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#136000 - 03/03/03 10:01 PM Isolating Transformers?
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Does anyone still use the isolating transformer, during thier normal daily work, for safety reasons?.
What does this device have over an RCD unit?.
Your input please-
Have you ever heard of a 3phase(Star-Delta)
type?.

[This message has been edited by Trumpy (edited 03-04-2003).]
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#136001 - 03/04/03 03:40 AM Re: Isolating Transformers?
Dapo Offline
Member

Registered: 03/04/03
Posts: 52
Loc: Australia
We used to use Isolating transformers, when we tested Armatures for shorted commutators and to sort the coil leads when we were rewinding them, Sure beat an RCD, you only got a shock if you touched both leads, can't say the same about an RCD. The thing I can't understand is that you were only allowed to use one appliance at a time with an isolation transformer, But a generator with floating windings acts the same as an isolation transformer, and it has no such restriction.

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#136002 - 03/04/03 08:12 AM Re: Isolating Transformers?
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
I use one regularly on the test bench when working on old AC/DC "live-chassis" radios and TVs.

It's almost essential for such work, as even if you make sure that the chassis connects to the supply neutral, you still don't want to connect grounded test equipment on to it and have parallel neutral currents causing problems.

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#136003 - 03/04/03 08:46 AM Re: Isolating Transformers?
SvenNYC Offline
Member

Registered: 08/19/02
Posts: 1685
Loc: New York City
I have a 1:1 isolation tranformer on the bench at home. However it puts out 125-127 volts when plugged across 120 volts.

Is this a normal manufacturing tolerance for these things? I thought they were supposed to pump out the same amount of voltage they were plugged in to?

I was looking at an iso transf. in another catalog and it cited the output voltage as 120 +/- 5 percent...so I'm starting to wonder...

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#136004 - 03/04/03 08:58 PM Re: Isolating Transformers?
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Dapo,
Welcome to ECN, mate!!, you have the honour of being the first Electrician from Australia, to join us!.
Greetings, from this side of the Tasman.
Hope you find ECN, to your liking,
if you have any questions at all about Electricity or what's connected to it, please, just ask away, there is more than enough collective experience, here, to solve the toughest of dilemma's.

[This message has been edited by Trumpy (edited 03-05-2003).]

[This message has been edited by Trumpy (edited 03-05-2003).]
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#136005 - 03/04/03 09:03 PM Re: Isolating Transformers?
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
Sven — It may be closer to 120V with rated load, which is usually the case for most all transformers. On larger [premises-wiring-type] transformers, the label is stamped with percent or per-unit impedance, which generally cooresponds the change in secondary voltage for no-laod to full-load.

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#136006 - 03/05/03 07:39 AM Re: Isolating Transformers?
SvenNYC Offline
Member

Registered: 08/19/02
Posts: 1685
Loc: New York City
Bjarney,

The spouse suggested that too. So I plugged in a 60-watt bulb (and then a 100) and measured the voltage across that.

Ditto the 35-watt tube radio I was working on...after it was properly working. It still read 125-127 volts when the entire setup was plugged into a 120 volt outlet (this was actual voltage at the receptacle).

I don't want to spend 50$ on a new isolation transformer if it's going to do the same thing, since the catalog listing for the device I was considering did say +/- 5% tolerance.

The primary and secondary aren't shorted to each other (I did a continuity test) so that's a good thing.

Wonder if I'm just better off plugging a lightbulb in series with the primary to drop the incoming voltage a bit...and save my money for a Variac

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#136007 - 03/05/03 01:19 PM Re: Isolating Transformers?
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Welcome Dapo! I went throught the threads rather quickly last night and didn't notice that you'd only just joined from Australia. Nice to have a new member from another country. C'mon in.... The water's fine.

Sven,
What's the VA rating on your isolation xfmr? If the xfmr is rated fairly high (maybe 2000VA or something in that range) then I wouldn't expect the secondary voltage to drop that much with just a 100W load.

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#136008 - 03/05/03 01:28 PM Re: Isolating Transformers?
SvenNYC Offline
Member

Registered: 08/19/02
Posts: 1685
Loc: New York City
Paul,

It's a 150 watt transformer -- big enough for small tube table radios that usually use around 30 to 40 watts of power.

I guess a 100 watt bulb was really going near the limit of the thing...since it did get slightly warm.

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#136009 - 03/05/03 06:41 PM Re: Isolating Transformers?
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
Sven — If it can be done safely, you may be able to reverse connections to the primary and secondary windings, hopefully dropping secondary voltage a little. “On paper” anyway, in the US, utilities and appliance manufacturers generally agree that 127V is the upper limit for 120V circuits.

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