In message , Ralph E
Lindberg writes
In article ,
mac davis wrote:

....

I was told (here and by Nova) that because of the design of the motor and it
being beltless, that 100 rpm was as slow as it can turn.. Not an engineer,
but I
believe it..

Well, I am (and EE) and I suspect that they meant "and retain enough
torque"

One of the issues of low speed is that their is insufficient rotational
speed so that the internal fan will provide enough cooling. In my case a
controller was added to a 3ph motor, and it was advised not to run at
less than 50rpm for periods in excess of 30 minutes without providing
extra cooling/ fans.

My own view on this is that the time/speed factor is a safety net, as
all motors will have slight varying performance, and the safety net is
probably to accommodate the worse case scenario of maximum heating. My
own motor exhibits very little heat externally, but internally things
are probably quite different.

As I also have a 10 speed gearbox on the lathe I can adjust motor speed
relative to gear ratio for best torque

--
John