Contactor Coil: Working principle, Troubleshooting, Coil Voltage

What is a Contactor Coil?

The contactor coil is an electrical passive device that has two terminals. When a control voltage is applied to its A1-A2 terminals it enables the main contacts of a contactor to change their position.

How Does a Contactor Coil Work?

The contactor consists of an on/off contact point of the electrical circuit which provides electricity to the motor load, an electromagnet completed from an on/off acting control coil, an iron core, and a connecting device sending the movement of the electromagnet to the contact point. If there is a standard voltage at the coil, the excitation current runs, and magnetic flux is derived from inside the fixed iron core. This causes the fixed iron core to become a magnet causing the moving core to be pulled toward the fixed iron core. In the holder connected with a pin to this moving core, the moving contact point is assembled and it moves together with the moving core, contacts with the fixed contact point at the frame, and then closes the circuit.

If breaking the voltage is permitted in the coil, excitation of the iron core is released and the moving core is repulsed again by the back spring. At the same time, the moving contact point escapes from the fixed contact point and the circuit is broken.

When the coil circuit is off

When the coil circuit is on

What is Coil Voltage in a Contactor?

The coil voltage of a contactor is the voltage that the contactor is supposed to receive in order to be energized. It is also referred to as control voltage. To have a stable operation of the contactor coil, the energizing voltage should be basically within the range of +10%/-15% of the rated voltage.

The coil voltage of a contactor can be AC or DC. For the operation of AC coils, the power source is almost always a commercial frequency (50 or 60Hz) with standard voltages of 6, 12, 24, 48, 115, and 240V AC. For the operation of DC coils, standards exist for power source voltage and current, with DC voltage standards set at 5, 6, 12, 24, 48, and 100V.

Do not use a voltage higher or lower than the rated coil voltage. High voltage may damage or destroy the magnetic coil, whereas low voltage may not give enough power to actuate.

The control voltage of a contactor is marked near the A1-A2 terminals and can be measured between the A1 – A2 terminals with a multimeter.

What Causes a Contactor Coil Failure?

The common contactor coil failures are:

Insertion fault

The coil is burned out by excitation rush current (10~15 times of holding current) flowing into the coil, if you operate it continuously without perfect inhalation after the power voltage decreases abnormal material penetrates the magnetic contact pole.

Overcurrent

Excitation current increases when the permitted voltage is too high for the coil and, the coil endurance is reduced. In case of high current occurs the coil will sometimes burn out.

Application mistakes of voltage and frequency

It can be the cause of coil burnout when the coil-rated voltage is higher or lower than the power voltage. The coil is burnt out when the frequency application is wrong.

Excessive switching frequency

It can be the cause of burnout when it is used more than the designated on/off frequency.

Closing installation

Heating emittance installation is accelerated for heating aging by the central coil is the highest temperature when the magnetic switch is used at a continuous flow current.

Chattering

If the contact point has severe chattering by control voltage variation or coil terminal release, it causes coil burnout by driving current flowing into the coil.

Coil terminal screw release

Connect the coil terminal with the appropriate torque as written in the catalog. Otherwise, there could be an electrical arc in the terminals.

Transients

Voltage peaks/transients are common, especially in poor-quality or weak networks. Too high voltage peaks/transients may cause damage to the coil.

Troubleshooting a Contactor Coil

To troubleshoot a contactor coil you need to use a multimeter to verify coil resistance. This resistance value should be between some Ohms to kOhms depending on coil voltage and contactor model.

• If you find a value close to or less than 0.1 ohms it means that there is a short circuit in the coil
• If you have an infinite value, it means that there is an open coil circuit.

For both situations, you have to replace the coil or the full contactor if you have no other possibility. Contactor coils are sold as spare parts and can be found in the catalogs of the manufacturers.

How to Replace a Contactor Coil?

If you want to replace a contactor coil, follow the below steps:

• Release the terminal screw by using a screwdriver between the mid-front and back of the contactor.
• Remove the upper frame.
• Remove the target coil to replace which is installed in the lower frame.
• Insert the new coil.
• Fix the position slowly with two screws at the front and back of the contactor.

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