Air compressor overload keeps tripping

Air compressor overload keeps tripping DEFAULT

In this second post in the ‘part of the day’ series, I will discus an important part that is found on almost all but the smallest (<2 kW) compressors: the overload relay.

The overload relay is also often called the ‘thermal block’ or ‘thermal relay’.

This part protects your compressor from self-destructing when things go wrong. It continuously senses the current that the electric motor draws. When the current is too high for a too long time, the overload relay trips. In other words: the overload relay protects your motor.

The overload relay

This is what a typical overload relay will look like, as found on industrial air compressors.

Example of typical overload relay

The overload realy or thermal block has a few settings:

  • dial knob for setting the rated motor current
  • Manual / automatic reset selector
  • Test button
  • Reset button

It’s common practise to set the trip current at 1.15 – 1.25 times the normal operation current of the machine. However, the setting should not be so high that the current couold damage the motor or wiring in case of a failure.

Most of the time, we want to use the manual reset setting. If the breaker tripped, there is something wrong with your machine, so we need to investigate further what the cause of the tripping was, before resetting and re-starting the compressor.

Overload relay / thermal block on industrial air compressors

Here are some photos of electrical cabinets of industrial air compressors. On each foto you can see the thermal relay.

The thermal relay is 99% installed below the main contactor. All industrial compressors that don’t have a VSD drive or ‘soft starter’, start up using start/delta starting.

On the picture below you see the star contactor on the left, the delta contactor in the middle and the main contactor on the left. The thermal block/overload relay ‘hangs’ below the main contactor.

Overload relay on industrial air compressor

Why ‘thermal block’ and how do they trip?

I know you might be wondering where the name ‘thermal block’ comes from.

Inside the thermal relay are bimetal tripping elements. The more current flows through the relay, the hotter the bimetal tripping elements become.In case you don’t know: bimetal bends when it becomes hot, and comes back straight when it cools down again.

When there is an overload, a huge amount of current is drawn, the bimetal becomes really hot and bends. The thermal relay ‘trips’.

Because of the way the overload relay is contructed, it will trip quickly at a huge overload (within a second). But it will also trip on a small overload, this could even ben minutes or hours.

Because of this, the thermal relay is perfect for protecting motors. When the motor stars, it draws huge amounts of current, but this doesn’t trip the overload relay. However, if the motor runs heavy at 120% current for example for a few minutes or hours, the overload relay will trip, to protect the motor.

Electrical connections and how the overload relay stops the motor

Here is a typical electrical diagram of an industrial air compressor.

As we have seen in the picture above, the overload relay ‘hangs’ (literally) under the main contactor.

When the overload relay trips, it does NOT open the main connection itself. Instead, a small auxiliary contact opens, that signals to the central controller than an overload has occurd. The central controllers then stops the motor.

On older machines, the auxiliary contact is connected in series with the excitation coil of the main contactor. If the overload relay trips, the main contactor loses power and opens.

Electrical connection of thermal overload relay on air compressor



I receive quite frequently questions about compressor tripping on the overload relay ‘spontaneously’, for no apparent reason.

My personal experience is that when these things get older, they tend to become too sensitive and might either trip spontaneously, or trip at a too low current. Often, it can’t be reset either (pressing reset doesn’t help).. which means it’s time to replace the thing.

That’s the ‘part of the day’ for today. Tomorrow we’ll discuss another air compressor part.




6 Reasons Your Air Conditioner Keeps Tripping The Circuit Breaker

AC tripped circuit breakerSo you've determined that your AC is tripping the breaker, but you're clueless as to why.


Well, an air conditioner usually trips the breaker because it's pulling in more amps than the breaker is rated for. That is, if you have a 20-amp breaker and the AC pulls 30 amps, the breaker trips.

That's why breakers trip: to protect you from overcurrents that can damage equipment and cause fires (yikes).

So DON'Tkeep resetting the breaker and letting it trip. Constant tripping can harm equipment and cause a fire. Find the cause of the problem first.

Common causes of an air conditioner tripping the breaker include:
  1. Dirty air filter
  2. Dirty outside unit
  3. Issue with the circuit breaker
  4. Motor has shorted
  5. Compressor has trouble starting
  6. Compressor is grounded

You can fix the first problem yourself, but everything else requires an AC repair technician.

Read on to learn why these problems cause an AC to trip the breaker.

Or you can schedule an AC repair with us right awayif you live in the metro Atlanta, GA area.

1) Dirty air filter

Imagine forcing a pillow over your face. Hard to breathe right? That's what a dirty filter does to your AC blower; the blower has to to work longer and harder to circulate air through the filter. This causes the blower to draw more electricity and trip the breaker.

