[TheForge] Fwd: Shop tools & layount (3-phase converter)

[Steve Smith]

xlch58@swbell.net wrote:
> A three phase motor has three windings.  With a rotary converter of the 
> type we are discussing,  two of those windings are being fed with the 
> same single phase source.     

I can see how you come to this if you are using a Y type motor. Say 
instead you have a delta wound motor. Then, you apply 240V single phase 
to one winding only. You aren't really applying the single phase to two 
windings at once, it just looks that way.

> The third winding is getting a second 
> phase that is created by the current  induced in the windings of the 
> rotary converter by the residual magnetism  in the rotor passing through 
> them.    So instead of having three individual phases seperated by 120 
> degrees, like real three phase, or even 180 degrees apart like real two 
> phase power, you have two phases seperated by 120 degrees.    

This is not correct. Here's the scope photo showing that the output of a 
converter is truly 3 phase, 120 degrees apart:

http://metalworking.com/DropBox/_1998_retired_files/FRW-5.jpg
Here is the text discussing the pictures:
http://metalworking.com/DropBox/_1998_retired_files/FRW-n.txt
The author of these (Fitch Williams) frequents rec.crafts.metalworking; 
it should be pretty easy to find many threads on converters he has 
participated in. He does a much better job of explaining things than I can.

The phase angles are fixed by the motor windings. The windings force the 
three phases to be 120 degrees apart in a converter. There is no way for 
  the converter construction to change this (assuming you are using a 3 
phase motor). You can change the amount of current flowing through the 
third terminal by use of capacitors, but you cannot change the phase angle.

> The second 
> phase will not have the current of the primary phase.  You can help 
> balance the current with a capacitor, but the capacitor will only be 
> dead on at a particular load(current).   Further, while the capacitor 
> will help shift the current forward, my understanding is that the 
> voltage will not move at the same time, so your power factor will be bad 
> for the generated phase even if you balance the current.   

By adding the right amount of capacitance at the right place in the 
windings, you can improve power factor quite a bit. Too much capacitance 
or too little and you will have the power factor problems you discuss.

So to sum up where
> the efficiency is lost, the rotary converter will draw some current to 
> keep it spinning since perpetual motion is unlikely.   The capacitors in 
> the circuit will dissapate some energy as heat, and the  problems with 
> power factor will result in a higher current draw for the same power 
> output.  

These losses are quite small compared to the power a loaded motor uses, 
these are fractional losses. If you have a bad power factor, the higher 
current draw will accentuate them, but they will still be a small part 
of the whole if you consider the power flowing when your motor is loaded.

> Additionally, the narrow and unbalanced phase seperation will 
> further erode efficiency.   The way that this works is that each winding 
> in the motor is dropped into the winding slots to create poles and their 
> position is optimized based on 180 degrees of seperation between the 
> phases.  

120 degrees for a 3 phase motor. This is why a converter produces true 
three phase.

In this configuration, two sets of windings are running on the
> same phase, and the third is running with only 120 degrees of 
> seperation.  Even if it had 180 degrees of seperation like a two phase, 
> then it would still be unbalanced since you have twice as many poles on 
> one phase as on the other.  I think I got all of that right, the EE 
> power engineers out there can step in and correct where I am wrong.

You really aren't powering two windings of the motor with the incoming 
single phase.

I'm sorry I haven't come up with a good way of explaining this in words, 
but it really is 3 phase. The only flaw is how well regulated the third 
terminal's voltage is, and for a specific load you can tune this. One 
way is to tune for a heavy load, when efficiency counts (and not 
overheating the motor counts!). Lighter loads are much less efficient, 
but are also a lot lower power, so the efficiency isn't very important.

Steve Smith