[Milsurplus] transformer

[Bob Camp]

Hi

A passive single phase to multi phase conversion box normally has a set 
of parts in it that match the impedance on the load side. A box that is 
set up for a 5 HP motor is not right for say a 2 1/2 HP motor. Prices 
are generally in the hundreds of dollars range. Their best application 
is on something like a milling machine where the motor is simply not 
replaceable with a single phase unit.

Normally with power supplies it's cheaper to send the transformer out 
and get it re-wound. That gets a bit odd (to say the least) with a 
three phase core. The result is no less efficient than the converter 
box though.

Here's a "simple" way to look at the converter box circuit. It's not 
exactly how it's done, but it's the right idea:

1) hook the "bottom" of the three phase Y to one side of the single 
phase line.
2) hook the "left top" of the three phase Y to a capacitor calculated 
to give the proper phase angle at the desired load current
3) hook up the "right top" of the three phase Y to a coil calculated to 
give the proper phase angle (the other way) at the desired load 
current.
4) Take the junction of the cap and coil and hook it to an 
autotransformer to boost the voltage so everything works out at the 
right voltages.

Normally you use a cap and coil combination in each leg to make an L 
network that eliminates the autotransformer.

As the current in the load changes the phase angle on the transformer 
changes. With a power supply this is normally a pretty significant 
swing.

The other way to go is to take a single phase motor and drive a three 
phase AC generator with it. That way all the phases are just right.

	Take Care!

		Bob Camp
		KB8TQ



On Mar 21, 2005, at 10:42 PM, Brad Latta wrote:

>  Hi all,
>             I am building a 28V, 50A power supply. The transformer I 
> have was a three phase 208Vac delta input , 20Vac star output. I am 
> hoping to connect the input winding to the single phase 230Vac mains 
> here, I seem to remember a circuit for converting three phase 
> connections to single phase, but cannot locate it. It's for a mil 
> project. Does anyone remember the circuit?
>              Regards, Brad
> ______________________________________________________________
> Milsurplus mailing list
> Home: http://mailman.qth.net/mailman/listinfo/milsurplus
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Milsurplus at mailman.qth.net
>