[mjohnsonsa]

About once out of every 10 times I start my CNC router the breaker trips. The reason is the inrush current from my Toroid power supply. Couple Questions:

1. Can I use a UPS (Uninterruptable Power Supply) as a means to limit the inrush current? or will I just fry the UPS?

2. Is there a commercial limiting device made just for this application? I saw one DIY circuit design on instructables...

3. What are you others with Toroid Power Supplies doing to avoid this problem?


Thanks!
Matt

[Eldon_Joh]

There is a NTC resistor in every computer power supply, about the size of a nickel. find one, put it in series with the toroid primary.

[RCaffin]

Yeah, toroids do that. You want a suitably-sized NTC resistor in series with the primary.
The NTC might start at 200 ohms (say), but it will quickly get very hot and the Negative Temperature Coefficient will drop that resistance down to (say) 1 ohm. This might take 1 - 2 seconds. A soft start.
Caution: an NTC runs very hot: mount using a ceramic terminal strip, NOT a plastic one!!!

Cheers
Roger

[cjmerlincnc]

You could also add a contactor wired across the NTC set with a 5 second delay to short out the NTC once it's done it's job as it doesn't need to be burning power all the time.

[handlewanker]

Hi.....I have the same problem with a 240 volt transformer dropping the voltage to 110 volts.......bought on EBAY as a bargain basement item going cheap......the trannie is huge and weighs about 75 Kg......rating approx. 50Kva

I want to use it for a steam oven I bought from the States which needs 110 volts at 10 amps etc, problem is the inrush current surge trips the breaker and needs at least 3 switch on goes before the trannie settles down.

I already have a 10 amp 110 volt transformer for the oven but the duty cycle only allows it to be used for 30 to 40 mins before it gets too warm and trips out

I'm thinking that to solve the problem I could connect a 100 watt incandescent light bulb in series with the live wire on the primary as this is basically a resistor running red hot in a vacuum etc.......this would limit the current flow to the primary windings until the voltage built up and the light bulb stopped glowing.

Having a resistor in circuit all the time would mean the current flow to the primary is limited to the resistance of the light bulb........about 1/2 an amp, so it could take a while to pass enough current before the primary windings are "full" etc.

I think there has to be a cut off point where the resistor value is high enough to slow the inrush current but large enough to pass the current for the secondary use.

To make the trannie fully functional once it's on line the resistor needs to be switched out of circuit with a make before break switch or a relay that drops out when the voltage is at the 240 volt mark.

I considered wiring in a heater element as a resistor and then switch it out when the trannie stayed on......any thoughts on this approach?
Ian.

[RCaffin]

Hi Ian

I could connect a 100 watt incandescent light bulb in series with the live wire on the primary as this is basically a resistor running red hot in a vacuum etc.......this would limit the current flow to the primary windings until the voltage built up and the light bulb stopped glowing.

Things will not work quite the way you want.
You need a resistor with a negative temperature coefficient for this, but a light globe, or the tungsten wire inside it, has a positive temperature coefficient. That means the light globe has a very low resistance when cold (check it with a meter), so it won''t do much to limit the inrush current, but the resistance will go up when it gets hot. That's why tungsten filament light globes work as well as they do.
See
https://en.wikipedia.org/wiki/Inrush_current_limiter

For power use, try
Inrush Current Limiters | Ametherm

Cheers
Roger

[mjohnsonsa]

What is the toroid primary? Is it one of the two 120v wire inputs?

There is a NTC resistor in every computer power supply, about the size of a nickel. find one, put it in series with the toroid primary.

[RCaffin]

Primary = input
Cheers
Roger

[handlewanker]

Drat.....just when I thought a simple solution was available..........dedicated current inrush devices are too costly.....gotta think of another simpler way.

I thought that a 100 watt light bulb could only pass a small amount of current even when it's cold.....supposing I wound a coil of Nicrome wire around a former and had 100 ohms resistance.....at 240 volts that would only pass 2 1/2 amps approx when cold........if that was in series with the live wire on the primary of the trannie, the other side of the resistor going to the trannie would be a virtual neutral source until the voltage and current built up whereupon it reverts to ma live wire.......I think?
Ian.

[RCaffin]

Hi Ian

I thought that a 100 watt light bulb could only pass a small amount of current even when it's cold

Nope, not a chance. They are all PTCs.

dedicated current inrush devices are too costly

What?
B57237S330M | Epcos 33? 3100mW NTC Thermistor, 90s, 15 Dia. x 7mm | EPCOS
$2.32
And it is DESIGNED for the task.

