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Old 20th Oct 2017, 5:16 pm   #1
IvorBlister
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Default Variac help please

I have recently bought a second hand Variac by Zenith Electrical, model Y20H 12.5A 250v. Although I have had it working it frequently trips the house circuit breaker on switch on. Supply cable to Variac is to terminals 2 and 4 and output is terminals 2 and 3. Pic attached. Anyone confirm that this is correct? I measured the resistance across the mains input terminals at only 1.5ohms. Is this about the right value or have I bought a faulty Variac? Many thanks in anticipation.
Nick
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Old 20th Oct 2017, 5:56 pm   #2
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Default Re: Variac help please

It looks to be connected up OK. 1.5 Ohms is reasonable as DC resistance. In operation it is the AC impedance which is important. The switch-on surge could well trip the circuit breaker. Maybe you need a circuit breaker with a higher surge rating. The fact that you've had it working implies that in steady state it's OK.
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Old 20th Oct 2017, 6:24 pm   #3
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Default Re: Variac help please

A couple of points for clarity, which circuit breaker is it tripping the circuit MCB or the ELCB on the supply. I would not be surprised if it tripped an MCB as the initial surge will probably exceed their rating, if remember correctly for industrial premises where you are likley to encounter large transformers and variacs they are different, if it's tripping the ELCB then it may be faulty with a leak to earth.
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Old 20th Oct 2017, 6:33 pm   #4
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Default Re: Variac help please

Thanks Stuart, that is reassuring. Chris, it is tripping the MCB only. I had wondered about an earth leakage but as the ELCB is not tripping I don't think that is a problem. It momentarily causes a dip in the brightness of the lights in my workshop when switching on the Variac. Could it be because it is a high rating Variac or would that not make any difference?
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Old 20th Oct 2017, 6:43 pm   #5
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Default Re: Variac help please

Quote:
It momentarily causes a dip in the brightness of the lights in my workshop when switching on the Variac. Could it be because it is a high rating Variac
Yes, it all sounds consistent with it putting a high load on the circuit breaker at switch-on. What rating is the present one? Perhaps it needs a C or D rating one.
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Old 20th Oct 2017, 6:50 pm   #6
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Default Re: Variac help please

Stuart, off the top of my head I don't know the rating but will check next time I reset it !!
Nick
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Old 20th Oct 2017, 6:56 pm   #7
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Default Re: Variac help please

Usual MCBs are "B" type, if this trips then you could use a "C" type which trips less easily on surges.
"D" type are known by sparkies as 'never trips', they stand massive short term overloads, a bit extreme for your use.
The alternative is a slow start circuit like used in simple microwaves. The load is connected through a hi wattage resistor of around 5 ohms, 10 watt. A mains relay coil is wired across the load after the resistor and the relay contacts short out the resistor when the relay energises. The short delay enables the transformer to build up a magnetic field through the resistor, limiting the surge. Think that's correct, its been a while.

OR try starting it up through your lamp limiter, then switch it full on by shorting the lamp out.

Last edited by Boater Sam; 20th Oct 2017 at 7:07 pm. Reason: added
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Old 20th Oct 2017, 7:22 pm   #8
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Default Re: Variac help please

Yes Boater Sam, it is a type B. Will get a type C and try that. The soft start idea had occurred to me but I didn't know how to do it so thanks for your suggestion.
Nick
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Old 20th Oct 2017, 8:09 pm   #9
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Default Re: Variac help please

I would suggest a soft start is easier than changing the MCB using a soft start with either a manual process with a lamp limiter, a power resistor with a 3 position mains Off/Start/On or using an NEGATIVE temperature coefficient (NTC) thermistor and a relay to make it automatic.
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Old 20th Oct 2017, 10:16 pm   #10
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Default Re: Variac help please

Chris, the manual soft start process sounds the one for me. Any thoughts on where I could obtain the circuit diagram and component values? I have enough ability to build something like that but not to come up with the circuit or values.
Nick
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Old 20th Oct 2017, 10:36 pm   #11
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Default Re: Variac help please

Type D MCB.

At my last workshop, we serviced BIG audio amplifiers. Despite the fact that all of these have soft very efficient start circuits, type B and C MCB's would trip - every bench had to have type D devices


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Old 20th Oct 2017, 11:34 pm   #12
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Default Re: Variac help please

I could never advise type D MCBs in a domestic situation. In industry maybe but only in exceptional very local situations controlling only the item that they are needed on.
I run a 180Amp welder on type B, yes, on occasions it will trip on first connection, but not when welding. This I will accept rather than risk the enormous surge current that a D or even a C will allow.

