My air compressor quit working. The motor just hums and will not start.
The belt and comporessor unit are free. I can easily spin it by hand,
which means it's not the motor bearings either. The motor is a 120V AC
about 1/2 HP. It has a capacitor on top, so I'm thinking that is a
starting cap. How likely is it that the cap is bad? I guess the only
other problem would be bad windings, but that often causes smoke. I
dont have smoke.

On 11/21/2013 04:47 AM, wrote:
How likely is it that the cap is bad?

3:1

On 11/21/2013 4:47 AM, wrote:
My air compressor quit working. The motor just hums and will not start.
The belt and comporessor unit are free. I can easily spin it by hand,
which means it's not the motor bearings either. The motor is a 120V AC
about 1/2 HP. It has a capacitor on top, so I'm thinking that is a
starting cap. How likely is it that the cap is bad? I guess the only
other problem would be bad windings, but that often causes smoke. I
dont have smoke.

Likely a run cap. And, yes, they are known
to go bad.

--
..
Christopher A. Young
Learn about Jesus
www.lds.org
..

wrote in message
...
My air compressor quit working. The motor just hums and will not start.
The belt and comporessor unit are free. I can easily spin it by hand,
which means it's not the motor bearings either. The motor is a 120V AC
about 1/2 HP. It has a capacitor on top, so I'm thinking that is a
starting cap. How likely is it that the cap is bad? I guess the only
other problem would be bad windings, but that often causes smoke. I
dont have smoke.


Could be a bad capacitor. My well pump quit a couple of weeks ago. An
above ground pump. It would just hum. I took off a cover and there was a
capacitor and a set of contacts that put the capacitor in the circuit on
startup. A piece of the contact block was broken and would not allow the
contacts to close. Replaced that and all was fine.

Does the electric motor output shaft turn.

If it doesn't, and the motor just hums, then my guess would be a bad starting capacitor. You don't have to buy a new capacitor from the company that made the motor or the compressor; any motor rewinding shop will have them for sale.

In a capacitor start motor, it's the capacitor that causes the magnetic field of the start winding to be offset by 90 degrees from the magnetic field of the run winding. It's that difference in timing of the magnetic fields that provides the torque to get the motor to turn. Without a working start capacitor, what the motor's rotor is seeing is an OSCILLATING magnetic field which doesn't provide the torque necessary to get the motor turning.

I'd definitely replace the start capacitor.

wrote:
My air compressor quit working. The motor just hums and will not start.
The belt and comporessor unit are free. I can easily spin it by hand,
which means it's not the motor bearings either. The motor is a 120V AC
about 1/2 HP. It has a capacitor on top, so I'm thinking that is a
starting cap. How likely is it that the cap is bad? I guess the only
other problem would be bad windings, but that often causes smoke. I
dont have smoke.


My compressor has the same problem, but the hum blows the overload button.
I can get it to run if I don't put a tool on it, but just put a brass
Quick Connect fitting on the tool end of the hose without a tool
attached. I start the compressor and the air blows out the open fitting.
I can then attach a tool.


--
Bill
In Hamptonburgh, NY
In the original Orange County. Est. 1683
To email, remove the double zeros after @

wrote in message ...
My air compressor quit working. The motor just hums and will not start.
The belt and comporessor unit are free. I can easily spin it by hand,
which means it's not the motor bearings either. The motor is a 120V AC
about 1/2 HP. It has a capacitor on top, so I'm thinking that is a
starting cap. How likely is it that the cap is bad? I guess the only
other problem would be bad windings, but that often causes smoke. I
dont have smoke.


It looks like everyone else pretty much nailed everything. Cap or contacts
on centrifugal cut-out.

Do you have a motor repair shop within a 10-mile radius? If so, take the compressor with the capacitor hanging on but mechanically loose to the repair shop. They can test the capacitor in about 10 seconds, and if it is bad they can hook up a replacement and you can see that it does work while you are there.

Alternative is to find someone with a multitester to test the capacitor. To do that, you will have to remove at least one lead from the capacitor so that it is connected on only one terminal. You then put a multitester set to ohms across the capacitor and connect it. The ohms will show a very low value and quickly rise to show an open circuit if the capacitor is good. Then if you reverse the leads and reconnect, the same thing should happen as your multitester reverses the voltage on the capacitor. If you don't get any indication that the capacitor is charging from the multitester, it may be open which would explain the non-starting.