Add "soft start" to Isolation transformer

[ekyle]

You could try using a solid state relay to power it up. Most of them turn on during a zero voltage crossing so you wouldn't have the problem of it being energized during the peak of a cycle. Also, turning off using a solid state relay won't cause the other problem of burning up switch contacts when it's turned off from the field collapsing. They can be had on Ebay for about $5.

[SeanB]

I added soft start to my variac, simply because of that inrush tripping a 20A breaker ( thermal one, old Fuchs that was likely 30 years old, and which had no magnetic trip at all, as that was not required at the time of manufacture, being handled by wire size selection only, so the wiring was 4mm for a 16A nominal load, as opposed to now being 2.5mm for a 25A load) so added a resistor and a shorting relay for them, along with a thermal fuse between the 2 10W 47R resistors ( what was on hand to use) to limit down, and then a 6.3A fuse on the primary and another on the secondary.

[IanJ]

Hi all,

I picked up an isolation transformer from the medical world which I have adapted for use in my workshop.

It's a 1000VA isolation transformer that was supplied as part of a urodynamic system, sold in 2011 namely a Duet Logic G2 Urodynamic system. It's mounted in the base of the cart and supplies isolated mains to the various components of the system and an addition 24V to connect to the patient unit module. The secondary output is switchable 120/240VAC.

I ripped out the 24VAC psu and re-wired the transformer as originally the earth in/out were connected, as was one side of the secondary AC from the transformer (neutral). Now it's fully isolated and has the option via two banana's I've fitted to allow interconnection of the earth in & out. I've also fused one side of the secondary.

Yes, there is visible inrush current (the lights in my workshop flicker).

Some pics, a mix of before/after.

Ian.

[Pjotr]

Those medical ones are often of the "low inrush" variety. That means they have a 50% (or even more) bigger core than needed. So saturation of the core at switch on is much less severe or does not occur.

[bitslice]

and replace those with NTC Thermistors...  Or should I just throw in a couple high wattage resistors?

Don't people just use a 100W incandescent in line with these things?

[bitslice]

For what purpose?

for this kind of thing I guess?

[staze]

But lightbulbs have low resistance on turn on, and then it goes up. So this would be the opposite, would it not?

[bitslice]

Oh yes lamps are just for shorts, but the OP seems concerned about breaker tripping. Just throwing it in there.

[staze]

I am the OP. :p I'm just not sure a lamp would help in any way. The inrush would still happen. :/ I'd want something that has a high initial resistance, and then a low ongoing one.

[ian.rees]

If the lamp is in series with the transformer primary, then the max inrush current is limited by the lamp - hopefully turning on a 100W lamp won't trip the breaker .  The lamp can be replaced by a short a few cycles later to get more power through the transformer.

[ian.rees]

Clearly there's more than one way to do it, and sure lamps aren't ideal (though they are a fine solution in this context).  I was answering the OP's question about how the lamp could help. 

[ian.rees]

No worries.

I think people use lamps for mains current limiting like this because lamps are simple, and everyone used to have a few around the house/shop.

If I had to deal with a similar transformer inrush issue today, it would be just about as easy to find an NTC as an incandescent lamp!  Of course the lighting at my home workbench is terrible, and it's chilly, so that lamp does have some follow on benefits. 

[Jeroen3]

The lamp also features as visual short circuit indicator when put in series with a dut.

[johansen]

But lightbulbs have low resistance on turn on, and then it goes up. So this would be the opposite, would it not?

since when do 100 watt lightbulbs trip breakers?

[staze]

Good point... Though I'm sure I could gang enough of them together.

Anywho, as someone said, it's hard to find 100W bulbs anymore. And the unit seemed perfectly happy powering on in my tests. Maybe due to what someone said about the medical units having oversized cores...

[Ian.M]

Load it with the maximum resistive load you are ever likely to use on it.  Switch it on and off a number of times, depending on the distance to and difficulty of resetting the circuit breaker feeding it - 20 times is probably plenty if its at the bench, but if its in a cupboard the other end of the building, you'd want to test it 100 times with no trips.

If it doesn't trip, all is good, provided your bench PC and other test equipment that doesn't like power being cut unexpectedly is on a different circuit.   If it trips, you'll need to consider implementing one of the soft start circuits that have been discussed.  If it needs it, I'd go for a timer relay shorting out a power resistor (with a manually resettable thermal trip strapped to it) + slave relay to hold off connecting the secondary side load so that the resistor only has to handle the transformer's inrush and magnetising current.

[uncle_bob]

Load it with the maximum resistive load you are ever likely to use on it.  Switch it on and off a number of times, depending on the distance to and difficulty of resetting the circuit breaker feeding it - 20 times is probably plenty if its at the bench, but if its in a cupboard the other end of the building, you'd want to test it 100 times with no trips.

