30 year old power tool capacitor replacement

Luke Rickert · May 2, 2017

Hi all I am working on a mid 80's Biax BL 10 scraper(Swiss made) that has some electrical issues. The motor windings test ok (with a multi-meter, maybe they should be tested with AC but I am a mechanical engineer rather than an electrical engineer.

what appears to be bad is a little capacitor with an extra wire coming out of it. I am guessing it might have something to do with temperature protection but really have no firm idea so I thought I would see if any of you more experienced folk recognize this thing.
there are a few photos here Biax cap - Google Photos

The company does not support these old machines. If I can figure out who made the motor or controller (Biax didn't) perhaps I can figure out what it is anyway. It would be easy enough just to get another capacitor of the correct size and rating but it would be nice to know what extra wire is about.

thanks

Luke

LiquidMetal · May 2, 2017

It is a dual capacitor. Simply buy 2 2400pF (2.4 nF or 0.0024 uF) and wire them in series. The black wire on the old cap is the common connection for the series connected new cap.

Bill D · May 2, 2017

I beleive this motor has brushes? If so the cap is only there to reduce radio interference. it should run fine without it. Was that extra lead connected to anything or was it floating. I agree it is a dual capacitor. Be aware in Europe the decimal and comma are reversed from US usage in writing numbers.
Bill D

adama · May 2, 2017

^ thoes caps help reduce arcing, yeah it helps massively with radio interferance, but it also greatly extends brush life!

Luke Rickert · May 2, 2017

Thanks guys, I will order some caps and see how it goes
L

dcsipo · May 2, 2017

whidbey said:
Thanks guys, I will order some caps and see how it goes
L

you can also check the cap with a modern digital multimeter, almost all have capacitance measurements

dee
;-D

6PTsocket · May 2, 2017

LiquidMetal said:
It is a dual capacitor. Simply buy 2 2400pF (2.4 nF or 0.0024 uF) and wire them in series. The black wire on the old cap is the common connection for the series connected new cap.

I don't think they are intended to be used in series because both caps share a common connection. The common is probably grounded and the caps supply spark or noise suppression for two brushes. BTW, two caps in series give you only half the capacitance of one capacitor. The only reason it is ever done is to double the maximum voltage it can handle. Many old tube radios had dual capacitors like this that were typically 30 and 50 mfd at 150 volts and shared a common ground connection. They were common in metal cans, mounted on chassis that had 3 and 4 capacitors in them but one common ground. They have virtually disappeared and are replaced with individual capacitors.

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Garwood · May 2, 2017

You really need a megger.

I was once under the false impression a good Fluke multimeter would tell you something about motor windings. Nope. No good at all really. Megger works totally different and is designed to show when winding insulation has broken down.

EPAIII · May 2, 2017

Yes, it is a dual, 2400 pF (0.0024 uF) capacitor. First, it is probably there to suppress brush noise as others have said. As such, it is not needed for the operation of the motor so, unless it is dead shorted, it is not the problem.

Second, this type of capacitor almost never goes bad without some form of gross abuse, usually physical, mechanical abuse. So I strongly feel that it is NOT your problem.

To see if it is at fault, just disconnect it and see if the motor runs. If it does, the capacitor is not the problem.

BTW, it is just two 2400 pF capacitors in one package with one lead of each connected in common. That lead is most likely the center one and it is probably connected to the power line safety ground but it may be connected to the neutral. If you really do have to replace it, you can just as easily use two separate 2400 pF capacitors and connect two of their leads together externally. No difference.

James H Clark · May 2, 2017

Yeah, what 6PTsocket said. Its been my experience that caps in series do give you half the capacitance of one.

JH

steve-l · May 2, 2017

The tool of choice for checking brush motor or generator armatures is called a growler. Before alternators became the norm for cars and trucks, these were used to check for shorted armature windings. Try to find an old generator repair shop, they will have one.

