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Air Compressor Troubles

Anyone here good with big electric motors?

The air compressor in the garage at my house all of a sudden quit working a couple months ago and suspension work is tough without my air tools!

I know for a fact that the compressor isn't siezed because I can easily (and bravely) turn it over.  It's a big , it has twin belts and I think it's, what's that called, a two-stage compressor. 

Here's how it's set up.  There's an old-school fuse box where the power comes into the garage (uses those old paper-ish fuses).  The lines run over to the compressor where they first go into a small breaker box.  The breaker seems to operate fine.  The wires then run to a motor control, which seems to be an electric-magnetic relay sort of thing that turns the motor off and on based on the pressure switch.  (Two small gauge wires run to the pressure switch on the tank.) 

Now like I said, as I understand it, the power runs to the motor control and that control acts like a relay that turns the compressor off and on based on pressure.  My multitester shows me that the two wires at the top of the motor control show approx 110 volts each.  When I manually operate the motor control, it doesn't show a change in voltage at the bottom of the control where the power goes out to the motor. (There are three wires going into the side of the motor coming from the control.)

Anyone have any guidance here?  I'm guessing the motor control is bad.  Any way to hardwire this to see if the motor itself is faulty?

Air Compressor Troubles

Reply #1 –
I'll ask a freind about it. I might even get him to stop by and check it out but no promises.

Air Compressor Troubles

Reply #2 –
Any chance there is a wiring diagram decal on it that you can take a picture of and post?

When you measured the input voltage to the motor control, did you measure from the inputs to ground or between the inputs?

If you measured to ground, try measuring across the inputs and see if you have 220 volts.

When you manually operate the motor control, do you think you are closing the contacts? Can you see the contacts, are they burned up?

I know you are looking for answers and all I am giving you is a lot questions. Sorry about that.

Air Compressor Troubles

Reply #3 –
I just got another mem card for my digital camera.  I'll try to get a pic of the schematic. 

When I measured the voltage, I measured it between the input and the ground.  I'll try measuring across both inputs.  What are they called again? Line?  Load?  If I understand it, Line is coming in and Load is going out to another box or to a motor.... ugh.  I need to read up on AC and two/three phase.

Air Compressor Troubles

Reply #4 –
I have a few books on electrical shiznit sitting on my bookshelf if you wana read them.
 

Air Compressor Troubles

Reply #5 –
The 'motor control' is it just a pressure switch? if so flip the breaker before it off, hardwire it (pos - pos, neg-neg, leave the grounds as is) them flip the breaker back on, it will either work or it wont.
1980 birds X 3, 1982 bird, 1984 XR7, 1988 TC

Air Compressor Troubles

Reply #6 –
Not quite.  I think the motor operates at 220 and the pressure switch (which checks out ok) runs at approx. 110.  The motor control, as I understand, acts like a relay so the 110v switch can turn on and off the 220v motor.

Air Compressor Troubles

Reply #7 –
Is there anything left of the schmatic left on the box? And did the book help at all?

Air Compressor Troubles

Reply #8 –
Quote from: Ifixyawata;111187
Not quite.  I think the motor operates at 220 and the pressure switch (which checks out ok) runs at approx. 110.  The motor control, as I understand, acts like a relay so the 110v switch can turn on and off the 220v motor.


I was wondering if it had a low voltage power supply for a control voltage or if it used a line voltage.

What is the brand of the compressor? How about the motor control, does it have a brand and model number?

Air Compressor Troubles

Reply #9 –
Most homes have 220v to the main breaker. There is also a neutral that unused current returns back to the power company. In addition to these you have an earth ground. If you look on your property you should find a copper wire leading to a buried stake in the ground. From there It's connected to your service panel. Earth ground is distributed through your house from the service panel. The power company generates 3 phases of current they're 90 degrees apart from one another electrically. Most homes don't have the third phase. That phase of electricity is added mostly in industrial sections of town. It's a cost savings for the power company. From the transformer on the pole that feeds your home you'll have 2 phases this is common for kitchen appliance, electric heat, garage home machine use. So at the pressure relay it's normal to have 120v from either of the conductors to the neutral conductor or to earth ground. If you measure across the feeds (the two black or red conductors depending on who wired it) you'll be measuring both phases of current which should read 220v. A typical problem is that the spring on the pressure switch will with age become more difficult to trip by a given pressure. two different settings are necessary. One causes the relay to close to start the compressor and the other opens the relay when the pressure is built up to a max setting. These are usually spring loaded tension screws. You back the screws in and out to set the start/stop settings for the relay. Examine the relay closely with the power disconnected. Make sure it's dead at the relay by taking measurements! across the feeds and from each feed to the white (neutral conductor) and your ground conductor (green). Once your sure that the relay has no power to it get in there and examine the pressure sensor relay closely look for loose connections burnt or bad contacts or conductors. If you bypass the relay the only protection, if none is built into the motor is the fuses or breakers. Breakers are only guaranteed for one operation by the manufacturer. The problems of electric motors open wiring, worn out bearings shorted turns or shorted to the case are typical problems. Many of these problems will blow your fuses or trip your breakers. If your compressor is siezed (kind of unlikely) the motor will humm. You might smell burning wiring.

