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Old 06-08-2012, 10:19 PM   #1
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Stepping up voltage


what happens when you hook a 208v/480v 75kva transformer up to 240v? 3 phase delta

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Old 06-08-2012, 11:09 PM   #2
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Stepping up voltage


You get 554 volts output. (480 times 240 divided by 208 since the output voltage is proportional to the input voltage.) Practical uses are limited because 554 volts is not a standard voltage.

Since the primary winding is rated for 208 volts, not 240, it may burn out from the "overvoltage" before you reach the 75 KVA draw.

(added later) The difference between 208 and 240 volts is not that great. Transformers may not be overvoltaged by larger amounts, for example you may not take a 120 to 12 volt transformer and connect it backwards to step up 120 volts to 1200 volts. Here the 12 volt winding might burn out immediately from the 120 volts given it even with no load drawn (drawn from the 120 volt winding now used as the output). It's the winding voltage rating, not the fact you turned the transformer around backwards, that leads to burnout at well under the transformer rating. Overvoltage of up to 20% is probably safe.

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Last edited by AllanJ; 06-10-2012 at 08:05 AM.
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Old 06-09-2012, 02:18 AM   #3
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Stepping up voltage


It's possible that the transformer may experience "core saturation", where the excess voltage causes the steel core to absorb more than the maximum magnetic field it can handle. The result is excess current draw and overheating of the transformer.
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Old 06-09-2012, 06:39 AM   #4
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Stepping up voltage


Thanks. I was planning on reverse wiring, still overload right?
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Old 06-09-2012, 07:27 AM   #5
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Stepping up voltage


Quote:
Originally Posted by Torchwork View Post
what happens when you hook a 208v/480v 75kva transformer up to 240v? 3 phase delta
240 3ph?

Like mpoulton said, core saturation is a possibility....

But also be aware of the current.....your going to need a lot of current at 240Vac to drive any kind of 480v load.

Sounds like your trying fit a square peg into a round hole....
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Old 06-09-2012, 07:37 AM   #6
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Stepping up voltage


Yes this is a square hole round peg situation.
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Old 06-09-2012, 11:27 AM   #7
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Stepping up voltage


As noted, the core will saturate. This will cause excessive heating and it'll hum a lot more than usual.

In time, the core will begin to delaminate and the hum will turn to more of a buzz. Then, it'll develop a turn-to-turn fault in the windings, rendering it useless.

The maximum heat will occur at no load, diminish somewhat as load increases and pick back up again with more load.

The voltage taps are on the 480 side, so they won't do much good.

Also, the initial current surge (also known as core charge) will be huge. It would very likely trip a 200 amp breaker and maybe even a larger one.

Rob
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Old 06-09-2012, 11:46 AM   #8
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Stepping up voltage


Thanks for the help guys. A couple of you have already helped with another post about wiring up the kiln I have. I don't need full power, just want to get as close as possible. I think I'm going to go with the 240v to 575v transformer. I won't have to reverse wire this one and if the power company can't handle it then I'll deal with that when it comes. Might need a new meter put in I guess.
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Old 06-09-2012, 04:55 PM   #9
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Stepping up voltage


Have you considered a VFD? Yes...the modern VFD's can be used to drive a resistive load.....
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Old 06-09-2012, 06:14 PM   #10
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Stepping up voltage


I need 600v 3 phase 60amps from 240 3phase. Or close to it.
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Old 06-09-2012, 06:37 PM   #11
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Stepping up voltage


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I need 600v 3 phase 60amps from 240 3phase. Or close to it.
That works out to about 150A for your 3ph 240Vac supply

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