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Daemon
07-16-2009, 05:37 AM
Does anyone have any knowledge of variacs?

I was wondering if they are stable enough to use to drive my 24VAC lighting.

I can get a big sucker 240v input(which we have here in New Zealand) and 0-270v output. 1.6KVA

Could I use this set o 24v as a transformer?

Apart from the big inrush current when you turn it on are there any other downsides to them?

prof
07-16-2009, 08:06 AM
Two major drawbacks.

As the variac is not isolated the lights will be at mains potential and need to be treated accordingly.

Variacs do not like to be run at very low percentage settings. You will not be able to get 1.6kVA of load at 24V out of it. (At full load, 1600VA / 240V = 6.6A drawn by the variac). As the variac is just a single winding you basically can only get about the same current out as you put in. i.e. 24V x 6.6A = 160VA. And not the 66A you think you should. The variac will try to supply this load but will only end up letting the magic smoke out.

Prof.

Daemon
07-16-2009, 07:09 PM
Thanks Prof

I knew there had to be a good reason they weren't being used.

Many thanks for the reply
Steve

piesrule58
07-17-2009, 05:22 AM
I have 2 big variacs that I used last year without a problem. 240v input - 0v to 270v output. I had one set to 24v and one set to 36v. Ran approx 10,000 lights from each one. Worked great.

mrpackethead
07-17-2009, 06:26 AM
I have 2 big variacs that I used last year without a problem. 240v input - 0v to 270v output. I had one set to 24v and one set to 36v. Ran approx 10,000 lights from each one. Worked great.

Sure it works, but you need to treat the lights as if they are at mains potential, because between the lights and Earth ( ie you ) they are at mains potential!

piesrule58
07-17-2009, 10:40 PM
Sure it works, but you need to treat the lights as if they are at mains potential, because between the lights and Earth ( ie you ) they are at mains potential!

Sorry, I am not an electrician and I do not understand what you mean by mains potential.
BTW, my son is an electrician. He got the variacs for me and set them up. So I assume that they are safe to use.

mrpackethead
07-18-2009, 12:55 AM
Sorry, I am not an electrician and I do not understand what you mean by mains potential.
BTW, my son is an electrician. He got the variacs for me and set them up. So I assume that they are safe to use.

THe voltage across teh load ( the lamps ) may be 24V, but the voltage between phase and earth could be at 240V ( mains potential ).. So if you were to come into contact with one of the wires, you may be in for a shocking suprize!

A variac only has one winding, with multiple taps. Your electrican son should know better than to do this. Are you sure you've got a variac?

sjwilson122
07-18-2009, 11:32 AM
Wouldn't an isolated variac eliminate this problem? I used one for years when I was an electronics repair tech. I think they changed the name now to variable transformer but years ago the were called isolated variacs.

ErnieHorning
07-18-2009, 02:49 PM
VARIAC is a trade name. It originally stood for ‘Vary AC’. ‘Variable Transformer’ is just another name for the same thing.

There are isolated VARIAC’s but they’re not very common. You could use a 1:1 isolation transformer on the input or output but they’re just as expensive as the VARIAC.

It is possible to use two transformers back to back, i.e. tie the secondaries together and use the primaries as input and output. The higher the secondary voltage, the higher the current will be for a given size. This will at least provide isolation.

Personally when I use a VARIAC while testing, I just know not to be touching anything that I wouldn’t if it was plugged in directly.

mrpackethead
07-18-2009, 11:04 PM
VARIAC is a trade name. It originally stood for ‘Vary AC’. ‘Variable Transformer’ is just another name for the same thing.


Check out "autotransformer" at wikipedia for a good explaination.

piesrule58
07-21-2009, 05:08 AM
My son has given me this information regarding the use of variacs.

The input and output are not electrically isolated in an auto transformer so if the "neutral" side of the input is not at ground voltage the "neutral" side of the output will not be either. Utilising the MEN ( Multiple earthed Neutral) system we have in Australia, the neutral is NOT a floating voltage and therefore remains at zero potential – ie Earth potential.

The only way possible to have full supply voltage across any wiring in your lighting circuits and earth is by – a. Catastrophic insulation failure inside of the autotransformer itself ( Highly Unlikely with a quality unit – Such as my dads – Built for the electrical supply industry for high voltage testing) or b. Somebody changing the output voltage manually by turning the dial. ( Also highly unlikely due to the units being stored away from wandering hands)

Situation a. would trip the RCD – See below. Let me add though, that the chances of catastrophic insulation failure on your transformer are right up there with winning the lotto .

Situation b. Multiple lightbulbs would blow immediately followed by a breakdown of insulation on the wiring itself unless the insulation was rated accordingly. This would hopefully be noticed pretty quickly and de-energised by the user or most likely a fire would result. IF by some unfortunate means it wasn’t de-energised and you happened to be touching an active and neutral wire during one of these unlikely times – you will receive a shock at the voltage dialed onto the unit – the chances of someone turning the dial, a whole lot better, hence a need to secure the unit properly.

