Sanborn 3HP Compressor - No Start Most Times - Only Hums

[Vet65te]

Have owned a Sanborn portable compressor for about 30 years with great service up until recently. It's Model: 112A300-22 running on 220VAC.
Past history - had some difficulty with the factory on-switch and replaced it with a Hubbell No. 7832 rated at 30A and 600VAC for single phase and it has worked fine since. Recently, when I flip the switch, the motor hums most of the time but starts occasionally and when it does fire up, runs to the expected ~120psi range. I'm not an electrician but after reading a number of similar threads on the GF, I thought it might be the start capacitor which I expected was the one sitting piggy-back on top of the Century brand 3HP motor. No smell from the capacitor which is a Mallory rated at 590-703 MFD and when I brought it into a local power tool repair shop, they tested it and it showed about 617 MFD so they declared it good. Any suggestions on where to check next, maybe that Hubbell switch?
Thanks, Mike

 

[OccupantRJ]

Centrifugal start switch inside the rear end bell of the motor. Any debris between the two contact points will cause this. I have found threads used to bind motor windings in them a couple times over the years. Remove the belt, spin the motor, then apply power. If motor takes off properly, this switch is likely the problem.

 

[dogdog]

Check the capacitor?
and continuity of that Centrifugal switch...

basically, take off the capacitor and test for continuity between the two leads. The Centrifugal switch usually is connected to start winding and the capacitor... It is a Normally Closed switch... when the motor starts up to speed, the centrifugal switch disconnects the start winding by opening the connections.
You can also test each winding if you have access to them they should be equal in resistance ( of cause dependent on your meter accuracy and configuration) usually equal or multiple of two......

there is a few good post here as well.... few months back.

There is a really good post in the littlemachineshop forum explaining this...

If the motor hums, usually it means it's not the electrical switch/pressure switch.... cause it's getting electricity already.

You should take the belt off the compressor to do the test, just make sure it's not a seized compressor or anything funky... I know you say it starts... this is to isolate your issue.

 

[6PTsocket]

Centrifugal start switch inside the rear end bell of the motor. Any debris between the two contact points will cause this. I have found threads used to bind motor windings in them a couple times over the years. Remove the belt, spin the motor, then apply power. If motor takes off properly, this switch is likely the problem.

Far more common than debris in the contacts is contacts that are just shot, burned up. When the switch opens there is an arc at the contacts, over the years it takes it's toll. They can sometimes be cleaned up with a fine file to extend their life for a while. Less common are mechanical problems with the switch like stuck weights or the springs breaking or falling off. The OP has had the compressor for a long time. If it is not the capacitor I would look at the condition of the switch contacts. I would bet they are fried.

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[Vet65te]

Since the existing capacitor checked out (~617 MFD out of a range of 590-703), I'll check out the centrifugal switch. I'll try to attach a pic of the back end of the Century Motor. Looks like besides disconnecting the wiring leads I'll need to remove the 4 nuts but is there anything else I'll need to be careful about when removing the back end casing of the motor like a bearing/bushing or ??? And, once I get a good look at this centrifugal switch and it's burned up or beyond a simple cleaning and filing to dress up the points, is it a simple procedure to replace?
Mike T - Prescott AZ

 

[dogdog]

Not sure where you would be finding a replacement if you can't clean it, each motor could be a little different. Maybe those motor rebuild shops would have a replacement for yours... or order from manufacture?

here is the pic of my 1/2 HP from the drill press motor... I mod to make it reversible... you'll see this particular Centrifugal switch is two piece... the contact piece has a tungsten contact points... usually.... Not 100% sure how yours would look like. BTW don't take apart the mechanical arm part.... that would be painful to put it back together if you don't have a reference picture of how it is assembled.

 

[bsaint]

Does the thing unload the discharge tube when it stops? Id check that first. Then check to make sure you have 220v coming in.

 

[6PTsocket]

Does the thing unload the discharge tube when it stops? Id check that first. Then check to make sure you have 220v coming in.

What is a discharge tube?

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[6PTsocket]

I can see the switch at the top of your picture, the thing with the springs on it but I can't see what the contacts look like or very much of the switch. Find the contacts and see what they look like. It wouldn't hurt to check them with a meter.

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[6PTsocket]

Not sure where you would be finding a replacement if you can't clean it, each motor could be a little different. Maybe those motor rebuild shops would have a replacement for yours... or order from manufacture?

here is the pic of my 1/2 HP from the drill press motor... I mod to make it reversible... you'll see this particular Centrifugal switch is two piece... the contact piece has a tungsten contact points... usually.... Not 100% sure how yours would look like. BTW don't take apart the mechanical arm part.... that would be painful to put it back together if you don't have a reference picture of how it is assembled.

Modern technology to the rescue. There are solid state replacements for centrifugal switches. They are in a little box that sits next to the motor. They have 4 leads. Two connect where the centrifugal switch was connected and the other two go in series with the motor (Into one of the power leads). The only spec you need is the current rating on the motor plate. They come for 120 or for 220. We had a Leeson that came with one from the factory. Instead of centrifugal force opening the switch, the leads in series with the motor measure the start current and open the solid state switch when the start current has dropped to near the run current on the motor plate. It sure beats trying to find a physical switch that fits some old motor. They are better than the original.

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[cgrutt]

I have an IR that was doing same thing. As others have said it was indeed the centrifugal switch. Bought motor into repair shop and they had it back and running a couple days later.

 

[usa#1]

Does it start every time when there is no pressure in the tank?

 

[dogdog]

What is a discharge tube?

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He might me pointing to the unloader valve.... that is why I suggested the op to test the compressor and the motor separately by just unhooking the belt if he could...

I can see the switch at the top of your picture, the thing with the springs on it but I can't see what the contacts look like or very much of the switch. Find the contacts and see what they look like. It wouldn't hurt to check them with a meter.

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Oh if you are talking to me, that is just a pic for the OP to get an idea.... Not too sure what his switch would look like either. I just took mine off, send it to the ultrasonic cleaner, file the contact with a fine sandpaper and smear it with a dap of electrical grease, test it for continuity when I had it off . I took mine off so I can re-wire it to be reversible last year....Brake cleaner also works if it is gummed up or greased caked.

Modern technology to the rescue. There are solid state replacements for centrifugal switches. They are in a little box that sits next to the motor. They have 4 leads. Two connect where the centrifugal switch was connected and the other two go in series with the motor (Into one of the power leads). The only spec you need is the current rating on the motor plate. They come for 120 or for 220. We had a Leeson that came with one from the factory. Instead of centrifugal force opening the switch, the leads in series with the motor measure the start current and open the solid state switch when the start current has dropped to near the run current on the motor plate. It sure beats trying to find a physical switch that fits some old motor. They are better than the original.

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Thanks, I'll keep that in mind when I cross that bridge Never knew that.