Re: [TCML] VFD motor

[FIFTYGUY@xxxxxxx]

In a message dated 1/16/09 12:12:10 P.M. Eastern Standard Time,  
william_b_noble@xxxxxxx writes:

>a VFD drives a three phase motor only, you cannot use "any old  induction 
motor". 
 
    You might be able to get one phase of a VFD  output to *narrowly* adjust 
the speed of a "single phase" motor, but it's  certainly inadvisable!
 
>I am unaware of ANY VFD that will do more than synchronize to the  60 hz, 
and those are exceedinly >rare - 
 
    I don't follow this statement at all. A VFD by  definition will not be 
synchronous unless it's running at the frequency of the  power line (nominal 
60Hz). If you're trying to say that VFDs aren't designed  to be *exactly* 
synchronous to the slight variations of the power line freq,  that's certainly true. 
But I imagine you could implement a tight control loop  (PLL?) to keep the VFD 
in synch.
    If you need to adjust phasing, it's just a matter  of getting the VFD out 
of synch slightly for just a bit before bringing it  back. Again, not too 
tough?
    And of course you will need a very accurate motor  model/tuning or 
(preferably) speed feedback from the motor itself to account  for slippage in the 
induction motor, since presumably the point of  this exercise is to get the 
*motor* running at synchronous speed. 
 
>if you want a synchronous motor, a VFD is absolutely the wrong way  to do it.
 
    You can use a VFD to run a synchronous motor, but it  very much depends 
on the motor and other circumstances.
    And how do you think they drive all  those "brushless DC" motors?
 
    If all you want is decent-sized synchronous  motor, the cheapest and 
easiest route is just to do the salient pole mod on a  spare single phase motor. I 
used only a file and angle grinder, and it turned  out perfectly. OTOH I run 
a VFD to a stock 3-phase motor on my pig coil now  and I think it offers much 
more versatility.
    I've also run a small Oriental on my medium  coil, and that's pretty much 
the out-of-the-box solution.
 
-Phil LaBudde
Center for the Advanced Study of Ballistic  Improbabilities



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