Rewiring a Transformer Secondary

[Andrew Leigh]

Hi,

I salvaged a 220V to 24V transformer from a broken UPS. I would like to rewire this to use with chemelec's inverter but need the transformer to be centre tapped at 12V - 0V - 12V . Can one calculate the turns or is it better to strip the winding and keep count?

Am I correct in stating that if I could find the middle of the winding and take a tap off from there that I would then have my 12 - 0 - 12 from a 0 -24?

Finally, what would the permissable current density A/mm2 be for such a transformer?

Thanks
Andrew

 

[ericgibbs]

Hi,

I salvaged a 220V to 24V transformer from a broken UPS. I would like to rewire this to use with chemelec's inverter but need the transformer to be centre tapped at 12V - 0V - 12V . Can one calculate the turns or is it better to strip the winding and keep count?

Am I correct in stating that if I could find the middle of the winding and take a tap off from there that I would then have my 12 - 0 - 12 from a 0 -24?

Finally, what would the permissable current density A/mm2 be for such a transformer?

Thanks
Andrew

hi,
I'm assuming the transformer is the 'EI' former type.

Usually the secondary is the outer winding on a transformer.

I would count the turns as you you unwind the 24V secondary.
When rewinding count half the number of turns as 12V, bring out a loop of the wire [centre tap] make sure the loop is insulated from the windings.
Wind on the remainder of the wire, always wind the second 12V winding in the same direction as the first half.

A rough rule of thumb is 6 turns per square inch of the central iron core.

eg: if the centre iron limb is 1 square inch, its 6t/volt
if the limb is 2 square inch its 3t/volt

Tell us how many turns you count. Also the wire gauge...

 

[Andrew Leigh]

Hi Eric,

Yes it is E&I.

Yup the secondary is on the outside.

The winding unwinding part should be fun. The wire must be 3,35mm in diameter 10 SWG. To make a loop of 3,35mm wire could be an issue. I think I will make two seperate winding and terminate the 0V connection outside the winding, could be neater.

I run a business that makes enamelled wire, I think I will get them to run a couple of meters, of the correct size, through the enameller at a reeeaaaal slow speed so that it annealed to death. That will make winding much easier.

Cheers
Andrew

 

[ericgibbs]

Hi Eric,

Yes it is E&I.

Yup the secondary is on the outside.

The winding unwinding part should be fun. The wire must be 3,35mm in diameter 10 SWG. To make a loop of 3,35mm wire could be an issue. I think I will make two seperate winding and terminate the 0V connection outside the winding, could be neater.

I run a business that makes enamelled wire, I think I will get them to run a couple of meters, of the correct size, through the enameller at a reeeaaaal slow speed so that it annealed to death. That will make winding much easier.

Cheers
Andrew

hi,
Now that you know the turns/volt ratio, you could use a smaller gauge wire to suit the current you require on the secondary.
What do you plan to use this rewound transformer for.?

If you wind two separate coils, keep winding in the same direction. [phasing]

 

[Andrew Leigh]

Hi Eric,

it will be used for a 500W 12V to 220V invertor. Assuming 10% inefficiency I think we will still be 46A or 3,55mm diameter wire.

The truth is I actually only need 160W (freezer element) + 180W (fridge element) = 340W. 500W just sounds like a nice round number, the design I have also has componentry to do 500W. This is not a train smash and I can go down to 350W, however this will only reduce the wire diameter 0,55mm, not significant enough to make winding appreciably easier?

The duty cycle I think will be about 50% and you could have the combination of both being on simultaneously.

Will measure the core tonight.

Cheers
Andrew

 

[chemelec]

Hi Eric,

it will be used for a 500W 12V to 220V invertor. Assuming 10% inefficiency I think we will still be 46A or 3,55mm diameter wire.

The truth is I actually only need 160W (freezer element) + 180W (fridge element) = 340W. 500W just sounds like a nice round number, the design I have also has componentry to do 500W. This is not a train smash and I can go down to 350W, however this will only reduce the wire diameter 0,55mm, not significant enough to make winding appreciably easier?

The duty cycle I think will be about 50% and you could have the combination of both being on simultaneously.

Will measure the core tonight.

Cheers
Andrew

Andrew, If its a Compressor type Freezer, You need to consider my inverter is Square wave output.
** Some Motors Don't like that.

Also you must consider STARTING Current of the motor.
It can be MUCH Higher than Running Current.

 

[ericgibbs]

Andrew, If its a Compressor type Freezer, You need to consider my inverter is Square wave output.
** Some Motors Don't like that.

Also you must consider STARTING Current of the motor.
It can be MUCH Higher than Running Current.

hi chemelec,
This came up in his other post, its a caravan type fridge, no motors, just heating elements.

 

[blueroomelectronics]

I would think winding your own power transformer would be a tedious experience at best. Wouldn't there be hundreds of windings?

 

[ericgibbs]

I would think winding your own power transformer would be a tedious experience at best. Wouldn't there be hundreds of windings?

hi Bill,

Have you seen the wire gauge, its over 3mm OD.!

Andrew also works for a enamelled wire company.