Author Topic: Universal motors help...  (Read 1225 times)

Offline 10KPete

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Universal motors help...
« on: March 17, 2017, 10:25:48 PM »
 Hokay, I've got one for ya'll: I have a hand held power tool I'm resurecting and it's missing a capacitor. See the schematic attached (I hope!)

Parts are not available for this unit. It runs off 120vac and draws 2.2A. The parts illustration I have for the tool show a 'triple' capacitor 'can' (I don't know what it's actually made of) and the space it has to fit in makes the can 20mm dia. x 30mm long. There are two wires coming from one end and a third on the other end.

I don't know if I can find a cap with the original physical configuration but I believe I can duplicate the the function.

What I need to know is what values the caps should be. I suspect the two across the AC legs should add up to about .25 uf but I don't have clue about the shock protection cap going to the field laminations.

The service rep for the company is looking into this but the tool has been obsolete for 15 years and the company that actually made the motor is no longer with us.

I've Googled with every word combination I can think of; nada.

With all the brains on this board I figure there's someone who can answer the question about the values! I hope!

Thanks guys,
Pete
Craftsman, Tinkerer, Curious Person.
Retired, finally!
SB 10K lathe, Benchmaster mill. And stuff.

Offline AOG

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Re: Universal motors help...
« Reply #1 on: March 17, 2017, 11:32:07 PM »
What your describing sounds a lot like a start/run cap. They are typically used in HVAC to run fan motors. It's been years since I've dealt with them but I seem to recall that they are sized by the stall power of the motor. For industrial equipment the sizes you need are usually printed on the motor data plate. If there is nothing written, there is a formula to calculate the size of a start capacitor and a run capacitor but it escapes me at the moment. Hopefully you can find them on the internet.  I hope that helps.

Tony

Offline PStechPaul

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Re: Universal motors help...
« Reply #2 on: March 18, 2017, 01:21:55 AM »
A universal motor does not require capacitors to run, but it may have low power factor and arcing noise from the brushes. Capacitors as shown may help with both. Power factor correction will reduce the current draw of the motor (inductive) by supplying current at a different phase angle. An approximate size might be calculated from the 120 VAC and 2.2 amps, which is an impedance of 54 ohms. You might choose a capacitor that draws about 1 amp at 120 volts, which is 120 ohms reactance. C = 1 / (2*PI*60*100) = 26 uF. That would be a rather large capacitor, and what you have appears to be much smaller, so they are probably more for arc suppression. In that case the value would be non-critical, and something like 0.1 uF to 0.47 uF should be good. They should be rated for across-the-line AC use and voltage at least 150 VAC or 250 VDC.

Here is something I found on universal motors and arc suppression:

http://www.uotechnology.edu.iq/dep-eee/lectures/3rd/Communication/machine/PART%203.pdf

http://www.panconcorp.com/PDFs/capacitors/QRL-Quencharc.pdf

https://en.wikipedia.org/wiki/Arc_suppression

https://www.allaboutcircuits.com/textbook/alternating-current/chpt-13/ac-commutator-motors/

http://robotics.stackexchange.com/questions/267/why-are-capacitors-added-to-motors-in-parallel-what-is-their-purpose


Offline 10KPete

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Re: Universal motors help...
« Reply #3 on: March 18, 2017, 01:33:52 AM »
Paul,

Yes, C1 and C2 are for arc suppression while C3 is what's called a shock protection cap in double insulated tools. I think you missed the decimal point in front of my number. So my .25 uf is about right.

I'll read the links.

Any thoughts on the shock protection cap? I can't find any info on that name or by any other word combo I've tried. Yet.

It will be at least a week before I'll be close to needing it, maybe more!

Thanks,
Pete

Another edit: I know the caps aren't necessary for running. It will run fine without them. But I think that the brushes might last longer with them.   :shrug:

Pete

« Last Edit: March 18, 2017, 04:33:01 AM by 10KPete »
Craftsman, Tinkerer, Curious Person.
Retired, finally!
SB 10K lathe, Benchmaster mill. And stuff.

 

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