[TheForge] that tired old phase converter question again

[Steve Smith]

Well, I'm afraid my EE training doesn't count. I almost flunked my 
motors course; what I do is design chips.

But I find motors pretty interesting. I've built maybe a dozen phase 
converters out of stuff from the scrap yard. I'll be happy to answer 
questions when I know the answer. The link I posted to the Metal Web 
News article is fairly decent at explaining things: 
http://www.metalwebnews.com/howto/ph-conv/ph-conv.html
A truly excellent reference are any postings by "Fitch" on 
rec.crafts.metalworking.

Simply put, there are two issues in running a three phase motor off of 
single phase power, 1. starting it and 2. maximizing the HP available.

Starting a three phase motor on single phase is straightforward. The 
problem is that three phase gives a motor a direction to start in; 
single phase is like the proverbial mule between two bales of hay. If 
you can get the motor turning in the right direction (fast enough), and 
then apply single phase power to just two terminals of the motor, it 
will run. You can do this with a rope (pull start), a capacitor across 
the right winding (this is what a static phase converter is), or by 
already having three phase running (i.e. a rotary converter). Note that 
if *all* you do is apply single phase power to two terminals (leaving 
the third open), you only have 1/2 to 2/3 of the HP available (you are 
only using two windings out of three).

A single phase motor has a capacitor and an auxilary winding inside that 
gives the motor a kick in the right direction, same as a static 
converter. Both of them usually switch the start capacitor out after the 
motor is started.

If you need more HP available than 1/2 to 2/3 of the rated HP, you need 
to pull current out of the third leg of your motor. Adding an idling 
three phase motor in parallel to your load motor will allow your load 
motor to pull current from the idler. This is a lot like how an 
alternator works on your car. The presence of the idler greatly improves 
the quality of the three phase. Another thing you can do is put 
capacitors between each side of the single phase power and the third 
leg. The capacitors will pull current through the third leg. The best 
solution is to have both an idler motor and capacitors. This not only 
makes the current in the load motor more balanced between the windings, 
but also quiets the idler, which can make a fair bit of noise if you run 
without capacitors. If you spend the time to balance the voltages on the 
three phases (by varying capacitors) under load, you can get 90-100% of 
the rated HP out of your load motor.

Note that all of the above need correct magnetic starters and switching 
with safe wiring. I have a fair number of 1-3 HP starters if anyone is 
in need.

Steve Smith



Adam Whiteson wrote:
> Steve  Smith,
> 
> If you find yourself with time on your hands, how about showing off some 
> of your EE training and posting an intro to the working principles of 
> static and rotary converters?
> 
> I would sure appreciate it, and judging from the frequency  (har har!) 
> with which this thread reappears, others would too.
> 
> Just a suggestion.
> 
> Adam
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