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Old 23rd Mar 2020, 7:15 pm   #1
woodchips
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Default Three phase transformer

Lots of clever people here, try this one.

Need 150V at 25A, as cheaply as possible so not new. Bought some 3ph 400V in to 24V 40A DC out transformers.

Removed rectifiers, split primary into three wye windings similarly the secondary wye windings.

Connect one primary to 230V, get about 12V from the secondary, yes, bit low, not certain why.

Connect two primarys in parallel to increase the flux in the core. But the primary winding on the third leg of the transformer has no, zero, voltage induced in it. Doesn't matter if the energised legs are a centre and outer, or both outers.

This was unexpected, didn't actually consider the problem before powering up, just taken by surprise when prodding around.

Why?

I thought the flux would go along the unused leg, was hoping it would induce a secondary voltage. No secondary voltage, the outer winding, until well into saturation and I think that is radiated induced voltage.

All secondary voltages off load, need to find a large resistor, 1kW.
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Old 23rd Mar 2020, 7:33 pm   #2
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Default Re: Three phase transformer

I am guessing there can't be magnetic coupling between L1 L2 and L3.
Surely if there were it present a short circuit when connected to three phases?
I would think you could treat it as three indepent transformers?
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Old 23rd Mar 2020, 7:44 pm   #3
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Default Re: Three phase transformer

Sounds like it's working as I'd expect. In fact some 3-phase transformers are just 3 single phase ones bolted together.
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Old 23rd Mar 2020, 10:03 pm   #4
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Default Re: Three phase transformer

The low secondary volts in single phase mode suggests that the transformer primary was delta connected giving a primary voltage of 415V.

Putting 240V on such a primary will give a lower secondary voltage than you might expect.

In any case a 3-phase bridge rectifier gives a dc output close to the peak L-L secondary voltage with only about 4% ripple so again, secondary voltages may not be what you would expect.

My three phase transformer theory is a bit sketchy- I only had to design power supplies using them to drive three phase bridges for the raw dc rails. Someone like Ed Dinning should be able to do chapter and verse for you if necessary ......
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Old 26th Mar 2020, 1:58 am   #5
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Default Re: Three phase transformer

Can we clarify details a bit please.

Are there multiple 400VAC to 24V 40A DC output rated power supplies?

Did each of those power supplies contain a transformer along with 3ph diode rectifier (and capacitor filter) and have a 24V 40ADC rating?

Was the transformer a single 3-leg transformer core, with a primary side wye winding (ie. each core leg has a winding, and the three windings are connected at one end with a star connection, and the other three winding ends connect to '400VAC' phase terminals? Did the power supply have a neutral connection, and was that connected to the star connection, or just used for control purposes (in which case the star connection was a floating star)?

How were the rectifier diodes connected to the secondary windings (supposedly three windings on the 3 legs).

I presume the transformer doesn't have a rating plate or any label.
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Old 26th Mar 2020, 10:03 am   #6
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Default Re: Three phase transformer

And you originally stated you wanted 150V at 25A. Is that AC or DC? What is it for, does it need to be isolated?
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Old 26th Mar 2020, 10:07 am   #7
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Default Re: Three phase transformer

What you have is relatively simple to explain and indeed Herald 1360 has more or less provided an accurate response, I refer.
Most three phase transformers are connected Delta/Star in which the primary Voltage is the phase Voltage and in this case will be 415 Volts. That is, each transformer winding will provide its nominal output if 415Volts is applied to the primary.
If we apply a 1ph 240V supply to the primary the output Volts will be 240/415 x 24V = 13.88V approx at no load.
We must then assess the VA requirements that you are seeking namely 150V x 25A which is 3.75kVA which is substantially in excess of the output of the transformer you have stripped down. In this I am referring to its current nominal output of 24V/40A DC output = 960VA.

The latter would suggest you put the item aside for a rainy day and seek another solution.
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Old 26th Mar 2020, 11:15 am   #8
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Default Re: Three phase transformer

Thanks for the comments.

Both primary and secondary are three wire Wye connected, so each primary is 230V, each secondary is about 14V.

I have four essentially identical transformers. Each just has rectifiers with some smoothing which I am not using.

The transformer is the normal three leg three phase type, each limb with its own primary and secondary. There is no connection to the star points. Each limb has a full wave rectifier, so six rectifiers in total. Rating plate says 400V +/-5% to 24V 40A.

