Re: [TCML] VFD motor

[FIFTYGUY@xxxxxxx]

In a message dated 1/23/09 11:54:21 A.M. Eastern Standard Time,  
sparktron01@xxxxxxxxxxx writes:

>Early in my career when darlington transistors had just  become
>widely available in VFD's (@ 230VAC) I worked on a 5HP  application
>where the input 480 was reduced to 240 and a second  output
>transformer elevated voltage back to VF,VV 480 to drive 3ph  motor.
>The input transformer was SMOKING (at least 20-30deg C  hotter)
>then the output transformer.  Input THD from bridge  rectifier
>(no input reactor) and third order harmonics were  saturating
>input transformer, where the output unit was within  ratings...


Very interesting! I wonder why they didn't just  use the motors at 240V 
connection, the price of pipe and wire was probably  insignificant in a 5HP 
application, the motor would have had more margin of  safety from transients, and the 
transformers represented another 10-15%  efficiency loss (probably a lot more 
if one was smoking!). I've also seen  older VFD installations - some you were 
lucky to get even a two-digit LED  display, and some you only got a single 
analog meter! Most had off-the-shelf  overcurrent relays installed downstream of 
them, since the drives themselves  were not trusted to prevent motor 
overcurrent conditions!
 
>So BE CAREFUL when using transformers OR motors  with
>VFD's.  Another tid-bit, don't run motors less then  30-40%
>of baseline speed nominal WITHOUT supplemental  cooling;
>you WILL eventually burn the motor up.  Ask me how I  know...  ;^D


    That's assuming a constant-torque load. Most  pump/fan applications won't 
draw as much current at low RPMs. Our rotary gaps  are much like those types 
of loads. Plus I suspect our rotaries induce extra  airflow over the motors, 
even at lower speeds, and we don't run our rotaries  very slow anyhow (bigger 
danger from resonant rise in the primary tank!)
    And - (and I know YOU know this, Dave!) - if  you're properly specing a 
new system, you'd be using a purpose-designed  VFD-rated motor(with a separate 
cooling fan motor) for a major VFD  project...
 
-Phil LaBudde

Center for the Advanced Study of Ballistic  Improbabilities
**************From Wall Street to Main Street and everywhere in between, stay 
up-to-date with the latest news. (http://aol.com?ncid=emlcntaolcom00000023)
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla