Power VA and secondary windings.

[davelectronic]

I recently acquired a toroidal transformer of single primary and single secondary. The numbers are 100VA at 30 Volts AC, for the use i have in mind, i need a voltage under 20 Volts if possible. The current is 3.3 Amps as the transformer is at 30 Volts at 100VA, i think that right. It looks reasonably easy to remove secondary windings to drop the voltage to something i can make use of. Would i be right in thinking that the power rating would increase as a result of lowering the secondary voltage ? Or is it the case that the power rating would remain the same, regardless of removing secondary windings. I'm asking as although the voltage is lowered, but the secondary wire gauge remains the same. So only design power rating remains the same, but with reduced secondary voltage  ?
Thanks for reading, i have googled this in a few different key words, but can't find anything specific to the question above.
Thanks again, thoughts on this appreciated.

[Zero999]

Reducing the number of windings on the secondary will not increase the power rating.

If you rewound the secondary with thick enough wire to take over 5A, then the power rating would remain the same. If you simply unwound part of the secondary winding, then the power rating would be reduced,  since the secondary winding will not be rated to carry the extra current, so you'd be looking at 20V at 3¹/₃A, which would be power rating of 66²/₃VA.

[madires]

When the OP removes 1/3 of the secondary windings he also reduces the wire's resistance by 1/3.

[GromBeestje]

When the OP removes 1/3 of the secondary windings he also reduces the wire's resistance by 1/3.

In total, yes. But the resistance per centimeter will still remain the same, so the wire itself will not be able to handle more current then it did before.

[wraper]

Power rating will remain the same at best. That's given that wire used for secondary winding have higher cross section than minimum needed, so you can safely pull more current at lower voltage (not exceeding total power rating). If wire thickness is bare minimum for rated current, power rating will decrease as output voltage will decrease but you cannot pull more current.

[Gyro]

The winding would no longer be evenly distributed either. The remaining section would be 'bunched' so the adjacent turns wouldn't be able to dissipate heat any better than they did before.

[madires]

When the OP removes 1/3 of the secondary windings he also reduces the wire's resistance by 1/3.

In total, yes. But the resistance per centimeter will still remain the same, so the wire itself will not be able to handle more current then it did before.

I thought the transformer's maximum temperature drives the selection of the wire gauge. With P=V*I and R=V/I we get P=R*I². To keep the power dissipation the same: R * (3.3A)² = 2/3R * I² -> 3/2 * (3.3A)² = I² -> I =4A. The maximum current changes from 3.3A to 4A after removing 1/3 of the windings.

[davelectronic]

When the OP removes 1/3 of the secondary windings he also reduces the wire's resistance by 1/3.

In total, yes. But the resistance per centimeter will still remain the same, so the wire itself will not be able to handle more current then it did before.

I thought the transformer's maximum temperature drives the selection of the wire gauge. With P=V*I and R=V/I we get P=R*I². To keep the power dissipation the same: R * (3.3A)² = 2/3R * I² -> 3/2 * (3.3A)² = I² -> I =4A. The maximum current changes from 3.3A to 4A after removing 1/3 of the windings.

Thanks for all your replys. That looks promising,  a drop of 10 Volts or there abouts would be fine. Having the same current of 3.3 Amps would have been ok, but a 1/3 winding removal, for an increase of 0.7 Amps and a total of 4 Amps would be a bonus.
I only wanted to drop the voltage and close the input to output for a linear voltage regulator. 30 Volts was just to high for my needs, and heat dissipation would be unreasonably high.

[Zero999]

I wouldn't recommend drawing more current than 3.33A from the secondary, after removing a third of the winding. Although the power dissipation in the rest of the transformer will be reduced, the power in the secondary winding itself will be increased by a factor of 4²/3.33² = 1.44 and to make matters worse it will be spread over two thirds of the volume.

[davelectronic]

Unless i rewind it evenly with the requisite turn for the lower voltage. Unlikely to be honest, I'm happy with the 3 Amps maximum at a reduced voltage. At least i can make use of it, rather than it collecting dust.

[Brumby]

In reducing the secondary from 30V to 20V, you can still pull 100VA from such a transformer - provided the gauge of the wire used in the secondary is capable of handling a higher current.

Temperature rise should also be considered carefully in doing so - since excess heat is one of the foremost causes of failure in components (from my experience).


