How to Check a Contactor Coil with a Multimeter

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Technology has advanced throughout the years. However, the primary means of regulating electrical circuits stay the same. Among those are contactors. This post aims to assess what a contactor and contactor coil is, how they work, and how you can check them by using a multimeter. 

What is a Contactor? 

When a relay is utilized to switch a big amount of electrical current through its contacts, it’s provided by a special name: contactor. In case you didn’t know, a contactor normally has different contacts, and those contacts are open, allowing the power to that load to shut off once the coil is de-energized. The most typical industrial use for contactors is the control of electric motors. 

A contactor is an electromechanical switch that functions to make or break the connection between the load and the power supply. It is electrically regulated and powered at a lower level compared to a switched circuit.

For instance, you would normally have a 24-volt coil electromagnet, which regulates a 230-volt motor switch. 

The application of a contactor may include:

  • heating
  • capacitor banks
  • lighting
  • thermal evaporators
  • electric motors
  • and other electrical loads

Take note that contactors vary in capacity and size. You have those, which can be simply lifted with your hand to big ones that measure at least a meter on the side. Further, you can have those contactors with a breaking current, which range from a few amperes to thousands, and those from 24-volt DC to many kilovolts. 

Are there Different Types of Contactors?

Yes, there are different types of contactors. They include the following:

  • Magnetic contactor – This is considered the newest contactor design and is the most innovative type of all. A magnetic contactor is mostly utilized for industrial purposes because of its features such as:
  • it uses the least amount of control current to close and open a circuit
  • it provides the safest operation
  • it works automatically
  • Manual contactor – This type of contactor was a substitution and improvement on the knife blade switch. Nonetheless, it still features manual operation. Some of its key features are:
    • smaller size
    • safer operation
    • it can correctly enclose unit protecting the inner parts
    • it has double break contacts that could open the circuit in two places at the same time, offering more current in tinier spaces
  • Knife blade switch – This is considered the oldest type of contactor and utilizes OFF and ON electric motors. It’s composed of a level and strip. The level functions by pulling the metal strip and down, which makes this type a manual operation. It has numerous weaknesses that led to its obsolete use, such as:   
  • double break
  • it was prone to dirt and moisture
  • had safety dangers
  • high occurrences of arcing led to the contactor’s short life span 

So, How Does It Work Actually? 

Would you like to understand the working of the contactor better? It would help if you understood the different parts of it. You will find three vital parts of a contactor:

  • coil
  • contacts
  • frame or enclosure 

The Coil 

You see, the contactor coil offers the driving force in your contactor, which closes the contact. It offers a coil wound in the electromagnetic core and acts as an electromagnet. That coil has two parts: a movable part with a spring connecting both parts and a fixed one. That structure makes a spring return system.

A rod known as the armature is attached to that movable component. When the force of the coil is more than the force of the spring, both contacts connect. It then disconnects when the spring’s force is more than the coil’s force. 

Take note that the contactor coil’s input could either be DC or AC. That current comes from an external control circuit for the contactor and acts to excite the electromagnetic core. 

In DC contactors, the electromagnetic core’s material is solid steel, as the problem of eddy current doesn’t arise.

For AC contactors, the electromagnetic material is soft laminated iron. It helps lower the eddy current loss. 

The Enclosure 

This component safeguards the contactor’s internal parts. It protects the contacts from oil, explosion hazards, bad weather, and dust. It also stops personnel from touching the contacts.

The Contacts 

Contacts do the current-carrying function in a contactor.  Its power has two types: movable and stationary contact.

How Do You Test a Contactor Coil Using a Multimeter?

  1. Unplug the power or turn off the circuit breaker that leads to your contactor. Pull out the wires in the line side screws. Use your pen to write L-1, L-2, or L-3 on the clear tape and place it on the respective wirings. 
  1. Pull out the wires from the terminal side and L-side. Determine and add marks on every wiring using T-1, T-2, and T-3 as names. Doing that will guarantee every wiring has its consequent side and stops any misinterpretation in the next processes. 
  1. Get your multimeter and turn it on. Ensure the ohm is positioned in front of the selector switch. Set the read lead into the ohm and the black lead on the common. Touch both leads together. Your meter should read 0 ohms. 
  1. Continue testing the L-side with T-side L-1 to T-1 with your readers to the L-side and the black lead to the T-side. If your contactor coil doesn’t not 0, it means the contact is bad and should be changed. 
  1. The reading must produce the same 0 ohms reading and direct short. If it doesn’t, check if the contactor coils are connected properly. If you can’t hear a clicking sound, check the coil’s voltage source. 
  1. Turn your multimeter to volts and set the read probe into the volts connector, and start power to the coil. Every lead should have direct contact with every coil connector. The reading should be the actual voltage supply. If the contactor coil doesn’t close, even if the voltage is correct, you should shut off the energy flow. 
  1. Rest the volt ohmmeter back to ohms. Touch every lead which leads to the coil connector. Ensure the multimeter reads between 10-100 ohms. If it doesn’t, your coil should be changed.

Final Thoughts

There are instances when you have a contactor in an application, but it’s not pulling in (or turning on). You assume that the entire contactor or the coil is bad. While that might be the case, you need to make sure your contactor gets the needed voltage to actuate the coil. 

Frequently, the contactor “not working” is the outcome of not receiving voltage to the coil. That could be a result of a faulty switch elsewhere. To fix that problem, grab your digital multimeter, and follow the steps above.

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