Having full supply voltage between yourself and earth and the nasty shock that would occur – Easily rectified by the use of an RCD ( Residual Current Device) or safety switch – which measures a current imbalance between the active and neutral conductors ( there should be no imbalance in normal conditions) – During a fault – ie. You becoming part of the circuit - , there will be a difference in the current going out through the active conductor and returning through the neutral, and therefore and imbalance and a subsequent trip of the breaker quicker than you can even feel the tingling sensation of a shock. BTW – RCD’s having a tripping current not exceeding 30 milliAmps for 30 milliseconds ARE mandatory on all power circuits installed in Australia since 1992. If you do not have them installed, you really should think about getting them put in.

mrpackethead
07-21-2009, 06:08 AM
Utilising the MEN ( Multiple earthed Neutral) system we have in Australia, the neutral is NOT a floating voltage and therefore remains at zero potential – ie Earth potential.

This assumes that you don't have an earth fault. :-) which is quite possible, issues range from the ground being so dry that your earth stake does'nt really work.. through to a cable fault..

piesrule58
07-22-2009, 02:38 AM
This assumes that you don't have an earth fault. :-) which is quite possible, issues range from the ground being so dry that your earth stake does'nt really work.. through to a cable fault..

Is that a round about way of saying that my son is right?

mrpackethead
07-22-2009, 04:24 AM
Is that a round about way of saying that my son is right?

Your son is completely correct.

(a) there is no isolation across the transformer.
(b) there are conditions which exist in which your load will be at mains voltage.

But theres more to the story.

An autotransformer is primarly a test tool ( thats a whole different topic ), and realistically the only things that should be plugged into it are things that are already themselves safe to plug into the mains.. Mostly example double insulated appliances. A low voltage light string does not normally fit into this category. And if for no other reason, an Auto-transformer is a very expensive way of getting low voltage.

So, while it works, its not a good idea, and its safety is dubious.

piesrule58
07-23-2009, 12:51 AM
I respect your opinion. So lets just agree to disagree shall we?

mrpackethead
07-23-2009, 04:42 AM
I respect your opinion. So lets just agree to disagree shall we?

I'm glad you respect my opinion. I hope you respect your safety and the safety of those around you. The use of auto-transformers with loads that are not double insulated, is dangerous. But just so this dialogue is based in fact rather than opinion, i pulled up the relevant parts of the electrical regs. ( of which both Australia and NZ share the same standard ).



Part 91(c)..

91. Extra-low voltage installations
Extra-low voltage electrical installations are deemed not to be electrically safe for the
purposes of regulation 69 where—
(a) Revoked
(b) The fault current density of conductors (other than circuits operating at
telecommunications network voltage) 0.5 mm2 or smaller is greater than 1
amp/mm2; or
(c) The supply to the installation is derived from the tapping of a stator of an electric motor, or
from an auto-transformer

for your reference. the interesting part of part 69 reads..


(69.2) Subject to subclauses (3) and (4), for the purposes of these regulations, ”electrically
safe“ means that there is no significant risk of injury or death to any person, or of damage to any
property, as a result of the use of the works, electrical installations, fittings, electrical appliances,
or associated equipment, or the passage of electricity through those works, electrical
installations, fittings, electrical appliances, or associated equipment, as the case may be.

WireWrap
07-23-2009, 07:31 PM
My son has given me this information regarding the use of variacs.

The input and output are not electrically isolated in an auto transformer so if the "neutral" side of the input is not at ground voltage the "neutral" side of the output will not be either. Utilising the MEN ( Multiple earthed Neutral) system we have in Australia, the neutral is NOT a floating voltage and therefore remains at zero potential – ie Earth potential.

The only way possible to have full supply voltage across any wiring in your lighting circuits and earth is by – a. Catastrophic insulation failure inside of the autotransformer itself ( Highly Unlikely with a quality unit – Such as my dads – Built for the electrical supply industry for high voltage testing) or b. Somebody changing the output voltage manually by turning the dial. ( Also highly unlikely due to the units being stored away from wandering hands)
...



Unfortunately, there is a third way that is very easy to occur and results in the exact hazard discussed. (Forgive the poor ASCII drawing, but I hope it will get the point across).

The power input is to M(ains) and N(eutral).

The reduced voltage is taken from W(iper) and N(eutral) to your lights.

IF, FOR ANY REASON, THE INPUT NEUTRAL LINE IS OPENED, whether by a poor screw connection, a broken spade lug, a broken wire in the power cord leading to the auto-transformer that was stepped on too often, the plug pulled partly out, ANYTHING -- Both sides of your light string are at FULL MAINS VOLTAGE!!! If you touch either side of the string, you are subject to a shock if you are making a path to ground.

It is for this reason that auto-transformers are considered a shock hazard and not approved for low-voltage lighting.

Edit: I forgot to add that no bulbs will blow in this situation, they will just be out -- waiting for you to touch the leads of one.




Auto-
Transformer
----------,
| )
| )
| )
| )
| )
> M--- | )<---------------W > >----------------
) | | | | Your Lights
) O O O O
) | | | |
> N--------------'----------------N > >----------------