How to Test a Magnetic Contactor

A magnetic contractor on a white background.
  • 1 hours
  • Intermediate
  • 10
What You'll Need
Clear tape
Volt Ohmmeter

A magnetic contactor or starter is uniquely made to produce power for electric motors. It has an overload relay which is attached physically and electrically. It can detect motor overload which triggers the supply voltage to open and start. It is a switch that’s run by electricity and is used to operate electric motors, lighting, heating, and other items loaded with electricity. This guide will help you test a magnetic contactor.

Step 1 - Identify the L-side and T-side

First, turn off the circuit breaker or unplug the power line that leads to the equipment to be tested, containing the magnetic contactor. After that, you may pull out the wires located in the line side screws. You must recognize these wirings by using a pen or a clear tape as markers. Use your pen to write L-1, L-2, or L-3 on the clear tape and post it on the respective wirings.

Next, pull out the wires from the contractor’s T or terminal side and L-side. Identify and put marks on each wiring using T-1, T-2, and T-3 as names. This will ensure that each wiring has its corresponding side and also prevent any misreading in the next procedures.

Step 2 - Connect the Wirings to the Volt Ohmmeter

Turn the control switch on to energize the coil in the contactor. You should hear a clear clicking and humming sound. Other magnetic contactors have a visual hint once the coil is energized.

Get the volt ohmmeter and turn it on. Make sure that the ohm is positioned in front of the selector switch. Then set the red lead into the ohm connector, the black lead on the common, and touch both leads together. The volt ohmmeter must read 0 ohms.

Proceed to test the L-side with its corresponding T-side L-1 to T-1 with the red lead to the L-side and the black lead to the T-side. Check if the wirings read 0 ohms. If it does not, then that means that magnetic contact is bad and therefore the contact must be replaced.

Step 3 - Observe the Test

The readings on the volt-ohmmeter should generate the same reading of 0 ohms and direct short. If it does not, check if the coils are properly linked or try to cycle the control switch of the coil if you can hear a clear clicking sound. If you cannot hear a clear clicking sound, then check the voltage source of the coil.

Turn the ohmmeter to volts and set the red lead into the volts connector of the meter and initiate power to the coil. Each lead must have direct contact with each coil connector. The reading of the meter must be the actual voltage supply. You should read the circuit specification to get the quantity of voltage. If the coil does not close even if the voltage is accurate, then you need to shut off the flow of energy.

At this point, you can proceed to remove the wires using a screwdriver. Reset the volt ohmmeter back to ohms. Then try to touch each leads that leads to the coil connector just like you read the voltage awhile ago although with the meter now in the ohms position. Make sure that the ohmmeter reads between 10 and 100 ohms. If it does not, the coil needs to be replaced.