Softstart for variac

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mikeyB

Well-known member
Joined
Apr 12, 2005
Messages
568
Location
Manchester UK
Hi folks, not used my variac for a while. It's a big 8 amp one, that on rare occasion would trip the breaker at my mains distribution board! Just fired it up but now it's tripping the board on most occasions, when it doesn't trip, the unit works fine as expected. I unplugged and took it apart, all looks good and clean, no dust present, bush on arm looks ok. What's the easiest or best way to arrest the surge on this thing. would a power resistor with a mains switch parallelled work ok? No load presented until the parallel switch would be "on". NTC? Soft start poweramp cct from ebay/aliexpress?

Your thoughts would be welcomed.

As usual, thanks in advance!!
 
Can you measure its magnetising current, as a way to be confident there is nothing operationally faulty?

If the variac is fed from its own cb on the MDB then you could get that cb swapped to the same current rating but a longer delay (C type to B type where I live).

And yes you could make up your own pre-start switch - that would be my goto, as a NTC may offer too high a resistance given your magnetising current may be a lot lower than 8A (which is reasonably what the NTC should be rated at least to).
 
Cheers for the replies guys.
Yes John, I'll inspect closely for faults. It's an 8 amp variac so I would need a higher rated switch then a lamp switch?

Trobbins, yeah I did think of the MCB type but that hasn't changed when the variac was fine, I'm sure it is something related to the variac, which I will be inspecting closely in the coming days. I'll be trying the pre start switch also.
Thanks once again guys!
 
Could be the cb starting to play up - I had somewhat similar situation for 40A 3ph variac connecting via a 32A cb, which became a hassle to get it to not trip a c-type cb. The safest way to measure the magnetising current would be through a commercial (but often cheap) voltage/current/watt modern piggy-back-plug-socket meter (and although not in true-rms quality it does give an indication of current level) that is quite usefull for the bench.
 
Just a thought , maybe blow out the variac with compressed air just in case anything conductive like strands of copper or bits of solder have got in there while on the bench .
 
Magnetization currents @ 50Hz and a cheep Variac the core is saturating upon turn ON. Place a 100 watt light bulb in series @ turn ON, then bypass the bulb after its on.

Get a 3 position AC power switch where OFF-ON-ON and the ON-ON is a make before break type.
Duke
 
Does it trip the breaker with the output down as far as it will go? Makes me think the brush on the wiper may be shorting a turn or 2. If that's ok then I have some CL100 surge limiters, .5 ohms cold if I can find them. They always show up when I don't look for them.

To give you an idea they were used in Hafler DH500 power amps. I used them for Moscode 600 power amps, 325 wpc both channels driven. 11 amps at full output and I think it would solve the problem. PM me if interested in mine. You can also get them from Mouser. I have put several in series to limit inrush with big cap banks.
 
I have used Variacs of all sizes over the years. i believe the "surge" you're talking about that trips your breaker is a direct result of the residual magnetic particles facing the "wrong way" during initial energization of the Variac iron core. Also, to make matters worse, expect a lot of third and fifth harmonic current content during start up. It;s not uncommon to see an eight fold increase in full load current during this brief time. So an 8 amp variacs' inrush current could be 50 to 60 amps easily. Same thing happens to Amateur Radio Operators when they energize their big giant 3 KVA plate transformers on their big old broadcast transmitters, so this problem is not just characteristic of variacs alone. The good news is that there are ways to fix this annoying problem.

The best method would be to incorporate a "zero crossing detector" that closes the relay at precisely zero volts. This method requires basic physical knowledge of how fast the relay you are using can close in conjunction with the main contactor, etc. The actual time the relay closes is typically measured in milliseconds and is programmed via microprocessor and added as part of the start circuit. This method is way too complicated and expensive for us regular guys who just want to plug a small variac into the wall.

Next would probably be one of those "soft start" circuits many folks talk about and use. I draw back to many of these simple circuits seems to be the power resistors used on the soft start boards have a tendency to burst into flames if left in the "full load" position too long. I've heard complaints.

I found a very reliable circuit that works very well for me. I use a small time delay relay board and adjust the operate time for about 1 second or so. I got these boards from BangGood for about $8 each (12 volt input with a 555 timer and a relay). I added a Mars relay (Available from air conditioning shop or A/C repairman.) The board may be energized with a small DC wallwart at start up and that's when the countdown begins. Here's a handy twist: Instead of using a power resistor across the relay contacts, use a run capacitor. 15 to 26uf at 240 VAC should be fine,. I grabbed one off of an old retired washing machine motor!

The beauty of using a run capacitor instead of a power resistor is that either way you chose to do it, the "random magnetic particles" in the iron core become "lined up" making large start up current inrush a thing of the past. Don't get me wrong, you still have the inrush, but it's "impedance controlled" much much smaller thus keeping inrush current to a manageable level.

One thing to remember is that in the unfortunate event that the contactor might fail (happens all the time) you won't end up with your power resistors bursting into flames, as there is no heat produced using a capacitor.

Regards, Hank
 
I know magnetization current can be high. I would think the internal resistance of the windings would keep it in check on a 8 amp variac. But I could be wrong.

I have a Powerstat 10A variac that I've not had this problem with in 40 years. I use it all the time to bring up equipment slowly when necessary and it has a standard toggle switch on it.

Those are all good problem solvers. The HAM radio 3KVA transformers have a fully discharged capacitive load at turn on which is where the inrush current would be, don't they?
 
Duke.. you mentioned a three way switch

"Get a 3 position AC power switch where OFF-ON-ON and the ON-ON is a make before break type.
Duke"

Wouldn't a series bulb in line with a spst switch across it (paralleled) serve the same purpose?
 
Nice but OWCH!!! £140 from RS!!! That's twice what the variac cost!! I'll go with the incandescent bulb and switch for now!!
FWIW back in the day I used a positive temperature coefficient fuse inside a power amp.... My recollection was only a few dollars back then.

Sadly incandescent lightbulbs may be growing scarce thanks to government nanny state.

JR
 
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