4000VA Toroid Transformer Problem

[Dr Wattson]

Hi everyone!

This is my first ever thread on here and I've just registered. I must say - this site is excellent and there are some brilliant people on here!

My question is as follows:

I'm buidling an amp who's rail voltage should not exceed +/-60DC for the rails.

I have a 4000va toroidal which has a primary voltage of 230vac.

It has two electrically isolated sets of secondary windings.

Each set has a centre tap and each set has 260vac and 120vac sharing the centre tap.

These secondary sets produce too high a voltage when rectified and filtered. +/- 90VDC unloaded is the lowest I can achieve with the 120vac winding.

What I was thinking was to use the transformer in reverse, by supplying 230ac to one of the 260ac sets of secondary leads.

I'm thinking that this would give me 230Vac/260Vac = 0.8846.

0.8846 x 120Vac = 106Vac (i.e. a drop in the second set of secondary windings by 14Vac).

I'd end up with a primary voltage reduction from 230Vac to 203.5Vac, but I'm not bothered about using that (they would end up being redundant).

This is the diagram of the transformer:

___________________130Vac
| }____________ 60Vac
| {________________ Centre Tap
| }____________ 60Vac
_________{ | {___________________130Vac
230Vac } |
_________{ | ___________________130Vac
| }____________ 60Vac
| {________________ Centre Tap
| }____________ 60Vac
| {___________________130Vac


I'm just wondering if this would work, in theory it should.

I could connect the the 120Vac secondaries in series and then just use the primary ( I think this could be better - the wires all seem to have the same thickness).

All comments welcome - 4000Va is death waiting to happen if I bugger this up!.

[Sch3mat1c]

What are the ampacities of the secondaries?

The diagram didn't work well... try drawing in Paint then attach.

Are the secondaries independent, with common CT, or are the +/-60V taps actually tapped off the 130?

You could also try a choke-input filter, if you don't mind having another rather large piece of iron. This will give you Vdc = Vrms * 1.4 unloaded, but closer to Vrms * 0.9 when loaded, and much better power factor than cap-input (you won't be blowing 8kVA breakers trying to run this thing at full power!).

Tim

[AndrewT]

Hi,
that size of transformer must be using quite few turns/Vac on the secondary.

It might be easy to rewind the secondaries as 40+40+40+40+40+40Vac for as much wire as you get from it. Assumming the secondary has been wound on the outside of the primary.
You can add 6Vac+9Vac+12Vac+15Vac windings for accessories using 0.6mm diam enameled copper wire..

[tinitus]

Originally Posted by Sch3mat1c

What are the ampacities of the secondaries?

With 4kw and 2x 60Vac, that would be something like 2x 30A, I suppose

If 4kw is really true its a huge monster
Hell, I have 2kw trafos, and can barely lift them, but they are not toroids

Anyway, I think the common mains around here can manage 10A with 230V

Where do this monster come from ?

Maybe use SS regulating to get it down
But its high voltage, and with that wattage/amp I think its pretty dangerous to handle
Its adviceable to stay well clear of the supply caps

[Dr Wattson]

Here is a better diagram.

There are two electrically isolated sets of secondary windings. It looks like each set has a shared centre tap.

I checked the two centre taps of the two secondaries with a meter and they isolated from eachother.

I am not sure if the 130Vac lines extend from the 60Vac lines or if they are completely separate but share the centre tap.

When powering up the transformer via (NTC thermisters to limit inrush) and placing the two separate 130Vac windings in series I get 540 Vac.

I decided to put 230Vac through the two secondary windings in series to see what I would get at the primary and at the 60Vac windings.

On the primary I get ~92 Vac

The 60Vac ones now give 50Vac peek-to-peek or 25Vac from centre tap to any of the other two on the same winding.

Could I use the 60Vac windings (now at 50Vac) and still use the centre taps as ground? Remember I'm feeding the transformer from the secondary longer windings. Each centre tap will be 0Vac in relation to the windings they are centred from. but would it be dangerous to earth the centre tap in this mode of operation? has the centre tap now got a percentage of the input ac therfore causing an earth trip if this is done?

