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Old 11th July 2003, 04:10 PM   #1
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Default Transformer surge current

Is there any way to find out the power on surge current of a torodial transformer?
I have a1.5KVA with 23,000uf per rail and sometimes when I power it up the breaker trips.

Also next month I plan to build a power amp that will use two 1.5KVA toroids, will I have to use a soft start circuit?

Thanks in advance
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Old 11th July 2003, 04:37 PM   #2
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best way is with a peak hold function on a digital ammeter
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Old 11th July 2003, 04:48 PM   #3
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Measure the DC resistance of the primary. Take top voltage 160/310 volt divided by the resistance. This gives the absolute maximum and is in fact not far from the real value! = huge current!

Your problem isn't the 23000 uF, it's the toroid.

You MUST have a soft start and I recoemmend that you have 20-100 ohms (10-50 watts) in series for 1 second and shortened by a relay. It's important that you have fast reset of the circuit otherwise a short glitch on the mains power can make your fuse blow.

Your whole problem is about building up magnetic energy in the toroid. If you switch on the toroid at top voltage yopu will saturate the core and this lasts for a while.
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Old 11th July 2003, 07:26 PM   #4
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Default Crazy current

Hey peranders,
I tried your suggestion, the resistance of the primary is 0.2 ohms.
So 160/0.2 = 800 amps that is some serious current.

So each time I turn on the transformer I pull 800 amps from the mains.

Are you sure this is right?
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Old 11th July 2003, 07:31 PM   #5
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No, not exactly but one time of ten according to my experience. But you will get huge currents statisticly rather often so you will need a soft start anyhow.

I guess your start current will be close the short circuit current in your wiring, not 800 A maybe, but much!

I got 77 A with a 500 VA toroid at 230 volt and no load at all. This was measured with a current probe (hall sensor) and oscilloscope.

The primary resistance was 2.5-3 ohms, don't remember the exact value.
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Old 13th July 2003, 12:05 PM   #6
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Quote:
Originally posted by lawbadman
Are you sure this is right?
Quote:
Originally posted by peranders
I got 77 A with a 500 VA toroid at 230 volt
In an article a few years back on soft start circuits in Audio Amateur magazine an American author was making measurements on a 120V toroid with a hall-effect probe and could routinely overrange it. The probe was rated at 100 Amps full scale.

The currents can be quite large at swich on but only for the first 4 or 5 cycles and decrease exponentially.

A soft-start circuit is highly recommended. Several designs have already been discussed in this forum.

James
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Old 13th July 2003, 12:20 PM   #7
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IIRC, turn on surge is minimum when you turn the amp on right at the moment where the mains is at max voltage (exact opposite to what you would think.... it's got to do with setting up the magnetizing current in the transformer core) so with a bit of thought you could make an electronic switch that turns on power right at the peak of the mains AC wave. That way, the soft start current limitter would have a consistent and relatively easy job. And yes, turn on surge with a large toroidal transformer can commonly be in the range of 100 Amps or more.
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Old 14th July 2003, 10:52 PM   #8
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> the resistance of the primary is 0.2 ohms. So 160/0.2 = 800 amps that is some serious current. So each time I turn on the transformer I pull 800 amps from the mains. Are you sure this is right?

Roughly right.

You should add the resistance of the wires in the wall behind the wall-outlet, all the way back to the generator. However as a first approximation, there will normally be less than 2% voltage drop at 20 Amps (in US residential work). 2% of 120V is 2.4V, 2.4V/20A= 0.12Ω. If you are close to the fusebox it may be less.

So you have from 0.2 to 0.3Ω, 840A to 560A first-cycle surge current.

> I have a 1.5KVA... will use two 1.5KVA

These are really too big for US residential use with a capacitor input supply. Use smaller (higher resistance) transformers, or rig some soft-start scheme.
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Old 5th September 2003, 01:44 PM   #9
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Default Soft Start

In the current issue (Q3/03) of Texas Instruments "Analog Applications Journal" there is a feature on Soft Start and LDO Regulators. The principals applied here can be ramped for your application:
http://focus.ti.com/lit/an/slyt047a/slyt047a.pdf
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Old 6th September 2003, 03:29 AM   #10
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Default Things that go "bump" in the night.

Quote:
Originally posted by AudioFreak
IIRC, turn on surge is minimum when you turn the amp on right at the moment where the mains is at max voltage (exact opposite to what you would think....
That is exactly right, especially if there is nothing connected to the secondary. The dc caps will pull a fair bit but nothing like the tranny.

What happens during normal running is the AC voltage swings from zero to 339v peak (240vac here) and the core magnetising flux (lagging 90 degrees) goes from *negative peak* to zero *at the same time*. Then as the voltage swings down from 339v to zero the mag flux goes from zero to max. Everybody is happy.

Buuuuut... if you turn on the tranny at the zero voltage crossing point of the AC, as it rises normally toward 339v - the peak voltage - the magnetising flux goes from **zero** (not peak negative value as before) to the safe working maximum at the same time. Then... as the voltage continues from 339v to zero the magnetising flux just keeps on going up and up until the core saturates, and then it goes *way* up. Takes quite a few cycles to settle down.

My suggestion? Use an NTC thermistor in series with the primary, and short it with a relay after a few seconds. Thermistors are really rugged, 100% safe for this kind of use, and as they heat up they reduce resistance, making life nicer for the relay. Also, if a momentary power interruption makes the relay contacts open, the thermistor will be cold and therefore high resistance, making for a really yummy restart.
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