What Is an Electromagnetic Contactor?
An electromagnetic contactor is a device that uses electromagnets to open and close loads, such as motors and heaters.
The electromagnetic contactor is energized when a movable contact is brought into contact with a fixed contact by an electromagnet inside the electromagnetic contactor. The mechanism is the same as that of an electromagnetic relay, but it is used to energize a larger current than an electromagnetic relay.
Uses of Electromagnetic Contactors
Electromagnetic contactors are mainly used as internal components in control panels.
Examples of use are listed below.
- Lighting control of common lights in condominiums
- Operation/stop control for pumps that sprinkle water on plastic greenhouses
- Temperature control of aquarium tanks
- Operation control of freezers in commercial freezers
As shown above, electromagnetic contactors are mainly used for automatic control.
Principle of Electromagnetic Contactors
Electromagnetic contactors consist of an electromagnetic coil, an iron core, a movable contact, a fixed contact, a return spring, and so on. When open, the return spring lifts the movable contact and separates the fixed contact from the movable contact.
When closed, current flows through the electromagnetic coil. When current flows through the electromagnetic coil, it generates a magnetic field that attracts the movable contacts together with the iron core.
The attracted movable contacts make contact with the fixed contacts and energize the main circuit. When the main circuit is to be interrupted, the current flowing through the coil is interrupted and the return spring lifts the movable contact, thereby interrupting the main circuit.
Other information on Electromagnetic Contactors
1. Difference between Electromagnetic Contactors and Electromagnetic Switches
Electromagnetic contactors are devices that open and close an electric circuit, but it does not provide overcurrent protection.
To provide overcurrent protection, electromagnetic contactors are combined with a thermal relay (thermal relay) to form an electromagnetic switch.
The thermal relay outputs a contact when an overcurrent occurs in order to protect loads, such as motors. Detecting the output of the thermal relay, the circuit is interrupted or an alarm is issued.
The difference between electromagnetic contactors and electromagnetic switches (magnetic switches) is the difference in functionality depending on the presence or absence of a thermal relay.
2. Life span of Electromagnetic Contactors
Electromagnetic contactors control the running and stopping of equipment. Therefore, they are durable enough to withstand the starting current of the equipment many times. If the electrical characteristics are below the rating, the mechanical life is estimated to be 5 million to 10 million times of opening and closing.
3. Precautions when using Electromagnetic Contactors
There are several types of electromagnetic contactors, including standard, reversible, and DC-operated types. Therefore, it is necessary to select electromagnetic contactors with an understanding of their respective characteristics.
- Standard Type
The contact operates only while the electromagnetic coil is excited. - Reversible Type
The direction of rotation of rotating equipment can be switched by interchanging the phase order of the contacts. - DC-operated Type
While general electromagnetic contactors operate the electromagnetic coil with alternating current, the DC-operated type operates the electromagnetic coil with a DC power source.
4. Consideration of Back EMF of Electromagnetic Contactors
Back EMF (surge voltage) is generated in the electromagnetic coil of electromagnetic contactors when they are turned on and off. The electromagnetic coil part is generally controlled by a control circuit.
Since devices with low withstand voltage and current are connected to the control circuit, a surge killer may be connected to the electromagnetic coil section to protect the control circuit.
There are three types of surge killers:
- Varistor Type
Varistors are used to suppress peak voltage. Peak voltage can be suppressed, but high-frequency components cannot be limited. - CR Type
This is a low-pass filter to limit high-frequency components on the surge voltage. - CR + Varistor Type (hybrid type)
This is a type of surge aspirator that combines a varistor and CR circuit. Both peak voltage and high-frequency components can be limited.
5. Burbling of Electromagnetic Contactors
One of the most common problems is a buzzing noise from electromagnetic contactors. When the coil is AC, the attractive force of the magnetized iron core changes with frequency. In response to this, the iron core is constantly vibrating slightly, and the sound associated with this vibration is called a “whirring sound.”
The whining sound is caused by foreign objects being entangled in the contact surface of the iron core. The fixed iron core and the movable iron core, which are normally in surface contact with each other, make point contact, resulting in a buzzing sound.
If the electromagnetic contactors are disassembled and the foreign matter on the contact surface of the iron core is removed, it can continue to be used. When disassembling the electromagnetic contactors, the power supply should be disconnected and, if possible, the electromagnetic contactors should be removed.
If the electromagnetic contactors cannot be removed from the control panel, use a vacuum cleaner to clean it instead of blowing air on it. This is to prevent foreign objects blown by the air blower from causing a short circuit elsewhere.