dc voltage from 3 phase ac

[mgyver]

our last building had a breakin and the thieves stole a transformer that powers a dc crane.the rectifier is just six power diodes( no capacitors)producing 250v dc (somewhat choppy). our electrician tried feeding 240ac to the bridge but got too much voltage dc.How do we figure out what ac 3 phase voltage we need to put into the diodes to get 250 dc volts out put?

 

[Zogzog]

Check the nameplate on the rectifier. Stole a transformer huh, thats actually impressive.

 

[edison123]

In 3 phase bride rectifier with six diodes, 100 V AC = 130 V DC app on no-load. To get 250 V DC, apply about 190 V AC. That stolen transformer probably stepped your 240 V supply to 190 V to feed to the rectfifier.

But a DC crane ? That's a new one to me.

 

[SteveWag]

What voltage did your electrician measure? Bet it was over 300 volts, maybe as high as 330 volts. (240 * 1.414). Assuming he was using the same meter that indicated 250 volts before the thievery, reduce the input voltage by the ratio of 250/XXX. My guess is 180 volts.
Steve

 

[edison123]

Steve

If I may, the Vdc to Vac ratio is 1.3 not 1.414. Just today tested a rotating 3 phase bride rectifier pack today confirming that.

 

[waross]

Hi edison123, if you are using a D'Arsenval meter movement, you have approximately a 1.1 form factor to consider. This is the ratio of the sign wave average to the sign wave RMS. If the wave form is not a pure harmonic free sign wave the form factor changes. This may account for the discrepancy.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mgyver]

thanks for info on the voltage. one more question.for the dc current amps supplied.if my 3 phase ac 190 volts can supply 100 amps per phase how many amps dc can the 6 diode bridge supply at the 250 dc (assuming diodes are capable).i'm trying to size the transformer.the original had no nameplate but looked about 45 - 75 kva.

 

[edison123]

The trafo KVA should match the crane DC motor HP since it is part of the power system feeding the motor. I would say add 10% more for a safety factor.

In 3 phase bridge rectifer, the ac line current is about 75% of DC load current, if I remember right.