Regavolt variac fuse.

[Line whistle]

Hi,
I have Regavolt 4 amp variac. The mains lead looks to be 13 amp but the wiring inside appears to be much thinner. There is a 1 amp fuse fitted for the output and a 13 amp fuse in the plug. Anything less than a 13 amp plug fuse blows on switch on. Nothing appears to be getting warm / hot with the 13 amp fuse. Please excuse my ignorance with this. Any ideas.
Thank you,
Des.

[G8UWM-MildMartin]

It's normal for them to have a high inrush current.
Martin.

[AlanBeckett]

Mine is a much bigger one - 15A - but the inrush current not only blows 13A fuses it trips ordinary Type B 20A Breakers. I now run it on a 16A Type C Breaker. It only trips that occasionally if I switch on with it turned up.
So, I wouldn't worry.
Alan

[Skywave]

At the instant of switch-on of the primary current, without a load connected to the "Variac", the only 'opposition' that the 240-v. mains supply 'sees' is the resistance of the primary of the "Variac". This will be very small - resulting in a large 'in-rush' of current - until enough time has elapsed for the back e.m.f. (caused by the primary inductance) to become large enough to reduce that current flow. It is this large in-rush current, when the "Variac" is first connected to the 240v. mains supply, that necessitates a large fuse to be fitted in the 3-pin mains plug.

Therefore, a case can be made for introducing a form of current-limiting device in series with the mains supply to the "Variac" to reduce this initial surge current; a device that, ideally, is then by-passed automatically after a few seconds. A manual switch will obviously also do this job, but the risk is then run of forgetting to manually operate this by-pass, producing poor regulation of the O/P voltage from the "Variac" and a substantially overheated current-limiting device.

Food for thought . . .

Al. / Skywave.

[Dekatron]

I have a circuit that I have used many times for just this occasion, I found it on the back on one of my previous Metrel variacs, which was built as an isolating transformer too.

Check the foto from the back of the transformer I have included.

You will have to calculate the inrush current to fit the power rating of the resistors to your transformer. To be sure not to blow the capacitor I used a 450V model.

[murphyv310]

My 40 year old 3KW variac has a very similar set up to yours, the limiter resistor is 8.2ohm and is 17watt. Unlike Martin's its not isolated, as its a variable autotransformer.

[Tractorfan]

Hi,
Just to upset the applecart, I've got a large ancient variac here and I can't remember it tripping or blowing anything here or in the UK. It does give a short, angry buzz on start up, though. I've used it quite a few times for dimming amateur theatrical lights. There's no fuse in the plug but the various dis. boards usually have MCBs. French electrics seem more tolerant to inrush current. Of course it depends on what part of the AC waveform that the load is applied whether it pops the fuse or not. Our tumble dryer pops fuses more often on start up than the variac does .
Cheers, Pete

[kalee20]

Switch-on surge can be a nuisance with Variacs (which are generally wound with as few turns as possible, for obvious reasons - hence the core is running at a high flux density). But that's what it is - a nuisance, not a fault.

Anti-surge fuses or Class 'D' circuit breakers are the way forward. If not, a surge limiter, as has been suggested, is the next best option.

Like Pete Tractorfan, I've hardly ever had one trip a breaker or blow a fuse. But if there is a tendency to do this, then once you are powered-up, leave the Variac running and wind up and down or switch the secondary, rather than switching off at the primary.

[Herald1360]

Originally Posted by Skywave

At the instant of switch-on of the primary current, without a load connected to the "Variac", the only 'opposition' that the 240-v. mains supply 'sees' is the resistance of the primary of the "Variac". .

That's not exactly what happens.... what causes the problem is switching on near zero voltage crossing. On the first half wave the core flux tries to build to twce its steady state peak value and at some point in the half cycle the core saturates at which point the inductance falls to that of an air cored inductor (ie a lot less than before) and a huge current spike appears around the top of the current waveform. This spike decays exponentially over the next few cycles as the core eventually "catches up" with the magnetising emf. Residual magnetism in the "wrong" direction will make this spike even bigger.

Interestingly enough, the best compromise point at which to hard switch a single phase transformer on is at the voltage peak of the supply. There will always be some surge, but it will be much less than the possible peaks from switching on at zero on the "wrong" half cycle.

http://relays.tycoelectronics.com/ap...fs/13c3206.pdf

provides a better description than my attempt......

[Skywave]

O.K. - thanks for the expansive exposition. My original Post was only intended to be a brief summary as a bit of guidance.

However, quote: "what causes the problem is switching-on near zero voltage crossing" has me a bit puzzled. Since - at that point in time - there is zero voltage across the inductive load (the primary of the 'Variac'), will there not be zero current? I would have thought that switching in the load when the mains supply is at the peak of its waveform was the worst moment in time to make the connection.

Al.

[kalee20]

Herald1360 is right.

If you switch on at zero voltage, then yes there will be zero current. But during the next 10msec the voltage will all be positive, so current will be increasing and will reach a maximum by the end of the half-cycle. That's what causes the trouble.

If you switch on at the voltage peak, then yes you'll have significant voltage, but current can't change instantaneously in an inductor so the current will still be zero. During the next 5msec the voltage will be positive so the current increases, but after that the voltage goes negative so the current decreases again - after 5msec negative, the current will hit zero (at max negative voltage), the next 5msec the current will reach its maximum negative value but the voltage will then be ready to go positive again.

You'll notice that at zero-voltage switch on, you have 10msec-worth of increasing current. At max voltage you only have 5msec-worth of increasing current. So 10msec gives you double the peak current - assuming that the transfomer behaves like a linear inductor.

In practice, it doesn't behave like a linear inductor and is likely to approach saturation towards the end of 10msec-worth of increasing current, especially if the core had some residual magnetism after the last time it was operated. This can cause a big increase in instantaneous current. The current spike occurs towards the end of a half-cycle, not at peak voltage, but it makes no difference to a fuse or a circuit breaker.

[Herald1360]

It would seem that the problem with zero voltage switching is that since under steady state conditions, the flux in the core lags the voltage by 90deg, as the voltage starts to rise from zero it would normally be swinging the flux from peak flux negative to peak flux positive over the next voltage half cycle, a swing of 2*peak flux in a positive direction. If flux is zero, as at switch on, it starts from zero instead of peak negative, but still tries to swing by twice the peak value positive. Somewhere between peak and twice peak the core will saturate leading to the inrush current problem.

Switching on at peak volts, the core is already in the steady state peak volts flux condition of zero flux, give or take any residual magnetism, so it all just continues as if it were in steady state, unless the core is driven so hard that normal peak flux plus residual flux is enough to saturate, in which case there will still be some surge, albeit a lot less than the potential surge under zero volt switching conditions.

For a simple lump of copper, iron and insulation, transformers can be fiendishly complex (certainly beyond my limited experience) to understand fully. (Try a peek at the J&P Transformer Book for a flavour of the art!)

Edit: Beaten to the draw by Kalee20 with a somewhat better explanation.....

[julie_m]

This is probable the clearest, most accessible essay I have ever seen on electromagnetics:

http://ludens.cl/Electron/Magnet.html .

Since I last visited the site, it's been updated with a new article on winding your own transformers from scratch; which you can see at http://ludens.cl/Electron/trafos/trafos.html . NB: What the author calls "spaghetti", we would know better as Vidaflex.

[Line whistle]

Hi,
Thanks for your replies.
Des.