The manual soft start with a light bulb is fine, using a resistor this way could risk setting the resistor on fire if held in start for more than a fraction of a second. Using a resistor I would use a relay, or really a contactor would be better. It only needs to connect the load via the resistor for a few cycles before going direct

Last edited by Boater Sam; 20th Oct 2017 at 11:39 pm. Reason: added
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Old 20th Oct 2017, 11:45 pm   #13
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Default Re: Variac help please

I would be very tempted subject to there being a spare location in the distribution board to fit a 16 amp industrial style outlet serviced by a 16A D type MCB.
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Old 21st Oct 2017, 12:11 am   #14
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Default Re: Variac help please

Quote:
Originally Posted by Boater Sam View Post
I could never advise type D MCBs in a domestic situation. In industry maybe but only in exceptional very local situations controlling only the item that they are needed on.
I entirely agree with Sam!
In my work as an electrician in light industrial/ commercial premises, I have never needed to use a type D. Even on motors of 20KW or so starting high inertia heavy loads, type C has been perfectly fine.
The required maximum loop impedance is radically different between type B 16A (2.40 Ohms) and type D 16A (0.60 Ohms). (Figures from 16th edition as that's all I have to hand at the moment!)
The loop impedance needs to be measured before it can be decided if the type of breaker can be changed.
Definitely some form of soft start option is the way to go I think.
In addition, I would say that type B 16A breakers are pretty sensitive. They will trip much quicker than a 13A plug fuse. My table saw trips the B16 in the garage the instant I press the start button. It's fine connected to the socket ring in the house which has a B32.
All the best
Nick

Last edited by 1100 man; 21st Oct 2017 at 12:18 am. Reason: extra text
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Old 21st Oct 2017, 8:13 am   #15
IvorBlister
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Default Re: Variac help please

There is a separate MCB on the supply to my workshop. Well I call it my workshop as it sounds better than shed!!!. I only plunge myself into darkness when it trips not the whole house so from what I gather from replies is that a type C would be acceptable to use. Have I got that correct? Thanks again for all the advice and guidance.
Nick
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Old 21st Oct 2017, 9:25 am   #16
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Default Re: Variac help please

If this is a remote shed away from the main house and consumer unit I would be very careful using an MCB higher than type B as the earth loop impedance on the supply to your shed may then exceed the limit allowed.
If so this will severely increase the fault tripping time of the breaker, jeopardizing your safety.

Why not use a soft start? All it needs is a filament light bulb in series with the socket you use for the transformer with a manual switch that shorts out the bulb. I have this arrangement permanently wired on an extension lead as my test supply for unknown sets.
You open the switch, plug in the transformer, the bulb will light, close the switch and you are done.

Last edited by Boater Sam; 21st Oct 2017 at 9:28 am. Reason: clarification
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Old 21st Oct 2017, 9:28 am   #17
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Default Re: Variac help please

We have a few mobile installations where it's been practice for many years to have an isolating transformer on board.

The inrush current of which always caused much grief with tripping of the mobile's MCBs when plugging-in. Changing to a D-curve breaker just meant you were more likely to trip something further upstream - usually in a cupboard requiring a phone call to building services followed by a long wait for the man with the key.

Somebody recommended we fitted these CAMTEC ESB101 devices between our 16A inlet connector/MCB and the transformer. No more nuisance tripping.

http://www.camtec-powersupplies.com/...62016-01-0.pdf

I've found an on-line a supplier offering these at 70 Euros.

Hope that's of interest. A good branded D-curve Double Pole MCB would probably cost half as much and only be masking the problem.

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Old 21st Oct 2017, 9:50 am   #18
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Default Re: Variac help please

At switch on this is a big inductance being placed across the supply. You could try altering the power factor by a suitable capacitor - something I have been meaning to do for a while as I have a big Variac which sometimes trips out the shack breaker!
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Old 21st Oct 2017, 10:15 am   #19
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Default Re: Variac help please

A low cost solution would be a zero crossing triac.
I have one that was BER due to a short that had blown a track off the PCB. The triac was still all right.
The parts to build one would come in at about £10 including the zero crossing opto coupler. Heat sink not included.
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Old 21st Oct 2017, 11:51 am   #20
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Default Re: Variac help please

No, a zero-crossing is about the worst thing you could do.

On switch-on, you'd then have the mains polarity stay the same for the full 10msec. So inductive magnetising current would peak at its highest possible value, meaning maximum likelihood of tripping the breaker. And this maximum is likely to be well into saturation of the Variac's core.

I have a 25A Variac which does the same thing. It even takes out 13A fuses on switch-on. It's a pain.

Definitely soft-start is the way forward, just as others have said! A 100W lamp with bypass switch is about the cheapest, albeit that it's manual. But, it's easy to set up, and by switching back IN the lamp, you get the lamp limiter you've always wanted!
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