How to Test a Contactor with a Multimeter (Step-by-Step)

Contactor

Contactor is a type of electromechanical switch. Through a Contactor , A power circuit can be controlled by just a minimal signal.

Working of single pole contactor is shown below; through a GIF. In a GIF, Contact move up and down. While a spring is sandwiched in between the contacts and solenoid coil(base). When solenoid coil get energized, It becomes electromagnet. Contacts get pulled downward. Contacts get closed. When coil get de-energized, Contact get upward due to the spring tension. Contacts get opened back to its normal initial state.

Through a signal to Contactor , Contactor’s contacts changes its position.

In normal controlling system, load is connected to one side. While power is connected to another opposite side. Supply to the load, can be control through a minimal signal at lower current rated, Control voltage can be at different voltage rating.

TABLE OF CONTENT

Parts of Contactor

• Contactor contains a contacts “NO “and “NC“. These contacts are mounted on above the stamping.
• Contacts are further categorized “Power contacts” or “Control contacts“.
• A coil with stamping, is placed at the bottom.
• A mechanical spring arrangement is sandwiched in between both the stampings.
• In heavy Contactor , Contacts are covered with metallic element to barrier the generated arc.

For better understanding, Contactor ML-3 by L&T is explained next !

1. ML-3 Contactor by L&T have rated current of 40 Amps.
2. Rated voltage is 415V AC; 50Hz.
3. Coil voltage 240V AC; 50Hz.
4. Contactor has total 3 main power contacts While it has 2 auxiliary control contacts.

Energized state of ML-3 Contactor by L&T

• Coil get a voltage 226 VAC.
• Contacts get pulled down and contacts get closed. (Clearly seen in an image belowI

De-energized state of ML-3 contactor by L&T

• Coil voltage disconnected.
• Contacts get pulled back to its initial normal state, Which is contacts opened. (Clearly seen in an image below)

Working of Contactor

There are two states of contactor

1. Energized state/ forced state : When electromagnet get energized, It pull the contacts downward. Contact’s state changes. NO contacts get closed. NC contacts get opened.
2. Normal state / free fall state : When electromagnet is not energized, It stays at its normal state of upward. Contacts retain its normal position. NO contacts stays opened. NC contacts stays closed.

Faults in contactor

“Contactors situation”, “Contactors Fault’s”, “Contactors causes” and “preventive solutions” are discussed in a tabular form below :

Situation	Contactor’s faults	Causes	Preventive Solution
Contactor is not energizing	Coil damage : Coil burnt or coil winding broken	Aging, material ingress, Long operation	Contactor should be compactible to the load, Its type and ampere rating | Electrical panel ingress protection should be designed according to the area.
Humming sound from contactor	Rust and carbon in stamping of electromagnetic coil	Material ingress, wrong ingress rating	Electrical panel ingress protection should be designed according to area.
Contactor’s contacts not moving	Contactor contacts get stuck, Contactor‘s contacts get welded	Aging, wrong rating selection, fault at load side	Contactor rating should compatible to load rating, right thermal overloading safety, right selection of MCB
2 phasing, 1 contact not working	One of the contact get stuck, one of the contact get burnt or damaged.	Aging, wrong rating selection, fault at load side	Contactor rating must be compatible to load rating. Right thermal overloading safety. Right selection of MCB

Method to test a Contactor

There are two method to test a Contactor . First, it should be tested visually. After that is should be tested electrically.

Visual testing of Contactor

Before visually testing the contactor, Make sure it must not be energized. It must be disconnected from power supply. Visual testing points are discussed below :

• Contact should be clean and rust free.
• Contacts should not be deteriorated or carbonized.
• Contact’s side by insulation should not be heated or burnt.
• Each and every contacts should be firmly fixed in its position.
• Contacts movement must be verified by pressing the contacts downward. After release, Contacts must move freely and rapidly.

Some of the images related to faults and solution are shown below :

Electrical testing of Contactor

Continuity Method

Check the continuity of the contacts in both the stages : When coil is energized and When coil is not energized.

1. NO contacts must be closed when coil is energized. While NO contacts must be opened when coil is not energized.
2. NC contacts must be opened; When coil is energized. While NC contacts must be closed when coil is not energized.
3. Check continuity/resistance value of a coil.

Voltage Method

First of all, Check contactor coil by giving the supply direct to the solenoid coil. Confirm the coil voltage first. What ever the voltage type is connected across the contactor, Contactor can be tested through voltage method.

Let suppose contactor is same ; ML-3-3 pole coil voltage is 230V AC. 3 Phase supply is connected across one of the side of contactor and another second side is open. When coil is get energized, 3 phase supply get available at its second side, if contactor is working fine. Other wise no voltage appear across second side.

Contactor of different Makes

There are multiple types of contactor made by multiple different companies. There look or geometry can be different but their basic function remains the same. Next some of the contactor of different ranges and types; made by different comapnies are shown below :

Summary

• A Contactor is an electromechanical switch that controls power circuits through minimal signals.
• Its operation involves contacts that shift positions when a solenoid coil is energized, pulling contacts(NO ) down to close and releasing them to open when de-energized.
• Contactors have “Normally Open” (NO ) and “Normally Closed” (NC) configurations, with power and control contacts.
• The ML-3 Contactor by L&T, rated at 40 Amps and 415V AC, illustrates this mechanism. Common issues include coil damage and stuck contacts, often stemming from improper ratings or aging.
• Regular visual and electrical testing is recommended to ensure functionality and safety.