Mind you, it all depends on what sort of power you are going to pull thru the toroid. If it is 5 kW - we find another solution! Like a motor soft-start unit, or this:
DIN rail soft start or soft starter and start up current limiter for transformers up to 1000W.
or this:
Same day despatch on Soft Starts parts | RS Components
although some of those may be going a shade overboard for your application :-)

Cheers
Roger

[A_Camera]

About once out of every 10 times I start my CNC router the breaker trips. The reason is the inrush current from my Toroid power supply. Couple Questions:

1. Can I use a UPS (Uninterruptable Power Supply) as a means to limit the inrush current? or will I just fry the UPS?

2. Is there a commercial limiting device made just for this application? I saw one DIY circuit design on instructables...

3. What are you others with Toroid Power Supplies doing to avoid this problem?


Thanks!
Matt

The task of an UPS is not slow starting, and it is also a very expensive solution. I have an UPS but that's to prevent short mains interruptions which happens once or twice a year.

I have a soft starter like this:

DIN rail soft start or soft starter and start up current limiter for transformers up to 1000W.

...and that prevents the current rush and the circuit breaker trips.

[handlewanker]

Hmmmm........possibly I have not made the situation clear (in my case0.....the transformer I mentioned is a conventional E I type frame with a 240 primary and a 110 volt secondary and a capacity of 50 amps or more.

When it switches on, with no load on the secondary, the lights dim and the breaker trips......in rush situation etc.

The above mentioned inrush limiter at 1000 watts is nowhere near the capacity to allow the transformer to get on line as the mains supply sees the transformer as a dead short to earth and in throwing the amps from the grid at it trips the breaker accordingly as it's been designed to do.......the trannie has to supply at least 2,000 watts for the steam oven it is going to supply.

BTW......the transformer is in good nick and isn't shorting to earth etc.,,,,it's just big,...... bought on EBAY for A$25 from a guy who had a house full of ex USA 110 volt equipment

I'm wanting to use it, as a comparative transformer rated at 4,000 watts to power the oven at half rating and a decent duty cycle is over A$400.

I still can't see how a resistor of 100 ohms when cold can pass more current than it's rated at.....240 volts/100 ohms = 2.4 amps.....even my electric griller toaster rated at 1,000 watts only draws 4 amps at start up......it's a basic resistance in another form.

There has to be a current limiting factor when a resistor is in line or the mains supply would go beserk supplying any domestic device rated at a KW or less when the mains breaker is rated to trip at 10 amps on a 240 volt supply.

That is what I'm basing my assumption on....the low current availability build up when a resistor is in series with primary live wire to the transformer.

Am I not seeing the current flow/resistance statistics right?

A 1Kw electric fire Nicrome element has a resistance of 58 ohms and will only draw 4 amps at start up.
Ian.

[RCaffin]

I still can't see how a resistor of 100 ohms when cold can pass more current than it's rated at.....240 volts/100 ohms = 2.4 amps

Short answer - it can't.
BUT that assumes the resistor stays at 100 ohms. An NTC resistor is designed to get hot when in use. 240 V @ 2.4 A => 576 W. So the NTC resistor quickly gets hot when you switch the power on, and its resistance drops to maybe 1 ohm over maybe 1 second. That means the current into the transformer starts at under 2.4 A, which is perfectly reasonable for a GPO, and then ramps up as the NTC drops in value.

But all this is hiding the real problem, or what is really happening.
Imagine you turn the power on just as the AC voltage crosses zero.The current through the transformer will ramp up as the voltage increases, but it will be limited by the inductance of the transformer. There will be a bit of a surge, but not too much. OK.
Now imagine you turn the power on when the AC voltage is at a peak. You have just put 380 V across a very low resistance plus an inductance. Instant current spike of huge size - and the breakers trip.

So you have several goes at turning the power on, and most times the breaker trips. That's because your chances of switching when the voltage is near zero are small. Eventually you luck out.

Incidentally, this reminds me that there is another way you could possibly try switching the trannie, but it is less cheap. You could try using a 'zero-crossing Crydom relay'. This is a solid state device which only switches as the voltage goes through zero. Useful, but not cheap. Frankly, I think the soft starter would be your best bet.

Cheers
Roger

[handlewanker]

Hi Rog.....correct me if I'm wrong........If I connect a single 100 watt light bulb to the 10 amp supply, the light comes on immediately.......low resistance at start up so no impedance to current flow that consumes .42 amps when fully lit up.

Justfor an example, if I connect a string of 100 X 100 watt light bulbs in series to the 10 amp supply the breaker does not trip but the light bulbs only glow dimly.........that indicates to me that the combined resistance of 100 light bulbs now passes so little current when in series that the breaker will not trip even though the load is quite huge .

If the light bulbs are wired in parallel the supply only see the resistance of a single light bulb but the current flow can trip the breaker because the combined wattage is now 10,000 watts which needs a current flow of 41 amps to light them to full brightness and so the breaker trips.

