Views: 4 Author: Site Editor Publish Time: 2019-09-25 Origin: Site
The control circuit of the electric hoist is composed of circuit breakers, contactors, transformers, terminal blocks, control handle buttons, thermal relays, wires, sliding wires and other electrical components. The following protective measures must be taken in the
1. undervoltage protection
When the line voltage drops, the motor torque is reduced, and the speed decreases accordingly, which will affect the normal operation of the motor. When the voltage is seriously reduced, the motor will be damaged. In the control circuit with self-locking, when the motor is running, the power supply voltage is reduced. The value has been set, generally lower than 85% of the working voltage, the magnetic flux of the contactor coil is weakened, the electromagnetic baptism is insufficient, the moving iron core is released, the auxiliary contact is normally opened, the self-locking is lost, and the main contact is also disconnected. The motor stops rotating and is effectively protected.
2. contactor voltage loss protection
The self-locking control circuit in the control line, when the power supply temporarily loses power and resumes power supply, since the initial self-locking contact has been disconnected, the control circuit will not be turned on, the contact coil has no current, and the normally open contact will not Closed, so the motor will not start running by itself, and accidents can be avoided. This protection is called power failure protection.
3. overload protection
The electric hoist may cause the current of the motor to exceed its rated value if the long-term load is too large or the operation is frequent or short-phase operation during operation. Sometimes the leakage short-circuit switch will not trip in this case, which will cause the motor winding to overheat, the temperature exceeds the allowable temperature rise, the insulation damage, affect the service life of the electric hoist, and even burn the motor seriously, so the electric hoist needs to be taken Overload protection. The most commonly used one is to use a thermal relay. The heating element of the thermal relay is connected in series in the two-phase main circuit of the control circuit. The normally closed contact is installed in the control circuit. When the current exceeds the rated value due to overload or other reasons, the heat is generated after a period of time. The component is bent by heat, and the normally closed contact connected in series in the control circuit is disconnected, the control circuit is cut off, the coil of the contactor is powered off, the main contact is disconnected, and the motor is stopped from the power supply, thereby functioning as an overload protection.