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 :D

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....

(http://www.famousparts.com/soststbo.html)




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.

[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.

[632 Regal]

or just replace the breaker wit a 30 amp...make sure to swap them out with the 10 amp breaker after the place burns down though.

[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.

[DanH]

You can run a 30 A breaker if you use 10 gauge wire from the circuit panel. You should have 12 gauge wire now with a 20 amp breaker, how far is the circuit panel away? If its a long distance then you should use 10 gauge wire with a 20A circuit.

Its better to use 220V service for compressors if thats an option with your model. You'll only pull half the current and have half the initial load spike. You also could attach a nice soft start circuit that adjusts the AC frequency on start-up, but that would be getting expensive for such a small motor.

As SRR2 stated... the 6HP rating is not totally accurate. Well it is if you calculate instantaneous power using the peak-to-peak voltage for Vrms = 120 V which is:

Vpp = 2 * (sqrt(2) * Vrms) = 2 * 1.414 * 120V = 339V

Peak power = P = I * Vpp * pf = 20 A * 339V * 1 = 6780W = 9.1 HP Peak power available.
Therefore, they could try to claim up to 9HP on a 120V, 20A circuit, assuming power factor of 1.

RMS(Root Mean Square) Voltage is more realistic number to use:

Assume:
Irms = 20A
Vrms = 120V
Power Factor (PF) < 1 for motors.

Real Power = P = Irms * Vrms * PF = 20A * 120V * 1 = 2400W = 3.2 HP
So, 3.2HP is the maximum real RMS power available on 20 A circuit.

Since motors are inductive loads, you will actually do better by adding a capacitor to your motor to get the power factor closer to 1, adding an inductor will cause less efficient use of power. You have to know a bit of info about the motor before you could calculate the right size cap to add, however, that may not solve your problem anyhow.

Most likely is much simpler than that. Thats your Electrical Engineering lesson for the day.





or just replace the breaker wit a 30 amp...make sure to swap them out with the 10 amp breaker after the place burns down though.

[SRR2]

You're too polite. The 6HP is total ********. And "instantaneous power" is right behind it on the ******** meter. The only meaningful rating is what the motor actually produces under load at the shaft, not some specious imputation of motor power based on input voltage and current.

There is no such thing as a motor that converts input power to output power at 100% efficiency, therefore any statement that the motor is "X" HP because it draws "Y" amps at "Z" volts is nonsense. It's well known in the business that motor manufacturers for consumer goods (vacuum cleaners, home-grade air compressors, etc.) minimize the use of iron and copper to the extent possible to produce the specified power output at the maximum acceptable temperature rise. And they don't give a crap about how much power it consumes (wastes as heat) within those parameters. Therefore it's disingenuous to assume that some interpretation of input specifications tells you anything meaningful about power output.

The figures I gave are probably pretty close to what the OP has. The pf of the motor is probably around .75 to .8 assuming that it uses a phase shifting capacitor. I have seen some thoroughly disreputable manufacturers (yes, there are those even worse than Sears) that use pole shading on these motors. Sure, they draw 18A and produce 2/3hp, and get as hot as the proverbial firecracker, but who cares when you can buy the compressor for $49.95, right?

I stand by my recommendation that he run a new branch circuit with a 20A breaker (sized for the outlet type, per code) and 10ga wire. That should be adequate to keep the motor happy. Under NO CIRCUMSTANCES should he follow the advice of these electrical hobbyists here who are recommending the use of a 30A breaker on 12ga or smaller wire. It's not only illegal, it's freaking stupid. If that were to catch on fire, the source can be easily traced to the branch and then to the capacity mismatch, whereupon his insurance coverage on the dwelling would be immediately cancelled. And the insurance company WILL want to know how the fire started before making a $250000 payout.

At the typical pf for one of these motors, adding a shunt capacitor to the line input for pf correction isn't going to make enough of a change in current draw to make it worthwhile. You might be able to reduce draw by an amp or two at best. Besides, the capacitor would be expensive. Assuming this motor uses a phase-shifting capacitor, adding a self-switching 'hard start' capacitor can help, but would require some insight into which to choose, not to mention some skill in installation to make it all safe in a situation where it wasn't planned-for in the first place.

For the OP -- get yourself a "Kill-A-Watt" meter (search Google). It costs under $35 and will give you accurate readings of voltage, current, VA, watts, and power factor. It also integrates power to measure energy consumption, which is pretty cool if you're trying to justify buying a new refrigerator. Anyway, the KAW will tell you exactly what you're dealing with so that you can make an intelligent decision on circuit specifications.