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Old 17th Sep 2020, 3:01 pm   #1
juanafati
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Default Graphite Brush for Zenith 2A Variac

I have recently bought a Zenith Variac (Duratrak 2A) from someone in Aldeburgh, Suffolk. It was a bit "stuck". It was hard to move the top adjustment knob to adjust the voltage. So, I decided to try to disassemble and soften it.

I opened it loosing the lower part of the shaft, as I could not loosen the 2 screw in the top of the adjustment knob because they had their grooves worn out because of a hard manipulation.

Though I cared my handling, unfortunatly, the graphite brush got broken. The result is a useless Variac.

Could anyone tell me if there are spare parts of that piece and where could they be obtained?
It is labelled V-5H.

In case anyone owns one of them and doesn't mind to get ride of it, I am interested on the object.

I will try to include a couple of photos in the message.

Thanks in advance and regards.
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Old 17th Sep 2020, 4:09 pm   #2
ajgriff
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Default Re: Graphite Brush for Zenith 2A Variac

Sadly original parts are unobtainable these days. However, I used one of these to replace the worn brush on my 3A Duratrak:

https://uk.rs-online.com/web/p/variacs/8902897/

I'm afraid I don't know whether or not these are available in Spain as my spanish is not up to searching the site.

I dug out the old graphite from the slot in the holder, carefully filed the new brush to fit and glued it in place. Additionally I drilled out the old braid and replaced it so that the copper threads were trapped against the brush when fitted in the slot. Works perfectly.

I'm sure you are aware that it's important to use a suitable planar graphite brush for Variacs in order to reduce the effect of shorted turns.

Hope this helps.

Alan
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Old 18th Sep 2020, 6:13 am   #3
Diabolical Artificer
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Default Re: Graphite Brush for Zenith 2A Variac

I've replaced carbon brushes in motorbike starter motor's with generic brushes then filed to shape. As Alan says you can glue wires in place, braid is easy to make out of wire, just like braiding hair or bracelets.

Andy.
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Old 18th Sep 2020, 6:49 am   #4
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Default Re: Graphite Brush for Zenith 2A Variac

Found it:

https://es.rs-online.com/web/p/variacs/8902897/

Alan
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Old 18th Sep 2020, 8:45 am   #5
Ed_Dinning
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Default Re: Graphite Brush for Zenith 2A Variac

Hi Andy, I'm afraid electric motor brushes will not do as they are basically a block of carbon and not laminar. You run the risk of burning out the variac.

Ed
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Old 18th Sep 2020, 8:46 am   #6
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Default Re: Graphite Brush for Zenith 2A Variac

Doesn't glue act as an insulator? I realise that current is ever likely to 'find its way through', but nonetheless, surely it's not good practice to mix insulators and conductors together when you just want one of them? I thought Variac brushes were held in place by pressure alone, not glue.
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Old 18th Sep 2020, 9:47 am   #7
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Default Re: Graphite Brush for Zenith 2A Variac

Quote:
Originally Posted by stevehertz View Post
Doesn't glue act as an insulator? I realise that current is ever likely to 'find its way through', but nonetheless, surely it's not good practice to mix insulators and conductors together when you just want one of them? I thought Variac brushes were held in place by pressure alone, not glue.
I do agree with all of this. However, when re-profiling a non-original piece of graphite it's difficult to ensure a tight fit. I only used a couple of spots of two part epoxy at each side of the brush and tried to keep it away from the trapped braid - just additional security really. On completion I ran the Variac at two amps for a couple of hours and there was no sign of excessive heat build up. Also the measured resistance between the end of the brush and the braid's tag was insignificant. The Variac's been in fairly regular use for the last twelve months without any problems.

Alan
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Old 18th Sep 2020, 10:54 am   #8
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Default Re: Graphite Brush for Zenith 2A Variac

I thought that motor commutator brushes were laminar as well?

You move the brush from segment to segment and they are shorted, so need the laminar carbon to stop the arcing and sparking?
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Old 18th Sep 2020, 11:34 am   #9
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Default Re: Graphite Brush for Zenith 2A Variac

These people may know: https://www.mrcarbonbrush.com/
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Old 18th Sep 2020, 12:17 pm   #10
ajgriff
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Default Re: Graphite Brush for Zenith 2A Variac

Most small motor brushes are made from amorphous carbon which is not a suitable material for Variac brushes as Ed highlighted in post #5.

Alan
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Old 18th Sep 2020, 12:19 pm   #11
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Default Re: Graphite Brush for Zenith 2A Variac

When I fixed one I got FOC with a broken brush the brush holder had a clamp with a screw to tighten it.
I filed a washing machine brush into shape being careful to retain the braided lead.
With the braid unfurled and laid over the part to be gripped clamped it into place.
The braid acted as a low resistance connection to the clamp.
It has been in use for quite a few years.
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Old 18th Sep 2020, 1:55 pm   #12
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Default Re: Graphite Brush for Zenith 2A Variac

This may solve a bit of a mystery for me.