Help needed - I'm happy with the voltages I am now getting, the transformer is not getting hot, there is no noise.

If I can not use the centre taps as gnd (and earth) for an amplifier - could I just cap them off and use two bridge rectifiers with two sets of filter caps and create a kind of DC based GND?

Thank you all kindly.

[Dr Wattson]

It came from the inards of an AH4000 amplifier, it was said to be able to supply 4000WRMS into 4 Ohms, inline with the primary is a 20A fuse!

[event horizon]

I can see what you are now on about

However, there could be a bit of a problem that you might not have even thought about. The isolation between the 230V proper primary of the transformer & the secondaries is likely to be rated far higher than the isolation between the two seperate sets of secondaries.

If i was going to do what you are contemplating i'd want to flash test between the two sets of secondaries. That'd involve at least 500V or better yet well over 1KV for at least 60 seconds. If there is any breakdown between the secondaries then your idea just isn't going to be safe

While i think about it you'll need to derate the transformer to 2KW as i doubt very much that each secondary will be happy doing near 4KW of power...

[Dr Wattson]

The transformer total is 4000VA, the two sepate sets of secondaries powered a 2000WRMS amplifier module each.

As far as I gather, the amp modules ran from the 60VAc lines and when the extra amplitude was required som IRF640s would rail switch to the higher secondary lines instead.

My way of thinking is that when I place 230 Vac on the primary I get 260Vac from one of the higher voltage secondary windings. So placing the two secondaries in series I would arive at 540 Vac. I instead place 230Vac into the secondary windings that are in series and arrive at 100Vac at about 40Amps at the primary.

The highest operating voltage in the transformer would be 230Vac in this way so there shouldn't be any problems with the insulation.

Thinking about it the higher 260Vac secondaries would be longer than the 230Vac primary windings so I think the mass and wire thicknesses still balance out?

[tinitus]

Hmm, maybe the 130V secondaries are used if the primary is connected to 120V mains

Have you measured with ohm meter, to be sure how they are connected
Those multi tapped trafos can be quite confusing

I suppose the AH4000 would be in bridged mode to deliver that much
Maybe that kind of wattage is very theoretical

btw, then its a special double trafo, with 4x 15A, for a double mono supply, or double stereo, or double bridged stereo

But if that AH4000 is a 2x 1000watt stereo amp, each secondary could just as well be 2x 10A
With 4x 10A and 4x 60V it would give you 4x 600watt secondaries, and 2400watt total
Just another theory

[Dr Wattson]

Originally Posted by tinitus

Hmm, maybe the 130V secondaries are used if the primary is connected to 120V mains

Have you measured with ohm meter, to be sure how they are connected
Those multi tapped trafos can be quite confusing

I suppose the AH4000 would be in bridged mode to deliver that much
Maybe that kind of wattage is very theoretical

yeah, the AH4000 needed to operate in bridge mode to do that, unfortunately the amp modules kept blowing up due to a very poor design, the manufacturer got lazey and used big fat dropper resistors to power the logic - the resistors would just turn into the type of heating elements you got in an electric fire!

The amp modules had 16 mosfets each. I tried to repair one of them that had blew up.. and it blew up again. I still have a working module from it but I don't trust it. It's bias system was poorly designed and I found that if the bias trimmer pot had an open circuit full current would rush into the output fets and make them all die.

Very dangerous amp! I was working on it with just the good module working and the bad one left out and one of the 100V 10000uF capacitors exploded!! there was shards of aluminium stuck in the wall and oiled paper shot out the top of the cap like a rocket. Strangely enough, the cap was from the working good side of the amp and it still continued to play music even after that!.

I've gutted the whole amp now and just have the transformer. I plan on making a 4x 1000WRMS amplifier and I need the voltages dropping (which I've done), but need to know if the tranny will operate correctly and if the centre taps can still be used when I'm powering from the secondary higher voltage windings in series.