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Replacing a motor cap

Uruniko

New Member
Hi all,
I'm looking to replace a blown capacitor in some workshop equipment.
The exact item is proving hard to replace and I was thinking of making a composite set of caps to replace.
This is not an area I am familiar with so would appreciate a sanity check.

The cap is a dual canister cap with 3 wires connecting each pole of an AC motor to ground.

Label reads:

0.1uf (x2)
0.005uf(y) + 0.005uf(y)
Delta 250v

I suspect this is a motor run capacitor, with built in noise reduction?
Although the values are a little low for a motor run cap.

I am really struggling to find an equivalent part. Would it be correct to connect a 100nf+5nf in paralel X2 and bond the grounds together?,
If so would non polarized (polypropylene) caps be appropriate in this application 0.1uf electrolytic is hard to find.
 

JimB

Super Moderator
Most Helpful Member
From your description this is simply a noise reduction filter.

There are three capacitors, one 0.1uF and two 0.005uF.

The 0.1uF cap is connected across the mains supply (line to neutral in a single phase supply)

The 0.005uF caps are connected one each from line to earth and from neutral to earth.

The "X2" and "Y" in the description of the capacitors relates to a (safety) rating for capacitors connected across the mains in this sort of application.
The rating "X2" applies to a capacitor connected from line to neutral, and "Y" relates to a capacitor connected from line or neutral to earth.

Look here:
and here:
for some insight into X and Y capacitors.

JimB
 

Uruniko

New Member
From your description this is simply a noise reduction filter.

There are three capacitors, one 0.1uF and two 0.005uF.

Ahhhhhh. of course. X2 is the safety rating not times 2.

Thanks jim. Most helpful. I have a little experience dabbling in guitar amps, so "death cap" comes to mind.

The unit I'm trying to fix is a single phase 2nd hand workshop dust extractor (glorified vacuum cleaner).

Presumably the filter Is to stop the unit introducing noise rather than the other way round.

I got the unit as part of a job lot, when i tried to use it, it was throwing a breaker switch,
I initially thought the power chord needed replacing but upon opening i saw the capacitor had blown its top.

Presumably from what you are saying pollyprop/ceramic caps in a 'Y' configuration would be fine as long as they meet the appropriate ratings including X2/y.

I'm also thinking for test purposes it would be ok to run the unit momentarily without the caps in place to confirm they were the source of the short?
Presumably no ground is necessary for the motor to run, just the chassis safety ground.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I'm also thinking for test purposes it would be ok to run the unit momentarily without the caps in place to confirm they were the source of the short?
Presumably no ground is necessary for the motor to run, just the chassis safety ground.
Perfectly fine to run it as long as you like, they are only there to reduce interference sent back down the mains.

To replace it, fit the correct device - don't try and 'bodge' something up - such items are freely available:


One of the first hits from google.
 

Uruniko

New Member
Perfectly fine to run it as long as you like, they are only there to reduce interference sent back down the mains.

To replace it, fit the correct device - don't try and 'bodge' something up - such items are freely available:
Ahha.. perfect Nigel!, thank you very much!

It's RS I was searching with but I I didn't know my search terms and I never thought to look under paper caps.

Certainly beats the £12 plus £5 postage Record power wanted to charge me.

I'd always assumed after hearing about "Death caps" that this kind of thing wasn't done anymore,
But looking at it it appears to be a requirement to meet standards and is perfectly safe so long as you have a proper ground.

In fact introducing one of these in to properly grounded amps would generally be quite safe and beneficial as long as your earth is good?
 

JimB

Super Moderator
Most Helpful Member
I'm also thinking for test purposes it would be ok to run the unit momentarily without the caps in place to confirm they were the source of the short?
Yes, that is what I would do.

Presumably no ground is necessary for the motor to run, just the chassis safety ground.
Correct.

Perfectly fine to run it as long as you like, they are only there to reduce interference sent back down the mains.
True, but, from an electromagnetic environment point of view sending wideband noise back down the mains is most anti-social.
Please re-fit some filtering.

To replace it, fit the correct device - don't try and 'bodge' something up - such items are freely available:
Agreed, 100%

JimB
 

Uruniko

New Member
True, but, from an electromagnetic environment point of view sending wideband noise back down the mains is most anti-social.
Please re-fit some filtering.
I will matey, I run a lot of studio recording / guitar amplifier kit.
I also record videos and tweak amps in my workshop.
Quite happy to filter the bejesus out of anything and everything.

I just want to quickly flick it on to make sure it was the cap causing the short before fitting another cap and wasting money blowing the cap.
In theory the cap shouldn't blow anyways.. but still.. also means I can test while awaiting the postman.
 

MaxHeadRoom78

Well-Known Member
The cap is a dual canister cap with 3 wires connecting each pole of an AC motor to ground.
Label reads:
0.1uf (x2)
0.005uf(y) + 0.005uf(y)
Delta 250v

I suspect this is a motor run capacitor, with built in noise reduction?
Although the values are a little low for a motor run cap.

I am really struggling to find an equivalent part. Would it be correct to connect a 100nf+5nf in paralel X2 and bond the grounds together?,
If so would non polarized (polypropylene) caps be appropriate in this application 0.1uf electrolytic is hard to find.
A AC motor run cap is NEVER electrolytic, just bi-polar start capacitor are electrolytic due to only being in circuit for a couple of seconds.
All caps used in this application when connected permanently are AC continous rated, If these are just for suppresion means, you should be able to run/test the motor without them.
Max.
 

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