Toroid inrush current testing

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zdr

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?

BWRX

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

zdr

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

BWRX

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.

AndrewT

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.

zdr

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.

pooge

A softstart may also help keep you switch contacts from fusing together.

AndrewT

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.

xiphmont

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.

AndrewT

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.

xiphmont

AndrewT said:
no.
the toroid holds the flux (or similar) that it was switched off at.

Only true if it's a superconductor and nothing else in the PSU is bleeding energy off. Neither is going to be true in practice.

Power is removed, and resistance and draw pull out all the energy stored in the magnetic field.

e_fortier

Hi,

The inrush current is measured via a shunt resistance of 0.02 ohm connected to a digital scope.

The inrush will depend on the total primary resistance of the unit, if your transformer has 2 primary windings wired in parralel for 120Vac then theorically will be much higher then for the 230Vac setting which normally uses the 2 windings in series. For 120Vac the overall resistance is 1/4 the one of the 230Vac setting.

Normally you can easily get 80Apk for a 300VA transformer, of course it depends of the primary windings impedance. In order not to dammage your mains switch it is best to get a TV rated (ex. TV-5 or TV-8) switch, these are made specifically to handle high surge peak.

An easy fix is to install a NTC, they are cheap and do the trick.

Regards,
Eric

TheMG

e_fortier said:
The inrush current is measured via a shunt resistance of 0.02 ohm connected to a digital scope.

And not forget to use an isolated probe or connect the transformer you are testing through an isolation transformer!

Bad things happen when a non-isolated scope ground gets connected to mains "hot" lead.

zdr

thanks for all the replies, I guess I will have to make the measurements myself (in order to calculate NTC parameters.

richie00boy

xiphmont said:
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.

As Andrew has said the core has a residual flux. Upon start-up this flux is added to by the action of the current passing through the windings causing the core to saturate briefly until it reaches a kind of equilibrium where normal operation occurs.

Soft-starts are simply essential for the good design and engineering of a product with large power supply. Try it yourself - get a 500VA toroid or bigger and apply power to it whilst it shares the same power outlet as an incandescent lamp. Watch the lamp dim as the inrush current cripples the supply and stresses the switchgear and associated components.

AndrewT

The NTC dominates the start up current.
You don't need to know the transformer only start up current.
The best bit of useful information is the primary winding resistance.

Nuuk

I prefer dual mono supplies because they simply sound much better.

But an added benefit is being able to use two lower VA transformers and not having to worry so much about this sort of issue!

zdr

Nuuk said:
I prefer dual mono supplies because they simply sound much better.

But an added benefit is being able to use two lower VA transformers and not having to worry so much about this sort of issue!

Akhm, I am really sceptical about how dual toroids can sound "much" better, since bigger toroids tend to have better regulation. Moreover, I don't think either parameter would be audible at all, unless if there is a serious manufacturing defect. Well, at least I don't think I could hear it.

Secondly, it seems you are implying that use of two 250VA produces much lower inrush current than 1x500VA? I believe this is only true if you switch 2x250VA manually one at the time, don't think that having them in parallel on the same switch could make a huge difference.

ratza

zdr said:

Akhm, I am really sceptical about how dual toroids can sound "much" better, since bigger toroids tend to have better regulation. Moreover, I don't think either parameter would be audible at all, unless if there is a serious manufacturing defect. Well, at least I don't think I could hear it.

Oh, yes you would. Do this experiment: on a single toroid PS, keep one speaker connected and on the other channel mount an 8 ohm resistor instead. Now put some music or a signal on this channel and hit the volume. Do you hear anything in the loudspeaker? Yup, I thought so. With dual toroids, this will never happen.

peranders

Paid Member
zdr said:
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?
My amp with 600 VA /230 VAC toroid generates 77 A, worst case.

Measure the primary winding, should be 3-4 ohms. Take (230*1.4)/3 = maximum theoretical current

Over 300 VA you will need a softstarter, to spare the mains switch and to let you have a fuse which protects better.

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