Air Compressor Troubles

Reply #10 –
If you bypass the pressure switch and it humms disconnect the power. Something is messed up in the motor or the compressor. If you bypass the pressure relay I would wire a double pole switch in its place mounted in a box you would find at home depot or a electrical supply house. If the compressor is a large stationary one you can mount the disconnect on the wall near the compressor to assist in future work with the compressor. Make sure to size the switch for proper current carrying capacity. Ask for help at the store when purchasing. At this point disconnect the motor shaft from the compressor and apply power if the motor runs fine the compressor is most likely locked up. If the motor still won't turn the motor is bad. Motors can be repaired but if it's under 5 HP it makes sense to just replace it. If the compressor is a portable you might be better off to take it back to the local rep. beyond replacing the motor. If I had to guess your problem is probably the pressure switch. If you have 220v as measured across the two hot conductors on one side of the relay and your tank is empty the relay should be closed in that case you will have 220v on both sides of the relay. Which means that either an interlocking safety device after the pressure switch has opened of you have a motor/compressor problem. If the switch won't close get the brand and look on the net for setting instructions if it won't set and work properly replace it. Best of luck. Let us know what you find.

musings a further

Reply #11 –
After reading Softouchs' post another method comes to mind. That is the control relay. Instead of using a direct pressure contactor a control relay may be used to switch the conpressor on and off. In this case a pressure switch would be wired in series with a control relay coil. The coil of the relay would use 120v to close a set of contacts carrying 220v to the motor. In this case a separate pressure sensor/switch would be installed on the compressor. If this switch went bad the relay wouldn't close. So the relay doesn't have to be the culprit. These switches don't have to have adjustable settings they either work or they don't.

Air Compressor Troubles

Reply #12 –
A lot of electric motors used with equipment have a start capacitor.  This allows the motor to start up with a stored/assisted charge.  If the capacitor is bad---the motor will not start.  I can think of 2 kinds I see at home here.  One capacitor type is a separate container----for lack of a better description--it is roughly the size of an eyeglasses storage container, somewhat that long but not as flat.  The other kind is mounted on the case of the motor and resembles a hump.  If you have a capacitor start motor, better check that.  If I recall correctly, some motors have them built in --inside--not sure on that one.  An electrician will know. 
Capacitors can be dangerous if not discharged before handling--if you begin fooling with one---make sure it is discharged first, or you can be knocked on your ass, or killed.  It doesnt matter whether the equipment is off or hasnt been running either.  The picture here is one out of a microwave.

Air Compressor Troubles

Reply #13 –
Quote from: JKATHRE;112960
A lot of electric motors used with equipment have a start capacitor.  This allows the motor to start up with a stored/assisted charge.  If the capacitor is bad---the motor will not start.  I can think of 2 kinds I see at home here.  One capacitor type is a separate container----for lack of a better description--it is roughly the size of an eyeglasses storage container, somewhat that long but not as flat.  The other kind is mounted on the case of the motor and resembles a hump.  If you have a capacitor start motor, better check that.  If I recall correctly, some motors have them built in --inside--not sure on that one.  An electrician will know. 
Capacitors can be dangerous if not discharged before handling--if you begin fooling with one---make sure it is discharged first, or you can be knocked on your ass, or killed.  It doesnt matter whether the equipment is off or hasnt been running either.  The picture here is one out of a microwave.


I think that a polyphase [more than one phase of current] motor doesn't use capacitors. Under high torque conditions resistors or inductors are switched in and out during operation. They can be within the motor. On single phase motors however you will find a capacitor for inrush control on startup torques. This brings up another place for a problem to exist which if you hadn't brought this up I wouldn't have remembered. Many motors have thermistors that open if too much heat is generated within the motor. Unless they go bad they will reclose after the motor has cooled but the underlying problem may still exist. It can be as simple as exceeding the duty cycle of the machine or a problem with the load. Man it has been so long since I messed with this stuff I've almost forgot it all. :dunce:

Air Compressor Troubles

Reply #14 –
Is any of you smart people close enough to be bribed to come and fix this for us?