[TheForge] Motor Problems - Thanks Steve

Sun Jun 26 22:46:45 EDT 2005

Steve,

Problem Solved !! And a Big Thank You!!

I should have known that without a hum -- when the motor is powered up 
something was open.  I checked the resistance across the motor and it was 
very very high -- basically a open circuit.  Checking in the pecker head 
again (I had opened it and didn't see anything strange.) I found one power 
lead pulled loose from the winding leads but still kind of stuck in the wire 
nut.  It had arced some and melted copper in the past but was now open. 
Checking the other power lead while that was a good circuit the wire nut was 
loose.  Just a plain poor job of installing the wire nuts when the motor was 
installed on the compressor and connected to the starter.  (For these big 
wire nuts I like the ones you can drive on with a nut driver.)

I replaced both wire nuts and measured the resistance again.  With my meter 
leads touching each other I read 0.3 ohms and across the motor after fixing 
the wire nuts I read 0.7 ohms.  The motor starts right up no problem.  "The 
tank was out of gas".  I have helped do trouble shooting on a lot of 
industrial systems over the years and it usually turns out to be something 
simple that bites you (or two something's at the same time.)

Well all I need to do now is put everything back and button this up -- but 
that can wait till morning and I can sleep well knowing I don't have to get 
a new motor, capacitors or anything.

Again a big Thanks, and once again I am amazed at the fine help here on 
TheForge.

Dave Smucker

----- Original Message ----- 
From: "Steve Smith" <sos at alum.mit.edu>
To: "Sponsored by ABANA" <theforge at mailman.qth.net>
Sent: Sunday, June 26, 2005 8:52 PM
Subject: Re: [TheForge] Motor Problems

> Dave, I'm not clear on exactly what happened. Was the motor spinning and 
> then quit, or did the compressor just stop cycling? You may not have been 
> in position to notice.
>
> You have to spin a motor pretty fast to pull start it. I had a 7.5HP idler 
> on my phase converter (1750 rpm) that with a mighty pull and hitting the 
> power switch just when the rope left the pulley, I could start it. I 
> wouldn't be surprised if you were unable to pull start a 3450 rpm motor.
>
> I'm surprised you don't get a hum. Don't leave power connected very long, 
> the motor should be drawing high current when not turning. Try measuring 
> your motor windings. They should be the next thing to a dead short on this 
> large a motor. You can short your DVM leads and see how many ohms that is, 
> then try the motor winding. What you're really after here is whether they 
> are open.... If the motor measures open, look through the wiring between 
> the motor and the starter to see if a wire has come loose from a wire nut 
> or something.
>
> The resistors across the cap terminals are to bleed off charge after the 
> power is removed from them. Without it, the voltage on the cap can stay 
> around a very long time and be dangerous to a repairman.
>
> I'm not seeing an easy way to test the caps or start switch.
>
> Steve
>
> David E. Smucker wrote:
>
>> While not a direct blacksmithing question this is shop related.
>>
>> Today my 7 1/2 hp compressor motor quit.  It is a single phase, 1.15 
>> service factor, 3450 rpm, 230 volt motor.  Capacitor start, capacitor 
>> run.  The unit is not very old, as if that makes any difference now days. 
>> At most the compressor has several hundred hours on it.
>>
>> I have been using it on a regular bases recently and was doing some spray 
>> paint work at the time.  It kicked in for a few seconds and quit.
>>
>> I have check the reset on the motor starter and this is not a issue.  The 
>> pressure switch is closed and when power is supplied to the motor starter 
>> the coil closes and power is present at the output of the motor starter, 
>> full 240 volts and no sign of a problem with the overload heater.
>>
>> At this point I an thinking it is a failed capacitor or a problem with 
>> starting switch / winding.  I get no smell by the way at the motor 
>> indicating a burned something and the motor was cool, showing no sign of 
>> having been working hard.  The compressor might have run two cycles today 
>> as I was just starting and the spraying I am doing is not very demanding 
>> of air.
>>
>> Also I get no heavy hum, typical of what I have seen in the past with 
>> failed starting switches on much smaller motors.  I have removed the belt 
>> and could not start the motor by spinning up the motor pulley, but I many 
>> not have gotten it moving fast enough.
>>
>> OK after all of that, is there an easy way to test the capacitors, or 
>> should I just order new ones and do the swap out testing.  I know there 
>> is a way to test capacitors with an VOM meter, but I only have a digital 
>> volt ohm meter. It has been a long time but I think with a "meter" type 
>> VOM you would measure the resistance across the capacitor and it would 
>> first be high and then drop down as the capacitor charged.  Is there a 
>> way to do this with a digital meter or some other method.  Second, as I 
>> type this I am thinking that for any test I want to do I need to 
>> disconnect the capacitor from the motor, is this correct?
>>
>> Also what I assume to be the start capacitors (there are two) each have a 
>> small resistor across the inputs, what is this for?  to bleed off the 
>> charge as the motor starts?
>>
>> Also any way to test the starting winding / switch?
>>
>> Thanks in advance for any help that is out there.
>>
>> Dave Smucker _______________________________________________
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