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

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Steve Smith wrote:

>> There are a lot of disadvantages to running three phase motors on 
>> single phase.  The only advantages are:  You can bump stop three 
>> phase motors, and often inductrial equipment comes with integral 
>> three phase motors that can't be easily replaced with standard single 
>> phase.
>
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> And if you're a good scrounge, they're lots cheaper than single phase 
> motors. 

And if your a really really good scrounge you can bury yourself in 
single phase motors as well.  I had  friend that had a metal building 
full of them.  Everytime I picked up another old piece of machinery that 
needed a motor I would dig through his lot and buy one off of him.  I 
told him I just wasn't lucky like him.  He told me the secret was to 
buy/grab them when you didn't need them.   I grab every electric motor I 
see, working or not.  The busted ones I pull bearings, capacitors and 
switches out of, or repair with parts from previous donors.  Learned to 
rewind them as well, but only  rewound one for education.   I have a 
stack of three phase as well, since for a while there I was making money 
building rotary converters and selling in the paper. 

>
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>> 3) Rotary Converter.....
>
> > This does not produce true three phase, it
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>> produces quasi unbalanced two phase with a hinky power factor.  
>
>
> Charles, I'm not sure what you mean here. Sure, the third leg isn't 
> quite as good as the others, it's voltage may sag a little even with 
> capacitor tuning, but the phases of the legs are all locked in to the 
> correct angles. The idler motor ensures this. I'm probably not 
> following what you mean. 

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.     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.    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.   As many 
rocket scientists and nuclear engineers we have on this list, I know we 
must have an EE or two, so they may do better at explaining it than me ( 
or even point out where I am flat out wrong)    The issue of power 
factor is why you can't get full rated power out of a three phase on 
converted single phase.   HP = watts = amps*volts     While most people 
understand that AC voltage varies in a 60 hz sine wave, what they don't 
realize is that current varies on this same sine wave.   When your power 
factor is screwed, the two sine waves don't track together.  Inductive 
loads like motors make the current lag, capacitors will make the current 
lead.   If you have a big shop with lots of motors, the Power Company 
will come out and install capacitors on the line to correct power 
factor.  The reason it is an issue is that in order to get the max hp 
out of the motor you need draw the rated voltage at the rated amperage.  
If they aren't delivered in sync, then you end up making up the 
difference by drawing more current, since the peak voltage isn't going 
to go up.   This will cause the motor to heat up.   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.  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.  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.


Charles

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> Steve Smith
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