What Protection Devices Does an Electric Motor Need? What Are Their Functions?"

Switches, AC contactors, motor protectors (or thermal relays), intermediate relays, etc. can be selected, but only motor protectors, air switches, and AC contactors can also be selected. Sometimes it is necessary to match a transformer.

Electric motor protection devices: Air switch

The automatic air switch, also known as a low-voltage circuit breaker, has good operating characteristics-- fast opening and closing speed, protection characteristics--overload, short circuit, overcurrent, loss of voltage protection, and arc extinguishing characteristics--the arc extinguishing chamber can extinguish the arc quickly. Due to its good functions of both circuit breaker and protection device, it is very convenient to use and is applied as a control and protection equipment in low-voltage distribution systems.

Elecrtic motor protection devices: AC contactor

The role of the AC contactor is to act as an executive component and to connect and disconnect the circuit or frequently control the operation of equipment such as motors. It consists of main dynamic and static contacts, arc extinguishing cover, dynamic and static iron cores, auxiliary contacts, and bracket housings. When the electromagnetic coil is energized, the dynamic iron core is attracted under the action of electromagnetic force, and the dynamic contact is connected with the static contact, thereby turning on the circuit directly or indirectly through lever transmission. After the electromagnetic coil is de-energized, the dynamic iron core automatically returns under the action of the resetting spring, commonly known as the release, and the contacts are separated, and the circuit is disconnected. The AC contactor for motor protection is the most commonly used low-voltage control device in electric drive and automatic control systems.

Elecrtic motor protection devices: Motor protector

The role of the motor protector is that if the load device is connected in series with a three-phase motor, when the motor is overloaded, the motor will experience temperature rise, and eventually the motor will burn out. After the motor protector is connected in series, the on-off point of the motor protector is in the control circuit, thereby controlling the power-off or (after reset) power-on (protecting the motor).

Elecrtic motor protection devices: Intermediate relay

The intermediate relay is used in relay protection and automatic control systems to increase the number and capacity of contacts. It is used to transmit intermediate signals in the control circuit. The structure and principle of the intermediate relay are basically the same as those of the AC contactor. The main difference from the contactor is that the main contact of the contactor can handle large currents, while the contacts of the intermediate relay in electric motor protection devices can only handle small currents. 

Therefore, it can only be used in the control circuit. It generally does not have main contacts because its overload capacity is relatively small. Therefore, it uses only auxiliary contacts, and the number of contacts is relatively large. The new national standard for intermediate relays is defined as K, and the old national standard is KA. Generally, it is powered by a DC power supply. A few use AC power supply.