Low torque from single phase cap run motor 2

[OperaHouse]

A friend asked me to look at his commercial floor buffer. It would run with if tilted up with no load, but didn't have enough torque to start when normal on the floor. In the stall condition it draws about 40A, start cap about 18A, and the run cap about 2A. The run cap current seem a little low, but it is only a 20uF and my experince is that these caps either work or open. Opened up the motor and no signs of burning (color and odor) and all the winding have continuity and measure about 1.2 ohms. My field is more electronics and usually there are more obvious signs of a problem. What am I missing here? Thanks for any help.

 

[bklauba]

Did you check to see if the start cap is de-energized when the motor approaches full speed?

BK

 

[itsmoked]

But... bklauba how would that explain low starting torque?

 

[bklauba]

If the centrifugal switch is stuck, the motor would draw excess current, but not produce usual torque at higher speeds.

operahouse: The 2A thru the 20 uF sounds about right, but what is the value of the starting cap? ( 18 A seems too high.)

BK

 

[stardelta]

A centrifugal switch thats open cicuit at start up would cause the fault thats described. The start winding would be out of the circuit and doing nothing, hence the lack of starting torque, its not uncommon to find switch contacts that have burnt away over a period of time. We generaly see more faulty 1ph motors with switch problems than capacitor problems. If the switch had stuck in the closed position with power on the motor for longer than about 15 seconds, chances are the start winding would be "no longer with us".

 

[OperaHouse]

The centrifical switch contacts are in nice shape and you can hear them snap whe it gets up to speed. The weigh mech had all the weights intact and has a nice snap action, I've seen these become cracked. Can't say that I verified that the contacts opened, but without load the run current is 6.5 amps. Can't check it till Monday.

 

[stardelta]

Have a really close look at the the contacts and verify the are actually circuiting in the rest position. You may find that despite looking like they are touching they are not actually making contact. The snap you hear is the switch opening but at this stage we dont know if its actually closed in the rest position. The arc thats generated when the contacts open tends to burn away the coating on the surface of the contact over a period of time and its possible that its happened in this case. Check that the switch is closed in the rest postion with a suitable meter.

 

[OperaHouse]

I'm all for simple solutions, but the facts tened to steer away from the start circuit. The redundant contacts were checked and showed no excessive burning. The 18A through the capacitor indicates that circuit was conducting and at least out of circuit it was not shorted. If the contacts were closed all the time the motor would at least start and it doesn't. I'll start subbing caps out tomorrow.

 

[Marke]

I would look at the start capacitor, it could be breaking down with voltage applied. This would result in a high capacitor current and not start torque.
The other common cause, is a problem with the rotor bars, but this would generally be accompanied by a low start current.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[stardelta]

If the switch is ok and the winding is ok it can only really be the capactors at fault. There is a simple go/no-go test for capacitors, but its not really recommended and can be detrimental if performed many times. Basically you charge it up with a Megger then short the terminals, (be careful because it will make you jump when you do it)! No spark indicates its dead but, a big fat healthy one indicates its holding a charge. This however this is no substitute for a proper capacitance meter if available. Look also for a little hole in the casing thats leaking electrolyte or other matter, if you find one the cap is dead. A lot of makers purposely engineer in a weak point in the plastic that is designed to give if the capacitor "Blows".

 

[aolalde]

Operahouse:

For a given winding the capacitance must match optimum phase angle.
Test with two voltmeters while the rotor is locked. Maximum torque will be developed when V2 is close to 1.1xV1.
If you are far of such a condition replace the starting capacitor (Cs). If the capacitor is good and your voltage is lower, increase the capacitance.
If the voltage is larger reduce the capacitance. See diagram below.

 

[itsmoked]

Wow aolalde. Nice bit of info there.

 

[OperaHouse]

It took a few more days before I got back. I substituted a a different capacitor for the run cap with no difference. The switch does close and open up when the motor gets up to speed with no load. Nice drawing, but from resistance measurments the run cap has a seperate winding or at least aditional turns on the start winding. All widings have continuity, no smell, no discoloration, nothing that gives a clue of a potential problem. The only thing left is the motor itself.

 

[aolalde]

Operahouse.
Can you provide a winding, capacitors, switch diagram?

 

[OperaHouse]

It's just a motor with two capacitors and there is only so much you can do for free. Just puzeled by the lack of some obvious sign. Maybe some day I'll connect each winding to my VAW meter and run some low voltage into it. Sort of a high tech growler to see if there are some shorted turns by comparing windings.