Variac (autotransformer) tripping breakers on power-on

[Radek Chwistek]

Hi,

I recently aquired a beefy variac/autotransformer (http://www.ebay.co.uk/itm/141274235054), without much data about it though: input 240V, <400Hz; output 0-306V, weight ~6kg, power rating: ?? (I guess it's around 1.5...2kVA).

The problem is every time I plug it in, it trips the breakers (B32 type). It also fried 5A fuse inside the plug; failed to blow 13A replacement, still tripping house breaker though.
My reasoning being that the culprit is high inrush current, I tried wiring some NTC resistors (thermistors), first alone (1Ohm...5Ohm 5W didn't work, I popped 20Ohm 3W one), then in series (1+2+5Ohm), only to burn 5Ohm 5W in the end. The best result so far was to get short burst of light of a bulb at the output of the transformer before the breaker tripped.

Any clues how to get it started? Should I wire more 5W rated NTC thermistors in series? Or use some other components? Definitely don't want to spend more than £10...£15 on that...

Cheers,

Radek

[Adrian Godwin]

Wind the input up slowly on a variac ?

You probably don't need one of similar power, as you're using it unloaded. You could also use a builder's transformer to run the whole thing off 110V which should make it a bit calmer. All just measurement methods, not a final solution.

If you have a current clamp (cheap AC type should be OK) and a storage oscilloscope you could measure the switch-on characteristics. At least then you'd know what was happening, and the effect of the PTCs.

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[Paddy Duncan]

I would presume the reason is that it requires 240v 400Hz, not 50Hz.

I hope this is not too depressing.

Paddy

 

 

From: london-h...@googlegroups.com [mailto:london-h...@googlegroups.com] On Behalf Of Adrian Godwin
Sent: 03 April 2015 08:22
To: london-hack-space
Subject: Re: [london-hack-space] Variac (autotransformer) tripping breakers on power-on

 

Wind the input up slowly on a variac ?

You probably don't need one of similar power, as you're using it unloaded. You could also use a builder's transformer to run the whole thing off 110V which should make it a bit calmer. All just measurement methods, not a final solution.

If you have a current clamp (cheap AC type should be OK) and a storage oscilloscope you could measure the switch-on characteristics. At least then you'd know what was happening, and the effect of the PTCs.

On Fri, Apr 3, 2015 at 3:57 AM, Radek Chwistek <radek.c...@gmail.com> wrote:

Hi,

 

I recently aquired a beefy variac/autotransformer (http://www.ebay.co.uk/itm/141274235054), without much data about it though: input 240V, <400Hz; output 0-306V, weight ~6kg, power rating: ?? (I guess it's around 1.5...2kVA).

The problem is every time I plug it in, it trips the breakers (B32 type). It also fried 5A fuse inside the plug; failed to blow 13A replacement, still tripping house breaker though.
My reasoning being that the culprit is high inrush current, I tried wiring some NTC resistors (thermistors), first alone (1Ohm...5Ohm 5W didn't work, I popped 20Ohm 3W one), then in series (1+2+5Ohm), only to burn 5Ohm 5W in the end. The best result so far was to get short burst of light of a bulb at the output of the transformer before the breaker tripped.

 

Any clues how to get it started? Should I wire more 5W rated NTC thermistors in series? Or use some other components? Definitely don't want to spend more than £10...£15 on that...

Cheers,

Radek

 

 

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[David LHS]

Hi Radek,

On 03/04/2015 03:57, Radek Chwistek wrote:
> I recently aquired a beefy variac/autotransformer
> (http://www.ebay.co.uk/itm/141274235054), without much data about it

Have a look at this: <http://sound.westhost.com/articles/variac.htm>

> The problem is every time I plug it in, it trips the breakers (B32
> type). It also fried 5A fuse inside the plug; failed to blow 13A
> replacement, still tripping house breaker though.
> My reasoning being that the culprit is high inrush current,

Try measuring the resistances of the input and output to see if there's
a short. Make sure the variac is unplugged from everything first as
these things can be lethal.

> Any clues how to get it started? Should I wire more 5W rated NTC
> thermistors in series? Or use some other components? Definitely don't
> want to spend more than £10...£15 on that...