We got a second hand Bosch washing machine for a family member same type and model as our own.

The brushes were well worn so I bought a replacement set from an online supplier.

They fitted fine but the motor would overheat on the spin cycle.

When they were worn out in just a couple of months I bought another set from CES (Exeter) who have always been very reliable supplier to me.

Cool Motor and carried on for years, the machine was replaced by a newer model but went to a friend without complaint.

It seems sometimes there is more to brushes than meets the eye

Cheers

Mike T
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Old 18th Sep 2020, 5:53 pm   #13
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Default Re: Graphite Brush for Zenith 2A Variac

Yes there is as Mike T says. Morgan Carbons, carbon brush suppliers to BR when I worked for them in 1960's, gave away a sizable book all about selecting the correct grade and type of carbon brush for traction motors and generator. It was very informative, but traction motors are handling hundred of amps when starting off a heavy train. Ted
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Old 18th Sep 2020, 10:11 pm   #14
juanafati
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Default Re: Graphite Brush for Zenith 2A Variac

Hello again.

Thanks for all your answers, tips and suggestions. I have been waiting for them before writing back.

It is a bit more difficult, in my opinion. I attach some photos to explain that.

I have the tip of the broken brush. It is cilindrical, of 2 mm. diameter and about 5 or 6 mm. length (photo). The face of the tip is the one touching the copper. It has a thin gap.
I have found a web-page which, apparently, has some similar holder (photo), but their brushes are rectangular, not cilindrical.
Taht makes any sanding of retangular or square ones somewhat difficult or nearly impossible. Furthermore, the holder has a small perforated tip, on the upper part, the braid goes through and which is crushed to keep the braid tight. It would make the restoring more difficult.

Maybe I am a bit hopeless but the only way I see is the use of any tipe of strong glue with is conductor and try to stick the broken piece.
Or an original one coming from someone or somewhere.

I wish i were wrong.

There is a proverb in my place which says something like: "Every problem has a solution, but death"
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Last edited by juanafati; 18th Sep 2020 at 10:33 pm. Reason: Uncomple explanation
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Old 18th Sep 2020, 10:30 pm   #15
ajgriff
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Default Re: Graphite Brush for Zenith 2A Variac

Could you post a photo of the end of the holder where the brush fits?

Alan
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Old 19th Sep 2020, 3:09 pm   #16
juanafati
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Default Re: Graphite Brush for Zenith 2A Variac

I'll disassemble the variac again and take a couple of photos of both ends.
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Old 21st Sep 2020, 10:30 am   #17
juanafati
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Default Re: Graphite Brush for Zenith 2A Variac

Here are some more photos as Alan requested.

Carbonbrush.com has aswered me they have no record on this one.

Juan José
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Last edited by juanafati; 21st Sep 2020 at 10:38 am. Reason: Add more information
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Old 21st Sep 2020, 11:14 am   #18
juanafati
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Default Re: Graphite Brush for Zenith 2A Variac

The metal holder measures are about: 9,4 mm. width / 14,7 mm. lenth _ 0,37 width / 0,58 lenth. Without the length of the braid and conector, of course.

Juan José
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Old 21st Sep 2020, 1:24 pm   #19
ajgriff
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Default Re: Graphite Brush for Zenith 2A Variac

Interesting that Juan José's V-5H has a cylinderical brush rather than the flat brush in a slot used on my 3 Amp V-6H. Other than that the design of the holder looks to be much the same.

Difficult to know what to suggest but using two part epoxy around the lip of the socket might secure the broken brush as long as the contact area between the the broken parts doesn't get contaminated. The only other thing I can think of is replacing the brush in the manner described earlier as I do think it might be possible to file down a flat RS brush (2.5mm thick) to fit.

I suppose less than satisfactory options would include using the lead (usually a mixture of clay and amorphous carbon) from a chunky pencil or filing down an ordinary motor brush. Running the Variac close to its current limit could induce problems regarding shorted turns and heat generation. The Variac would probably still be ok with low currents for short periods of time. Personally I'd avoid such bodges.

Let us know how you get on.

Alan
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Old 22nd Sep 2020, 11:35 pm   #20
juanafati
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Default Re: Graphite Brush for Zenith 2A Variac

I am considering the adquisition of a new brush from the ones showed in the photo of the brushes shop in the first post. But I have a great doubt about the correct material.

Could anyone, please, tell me which material is the apropriate for the Variac brush: Gray E, Black E or Graphite G?

Thanks

Juan José
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Last edited by juanafati; 22nd Sep 2020 at 11:55 pm.
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