VFD's - Variable Frequency Drive

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.

(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?

Tom M.