Re: [TCML] VFD motor

[FIFTYGUY@xxxxxxx]

In a message dated 1/16/09 9:32:54 P.M. Eastern Standard Time,  
bartb@xxxxxxxxxxxxxxxx writes:

>A VFD is rated to drive 3-phase motors of a given horse power.  Don't try 

>running a 2HP motor with a 1HP VFD.
 
    Bart is correct in that you generally don't want  to run a higher HP 
motor from a lower HP drive. But you can usually get away  with a drive twice the 
HP of the motor. Just rules of thumb, and some  manufacturers are even more 
conservative with their recommendations. 
 
> Also, the motors should be rated for frequency drive use  (winding 
insulation). 
 
    It's not easy finding a motor with "inverter"  rated windings in the 
fractional HP range. However almost any *domestic* brand  of 3-phase motor will 
have high enough quality insulation that it shouldn't be  a problem. Keep in 
mind this applies to relatively *new* motors - old motors  have degraded 
insulation, and much older motors had lower quality insulation  to begin with.
    One thing that helps greatly in our low voltage  application (240 VAC 
input) is that a 240V drive will have a greater headroom  with regards to the 
motor's insulation system than the same motor driven by a  480V drive. Insulation 
failures in motors from inverter duty tend to occur in  the first parts of 
the windings close to the external connection points. 
 
> They are "not" synchronous motors but vary their speed to  maintain a given 
frequency. 


    Even a synchronous motor can be driven at  different speeds - permanent 
magnet rotor synchronous motors are just stepper  motors with a different 
label. Different step freq = different rotor  speed.

>BTW, the VFD's which accept 1-ph in and provide 3-ph out are  usually 
>lower HP (roughly 1.5 or less).



    I've seen some in the 5 HP range, but you're right  that they do tend to 
be on the smaller size. Of course, really *big* multiple  VFD's often run 
directly off monstrous common DC busses...
 
-Phil LaBudde
Center for the Advanced Study of Ballistic  Improbabilities
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