Solution: Change the air filter.

2) Dirty outside unit

Your AC system has an inside unit and an outside unit. The inside unit absorbs heat from your air using refrigerant. That refrigerant flows to the outside unit to disperse the heat.

However, the outside unit can't disperse heat well if it's super dirty because dirt is an insulator. So the AC runs and runs trying to disperse that heat outside until-click-the breaker trips due to drawing too much current.

Solution: Get an AC tech to properly clean the outside unit. You may be thinking, "Why can't I clean it myself?"

  1. You lack the cleaning equipment and cleaning chemicals to clean the condenser coil professionally (and buying the wrong type of equipment can actually do more harm than good to the condenser).
  2. Cleaning it improperly could damage/flatten the fins wrapped around the coil. This blocks airflow over the coil and is just as bad as having the coil matted with dirt.

3) Issues with the circuit breaker

The issue may not be with the AC itself but rather with the breaker.

Wires connected to the breaker may be loose or the breaker itself may be bad and needs replacing. This is a relatively inexpensive fix.

Solution: Have a tech tighten connections to the AC's breaker or replace the breaker.

4) Motor has shorted

Electric motors in your AC can run for hours and hours and can take quite a bit of abuse.
But if a motor runs hot for too long, the wire insulation can break down, leading to an electrical "short."

A "short" is where electricity bypasses its normal path, (so it's taking a "shortcut"). This shortcut allows more electricity to flow than the wires can handle, causing the wires to overheat, melt and cause a fire.

Of course, before the fire happens, the circuit breaker trips.

Solution: Call an AC repair technician to find the source of the short.

5) Compressor has trouble starting

The heart of your AC system is the compressor. It pulls tons of electricity when it starts up.

Unfortunately, as the compressor ages, it has trouble starting (called hard starting) and pulls even more electricity, causing the breaker to trip as a result.

Solution: Have an AC technician install a "hard start kit" which is a capacitor that gives the compressor motor an extra electrical "jolt" to get it moving.

However, the compressor may be old or damaged and need to be replaced instead.

6) Compressor is grounded

A "grounded compressor" or "compressor short to ground" means that an electrical winding inside the compressor has broken and hit the side of the compressor. This causes a direct short to ground, igniting the oil and causing a burnout. And, of course, the circuit breaker trips due to the sudden current surge.

Solution: This is the worst case scenario. An AC tech will have to replace the compressor and clean the refrigerant lines.

If your compressor is out of warranty (or your warranty never covered it), you might as well replace the entire outside unit. Cheaper that way. (Compressors are pricy.)

Need an AC repair in metro Atlanta? Call Ragsdale Heating and Air

If you've changed the filter and that didn't help, call Ragsdale to schedule expert AC repair. If you live in the metro Atlanta, GAarea, schedule an AC repair with Ragsdale Heating and Air. We serve cities all over the Atlanta metro area and beyond including Dallas, Alpharetta, Marietta, Roswell, Woodstock, Rockmart, Rome, Kennesawand more.
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When a  compressor is tripping and very hot the term used is: thermal overload. There are two usual suspects. 1. The outside coil is dirty or folded over so that the system has trouble rejecting heat, this causes the compressor to have to work harder and hence get warmer. 2. The compressor is 'short cycling'. It comes on runs, shuts off and then tries to restart in under 5 minutes. This causes the compressor to have to start while the refrigerant tubes have not had their pressure equalized. This makes starting the compressor much more difficult and often causes the thermal overload to trip. A compressor on thermal overload could eventually be a shorted compressor. If your compressor feels like it is constantly trying to start and can not start it is time to call a technician to diagnose what is causing this. Give us a call if you have any questions: 239-596-5855.

Compressor Overload Protector Testing! How it Works!

With just a couple of minutes running time, or sometimes after ten or fifteen minutes or so , the air compressor reset button always trips on my air compressor.

If this is your compressor problem here are a few things to look for and perhaps fix if your compressor reset button always trips.

First off, let’s make sure we’re all on the same page and what we are talking about when we are speaking of the compressor reset button.

Table of Contents

Air Compressor Trips Reset Button

You might know it as the thermal cut out or thermal safety switch. Others know it as the reset button. You can see one of them on the end of the electric motor in the image below.


It may be a small red button on the shroud of the compressor, it may be a button on the end of the motor if that is visible on your compressor, and I have seen them on the side of the motor at the shaft end, on some compressor motors.

Some are inside of the wiring cover on the motor, others are an inline switch on the wiring. The locations of this important piece of equipment on the air compressor sure do vary.