I don't think you need to add any additional components. I'd expect it
to work fine plugged in with no load unless it's faulty.

David.

[Paddy Duncan]

Er no.
As it says in the original email and in the pics, it is designed for 400Hz not 50Hz.
As such it is behaving exactly as expected, and if you persist with trying to drive it with 240V 50Hz and managed to give it enough current, it will go up in flames.
The max input voltage for this at 50Hz is ~30Vac. Above that its inefficiencies will melt it.

-----Original Message-----
From: london-h...@googlegroups.com [mailto:london-h...@googlegroups.com] On Behalf Of David LHS
Sent: 03 April 2015 12:05
To: london-h...@googlegroups.com
Subject: Re: [london-hack-space] Variac (autotransformer) tripping breakers on power-on

[Alexander Wright]

Yep, one of the images on the ebay listing shows it as being rated for 400Hz. This is quite common in aviation as using 400Hz electrics on planes allows magnetics to be made smaller. On 50Hz, the core would saturate in 1/8 of a cycle. If you're feeling adventurous, you could make an inverter :P

Alec

[David LHS]

On 03/04/2015 12:15, Paddy Duncan wrote:
> Er no.
> As it says in the original email and in the pics, it is designed for 400Hz not 50Hz.

Ah right, can't read the original pics on this machine. Though I wasn't
recommending plugging it in to the mains.

David.

[Radek Chwistek]

well done me, then....

I intended to crank up the fan's airflow by increasing it's voltage with that variac. Any ideas what (useful) it can be turned into? Anyone interested in having it (even for parts)?

Radek

David.


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[tek...@hotmail.com]

As mentioned by others the impedance of a dedicated 400hz transformer would typically be very low at 50 Hz.
However..... I have seen Mil spec transformers that work over this frequency range without any modifications.
No idea what pixie dust was in the laminations.

As a rule of thumb, dedicated 400Hz transformers are tiny for their stated power. Wide band versions are about the same size as a 50Hz of equivalent power.

If you still want to test it, there's a Kikusui PCR1000 variable frequency mains power supply under my workbench. It will happily output a Kilowatt at 240V and 400Hz.

For future reference if you have an issue with an unknown device tripping circuit breakers, wire an incandescent light bulb in series with the mains input.
I have 40W, 100W and 250W bulbs for this. If your device under test is shorted or otherwise compromised, the lamp will act as a current limiter.

Hywel

[Radek Chwistek]

Thank you all for your input, especially Hywel! Impedance mismatch didn't cross my mind ... until now, when it makes 8-fold difference if not taken into account.
And yeah, 60W incadescent is much simpler than NTC thermistors. Variac works, hums, doesn't get hot and doesn't trip any breakers.
Now as a long-run solution to power my 245W blower fan, I found "coldstart inrush current limiter" - http://www.ebay.co.uk/itm/121607774944. Hope it'll work.

Radek

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[Alexander Wright]

This won't work - the problem isn't inrush current, it's that the transformer will simply saturate every cycle.
Connecting a lightbulb in a series is just a diagnostic thing: it stops the thing from blowing up. It won't help with running a load. It might work a little bit, but the majority of the power will be lost in the bulb rather than delivered to the fan. If you're doing this, you may as well just have a load of light bulbs in series with the fan to vary the speed. The inrush current limiter would likely just catch fire or trip out, depending on what protection it has.

Alec

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[Radek Chwistek]

What if I used an inductor in series with the transformer to limit the current to 2A? According to the formula ~380mH should do that (minus some unknown variac's impedance 25...50mH). I though about using rewound microwave transformer (secondary winding removed). Will it prevent the variac from saturating @50Hz?

Radek

[Alexander Wright]

You'll be able to prevent the variac from saturating in that way, but it will essentially do so by acting as a potential divider - so the variac would only have 29v across it, and the most you'd see at the output would be about 38v.

Or to put it another way, you'll be increasing the self-inductance on the primary, but the mutual inductance between the primary and secondary will remain the same.

Alec

[Radek Chwistek]

Well, I'm cutting my losses now! Can't add more iron to the core, can't connect additional impedance in paralel with the primary.... Pricey lesson of transformer theory;) Thanks a lot for illuminating the case!

Radek