR is on. After disassembling the motor, it is found that one of the rotor bearings is damaged and stuck, which makes it difficult to rotate. This not only causes the rotor to rub against the stator and make an abnormal sound, but also causes the motor load to increase, the motor housing to heat up, and the protector to operate. Replace both rotor bearings together, manually reset the protector, close and reset the normally closed contacts ((95), (96)), and the fault indicator HLR goes out, then the motor can be restarted.