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Old 21st Oct 2017, 12:00 pm   #21
Neutrino
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Default Re: Variac help please

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Originally Posted by Refugee View Post
A low cost solution would be a zero crossing triac.
A zero crossing triac would ensure maximum inrush current each time the circuit is switched on. To minimise inrush current this way it is necessary to turn on at peak mains voltage so that the current only increases for one quater cycle before the polarity reverses causing the current to decrease:

http://www.st.com/content/ccc/resour...CD00003867.pdf
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Old 21st Oct 2017, 12:39 pm   #22
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Default Re: Variac help please

I have several isolating transformers and variacs and it was quite hit and miss whether an MCB would trip at switch on. I suppose it depends on the point of the AC cycle, ie the voltage at switch on.

The separate fuse box feeding my work-area outlets is fed via a B16A CB from the main fuse box. Individual outlets in the work area are fed via C6A breakers. However the selectivity is insufficient and more often than not, the 16A breaker would trip. If the 6A breakers tripped it meant crawling under the work bench to get at them - a PITA.

From eBay, I purchased a few soft start modules which have size and form of a normal CB and clip onto the mounting rail in the workshop fuse box. They were not exactly cheap, but since fitting them I have never had a CB trip. As far as I am concerned. Money well spent.

My 2 Euro cents worth.
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Old 21st Oct 2017, 2:56 pm   #23
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Default Re: Variac help please

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Originally Posted by Boater Sam View Post
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.
I resolved that one - my partner has a 22A single-phase kiln in the remote studio which could trip the 40A type B at the house end. An earth spike close to the consumer unit in the studio sorted that.
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Old 21st Oct 2017, 3:18 pm   #24
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Default Re: Variac help please

Many thanks again for all the advice. I will go with the lamp limiter first to see how that goes. I have just put one together so it will be it's first outing!! This is a great forum with so much advice on hand, some contradictory but, at least on the threads that I have read, usually backed up with sound arguments. I belong to a couple of other forums, not radio, where contradictions are often just a snipe at the previous poster.
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Old 21st Oct 2017, 3:57 pm   #25
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Default Re: Variac help please

I have got a 1000VA isolating transformer with an 8A Variac more or less hard wired to it that has welded a 13 amp socket switch to short circuit. We have still got wired fuses in the consumer unit.
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Old 21st Oct 2017, 6:23 pm   #26
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Default Re: Variac help please

I have a 1kVA isolating transformer and an 8A variac in the workshop. They both worked fine in the UK on a ring main with 32A type B circuit breakers, and I'd had no tripping problems in years and years of frequent use.

Then I brought them both to Poland where sockets are wired to poxy little 16A breakers and the isolating transformer and variac basically became unusable: either of them would trip the breaker at switch-on almost every time.

I fitted both with soft-start circuits and the problem has gone away. I still have to be a bit circumspect with my Tektronix 535A scope which has an enormous mains transformer, but I haven't modified that for soft-start. It lived in my office for a while and the security guard at the front desk got quite used to me coming and asking for the key to the electrical cupboard on our floor...

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Old 21st Oct 2017, 6:41 pm   #27
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Default Re: Variac help please

I have a little 2.5 amp 575VA variac that followed me home from work ( with the bosses blessing) and even that can sometime make the shack lights dip at switch on same as my big solotron scope which makes a big thump st switch on. Fortunately they've never tripped the c16 breaker so I guess I'm Lucky. Also when I worked at a transformer manufacturers years ago every time we tried to switch on our 3phase testing variac it made all the fluorescent lamps across the whole factory dip my boss said that was one of the reasons we still had fuses instead of MCBs! If I remember right its output wad protected by 3 60 amp rewireable fuses
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Old 21st Oct 2017, 9:27 pm   #28
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Default Re: Variac help please

The supply to my garage is a Type C. Before I rewired the garage and put a proper supply in, the type B would trip when my tool transformer was sarted. I don't suffer any trips with a Type C. Earth loop impedances are within max permitted.
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Old 21st Oct 2017, 10:11 pm   #29
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Default Re: Variac help please

I'm not surprised that this 12.5 Amp Variac trips its MCB, it's a bit of a huge one for a radio/TV enthusiast.
I remember when we finished a sauna installation with a 15kW connected load in a private house. On switch on the fluorescent lights in the kitchen went out completely and restarted!
We later found out that the site with 4 new houses were all connected to the 60 year old feed to the house that had stood on the site previously!
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Old 22nd Oct 2017, 11:53 am   #30
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Default Re: Variac help please

I agree, Boater Sam, that it is a bit large for my needs. I bought it off a guy on Chesterfield Market who sells second hand tools and engineering bits and pieces for £12. One of the carbon brushes was worn down but with adjustment it is now ok. Having tried it with a lamp limiter this morning it managed to power up without taking out the MCB.
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Old 22nd Oct 2017, 12:42 pm   #31
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Default Re: Variac help please

I have seen Variacs for sale at Chesterfield too several times. If you want one there will be a famine for months.
There are radios there too from time to time.
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Old 22nd Oct 2017, 3:11 pm   #32
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Default Re: Variac help please

Quote:
Originally Posted by Neutrino View Post
A zero crossing triac would ensure maximum inrush current each time the circuit is switched on. To minimise inrush current this way it is necessary to turn on at peak mains voltage so that the current only increases for one quater cycle before the polarity reverses causing the current to decrease:

http://www.st.com/content/ccc/resour...CD00003867.pdf
I don't quite follow the logic here. The breaker drops out when the switch-on occurs at a point where voltage is at a significant value. This causes a rapid change of flux (L di/dt) which in turn produces a large opposing back emf. If the device is connected at zero point, no energy can be transferred until the current builds up (relatively slowly) following the cycle thus much less di/dt. Have I missed something?

