How to feed resistive load with 3-phase power without using delta or wye connection?

in article snipped-for-privacy@posting.google.com, Default Name at we_all_do snipped-for-privacy@hotmail.com wrote on 8/2/04 9:07 AM:

Use the three phase power to drive single phase motor-generator set.

Yeah--life is tough.

Bill

Oh yeah, I didn't specify it needs to be light weight and power-efficient (i.e. motor-generator is NOT an option!)

I bet free would be nice also. As long as you are asking for the impossible.

You want to supply with three phase...then you need to wye or delta connect it. You say you can't (possible bought the wrong part did we?)

Otherwise you have to convert the three phase to something else: Use inverter to make DC....you say you can't because of harmonics. Use motor generator...you say you can't because of weight.

Given those limitations, the answer is simple...you can't.

Charles Perry P.E.

| I need to use 3-phase power to feed a resistive load. The load only | has two terminals (i.e. it can't be connected in wye or delta | configurations). The load could be modeled as a single resistor. | | I cannot connect this load to only one of the phases of the input | power because of phase imbalance that would result. | | There are strict regulations on current harmonic distortion on the | input lines so I can't use a 3-phase rectifier for this purpose. | Furthermore, I don't need a rectified waveform. Any shape would work | since the load is purely resistive. | | Would anybody know an easy way to draw even power from the three lines | of a 3-phase source without using a rectifier?

The current waveform in your load is going to have one of the following forms:

1. smooth DC
2. DC with ripple
3. rectified AC (DC with radical ripple)
4. AC with harmonics
5. purely harmonics
6. nice clean AC sine wave

Rectification and harmonic issues rule out numbers 1-5 fairly quickly. That leaves you with the ideal number 6. But the problem with that is the clean AC current sine wave is going to have some particular phase angle relative to the voltage phase angles of the 3 phases. No matter how you juggle it (for example with transformers configured with double delta secondary), you have ONE current phase angle, and that's going to result in a terrible power factor on one or two of the phases.

The ideal is to have a 1.0 power factor on each phase, no harmonics, and equally balanced. Real life sucks.

I don't know what kind of load you actually have. But if there is any way you can change the model of one resistor into a model of three resistors, you can have it solved. It's obviously not a heating load since that's easily done with split elements in some multiple of three (large electric kilns have this, as do some water heaters).

I've pondered how one might do an arc lamp on three phases, and still get the light source at a single point. Three elements converging at

120 degree angles potentially could do that. But it would be less of a point than two elements on single phase.

in article snipped-for-privacy@uni-berlin.de, Charles Perry at snipped-for-privacy@hotmail.com wrote on 8/3/04 7:58 AM:

Life is tougher on some than on others:=)

Just wire two of the three phases and close the breaker.