If you really want to maximise the current output capability to 5A @ 20V avoiding thermal and current problems, you can add another secondary winding of your own and connect it in phase and parallel to the shortened one.  Just make sure you use the number of turns to exactly match the voltage of the (shortened) existing secondary and use a wire capable of carrying around 2A or so.

[T3sl4co1l]

The offset in primary winding power dissipation will be enough to take, eh, maybe 7% more from the secondary.  (~30% reduction in VAs --> ~15% reduction in primary dissipation, ~7% increase in current because (7% increase)^2 ~= 15% increase.)

There is a nonzero gain available... but it's nothing important.

Reminder that, if you're connecting this to a FWB and cap-input filter, the power factor sucks (~0.6 at best) so for say 3A RMS current rating, you can only draw maybe 1.8A DC.  (This partly adjusts for the fact that output DCV is higher than ACV RMS, but it's worse than a one-to-one trade so you are losing some capacity.)

Also consider using a buck transformer: get a ~40V transformer, wire it to subtract from 120V so you're supplying 80V to the primary, and you get 20V out of the 30V secondary.  (Or 80V for 240V mains.)  This transformer only needs to handle the bucking VAs, so it can be smaller than the main transformer is.

Though, not much smaller in this particular case.  It's definitely a more useful strategy for small changes.  Also needs an oddball 40V transformer, though 36 is probably common enough.

Note that, although the current rating is not changed in this configuration, you still have all the impedance (DC and AC resistance, and leakage inductance) of the full-voltage transformer, but its drop is a bigger fraction of the total: in other words, regulation will be about 30% poorer.  If your load expects a stable voltage (say +/- 10% over full current range), you will be further restricted to a proportionally smaller current range as well (i.e. 20V 2A).

Upside to the bucking approach: core loss is much lower, so the transformer runs very cool and draws less idle power.  This is just about mandatory to reuse highly-saturated designs like microwave oven transformers (which are still perfectly good for say 300 VA when rewound with a safer secondary voltage).

Tim

[davelectronic]

Some very interesting facts there, things i didn't think of or take in to account.  Thanks for giving these stats a mention, if its not a ridiculous amount of turns, i might consider just rewinding the complete secondary instead of just part secondary windings removal. The only slight off putting is the closed loop of the transformer makes it harder to wind than an I E bobbin of a chassis laminated transformer.

[Gyro]

Ha, yes, it can be a pain. You can construct a long thin shuttle (basically a strip of wood with a notch at each end to hold the wire) to thread through the core.

Last time I wound a secondary with thick wire though, I ended up doing the winding while my wife ran backwards and forwards from the other corner of the room with the other end to avoid kinks! 

It's surprising how quickly the wire length builds up, make sure you measure correctly to ensure that you allow enough.

[Gyro]

Also consider using a buck transformer: get a ~40V transformer, wire it to subtract from 120V so you're supplying 80V to the primary, and you get 20V out of the 30V secondary.  (Or 80V for 240V mains.)  This transformer only needs to handle the bucking VAs, so it can be smaller than the main transformer is.

...

Upside to the bucking approach: core loss is much lower, so the transformer runs very cool and draws less idle power.  This is just about mandatory to reuse highly-saturated designs like microwave oven transformers (which are still perfectly good for say 300 VA when rewound with a safer secondary voltage).

A little OT, but why does everyone refer to (and configure) these transformers as bucking transformers. To me, it makes much more sense to wire them as autotransformers instead. It doesn't need to be any bigger, the load current still only runs through the secondary winding, but the voltage across the primary is lower, reducing core temperature and losses there too.

[T3sl4co1l]

Good point, autoformer also gives lower impedance -- not much difference (though still in favor of auto.) for small changes, but would be worthwhile here.  You'd use a 120:60V transformer (probably easy enough to find?) and wire them in series for 80V primary and 40V secondary, thus giving the requisite 80V to the larger transformer.

Tim

[davelectronic]

Ha, yes, it can be a pain. You can construct a long thin shuttle (basically a strip of wood with a notch at each end to hold the wire) to thread through the core.

Last time I wound a secondary with thick wire though, I ended up doing the winding while my wife ran backwards and forwards from the other corner of the room with the other end to avoid kinks! 

It's surprising how quickly the wire length builds up, make sure you measure correctly to ensure that you allow enough.