I thought I sort of understood 3ph transformers and motors, taken a bit of a dent! No books I have, 200 or so, think anyone would be silly enough to do what I am trying to do. Just energizing one limb and I get voltages on the other two, about 2/3 on one, 1/3 on the other, which is understandable. Connecting any two limbs in flux series aiding works happily. Run from a variac to stop any unexpected blue smoke and the result takes about 0.2A magnetizing etc current at 230V. But it is the total lack of any voltage induced in the third limb that took me by surprise. Read books on magnetism and they talk about leakage across the air gaps, I assumed that having a lump of core would increase this leakage many times, nope.

I am making a static single to three phase converter rated at 8kW, the transformer I am using isn't really up to it so bought these to boost the power. Need the 150V to add to the generator 230V to get 380V for the motor. Because the 230V in actually provides all the three phase current it is not obvious what the various currents circulating are. Got lots of spare time to think about it at the moment though.
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Old 26th Mar 2020, 11:32 am   #9
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Default Re: Three phase transformer

Quote:
Originally Posted by Bookman View Post
Most three phase transformers are connected Delta/Star in which the primary Voltage is the phase Voltage and in this case will be 415 Volts. That is, each transformer winding will provide its nominal output if 415Volts is applied to the primary.
Referring to just "phase voltage" can be a bit ambiguous. For clarity, a delta input with 415Vac refers to the phase-to-phase voltage, and it is that voltage across each primary winding on each core leg (limb).
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Old 26th Mar 2020, 11:59 am   #10
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Default Re: Three phase transformer

That diode configuration (of 6 diodes) is typically referred to as a full bridge.

Did you energise just a side leg winding when you measured the 1/3, 2/3 ratios?

If you energise two adjacent legs equally and in phase addition, then I'd expect the third limb to effectively have no flux through it. With 230V on two windings in phase addition, the flux level in each energised limb is about 15% more than rated.

Are you saying you need a 150Vrms, 25A supply that will be in phase with a 230V generator supply, to ultimately provide a 380Vrms single phase supply?
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Old 26th Mar 2020, 12:11 pm   #11
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Default Re: Three phase transformer

Quote:
Originally Posted by trobbins View Post
Referring to just "phase voltage" can be a bit ambiguous. For clarity, a delta input with 415Vac refers to the phase-to-phase voltage, and it is that voltage across each primary winding on each core leg (limb).
Agree! To be absolutely clear, I always ask, for instance, the "3-phase 200V supply" is that 200V line-line, or line-neutral? It is so easy to get things wrong by a factor of √3.

Quote:
Originally Posted by woodchips View Post
Both primary and secondary are three wire Wye connected, so each primary is 230V, each secondary is about 14V.

The transformer is the normal three leg three phase type, each limb with its own primary and secondary. There is no connection to the star points. Each limb has a full wave rectifier, so six rectifiers in total.
That slightly worries me - with no connection to star point, either primary or secondary, there's nothing to 'fix' the voltage at the star point.

Connected to a rectifier as load (or indeed, any load where the currents don't sum to zero at every instant), the star point gets 'pulled' one way or another so the winding voltages don't stay at 230V each. Some transformers have a set of isolated identical windings, connected in delta, to prevent this. Others, with either pri or sec in delta will self-balance. And of course if the primary star point can be connected to supply neutral, everything is happy (and unbalance currents flow through this star-point connection).

The conundrum, why is there no voltage on the third winding? Think of it by symmetry. Two identical windings on the straight legs of a stadium-shaped core, one going one way, one going the other, energised in series. Everything happy. Voltage divides equally.

Now add an iron 'bridge' with a winding on it, between the bare, curved parts of the stadium. Assume this winding has a voltage induced in it. What will its polarity be? Relative to one winding, you'd get one polarity. Relative to the other, you'd get the other polarity. The only way to reconcile this, is that both are correct, so the induced voltage must be zero.

No flux travels through the bridge, or the third limb, in this scenario.
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Old 26th Mar 2020, 1:20 pm   #12
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Default Re: Three phase transformer

Quote:
Originally Posted by woodchips View Post
I am making a static single to three phase converter rated at 8kW, the transformer I am using isn't really up to it so bought these to boost the power. Need the 150V to add to the generator 230V to get 380V for the motor. Because the 230V in actually provides all the three phase current it is not obvious what the various currents circulating are. Got lots of spare time to think about it at the moment though.
The transformer in question would seem to be a 1kVA unit that has three secondary windings capable of providing 40A at 24V with a 400V 3ph input.
Your discussions concerning connectivity and Voltage values notwithstanding would need further investigation as clearly if we reduce the primary Voltage by root 3 (230V) we would get 24V on the secondary less the root 3 value which would be 13.8 Volts which is what you are reading?

Regrettably however confirmation of the latter would be academic as there remains one great obstacle. You cannot add or superimpose a Voltage upon another equivalent Voltage without incorporating some form of synchronising device. Rate of change of Frequency (ROCOF) devices may also be desired if some form of external regulation may be required.

You could get a 230V transformer with a 24V secondary to connect to your generator and then feed the 24V into the secondary of the items in question?
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Old 26th Mar 2020, 9:23 pm   #13
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Default Re: Three phase transformer

Quote:
Originally Posted by trobbins View Post
If you energise two adjacent legs equally and in phase addition, then I'd expect the third limb to effectively have no flux through it. With 230V on two windings in phase addition, the flux level in each energised limb is about 15% more than rated.
I mucked that up. If the two outer limb windings were energised, the flux from each would take the shortest path through the centre limb and effectively cancel in that limb.
A 40A DC output from a typical delta secondary with full-bridge rectifier would have each secondary winding rated for about 33A. But not sure about a wye secondary design.
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Old 27th Mar 2020, 12:52 pm   #14
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Default Re: Three phase transformer

Just to clarify things for once and for all, is the transformer as described, wye:wye (star:star in UK speak) with floating star points feeding a 3-phase bridge (6 diodes in 3 series pairs with centre points to the 3 secondary phase connections and the outer 3 cathodes strapped for +ve out and outer 3 anodes strapped for -ve out?

If so, each primary will be 230V line to star point and each secondary about 10.6V line to star point. So one primary energised at 240V line to star will produce about 11V line to star on its corresponding secondary.

From this point it may be possible to rearrange the windings on several transformers to end up with a multiple of 11V from a set of series connected secondaries.

Over to the serious transformer guys now......
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Old 27th Mar 2020, 1:21 pm   #15
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Default Re: Three phase transformer

The secondary winding voltage will depend on who designed the power supply, and what they allowed for mains supply regulation (eg. -5%), transformer regulation (eg. 90%), and diode voltage drop (eg. 2V). If it was a delta secondary, that would give about 22.5V per winding with my design tool, so likely about 13V for a wye configuration.

As far as I can see, with a single phase 230V supply energising two of the primary windings (connected in parallel across the mains supply), that would provide about 26V of unloaded 'secondary voltage' per transformer, so 104V unloaded from four transformers.
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Old 27th Mar 2020, 7:04 pm   #16
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Default Re: Three phase transformer

Not sure I have full absorbed this, but maybe. You have a 3 ph trans with Y connection. Output side likewise, with added three ph rectification.
You have tried energising across two phases and hoped to find voltage across the third, but nothing. Try adding a capacitor AND a slave motor, to produce a simple rotary converter. (If you manually spin it -not advised - you may not need capacitors) I suspect you will now get the sort of output you require.
Seven years ago I developed a 230v to 415v single to three phase converter using a slave motor, without a transformer. You can see in action here:--
https://vimeo.com/130990021
Les.
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Old 28th Mar 2020, 8:44 pm   #17
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Default Re: Three phase transformer

Hi Folks, I've come to this one a little late.

There is no simple way to run a 3 phase transformer from single phase.

The transformers you have look as if they have been used for a low (ish) ripple DC supply that did not use smoothing caps . It is a 3 phase (6 diodes) full wave bridge. Not liked these days due to the poor power factor (current distortion) reflected back on to the mains supply

It would be possible to run these transformers in the manner of simple phase converters with a capacitor feeding the 3rd phase. I'm guessing it would need to be about 20uf and will be very dependent on the current drawn (so could be OK after some experimentation for a fixed current load).

It would be best if you disconnect the 3 secondaries and wired then in series and or parallel BUT TAKING GREAT CARE OF THE PHASING. This may get you somewhere towards what you want (easier if you just wanted DC).

2 windings that are not in phase can be connected in series and will give less that twice the voltage out (assuming equal winding) by an amount depending on the phase displacement (this is used on 3 to 18 phase transformers).

Not efficient, but as you say you have plenty of time to play.

Have fun and don't blow too many fuses,

Ed
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