Multi-output transformer in series & parallel ?

[mkeen]

connecting Multi-output transformers in series & parallel

hi,

I have two identical transformers with output of :
22.5V@10A,
12V@3A,
11V@1A,

Is it possible to connect the outputs in series and parallel to get:

45V@10A (series)
12V@6A (parallel)
22V@1A (series)

Mike,

 

[Genovator]

Yes, in series. But only voltage will add up. You have to stick with the lowest current rating.
No, in parallel, as transformers are of different ratings. So obviously, they will have different turn ratio and impedance. The trafo with high impedance will provide maximum current for the load, and will get hot. Only identical transformers should be used in parallel.

 

[mkeen]

hi thanks for your reply

but the two transformers I'm using are identical so there is no rating difference.

 

[godfreyl]

Should be OK then.

 

[chuckey]

Because of the low impedances of the windings a small (<100mV) voltage differential between the windings can sen a large current through the loweer voltage secondary when they are wired in parallel. How to proceed, connect one end of each winding to the other, measure the voltage across the two free ends, if its ~0V then OK, if its 2 X the voltage change the ends of one winding. When the voltage is 0V. Change the meter to measure 10A current and see how much current is flowing, then turn the meter to a lower current range. Then make up your mind.
Frank

 

[mkeen]

Because of the low impedances of the windings a small (<100mV) voltage differential between the windings can sen a large current through the loweer voltage secondary when they are wired in parallel. How to proceed, connect one end of each winding to the other, measure the voltage across the two free ends, if its ~0V then OK, if its 2 X the voltage change the ends of one winding. When the voltage is 0V. Change the meter to measure 10A current and see how much current is flowing, then turn the meter to a lower current range. Then make up your mind.
Frank

Thanks,
I will check it out, soon as i get chance to.

 

[FvM]

Because of the low impedances of the windings a small (<100mV) voltage differential between the windings can sen a large current through the loweer voltage secondary when they are wired in parallel.

This can only happen, if the transformers have (at least partly) different windings ratio. This won't happen for identical (same part number) transformers. 100 mV difference for 10 V output is about 1%. Assuming 5 % percentage impedance for both transformers (a higher value would be likely), the circulating current isn't higher than 10% of rated current. Unpleasant, but not dangerous.

 

[mkeen]

I heard somewhere that the transformers shouldn't be placed to close too each other, as it will cause them to heat-up.
is this true, if so what would be the recommended distance between them ?