Single phase motor capacitors!

[quatfro]

Hi all,

I have an old Wolf valve grinding machine and after buying some new belts for it, I ran it up and switched it off - shortly afterwards the main capacitor exploded and I'm kind of stuck!

The thing that's confused me the most is the dual / triple rated capacitor at the centre of this whole debarcle.

http://files.engineering.com/getfil...70e-4f4ad1fe23b9&file=2012-09-02_14.33.04.jpg

As can be seen, AC power is fed immediately into the back of this capacitor, confusingly, another capacitor, 0.1uf is parallel to the main capacitors input. The two conductors out the other side run off to the two switches, one for each motor. This machine has one 1/8hp and one 1/16th hp 2-pole induction motors.

Large main capacitor reads 0,07uf + 2x2500pfXY +2x2mH 250v 3,6A, possible part number LC 7022 - 2R20 3,6RM/5.

What I'd like out of this is to understand why the machine was wired up the way it was and whether there's any reason why I can't just get suitable capacitors for each motor and wire it up more conventionally.

2nd, if I did so, what kind of farads would I need for the above rated motors? I've searched and searched for tables but I can't seem to find a thing!!!

Cheers,

Tom.

 

[ScottyUK]

It's not a capacitor, it's an LC noise filter. Similar to

http://www.schaffner.com/uploads/tx_w4products/DS_FN2010_20110616_web42.pdf

for example, although the one in the link is a more modern package style. The chances are the motor is a brushed universal type which makes it a fairly good source of broadband electromagnetic noise; the filter is the stop most of that getting back out into the utility supply.

 

[quatfro]

Ah right!

That makes life an awful lot easier. I'll get searching LC noise filters and see where it gets me...
So I assume the capacitor in parallel with that is an attempt at a start / run capacitor, yeah?

 

[waross]

What Scotty said.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[quatfro]

I understand Scottys reply, but I'm still not sure of the necessity of the capacitor which was parallel to the feed of the filter.

It too appears blown up like the LC filter, though the machine appeared to run fine with it in that condition!
Whether it was suffering a lack of torque as a result I couldn't say, nor whether it contributed to the demise of the LC filter...

 

[ScottyUK]

That capacitor appears to be intended for noise supression too. It isn't a motor start or motor run capacitor.

Can you get to the motor itself and post a photo of it and of its nameplate?

 

[quatfro]

Yes, I'll get on the case shortly...

 

[waross]

If you compare the circuit of the filter with the photograph you will see that that capacitor is part of the filter circuit.
The filter cct shows three caps, that's the third cap.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[quatfro]

As requested.

Photo 1.

http://files.engineering.com/getfil...643-4cd9f49c4187&file=2012-10-11_17.39.14.jpg

Photo 2.

http://files.engineering.com/getfil...2dc-44f088acfd39&file=2012-10-11_17.40.50.jpg

 

[ScottyUK]

As I originally guessed - it's a brushed universal motor. It has no need for a capacitor, and is unusual in that it will run quite happily on both AC or DC. The commutator looks a little dark but in reasonable condition. Don't do anything to the commutator, especially don't polish it. The uniformly dark band is a fairly good sign.

There are probably two leads from the motor - anchor the motor securely to something or re-fit it to the machine, then apply power to them. If you have more than two leads then it may be a reversible type, or possibly a variable speed type. Both seem unlikely given the application and obvious age of the motor. The motor will run without the filter, but it will cause interference with radio reception. If the motor is runnable you really should get a new filter. Domestic washing machines usually have one fitted inside the rear panel.