Re: VFD/Potentiometer/Motor Questions

On Fri, 11 May 2007 01:50:01 GMT, Dave Young
<daveyoung@xxxxxxxxxxxxxxx> wrote:

I'm hooking up a VFD to my 1961 vintage 9" South Bend lathe (with .5 hp,
208 vac, 3 phase motor) and I have a few questions.

I can hook up an external pot to control the speed of the lathe. The
specs says the pot has to be between 3K - 5K ohms. What would be the
difference in operation if I were to use a 3K ohm pot vs a 5K (or vice
versa)?
No difference. The pot is just a voltage devider. Be sure you use the
right "taper" pot - likely Linear, not logarithmic. A higher
resistance pot loads the control circuitry a little less, so I'd
likely use an industry standard 4.7L linear pot, or a 5K

What would be a reasonable maximum speed for the motor? The
nomenclature plate says 1725 operating rpm, but I understand with a VFD
you can go much higher. I don't want to hurt the motor, but the extra
speed would be nice when using carbide. I've also got the ramp up and
deceleration speed currently set at two seconds; is this reasonable?

Generally not much more than 20% overspeed.

I knew nothing about VFD's when I started the installation (I'm still
not quite finished), but I have to say that it sure seems slick. Not
sure of the advantages/disadvantages of using a VFD compared to a static
phase converter (which is what I was using before). I understand with a
static phase converter you lose 1/3 of your horsepower, not sure if
the same thing happens with a VFD. I'm hoping not...

The VFD is significantly more efficient and has the extra plus of
variable speed.

Thank you!

Dave Young


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