Regardless of where the compressor thermal reset button is on your compressor – on the motor or on the shroud of the compressor – its purpose is to cut the power to the motor in the event that the motor itself gets too hot.

Without the air compressor thermal overload switch, if there were a condition in the motor that was causing it to get hot, eventually the heat itself would destroy the compressor motor.

If the compressor reset button always trips on your compressor, the motor is getting too hot and you may need a replacement. For information on How to buy a compressor motor reset switch please visit our guide!

Why A Compressor Reset Button Always Trips

There are a few reasons as to why your reset button may be tripping on your air compressor and these are:

  • The power supply
  • Compressor runs too long
  • Centrifugal switch

The Power Supply

Check to make sure that your air compressor is plugged right into the wall socket, and it is not being powered through an extension cord or power bar. Reason being is that the wire in the extension or power bar may not be large enough to allow the flow of power that the compressor motor needs.

The power cord of the compressor is sized to allow sufficient power to the compressor motor. Restricting it through a smaller wire will cause the motor to over heat. If the motor overheats, the thermal cut out will shut it down. This is a very common issue.

Compressor Runs Too Long

If you are using your air compressor for extended periods, more than, say, ten or fifteen minutes at a time, the typicaly DIY air compressor will overheat. The duty cycle of the compressor may be less than the time you are requiring it to run, and the motor will overheat. If you think this is happening to your air compressor, try only running it for ten minutes and then letting it sit for at least that time so the motor can cool, before firing it up again.

Older air compressors – and some not so old if the compressor has tipped over for some reason – can develop a tank check valve leak. If that happens, air will be bleeding out of the tank all the time, even when the compressor is running. If that is happening on your air compressor, it will take much longer for the tank to fill – if it ever does – to cut out, and the by product of that is a motor that overheats.

See the troubleshooting pages on this site about how to fix a leaking tank check valve.

If your air compressor has or is developing a mechanical issue – perhaps a weakened or broken intake or pressure valve or a blown gasket in the pump, the compressor may have to run much longer than normal to fill the tank to cut out.

It may run so long that the motor will shut down on thermal cutout before the tank is full. If you air compressor seems to be undergoing this scenario, it may be time to add a valve and gasket kit. The troubleshooting pages hyperlinked on this page address these issues.

Centrifugal Switches?

Our thanks to Waite Mccormick for providing the following information on centrifugal switches and their role in thermal overload:

Another issue, the  centrifugal switch can stick which will keep the start windings energized tripping the thermal switch within 30 sec.

centrifugal switch

That centrifugal switch is kinda like 1st gear sorta speaking, and when motor get almost 3/4 of its design r.p.m., it shifts the motor windings to the run windings , kinda like running drive gear. I’ll spare you from the more complicated answer… the start windings are much smaller and can’t withstand the heat (of running full rpm) and results in the kicking of your thermal overload.

Air Compressor Reset Button Fix Demonstration

I have picked out the below video as a useful demonstration of how to fix your air compressor reset button.

FAQs (Frequently Asked Questions)

Why does my compressor keep tripping out?

It is likely that your compressor is tripping due to your power supply not being large enough to power the compressor or the compressor is overheating from working for too long and so the reset button trips.

Where is the reset button on my compressor?

The reset button can be located on the motor or the shroud of the compressor, it typically depends on the make and model of your compressor.

Where is the reset button on a air compressor?

It really does depend on the make and model of the compressor. Some compressors will have their reset buttons wired into the motor, whilst others are located at the end of the compressor.

Do air compressors have a reset button?

Reset buttons, or otherwise known as thermal switches or thermal reset buttons are important parts of air compressors as they ensure that air compressors do not overheat to prevent damage to parts and the overall system.

How do you reset an air compressor?

Look for the reset switch, typically a small red or black button near the side of the motor near the power cord and click this. The location will vary depending on the make and model.

Why do I have to keep resetting my air compressor?

The sole purpose of resetting your air compressor is because it is getting too hot. Therefore it cuts power to the compressor to ensure that it doesn’t overheat any further and subsequently preventing damage occurring.

Existing Reader Reset Button Tripping Problems and Responses

Here are a number of existing reader problems and helpful responses to reset button problems with various brands of air compressors:

Makita MAC5200 reset button keeps tripping

Kobalt Compressor tripping reset

Central Pneumatic air compressor keeps popping reset

Central Pneumatic 67847 compressor only runs for a few seconds

Central Pneumatic model 61454 overload

Champion compressor runs only when you hold the RESET button

Reset button on DeWalt D55168?