I did get the misses to do that with a MOT, i wanted as many turns on it as i could manage. She left me back earlier this year... maybe it was winding that transformer. The long thin shuttle is a great idea, as long as it fits through the centre, and built up winding slowly shrink the centre. But done carefully to factor in measurements, its doable with all but the heaviest gauge wire.

[Gyro]

Sorry to hear that.   I've always thought crappy MOTs were more trouble than they're worth!

It would be fun to try and mimic the way commercial automated toroid winders do it - a gapped ring shuttle that slips over the core and is then rotated and filled with wire. Once full, it can then wind the wire onto the core. Being a large diameter, it can hold loads of wire and still fit through a small centre hole and it avoids the inevitable wire folds that occur at the end of a 'stick' shuttle.

Of course, paying out the wire at a fixed tension while rotating the ring shuttle at a slower speed than you're winding would probably result in an almighty tangle! It might be doable though.

[davelectronic]

Yes I've watched a few toroidal transformer winding videos on YouTube, fascinating how it works. I've got two MOT's laying around, one that's wound gives a solid 14.70 Volts. The other is stripped of its secondary, but unwound at the moment. They run very hot, the wound MOT i used silicone sheathed wire, and shrink wrapped the entire length that was wound on the core. I like the idea of picking up cheap toroidal transformers, and possibly rewinding the secondary with a usable voltage. Its a shame enamelled copper wire is so tough to wind on the core.
The transformer that i started this thread with, i might attempt to rewind the whole secondary with a slightly heavy gauge wire.

I would only go up one gauge for that, the idea of taps also sounds like something to try a some point in the future. Recently i purchased some isolation varnish for pcb and transformers / inductors etc. The data sheet for the varnish has a 27 page saftey process conformity of rules to follow. You would need less protective gear on a trip to Chernobyl, the mention of carcinogens and more in that product data sheet.

[Jwillis]

MOTs are built to be as cheap as possible. Core area and copper area is kept as low as possible to minimize materials . Because of this they run very close to saturation resulting in high core losses so they run hot . The primary side has nowhere near the amount of copper needed for an ideal transformer resulting in high copper loses also resulting in extra heat. This is why there s a forced air fan inside the microwave ,just to keep the transformer and magnetron cool. If your just mucking about using the original primary is fine but expect  things to get warm. If you want to rewind a proper transformer then both the primary and secondary need to be wound according to the core size and material used for the core .A typical MOT core will have at most 50-60 VA from a 500-700 watt microwave and maybe 80 -100 VA from a 1200W microwave oven.But only if the primary is wound correctly to minimize copper losses.About the only thing that's any good on a MOT transformer is the core it's self .
For your toroid of 100Va I'm guessing the outside diameter is around 4 inches. I'm guessing of course but your looking at around 60 -70 turns on the secondary which works out to around 7- 8 feet of wire.
Now you can rewind the secondary but you also need to consider the primary winding as well . As you load the secondary the primary will load as well. So the wire on the primary side also needs to able to handle the extra load. The current draw on the primary is proportional to the current draw on the secondary. Now the wire on the windings won't suddenly burst into flames if you reduce the voltage on the secondary side for more amps but it will get a bit warmer. After all your only talking about 2 extra amps. I've done this to a few myself because I couldn't find the correct secondary voltage.
Wire will handle much more current than it's rated for. Usually the rated current handling of wire is around 10 - 15 % the fusible rating . This keeps the transformer at spec for a class B which is around 25 degrees C . You can check the fusible rating of the wire online if you know the gauge. 
If your only going to draw the original amount of current then I wouldn't worry at all. Ideally you only want to draw at most 80% of the rated current potential of the transformer at 20 degrees C ambient anyway . This allows a little overhead for ambient temperature changes without the need for active cooling.
Antek has very good quality transformers at reasonable prices but living in the UK the shipping may be prohibitive. I have a couple of these .One 800VA and a 1000VA and I can tell you they really kick out the current and stay really cool.   

[davelectronic]

The MOT was just a bit of fun, just to see if a linear power supply with series pass trasistors would be possible. It worked ok, but still very warm, and power hungry. I would imagine the MOT primary as stock is not to disimilar to a heating element due to few windings. But they do the job inside a microwave oven.

I've read a fair bit on isulation class, and other transformer characteristics. Finding a feasible method for rewinding is a bit of a challenge,  mainly due to enamel copper wire is hard to work in small areas. UK prices on large power transformers isn't terrible, not cheap, but shop round and there are a few reasonably priced chassis IE and toroidal transformers.