20 Reasons Cause AC Contactor Failure

20 Reasons Cause AC Contactor Failure


It’s no secret that an AC contactor won’t last FOREVER.

They go bad after their stated lifespan – as any other electric device would.

But …

Whenever an AC gets on ‘NO COOL’ mode everyone wants a quick fix like there is no tomorrow, right?

For that, you need to identify the core problem first. And AC contactor failure is one of those major issues.

Several reasons may cause AC contactor failure.

To recognize all those wide-range causes that may disable your AC’s contactor, you’ll have to use your spidey sense.

Seems like hard luck?

No worries! I am going to take the guesswork out and reveal all the possible reasons for AC contactor failure in this blog post.

So, get set and go (right into the details)!

          20 Reasons to why AC Contactors Fail

Normally, an AC contactor has a pretty long shelf life of somewhere between 5-10 years – if taken care of properly with regular maintenance!

Still, all is not well.

Multiple reasons can make an AC contactor go wrong.

And it’s always better to know about them if you want to stay ahead of the game (who doesn’t?)!

Let me shed light on those icky reasons.

#1.  Wrong Product Selection

For an outlasting function, you MUST select the right product according to the current load it has to defy.

Some standard utilization categories define the current voltage that a contactor has to withstand. These current voltages depend on two factors:

  • The type of load being switched
  • The conditions a contactor has to work in

So, make sure that the rated voltage of the contactor’s coil is accordant with the operating voltage of the application.

There are different categories of contactors.

Such as AC-1, AC-2, AC-3, and AC-4.

Among all these categories, AC-3 is used in air conditioning units.

#2. Size

Choose an appropriate size of the contactor. While determining the size you can keep two things in mind.

  • The current closing
  • Interrupting Capacity

#3. Aging

Age is another major factor of contactor failure.

The winding in the coil of the contactor is stuck together with a varnished encapsulation. This keeps the coil in its place when the current flows through it.

As the contactors get older these coils can crack or move, which, in return, can break the insulation.

 #4. Product Coordination

If the selected contactor goes unsuccessful in coordinating with the application mechanism where it is installed, this results in welded contacts.

The other outcome can be worn-out contacts.

#5. Incorrect Cable Selection

If you want your contactor to work longer, choosing the RIGHT cable becomes mandatory.

An incorrect cable, in terms of passing on the current levels, means increased chances of cable overheating.

Outcome: Overheated contactor.

#6. Connection Mistakes

A torque value is recommended for the proper connection of the contactor in the instructions provided by the manufacturer.

Make sure to follow the guidelines and give the proper torque value – depending on the application.

If the connection becomes loose, it creates an overheating condition.

In another case, if the torque goes beyond the required limit, the terminal screw can get damaged.

When connecting the wires…

Make sure you don’t leave any metal parts exposed.

#7. Wearing out of the Poles

The most common reason for the contactor’s failure is worn-out poles.

Luckily, you can replace the deteriorated poles with a set of spare contact kits.

If the contact impairment happens too early after the engagement, it’s maybe because the contactor’s capability of passing on the current and its application don’t conform to each other.

#8. Chattering

The contactor’s plunger chatters when the contacts are dirty, or the coil has become weak.

Whatever the reasons, if the AC contactor keeps on chattering for an extended period, this can form an electrical arc. Also, it can damage the coil.

#9. Overcurrent in the Main Contacts

Sure, electricity is our friend.


It turns into the worst enemy in a matter of seconds – when carrying a higher current load.

The excessive current flow heats the main contacts of the AC contactor. The inflamed contacts of the contactor fuse together in the closed position.

You can avoid this overcurrent situation by choosing a contactor that complies with the full load current and utilization category.

#10. Power Quality

The quality of the electrical power also influences the life of a contactor. A contactor must be used in its defined operating parameters.

Transients and voltage variations harm the coil of the contactor badly.

#11. Overvoltage or under voltage in the Coil

The operational constraint of the contactor ranges between 85-110% of the rated coil.

A little increase or decrease in the voltage – as little as ±5% – will raise the contact bounce. And that eventually leads to wear and tear of the contacts.

The reason?

Higher voltage arouses the electromagnetic speed at the closing.

