Re: [CAD_CAM_EDM_DRO] VFD's - Variable Frequency Drive

mayfieldtm wrote:

>I just ordered a "Variable Frequency Drive" for
>use on my 14" Lathe.
>
>In hopes of keeping this on topic, I plan to convert
>the Lathe to CNC and control the Spindle Speed.
>
>The Lathe Motor is a 2 Speed, 3HP, 3 Phase, 220V motor.
>
>When I inquired with the Lathe Sales Person on the
>Amperage Rating of my Motor. He said that I should
>never run the Motor under 40Hz, in that it could
>damage the Motor.
>
>Is this true?
>
>I mean I would expect that very slow or fast speeds
>are not desirable, but, 40Hz is only 67% of normal.
>
>

Running at 40 Hz at full rated torque will eventually fry the motor.
Running for a long time at light load at 40 hz probably won't hurt it.
You can generally tell just by feeling the motor. If it feels a lot hotter
than normal operation at 60 Hz, then you need more cooling. If you
fit a small cooling fan to it such that it forces air through the motor in
essentially the same fashion as the built-in fan, it will help greatly.

>(I was planning to use a range of about 20Hz to 90Hz.)
>(3 times slower to 50% faster)
>
>After all the glowing comments that I've read by "happy"
>VFD users, now I have doubts about my purchase.
>
>Any one have a VFD, and any comments?
>
>

Yes, I have 2, now, and am very happy with both. But, there's no doubt
that running a motor at well below rated speed for extended periods
will lead to overheating. One thing to note is that the motor current only
changes a little from no load to full. The power factor changes
dramatically,
so that while the motor is drawing almost totally imaginary current, and
hence little real power at light load, it is drawing close to 100% power
factor at full load. For instance, my Bridgeport mill (using the current
readout on the VFD) draws 3.2 A at no load, but 3.5 A under heavy load.
So, the heat generated by the current in the windings changes only slightly
from idle to full load, even though the real power drawn changes a lot.
That's why even an idling motor run at low speed can be harmed, and
why additional cooling helps a lot.

Jon