[TheForge] Motor Problems

[Steve Smith]

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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