If it doesn't trip, all is good, provided your bench PC and other test equipment that doesn't like power being cut unexpectedly is on a different circuit.   If it trips, you'll need to consider implementing one of the soft start circuits that have been discussed.  If it needs it, I'd go for a timer relay shorting out a power resistor (with a manually resettable thermal trip strapped to it) + slave relay to hold off connecting the secondary side load so that the resistor only has to handle the transformer's inrush and magnetising current.

Hi

Or .... ummm ... plug it in... leave it plugged in and don't mess with all this stuff. That of course *assumes* you have a problem, which you will not with a laminated transformer (like the one in the picture) that has been properly designed.

Bob

[raptor1956]

Just to be clear on the soft start thing ... what level of peak inrush current should one expect from a toroidal transformer rated at, say, 120V primary and 750VA?  If it's 10X the 6.25A then we're talking 62.5A give or take IF there's no series limiting resistance.  Let's say we use a 10 ohm resistor in series with the primary then the max current would be about 12A but decline as the core sorts itself out.

So, if the outputs were disabled during this time and then, once the core was OK the input resistor is bypassed and the outputs enabled.  The input resistor would not need to be rated at 1440W as the length of time the resistor would see that level of power is small and over the 10 cycles, assuming the current drops to near zero, the resistor would likely see an average of about 800W for 1/6 second or about 133 joules.  Many camera flash units dump more than that energy during a flash.  I would think even a 5W resistor would be enough.

So, to layout the components needed for a softstart circuit you'd want a time delay relay at the input that picks in, say, half a second, and a 5W 10 ohm resistor in the hot leg of the input to the transformer that will be bypassed when the input relay picks.  The output could be controlled by a time delay relay and pick a half second after the input one picks OR the output could be turned on manually by pressing a button that picks it -- perhaps using a circuit in the input relay to inhibit turning on the output until after the input relay has picked.  Sort of a modified EMO circuit...


Brian

[uncle_bob]

Just to be clear on the soft start thing ... what level of peak inrush current should one expect from a toroidal transformer rated at, say, 120V primary and 750VA?  If it's 10X the 6.25A then we're talking 62.5A give or take IF there's no series limiting resistance.  Let's say we use a 10 ohm resistor in series with the primary then the max current would be about 12A but decline as the core sorts itself out.

So, if the outputs were disabled during this time and then, once the core was OK the input resistor is bypassed and the outputs enabled.  The input resistor would not need to be rated at 1440W as the length of time the resistor would see that level of power is small and over the 10 cycles, assuming the current drops to near zero, the resistor would likely see an average of about 800W for 1/6 second or about 133 joules.  Many camera flash units dump more than that energy during a flash.  I would think even a 5W resistor would be enough.

So, to layout the components needed for a softstart circuit you'd want a time delay relay at the input that picks in, say, half a second, and a 5W 10 ohm resistor in the hot leg of the input to the transformer that will be bypassed when the input relay picks.  The output could be controlled by a time delay relay and pick a half second after the input one picks OR the output could be turned on manually by pressing a button that picks it -- perhaps using a circuit in the input relay to inhibit turning on the output until after the input relay has picked.  Sort of a modified EMO circuit...


Brian

Hi

Plan for at least a million amps for a few hours based on some of the tales ....

It's not rocket science: measure the primary inductance and model it. Unless you have a really weird transformer, it will all sort out in less than two cycles. If you can not measure it take a look at a paper on transformer design. That will give you a pretty good idea.(secondary open, the primary inductance is what " takes the current to zero " ).

Bob

[raptor1956]

Just to be clear on the soft start thing ... what level of peak inrush current should one expect from a toroidal transformer rated at, say, 120V primary and 750VA?  If it's 10X the 6.25A then we're talking 62.5A give or take IF there's no series limiting resistance.  Let's say we use a 10 ohm resistor in series with the primary then the max current would be about 12A but decline as the core sorts itself out.

So, if the outputs were disabled during this time and then, once the core was OK the input resistor is bypassed and the outputs enabled.  The input resistor would not need to be rated at 1440W as the length of time the resistor would see that level of power is small and over the 10 cycles, assuming the current drops to near zero, the resistor would likely see an average of about 800W for 1/6 second or about 133 joules.  Many camera flash units dump more than that energy during a flash.  I would think even a 5W resistor would be enough.

So, to layout the components needed for a softstart circuit you'd want a time delay relay at the input that picks in, say, half a second, and a 5W 10 ohm resistor in the hot leg of the input to the transformer that will be bypassed when the input relay picks.  The output could be controlled by a time delay relay and pick a half second after the input one picks OR the output could be turned on manually by pressing a button that picks it -- perhaps using a circuit in the input relay to inhibit turning on the output until after the input relay has picked.  Sort of a modified EMO circuit...