The easiest zero-crossing device would probably be one of those solid-state relays with a bit of DC derived from the mains to turn it on.

Les.
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Old 22nd Oct 2017, 4:44 pm   #33
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Default Re: Variac help please

In an inductive circuit, current is 90degrees out of phase with voltage. So maximum current occurs at the voltage zero-crossovers.

If you switch on at one zero-crossing, no current will flow, true. But by the next zero-crossing, current will be at a maximum and that's when it will trip, ie 10msec after switch-on.

Under steady-state conditions, the magnetising current will be, maybe, -1A to +1A, a swing of 2A. But if you switch on at the voltage crossover, before steady-state has built up, you still need the 2A swing, so current will be 0 to 2A, double normal. After a few cycles, this will settle down and the 1A shift will die away. Everything is happy.

The trouble is, Variacs are designed to close limits and 2A of magnetising current is likely to saturate the core... Inductance falls, and the current actually shoots up to many times this, by the end of the half-cycle. That's when you hit problems. It can be made even worse if the core happens to retain magnetism from the instant when it was last switched OFF!
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Old 22nd Oct 2017, 6:30 pm   #34
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Default Re: Variac help please

Hi,
The simplest "soft start" is for a, loaded, Variac to turn it to zero out voltages and switch it on...
Karl
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Old 22nd Oct 2017, 6:53 pm   #35
Boater Sam
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Default Re: Variac help please

Sorry Karl but that makes no difference. Even with no output load, it will still trip on current in rush. That's the problem with microwaves, the heater in the magnetron is irrelevant, its the transformer primary on its own that necessitates a soft start circuit.
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Old 22nd Oct 2017, 7:07 pm   #36
karesz*
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Default Re: Variac help please

Hi,
PHP Code:
..the transformer primary on its own that necessitates a soft start circuit
Yea, I did understand you, but Im wroting over an unisolated(!) or usual Variac if its tourned to zero for switch it on-it hasent "primary"_or Im missed some?
Btw; some years ago I have had same situation with an Variac in Lab, and my proposal brought the solution...
Karl
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Old 22nd Oct 2017, 7:18 pm   #37
lesmw0sec
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Default Re: Variac help please

Quote:
Originally Posted by kalee20 View Post
The trouble is, Variacs are designed to close limits and 2A of magnetising current is likely to saturate the core... Inductance falls, and the current actually shoots up to many times this, by the end of the half-cycle. That's when you hit problems. It can be made even worse if the core happens to retain magnetism from the instant when it was last switched OFF!
I must confess to neglecting the effect of core saturation...
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Old 23rd Oct 2017, 10:49 am   #38
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Default Re: Variac help please

No extra solutions or anything to contribute but just popping in to say I am watching this with interest as my 8amp variac regularly (but not always) trips out the house ring main MCB on start up.

I know that reducing the output voltage on the dial shouldn't make any difference but my experience is that it does seem to make a difference. Maybe (probably) it has just been luck and accidental when I have had the output dial at zero.
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Old 23rd Oct 2017, 12:46 pm   #39
kalee20
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Default Re: Variac help please

Quote:
Originally Posted by karesz* View Post
Hi, Yea, I did understand you, but Im wroting over an unisolated(!) or usual Variac if its tourned to zero for switch it on-it hasent "primary"_or Im missed some?
Btw; some years ago I have had same situation with an Variac in Lab, and my proposal brought the solution...
Karl
A Variac turned to zero still has a magnetising current, and sizeable switch-on current. It's just like the primary of a toroidal transformer.

If it's not at zero, and you have a load connected, you will have to cope with the switch-on surge of the Variac, plus the switch-on surge of the load. So yes, reducing to zero will help, but won't eliminate entirely.

Of course, many people want their Variacs to wind-up the load slowly, so they always reduce to zero anyway!
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Old 23rd Oct 2017, 1:41 pm   #40
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Default Re: Variac help please

HI,

I agree with Kalee20, the last thing you would want is a zero crossing triac/switch. This is because basically the variac is an inductor off load, as you know in the inductor the current lags the voltage by 90 degrees. So when its connected and running the current is zero when the applied voltage is peaking (not at zero crossing). So if you could synchronously switch it in, counter intuitively, for minimal current, it would be when the applied voltage was peaking, certainly not at zero crossing. Zero crossing switching is only any chop for switching resistive loads, not reactive loads.

It is very simple what you need, just get one of these (SL32 10015) from element 14 (farnell) mouser or digikey:

https://www.ametherm.com/datasheetspdf/SL3210015.pdf

You can put two in parallel if you like to double the current rating. This is the common way to prevent initial surges by using power NTC resistors. This is done in Air con units for example, these are put in series with the compressor motors.

One trick, if you want to get fancy, is to have a timer with a relay contact that shorts the NTC out a few seconds after switch on. You can make a timer just from a diode, resistor, capacitor and a relay, no semiconductors needed.

Last edited by Argus25; 23rd Oct 2017 at 1:46 pm.
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