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Using 240vac to drive HV experiments

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IamSmooth

Thu Jan 21 2010, 08:58PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I am thinking about getting a 240vac input variac for some HV experiments. Would it be prudent to send the output through an isolation transformer before using it? Both lines are hot. What do you all do when using this as your power source?

Any recommendations for an isolation unit other than being 240vac input, and handling the required amperage?

Proud Mary

Thu Jan 21 2010, 10:19PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

You should put the isolation transformer and its fuses before the variable transformer.

Steve Conner

Fri Jan 22 2010, 10:13AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, we don't have anything BUT 240V over here.

If you have isolated differential scope probes, then you don't need an isolating transformer, you can scope the circuit without it. Just remember that everything will be live.

In Europe, we have 240 the way you have 120. Only one line is hot, and the other, "neutral", is at a potential near to ground. But you're not supposed to rely on the fact that neutral is near ground. In many parts of Europe, the plugs will fit in the wall either way round, so you get a lucky dip!

I always connect the isolating transformer AFTER the variac, myself.

My isolating transformer can be switched between 240V and 120V output, and in the 120V position, the current rating of the variac is doubled. And, I fuse the variac brush, because it seems to me that's the easiest part to burn out.

If you were using a voltage doubler to get a 320V DC bus from your 120V mains, then you can swap it out for a bridge rectifier. You still get 320V, but the line current is halved for the same DC bus current.

Proud Mary

Fri Jan 22 2010, 03:41PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

My approach is clearly different toi Steve's, as I have everything on my bench - instruments, tools, power outlet sockets, lights and so on - all isolated from the mains by a hefty 230-230 industrial isolation transformer, so that the entire bench may be closed down at the throw of a single switch.

radiotech

Fri Jan 22 2010, 04:59PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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The question is exactly how the experimental area is powered? What is your grounding system? Are you having a new circuit installed as per local regulations?
In high voltage work, the lines that bring the power in also can conduct energy out.

IamSmooth

Fri Jan 22 2010, 05:05PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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In the past I had run a 240vac line to my garage for welding - two hots and a neutral. I would like to tap into this for powering an inverter over 5kw. I would prefer if none of the rails could conduct to ground.

I am finding out that a large isolation transformer is expensive (and very heavy).

Proud Mary

Fri Jan 22 2010, 05:43PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

IamSmooth wrote ...

I am finding out that a large isolation transformer is expensive (and very heavy).

Mine is one of the heavy yellow boxes certified for use on construction sites etc. Most of these are 230/110V, but 230/230 single phase types like mine are manufactured and do appear on the second hand market from time to time. I bought mine on ebay a few years ago for Â£70 (~$115).

As for the Earth, I have the steel chassis of my workbench (plus ESD mat etc) connected to the mains Earth, the plumbing pipes, and an external Earth buried in the garden, a set up which has been adequate so far.

radiotech

Fri Jan 22 2010, 06:10PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Will the output of the 5kW inverter provide the ungrounded 240 volt source? Is this a UPS with an ATS (automatic transfer switch) ?

Some isolation transformers have a shield between the primary and secondary. Variacs are good for experimental work but as a permanent solution (ie- adjusted once and left for a long time) can be a source of trouble- (That sliding contact) The best solution is a transformer with hard wire taps if you need to correct voltage. Tap changing switches on transformers do need maintenance and we have seen some that fail. They are tested with a high amperage
ohm meter and exercised through all positions on regular maintenance rounds.

If you live in a 120/240 grounded country and you change to an ungrounded system you need to have a ground detector if in a residential setting.

All your high voltage stuff needs a safe ground.

IamSmooth

Sun Jan 24 2010, 07:15AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Steve McConner wrote ...

If you were using a voltage doubler to get a 320V DC bus from your 120V mains, then you can swap it out for a bridge rectifier. You still get 320V, but the line current is halved for the same DC bus current.

Steve, is there any problem using a voltage doubler to get 640vdc with two hot lines?third one

Steve Conner

Sun Jan 24 2010, 10:24PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

No, it'll work fine, just remember that all parts of the circuit are live with both AC and DC.

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