Brian

Hi

Plan for at least a million amps for a few hours based on some of the tales ....

It's not rocket science: measure the primary inductance and model it. Unless you have a really weird transformer, it will all sort out in less than two cycles. If you can not measure it take a look at a paper on transformer design. That will give you a pretty good idea.(secondary open, the primary inductance is what " takes the current to zero " ).

Bob

It is pretty amazing that this issue has such divergent views.  My first thought on this was to use a zero crossing solid state relay but TE explicitly stated that that would be just about the worst thing you could do.  Toroid's are a different beast with no air gap and the videos I've seen say they really do have a turn on issue so I'm not going to pretend they don't. 

So, if you're going to go to the trouble of building a soft start into it you might as well go the full nine and have an output that can be switched on and off separate from the transformer.  Parts list then gets to: toroidal transformer, time delay relay at input, 5W 10 ohm resistor, relay for output, main on switch, circuit breaker or fuse, output on switch and output off switch, some indicator lamps.

Look like a fun project...


Brian

[Circlotron]

The way I understand this transformer inrush current thing is like this: when the transformer is running steady state, one complete half cycle of mains voltage above the line pulls the core magnetism from negative max to positive max. No saturation. If though the transformer is switched on at the voltage zero crossing the core magnetism starts from zero, not negative maximum, and so somewhere during the following half cycle of mains voltage the core magnetism hits saturation and the current is only limited by the DC resistance of the windings. Plus other remanence factors too, but I think that's the guts of it.

[raptor1956]

Yes, with larger transformers and most certainly toroids, the magnetic effects can't be avoided.  For most things we as hobbyists work on magnetic effects are generally insignificant beyond the need to calculate and control for inductance etc.  But with bigger motors and transformers a whole new realm of factors come into play and few of us have the experience to know ahead of time what the pitfalls might be.

I will probably be building an isolation transformer kit in the coming months and I'll probably use a toroidal transformer as it's core.  I could make do switching on and crossing my fingers that the CB doesn't trip and reset it when it does but rather than deal with that I plan to incorporate a soft start into and while I'm at it make it a more complete and professional job as described in my priors.  I should be able to build a complete isolation transformer with 750VA output for about $250-$300 USD which is about what you'd pay for something from Amazon that is less capable and less usable.  And again, it looks like a fun little project...


Brian

[FlyingHacker]

I have a 1000 Watt isolation transformer that would trip the breaker every few times you turned it on. I added an Anatherm NTC in line with the primary a year ago, and it hasn't tripped since.

Simple solution. Not much downside, since this transformer had a slight step up anyway (120V to 124V). I never saw any reduction in output voltage anyway.

[Siwastaja]

It is pretty amazing that this issue has such divergent views.

Nah, it's just everyday stuff on internet forums (or with humans, anywhere). For some people, it's so hard to admit being wrong in such a simple, beginner-level, well-known subject, that they easily end up in a defensive loop (even after such a long time!), which, of course, only makes them look funny. It often happens that the original "being wrong" was not embarrassing at all -- it happens to all of us, all the time; but the later "explanations" are hilarious.

Such as in this case talking knowingly about inductance for modeling the inrush current; in reality, inductance value is meaningless because as described by others, the core saturates, which by definition reduces (or nearly removes) the inductance, causing the issue to start with. Resistance defines the maximum upper limit.

If the inrush current is not specified by the manufacturer, then you have to measure to be sure. But it should be on the same order of magnitude as the nominal maximum current.

[edpalmer42]

I have a worklight on my bench that uses a 300W projector bulb.  To protect the bulb from the turn on surge, I used a technique from the ARRL.

If you look at this document https://archive.org/download/arrl-1981-radio-amateur-handbook/arrl-1981-radio-amateur-handbook_text.pdf , on page 153 of the pdf (page 5-12 of the document) figure 20 shows a surge-suppression method that can be scaled to any situation.  It's described in the section called 'Surge Protection'.  Basically, put a power resistor in series with the load.  Size the resistor to fit the requirement.  Put a relay coil across the load and use a normally-open contact on the relay to short out the resistor.  When the surge hits, the load tries to draw so much current that the voltage drop across the resistor reduces the voltage across the relay and the load to the point where the relay won't operate.  As the surge passes, the voltage across the load increases until the relay pulls in and shorts out the resistor.

For my 300W bulb, I ended up using a 20 ohm, 25W resistor.  It needs to be high wattage because it has to carry the initial current surge.  The actual dissipation in the fraction of a second that the surge is there is minimal.  Remember that the relay contacts also have to be rated for the current surge and/or the system's full load current.

This is better than an NTC because if you turn the power off and back on, the NTC doesn't have time to cool off so you get a large surge.  The relay drops out immediately and is ready for another cycle.

Ed