If the breaker holds with a single light bulb then my assumption is that the amps are flowing slowly through the resistance and any load connected in series with the single light bulb will come on load slowly too as the current is so restricted.

The single light bulb cannot blow up as it cannot draw enough current to do so, but it will restrict the current flow which is my assumption for a current limiter to the trannie.

The trannie in series with the light bulb is a virtual neutral source until it's primary is saturated and up to 240 volts, whereupon the light bulb then ceases to glow and goes out........it will come back on again if the secondary of the trannie is loaded and causes the primary to pass more current which is limited by the value of the 100 watt light bulb, so the light bulb, once it has lit up and the trannie is on line, needs to be switched out of circuit with a make before break switch or relay.

I would think, in place of a manual switch, a Triac across the light bulb contacts etc, can have it's gate with a suitable value resistor to enable it to switch when the current rises in the primary and thereby switch the light bulb out of circuit.....the Triac needs to be of a high enough value to pass the amps that the primary can draw......10 amps in this case.

Is there a flaw in this assumption?

The toroid is also a transformer but wound around a circular iron circuit instead of the E l lamination version.

BTW.....what in effect is a soft starter....IE, what is the design theory......how does it work to slow the current flow/
Ian.

[RCaffin]

Hi Ian

Any resistor will limit the current thru a trannie, but it wll need a relay across it to get full power to the trannie. This is what an NTC does: it effectively shorts out in about 1 second as it gets hot. All in one neat little package.

The problem with toroids, as opposed to EI trannies, is that the stray inductance of the toroid is much lower, so the current spike will be much bigger. The compensation is that the toroid has lower losses.
Cheers
Roger

[handlewanker]

Hi Rog.....a thermistor......hmmmmmm.....I've worked with a number od those in various small sizes mainly in temperature sensing devices.

The problem I see with such a device for this purpose, which is current limiting, is it will always be in circuit and if it's hot in the process then it's a possible fire hazard I don't want to have.....perhaps over cautious on that score, but there is a large amount of amps in the picture.

True, a wire wound type resistor ....whatever.....also gets really hot,.....for a short period of time..... but as I proposed to switch it out of circuit when the pot boils........that is how I envisage it.

If the thermistor also gets hot.....that's no problem as it can be switched out too when it's done it's job.....just need the right value of resistance when cold to apply for this purpose.

A 100 watt light bulb is not an elegant solution, just an example, as it's quite large and fragile, also currently in short supply with the LED revolution.........maybe a 500 watt quartz halogen light source would be more practical as it's very compact and comes in a pencil shaped form about 150mm long.

BTW, we have a large toroid on the SVM-0 mill but it doesn't affect the mains at all on power up, even on a 240 volt 15 amp supply......the toroid the OP has must be pretty big or his breaker is a low amp one......a lot will depend on how much the secondary is drawing at switch on......also if there is any other current using devices on the same ring main circuit......mine is on a separate 15 amp supply line.

If that is the cause of this problem, then a separate supply line and breaker is needed specifically for the mill alone.
Ian.

[RCaffin]

Yes, an NTC gets hot, but not THAT hot. I have them in the attic on what incandescents I have left, to prevent burn-out at switch-on. They work fine, with the old light globes lasting 2 - 3 times as long.
BUT - use a ceramic terminal block, not a plastic one, just for safety.

100 W light globes are just the WRONG idea for this. They have positive temp coeffs: they get a higher R as the temp goes up. That is the OPPOSITE of what you want.

Both NTCs and PTC can be used for temperature measurement, to very good accuracy too.

we have a large toroid on the SVM-0 mill but it doesn't affect the mains at all on power up, even on a 240 volt 15 amp supply..

Ha - better look inside. There may be an NTC in there already. That would be quite normal for a large toroid. You might have seen it and not recognised it.

Cheers
Roger

[handlewanker]

Ha....now you have me worried.......a virtual sausage warmer in the cabinet.......I'd better check to see if any wiring is in a near proximity.

Judging from what you say, the device will only get hot after the initial switch on....if it is there and in circuit all the time,.

At switch on it should only get a quick warm up as the amps surge into the toroid and then cool off, and then warm up again as the amps flow with motor use etc.
Ian.

[RCaffin]

Don't worry, be happy ...
But yes, you have the idea. It won't be very hot.
Cheers
Roger

[handlewanker]

Thanks for the enlightenment.....never too old to learn....LOL.

BTW, when the smaller 110 volt trannie I have for the oven comes on it gives a loud THUNK sound.....that's a sign it's not current inrush protected.

The SVM-0 also gives a BAROOMP-CLUNK sound at switch on which would indicate there's no inrush device attached to it's toroid either......not that it needs one I think.

According to the label on the side of the toroid it's rated at 400VA.....220 volts at 1.8 amps?????.....seems small for a toroid that size.....probably rated at half for longevity.
Ian.