Universal motors...

[10K Pete]

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

 

[gunna]

By the look of your diagram, none of them are important for the running of the motor. They are all only noise suppression caps to try and cover some of the natural arcing at the commutator. I can't try to guess at the correct values, but I am sure the motor would run without them if neccessary. I think anything around 0.1uF to 0.25uF would be fine, mains rated caps of course.
Ian.

 

[10K Pete]

I agree that the caps aren't necessary. I would like to have them "just because"....



Pete

 

[BaronJ]

Hello Pete,

There are only two capacitors on that motor ! The motor body should be earthed and where you have drawn a capacitor to the motor body is not a capacitor, it is actually the common wire from the two capacitors. The value is around 0.1 uf for each one, they are only there for noise and spark suppression on the commutator. Suitable replacements capacitors would be the type found in the mains filter of just about any PC power supply.

Hope this helps.

 

[10K Pete]

Hello Baron, I copied the schematic from the factory manual. I've also seen that cap in other schematics and referred to as a shock protection capacitor. These seem to be unique to double insulated tools. That third cap connects to the field laminations, and none of the internal metal parts of the motor are exposed or connected outside of the plastic tool body.

Pete

 

[BaronJ]

Hi Pete,

Yes on double insulated tools there would not be an earth connection. I don't think that there is any point in having C3, the schematic could be wrong, its not a notation that I've come across. I've seen many applience universal motors with metal cased capacitors, where the metal case is secured to the motor frame and the power is supplied at one end to a pair of push on tags and the other end supplies the motor via a pair of wires directly exiting the capacitor. A lot of the Hoover and Electrolux cleaners were like that and so were the spin dryer motors.

I might still have one or two capacitors kicking about from the good old days, they didn't fail very often. Usually when they did there was a loud bang as one of the end caps shot off.

 

[10K Pete]

Hi Pete,

....loud bang as one of the end caps shot off.

Rof}

I can remember that happening a few times in the past. Scares the pants right off when the shop is nice and quiet and then.... BANG!!

Remember the smell of the old wax caps when they shorted?

Pete

 

[BaronJ]

Hi Pete,

Remember the smell of the old wax caps when they shorted?

Yes you bet I do, and the stink and mess when the electrolytics went bang.