If you have any questions regarding air compressor reset buttons then please leave a comment with a photo if applicable so that someone can help you!


Compressor tripping air overload keeps

Any air compressor gurus out there? FIXED!!!

Re: Any air compressor gurus out there?

tryingtokeepmyfingers said:

I considered the pressure switch as well, but it doesn't adequately explain the overload in the absence of excessive tank pressure. If his tank was charging to the point the safety relief valve opened and the motor ran forever then I'd lean more heavily towards suspecting the pressure switch. I would also suspect the pressure switch first if the unloader valve were simply opening and the pump simply continued to pump air through it indefinitely -- for me, everything keeps coming back to the overload(and stated absence of an extension cord).

But, something is causing the motor to overloadand, if an extension cord is not being used, the most likely culprit, in the absence of abnormally high tank pressure, would be the check valve. A fault in the check valve will result in abnormally high pressure in the length of line between the pump and the check valve. This abnormal back-pressure will place a strain on both the valves and, especially, the compressor motor because the motor is having to strain against the back pressure. This will then result in the motor either overloading or overheating (whichever is triggered first) and cutting off.

It could still turn out to be the pressure switch, it is just that it does not seem to adequately explain the stated overload without abnormally high tank pressure (or the pressure relief valve activating). As such, I would still replace the check valve before the pressure switch.


overload air compressor

Compressor Trips The Breaker On Restart

You’ve got a full tank of air. The air tool ramps up nicely and is working well. Then, and as the pressure in the tank drops to the cut in pressure setting, rather than the compressor starting to rebuild the air in the tank, when it tries, the compressor trips the breaker on restart. If it’s a fuse in the circuit instead of a breaker, when attempting to restart, the compressor blows the fuse. The information on this page includes:

-what’s happening when the compressor trips the breaker or burns the fuse on restart?

-what needs to be checked on the air compressor when that happens?

-what are the compressor components involved and how to check them?

What’s happening when a compressor trips the breaker / fuse on restart?

When the air compressor tank is drained down to the cut in pressure level, the pressure switch reacts to the drop in tank pressure, it trips to “passing” (as in, the current now flows to the compressor motor) and the compressor should start and run until the compressor tank pressure has reached the normal cut out pressure setting, at which time the compressor should stop and wait for the tank air pressure to drop to the cut in pressure level again.

At the cut in pressure setting the the current will flow first through the pressure switch and then through the motor start circuit, and for a huge number of different makes and models of air compressor, the current then flows to a start capacitor.

If a particular compressor motor does not have a start capacitor, often that’s a small, sub-fractional motor, and there is sufficient current in the electricity flow to kick that motor into turning.

Larger compressor motors require a “kick start” to get going, as there is not enough power in the circuit to to feed what the compressor motor needs on start up, and that’s called the “inrush current”.

If for some reason the motor cannot start, it will continue to draw power, and may pull enough amperage to exceed the capacity of breaker or fuse and as a result the compressor trips the breaker or fuse on restart.

Compressor breaker trips on restart - tripped electrical breaker

What are the things on the compressor to check when the air compressor trips the breaker or fuse on restart?

Did the unloader valve work?  Sometimes the unloader valve fails, and as a result, some compressed air is trapped over the cylinder piston. That trapped air adds to the load the compressor motor experiences on start up, and that additional load may be enough to force the motor into pulling too many amps and popping the breaker, or blowing the fuse.

To test for this, unplug the compressor, open the tank drain and drain all the air out of the compressor tank (which you should do after every use of your air compressor as part of the regular preventive maintenance), plug the compressor back in and if your compressor has one, flip the ON/OFF switch to on.

Square-D compressor unloader valve

If the compressor starts this time and the breaker doesn’t pop, that suggests that your air compressor unloader valve is not working properly, one of which made by Square-D is shown in the image above, and you need to either fix it or replace it. To be sure, shut off the compressor, drain the tank again, and try to restart it with an empty tank. If the compressor starts easily again, then it’s pretty much certain that it’s the air compressor unloader valve failure that is causing your air compressor to trip the breaker on restart.

Are you plugging the air compressor into a power bar or extension cord? Home use extension cords are notorious for throttling power, as is a power bar. You may be under powering your compressor motor, and the resulting overheated motor may be contributing to the breaker tripping when the compressor starts.

Leave the compressor to cool for 15 minutes or so, and then try plugging the air compressor power cord directly into a wall socket, preferably one that has a 20 amp breaker, and has no other electrical devices using power at the same time as the air compressor is trying to start.

