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Transformer in reverse

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John Rudd:

For the sparkie types among us.....I need a 400v ac supply.....

I have a toroidal transformer with 240v ac pri and 110v ac secondary....
If I feed it via variac on the 110v side to give me around 400v ac on the primary any problems in doing this?  :zap:

chipenter:
Step up step down workes both ways , step up can get hot if you draw to mutch current .

Joules:
I would seriously downrate the transformers capacity if you use it in step up mode.  Your old primary winding was wound with lighter gauge wire.  My guess would be the 110v side will also have a lower impedance than desirable, could result in a high inrush current.  Play carefully. 

John Rudd:
Thanks for the advice guys....

I have a 3ph inverter that obviously needs a 415ac supply, if I can generate 415v single phase then I should be able to test it.... :zap:

I have a test rig that comprises a light bulb in series with the Live leg, then a switch, then a 13A socket, using my variac ( 500w rated) I can gradually wind the juice up using the lamp bulb as a current limiter....ok back to it.... :bugeye:  :zap:

awemawson:
John, if I understand what you are doing correctly you are going to feed the 110 winding with pretty well twice the voltage it was designed for and it WILL draw too much current and over heat.

Although the transformer ratio is about right, when they designed the primary and secondary windings it was done to avoid magnetically saturating the core. If it saturates, impedances drop dramatically. fuses blow and the cursing starts.  :zap:

Some useful guidance here:

http://www.allaboutcircuits.com/textbook/alternating-current/chpt-9/practical-considerations-transformers/


If you don't need the transformer to isolate, then you can connect primary and secondary in additive series (ie start of the 110 to the finish of the 240). Then applying 240 to the 240 winding will give you 240 + 110 = 350 across all windings which may be in the range of operation of your VFD ?

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