Earl Sigurd · May 3, 2017

I have 2 portable power tools, Swiss and German, that suddenly exhibited gross sparking at brushes. I assumed armature winding failure. The cost of new parts writes these off.

I wonder if I have ASS U MEd wrongly and simple cap failure?

Luke Rickert · May 3, 2017

I won't be surprised if it is the windings but I figure swapping out the cap, which is bulging, is worth the try. I wasn't able to get any measurement of the cap with a meter and just ran into another old motor that had a bad cap so I think it might be bad. I think the higher voltage in Europe might be a factor as the 110v version of the machine did not have a cap from the factory but I suppose the would be no harm in testing it without the cap in there.

This thing has run a massive number of hours by the look of the worn commutator which I re-turned. The brushes are also new. These little scrapers are insane money, I think I will adapt a new motor (die grinder perhaps) on the mechanism if this motor is dead as a replacement for the current one is not going to be available.
L

JST · May 3, 2017

Earl Sigurd said:
I have 2 portable power tools, Swiss and German, that suddenly exhibited gross sparking at brushes. I assumed armature winding failure. The cost of new parts writes these off.

I wonder if I have ASS U MEd wrongly and simple cap failure?

Does that sparking seem to follow the commutator around in streaks?

If so, there may be an open connection on the commutator. That causes "ring fire", sparks in streaks around. With an ohmmeter you can often find the bar that has an "open", find and fix the bad connection. or replace the tool, your choice.

Can also be a suppression capacitor that is bad, but that usually just increases sparking a little.

Or it can be a brush that has worn down short and is bouncing or misaligned in the holder due to being so short. Brushes are replaceable in general, and are a cheap save if you like the tool..

EPAIII · May 3, 2017

I don't know what the price of that cap is, but it is not a "RUN" or a "START" capacitor. It is just too small to be either. It is most certainly for RFI suppression and, as such, it is not needed for the motor to run. Just disconnect it and see if the motor starts. If it does, then yes, the capacitor is bad. If it does not start, then there is most definitely something else wrong. This is a simple, no cost test and the results should be totally clear.

whidbey said:
I won't be surprised if it is the windings but I figure swapping out the cap, which is bulging, is worth the try. I wasn't able to get any measurement of the cap with a meter and just ran into another old motor that had a bad cap so I think it might be bad. I think the higher voltage in Europe might be a factor as the 110v version of the machine did not have a cap from the factory but I suppose the would be no harm in testing it without the cap in there.

This thing has run a massive number of hours by the look of the worn commutator which I re-turned. The brushes are also new. These little scrapers are insane money, I think I will adapt a new motor (die grinder perhaps) on the mechanism if this motor is dead as a replacement for the current one is not going to be available.
L

DDoug · Jul 2, 2021

Wiziard said:
It's better to go and give it to a specialist. I would add an impact drill to that list. I recently bought an impact drill that I found on spamguru.com. I can tell you from experience that this drill is an indispensable work tool. It is much easier to use than a classic drill. I feel that the battery on the impact drill lasts longer, which is also a significant advantage. But it's too bad that your bosses aren't going to buy you new tools. First, it's only a detriment to business, and second, it's a bad environment for workers. But it seems to me that they are going to make you feel guilty.

Spam I yam.....

EPAIII · Jul 5, 2021

Never mind, I already posted above.

And this is a four year old thread.

6PTsocket said:
I don't think they are intended to be used in series because both caps share a common connection. The common is probably grounded and the caps supply spark or noise suppression for two brushes. BTW, two caps in series give you only half the capacitance of one capacitor. The only reason it is ever done is to double the maximum voltage it can handle. Many old tube radios had dual capacitors like this that were typically 30 and 50 mfd at 150 volts and shared a common ground connection. They were common in metal cans, mounted on chassis that had 3 and 4 capacitors in them but one common ground. They have virtually disappeared and are replaced with individual capacitors.

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