If the compressor starts this time, and after you have used some air and the compressor successfully restarts when the tank pressure drops to the cut in pressure setting, then you’ve nailed down the problem.

As a general rule we suggest that you add air hose to get the compressed air to your work station as opposed to using an extension cord to move the compressor closer to where you want to use compressed air. Nothing gets damaged if it doesn’t get enough air through a long hose. On the other hand, electrical devices are overworked if their power supply isn’t up to what is required leading to electrical maintenance issues.

Is it the pressure switch itself that has failed? Less likely, but still possible, that your air compressor trips the breaker on restart can be cause by a pressure switch that has shorted out.

When the air pressure in the tank drops, a diaphragm inside the pressure switch moves, and ultimately, that moving diaphragm forces the points inside the pressure switch to touch, a circuit is made, and power flows to the compressor motor starting it, and driving the pump to pump more air into the compressor tank.

Turn off or unplug the compressor.

Drain the compressor tank and then close the drain.

With due regard to the presence of live terminals inside the pressure switch, pull the cover off,  reset the breaker, and watch inside the pressure switch to see if there is any arcing or sparking when you turn the compressor back on.

Since the pressure switch points should already be touching with the absence of air in the tank, there should not be any arcs or sparks, and if there are, that’s a pretty good indication that you pressure switch may be the problem.

You will need to replace the pressure switch.

Has the compressor capacitor failed?

If the compressor has a start capacitor, or a start / run capacitor, or two capacitors, the failing of one may mean that the compressor will blow a fuse or circuit breaker when attempting to start.

This is what a typical motor capacitor will look like:

Capacitor - new, for compressor motor

The correction of the problem of when the air compressor trips the breaker on restart becomes increasing complex once these earlier tests have been made. One of the more complex tests is that of testing the capacitors.

If useful for you, please follow this link to our page that describe what the capacitors are and about how to test them.

Compressor motor windings shorting? In our opinion the next component that will be suspect when the compressor trips the breaker on restart are the motor windings shorting, or any of the electrical connections inside the motor that may be shorting and the sudden flow of electricity to ground pulls too many amps.

If you have reached this state in your compressor problem diagnosis, and you are not skilled in electrical motors or electrical circuitry, we suggest that you get your compressor motor to an electrical motor shop for a load test to try and determine what, if any, part of the motor is shorting.

These are a few of the typical reasons why an air compressor trips the breaker on restart. If you have experience with other scenarios, why not share them with visitors using the comment box below? If you see a comment and you would like to add to it, please feel free.

It is always helpful if you identify the make and model of your air compressor in any post.


Now discussing:

Air Compressor Reset Switch Problem

There must be hundreds of different types of air compressors that might be varied in sizes and capabilities but all of them have the same functional similarities that allow us to check the problems ourselves and solve rather than taking it to the shop and giving a crapload for petty problems.

Isn’t it frustrating when you have a fully loaded air tank and you start using the air tool and as the pressure starts to build, the air compressor suddenly trips the circuit breaker and the reset switch is good for nothing?

So, the things are what happens to the air compressor when it suddenly stops and the reset button isn’t working?

Either the reset switch on the air compressor also known as thermal cut or safety switch located at the end of your air compressor or on the sides or some of the reset, switches are located inside the wiring of the motor. Regardless of the location of a reset switch, its prime function is to automatically turn off the power of an air compressor when it gets excessively hot. Without a reset switch during this condition, the unexceptional heat could damage the motor of the compressor and if the switch is tripping this means that the motor is already getting hot.

There are certain reasons why this happens; we have listed down some of the possible causes and their solutions to help you fix the problem.

1. Check the Power.

Make sure that your air compressor is rightly plugged into the socket instead of a power extension or power bar because the air compressor requires a high power supply, which could not be sufficient through an extension. Plugging it to a small power supply tool could overheat the motor and the reset switch will shut down.

2. Excessive Use of compressor

A reasonable time to run an air compressor is not more than 10 to 15 minutes. Excessive duty cycle would overheat the motor, try running it for a small amount of time let say 10 minutes maximum at a time so that it can have enough time to cool down before starting the cycle again.

3. Oil Leakage

Check the valves of your air compressor, there might be some oil leakage and if that is the case, the tank of your compressor will take longer to fill and if it gets filled the reset button will trip and the motors get overheated.

4. Mechanical Issue

Make sure that your air compressor is not having any mechanical issue such as the broken pressure valve or a broken gasket, trying changing the valve or gasket kit because this would cause the compressor to take more time to fill the tank or it might shut down the reset switch before it even is full.

Fixing these issues by using a rest switch can result in major cost-saving and would downtime further risks and damages.


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