On the other hand, lower voltage reduces the electromagnetic speed at the closing.

Both of these situations cause a higher level of contact bounce.

Higher Voltage and the increased electromagnetic speed play their part in emitting the noisy sound.

In the other case, when the voltage goes less than 85% during lifting the contactor won’t be sufficient to close the contactor. Consequently, the coil may melt due to overheating.

You might be thinking, why will the coil melt when it is designed to withstand the current inflow?

You are right. A coil can hold out the pouring current but for a limited time only.

When the current flow exceeds that time limit – it melts!

#12. Voltage Drop

The voltage drop – less than 85% – or other disturbances in the control voltage can increase the wearing off of the contacts or can cause welding of the main contacts.

These other disturbances in the control voltage include Poor relay control or too small control.

#13. High Current Peaks

If the contactor undergoes a high current peak during startup that high current may also lead to the failure of the contactor. Therefore, it’s crucial to watch for the contactor’s voltage handling capacity.

It has to be capable of withstanding such high current surges.

This type of malfunction occurs with the contactors of heavy-duty machines.

A big fan or a pump, for instance.

#14. Transients Affecting Coils

In weaker voltage networks voltage transients are common.

Higher voltages may damage the coil.

To avoid such transients and protect the coil damage, you may use RC filters or Varistors.

#15. Restarting the Motor

This kind of contactor failure again occurs in heavy-duty machines.

For restarting the motor, the rule of thumb is to let it shut down first. Completely!

Otherwise, restarting the motor before it fully stops will cause a current peak.

This current spike will be two times higher than the current it would produce for starting the motor from an idle position.

And you’ll end up with welded contacts.

#16. High Temperature

Temperature can also be the reason behind any contactor’s failure.

Most electrical devices need proper ventilation. So does an AC contactor.

If an AC contactor is installed in a place where the temperature is too high and there is no sufficient ventilation, it will go bad.

Greater temperature results in two consequences. Contact welding and coil melting.

When you suspect that overheating is causing the failure of your contactor, you should check the following:

  • See if the connections are properly torqued.
  • Ensure the ventilation of the electric panel.
  • Check if the cable size is correct or not.
  • Also, look for the coil. Is it correct as per the application requirements?

Therefore, avoid hot spots while installing a contactor.

#17. High Vibration

If a contactor keeps on vibrating for a longer time . . .

This vibration will burn the internal coil.

To avoid this situation, you should consult the manufacturer’s manual to check some parameters. Like:

  • Vibration
  • Mechanical Shock

Want to overcome this problem?

You can opt for a special contactor – designed for high-vibrated applications.

#18.  Environmental Factors

Dirt and debris build up on the surfaces of the poles can cause your contactor to make humming noises. Another cause of such noises can be deformed poles.

Hearing louder humming noises?

Maybe the shading coil of the contactor has broken.

 A corrosive environment where damaging chemicals or vapors are present can also affect the coil of the contactor.

#19. Lack of Maintenance

Maintenance-wise, contactors are low-key.

But regular maintenance can increase their shelf life. Without a shadow of a doubt!

A great way to ensure maintenance is: Scheduling.

Plan your maintenance program right after your contactor is installed and is put into operation.

After the early inspection, you can decide on the frequency of regular maintenance.

Your inspection schedule will depend on two things:

  • Condition of the contactor
  • The severity of its duty

#20. Using Fake Spare Parts

When the need to replace any spare parts of the contactor arises, you should only use genuine spare parts. There may be cases when you won’t find a genuine spare part replacement. If that happens, it’s okay to use a replica.

However, you should check for a few factors (They must be compatible with the original product). Like:

  • Number of poles of the contactor
  • Terminals (should match the original)


Now you know the reasons for AC contactor failure – all of them.

The key to enjoying an optimal performance of the AC contactor is:

  • Correct installation
  • Appropriate use (Don’t stress it beyond its rating and operational capacity)
  • Timely maintenance

Next time your AC stops functioning, and the culprit is contactor failure, the first thing you need to do is know the real cause.

And then come up with the solution.

Want to keep the repairman away? Examine your AC contactor often!

Did you know all these reasons before? If not, which one was NEW for you?

Let me know by dropping a quick comment below.


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