OT - Air compressor keeps tripping circuit breaker

[Dave Rutkowski]

Any electrical engineers out there who have ideas on this? I have a Craftsman 6hp air compressor in the garage, running off of a 120V, 20A circuit that doesn't really have much else on it. About half of the time when the compressor kicks on, it immediately trips the circuit breaker. I'm assuming there's some momentary, abrupt resistance drop that's causing a spike in the current draw.

Is there some sort of inductor I can build/buy to smooth out the spike in current draw? If it were a DC circuit I could probably just get a big nail and coil some of the compressor's power wire around it, but for AC I'm guessing it's more complicated....Anybody have ideas? I know it was greatly annoying Scott H during his wagon project, so I'd like to get it fixed.

[Scott H]

It was annoying me? The only other thing on that line.....your wireless router....was shutting down while you were working. I'm not so sure I was the one who was annoyed.....maybe the power supply for the router is causing it

I hand torqued every nut and bolt on that car anyway.....who needs a compressor.....

I know it was greatly annoying Scott H during his wagon project, so I'd like to get it fixed.

[Anthony (M5 in Calgary)]

I had the same problem except it always tripped the breaker when the compressor cycled off. I re-wired the motor for 240V and installed a 240V circuit: no more problems. AC electric motors prefer running on higher voltage.

[Bill R.]

If its not working then it will frequently trip the breaker because of the greatly increased amp draw... Is it a single stage compressor or a 2 stage? Ones that are equipped with an unloader make a hissing sound for a few seconds after the compressor shuts down, This is the cylinder on the compressor releasing the pressure in it so that the compressor will start up not pulling the load of the cylinder full of pressure right off the bat.

Any electrical engineers out there who have ideas on this? I have a Craftsman 6hp air compressor in the garage, running off of a 120V, 20A circuit that doesn't really have much else on it. About half of the time when the compressor kicks on, it immediately trips the circuit breaker. I'm assuming there's some momentary, abrupt resistance drop that's causing a spike in the current draw.

Is there some sort of inductor I can build/buy to smooth out the spike in current draw? If it were a DC circuit I could probably just get a big nail and coil some of the compressor's power wire around it, but for AC I'm guessing it's more complicated....Anybody have ideas? I know it was greatly annoying Scott H during his wagon project, so I'd like to get it fixed.

[Bill R.]

too low of a current draw... or you can put in a slow blow breaker that will tolerate a greater amp draw for a short period of time. Or you can put a start capacitor on it such as we used on a/c compressor , here's a link to a number of them depending on what type of motor you have on your compressor now....

If its not working then it will frequently trip the breaker because of the greatly increased amp draw... Is it a single stage compressor or a 2 stage? Ones that are equipped with an unloader make a hissing sound for a few seconds after the compressor shuts down, This is the cylinder on the compressor releasing the pressure in it so that the compressor will start up not pulling the load of the cylinder full of pressure right off the bat.

[632 Regal]

change the breaker and see if that helps, 220 would be better than 110 too for the loadup.

[Tiger]

I concur on changing the breaker... Otherwise, I would simply add another 20A circuit to your garage just for the compressor only... very easy enough if the panel is in the garage and you have one extra slot available.

Majority of the time in the residential application... they do not have dedicated circuit to each room... multiple rooms sharing is more common.

[Jr ///M5]

A 6hp motor pulls about 16-18 amps at 110v once it's running. At startup, it will pull 5 times that amount for a milli-second, enough to trip the breaker though. It won't do it all the time because it depends on what position the crank on the compressor is in the last time it shut off, causing the motor to work harder to get the compressor spinning. The unloader releases any air build up in the head of the compressor so it won't start up under load and increase the motor load. That's why you hear air hissing momentarily after the compressor shuts off.

Most electrical motors are dual voltage, meaning they will run on 110v or 220v. You will have to look on the motor label to see what this particular motor draws. It will say something like......(110v- 16amps) and directly under that it will say something like...................(220v- 8 amps) these are the running amps that the motor draws while it is running, not starting.

There should be a wiring diagram on the motor label to show you what wires you have to change on the terminal block inside the cover at the end of the motor. Usually, you only have to switch around a couple. Then check it for rotation, and if it's backward check the wiring diagram again to correct it or switch it. The compressor will pump either way, but the wheel on the compressor is designed to blow air across the cooling fins to help keep the compressor cool during cycles. That's why we want to make sure rotation is correct.

Your best bet is to change the motor wiring and install a 220v outlet or hard wire your compressor. Is there 220 in the garage or is it just a small 110 panel?

[Scott H]

it was an upgrade option. There is 220 service in the garage. I'll let Dave leave the grin smiley on that one.....

Is there 220 in the garage or is it just a small 110 panel?

[SRR2]

One HP = 746 watts, so a 6HP motor would draw 4476 watts. At 115VAC and unity power factor -- the best case and rather unlikely -- that would require 39A. Starting currents of a motor that size can easily run 10X to 20x that value for as much as 1/4 second. (You would have to refer to the "LRA" rating of the motor to determine actual peak starting current. Or, you could use a breakout box and clamp-on ammeter with a peak hold feature to see how high the current goes.)

Conclusion? The HP rating of your Sears compressor's motor is pie in the sky hogwash. You simply can't believe anything Sears says about power ratings whether it's stereos, vacuum cleaners, garage door openers, or air compressors. A compressor that delivers 4 to 5CFM at 90PSI will need about 1.5 to 2HP.

Here's something closer to reality: A 1.5 to 2HP motor will draw around 15-18A running into the compressor's maximum head pressure at the power factor it's likely to present. At the end of a typical 20A (12ga) branch circuit you could expect a best case voltage drop of 3-6V from the main panel while running, and even higher if the power factor is lower causing higher total current draw. At startup, even with an unloader, the drop could be as high as 30-40V. It's true that some motors are equipped with dual-voltage windings but most aren't, and if this is a consumer-grade piece of equipment it's unlikely that it has a 240V capability. Even if you could rewire it, you would have to make sure that the limit switches and power switch on the device are rated for 240V operation, and you'd have to change the plug on the line cord.

Ordinary branch circuit breakers are already designed to prevent nusiance tripping on motor startup. If your breaker is tripping it probably means one of two things, either the breaker is defective or the branch circuit is unusually long and causes excessive drop (thus very low voltage) at motor startup. The low voltage at startup means that the motor will take longer to reach speed and the starting current-time product will be too high for the breaker to hold.

Rewiring the motor for 240V operation is a good solution, but improbable in consumer-grade equipment, so your next best option would be to install a new branch circuit, dedicated to the compressor, using a 20A breaker and 10ga wire. You may have to use a GFI breaker in the panel or outlet to comply with code. It's legal to use an oversize conductor as long as the outlet style/rating is consistent with the breaker on the branch circuit.