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Old 27th February 2008, 10:24 PM   #1
zdr is offline zdr  Belgium
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Default Toroid inrush current testing

I decided to play a bit with inrush current. I implemented earlier blindly softstart circuit for two 230VA toroids with 4x10mF behind, thinking that they must need it. I have 16A automatic circuit breaker on the outlet where amp is connected, so I went ahead and bypassed softstart. Bottom line, I could not make breaker jump. I then placed another breaker of 10A in series and still could not jump anything at all. I presume that 2x230VA is not exactly equivalent to 1x460VA in terms of inrush current, but they are probably well over 1x300VA.

Anyone has some inrush current data measured for various trafo sizes?
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Old 27th February 2008, 10:30 PM   #2
BWRX is offline BWRX  United States
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Someone may have, but the easiest way to test it for yourself is to place a current sense resistor in series where you want to measure the current, then measure the voltage across it and use Ohm's law to calculate the current. You will need some kind of data acquisition unit or a storage scope that can capture the voltage across the resistor over time.

You must be very careful if you plan on working with mains voltages, and it is not recommended unless you fully know what you are doing
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Old 27th February 2008, 10:33 PM   #3
zdr is offline zdr  Belgium
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I could, but the measurement would be heavily affected by DM speed, and would probably miss the main value - the peak. Most of all, it would be affected by my slow hand writing of the values
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Old 27th February 2008, 11:23 PM   #4
BWRX is offline BWRX  United States
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Exactly, there's no way you could catch that by hand even if your meter were fast enough, so you need a scope or data acquisition unit. Or, if you're feeling adventurous, you could rig up a peak hold circuit to capture the maximum voltage across the resistor so you can figure out the peak inrush current.
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Old 27th February 2008, 11:38 PM   #5
AndrewT is online now AndrewT  Scotland
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Hi,
a 10A MCB passes the equivalent of 2300VA indefinitely.
That is no protection for a 230VA transformer that is going faulty.

If your 230VA transformer were the equivalent of a resistive load then at maximum power it would run indefinitely on a 1A fuse.
But, it won't start up on a 1A fuse not even a T1A (=slow blow = time delayed).
The usual rule for inductive loads like motors and transformers is to use a fuse of about 3times the maximum current rating. That would need around T3A for your single 230VA transformer. Theoretically a pair of transformers would require T6A. That's a big fuse and a lot of fault current to pass before the fuse even thinks about blowing.

Close rate your fusing and fit a soft start for better protection.
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Old 28th February 2008, 02:00 PM   #6
zdr is offline zdr  Belgium
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I was not after the protection of toroid. I was under impression that softstart is mainly to avoid <=10A MCBs to jump, and mine is not doing it... Toroid manufacturers always suggest softstart for 300VA and above for that reason, so I wanted to see for myself. I will get my new 500VA soon and will put on the same test as well. I just remembered that my DMM had some sort of peak memory, so I might capture something useful with a shunt.
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Old 28th February 2008, 02:12 PM   #7
pooge is offline pooge  United States
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A softstart may also help keep you switch contacts from fusing together.
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Old 28th February 2008, 03:56 PM   #8
AndrewT is online now AndrewT  Scotland
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It seems that in my effort to give information I missed stressing the main point.
Do not rely on the 10A MCB to protect the downstream cables and equipment when they become faulty.
Aim to use close rated fuses that allow correctly operating equipment to run without false or nuisance blowing of fuses.
for a 230VA transformer this is of the order of 1A (and could be much less if the amp is only 100W).
If it won't start on 1A then choose whether to abandon close rated fusing, but make this an informed decision, not just trial and error until the MCB stops triggering.
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Old 28th February 2008, 04:32 PM   #9
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A toroid is a huge inductor. Inductors by their very narture resist changes in current. Why do you think the toroid has an inrush problem? A transformer is usually its own soft-start.
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Old 28th February 2008, 04:43 PM   #10
AndrewT is online now AndrewT  Scotland
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no.
the toroid holds the flux (or similar) that it was switched off at.
When next switched on, the flux can oppose the start up current or make it worse or be near zero and give a starting current in between the extremes.

Once the mains has established the core flux then the current returns to normal operation, i.e. charging the capacitors.
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