In article ,
Charles A. Sherwood wrote:

A while back I bought a Rockwell 11 inch lathe with a mechancial variable
speed drive. This drive works well but seems to have a lot of friction.
The lathe came with the original 1HP 3PH motor. I installed a 1HP VFD
and I managed to make the lathe run. The VFD could not start the lathe
unless the Mechancial VS was set to less than 1/2. I Adjust some parameters
in the VFD to increased the starting torque which pretty much solved the
problem. However the VFD indicates the motor is drawing nearly full
current just turning the spindle at max RPM. And yes the spindle bearings
are fine!

Hmm ... mechanical variable speed mechanisms are not very
efficient -- lots of friction losses in that variable-speed belt.

The original motor starter was set up for a 2HP motor even though the
lathe only has a 1HP motor. Its very difficult to read the motor tag
inside the lathe cabinet and the front swithc say 2HP, so the electrician
probably put on a 2HP contactor with 2HP heaters.

So anyway I am considering installing a larger motor. I found some 1.5
HP 1140RPM motors fairly cheap. I think the lower RPM motor will develop
more torque and the VFD will still allow me to achieve full spindle speed
by increasing the freq to 90.

I am considering a 1.5HP to replace the exisiting 1HP motor. My logic is
that the lower speed motor will develop more starting torque and I can
still get the same spindle speed by increasing the freq on the VFD.
For example the 1140 RPM motor will develop 1HP at 900 RPM (900/1140x1.5)
where the existing motor will only develop 1/2 HP(900/1800) at the same
RPM (assuming constant torque). I think a 1.5HP 1140 RPM motor will
develop more low end torque that a 2HP 1740 RPM motor that I was
originally considering.

what do you think?

Well ... my 12" Clausing step pulley lathe has a 1-1/2 HP motor,
and I think that is a good fit for it. Your 11" would not need quite as
much horsepower, but the variable-speed pulley will require more to
drive it properly, so I think that I would consider the 1-1/2 HP at a
minimum, with perhaps the 2 HP which your switch and contactor suggest
being the better choice.

Good Luck,
DoN.
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