DC spindle motor

Hello,
I recently picked up an older milltronics cnc mill. This was someone else's
half started project. One of the first things that I thought was strange
when I purchased this machine was the spindle motor setup. It looks like it
originally came with an adjustable sheave pulley system. This original
design is no longer there. It has been replaced with a 1:1 twin pulley set
up. The original AC motor that came with the mill is a 2.5hp. I hooked up a
vf drive to see how it worked and it doesn't work, at least not very well.
There is no low end torque what so ever. Has anyone ever done a similar
modification? I think a lot of the later model bridgeports are set up with
the same type of sheave system. What is the best way to get back the low
torque? Use a much larger AC motor and drive? Or maybe go with a DC motor
and drive setup? I can pick up a 3/4 horse 1750 DC motor with a KB regen
drive for a round $200 surplus. The base speed is the same as the original
motor, but I'm not sure how the torque will rate compared to an AC motor. I
Know horsepower is horsepower regardless of AC or DC, but the torque
acquired from an AC motor at low speeds is nothing compared to DC motors. My
goal is to use the features I have with my Ajax cnc control such as rigid
tapping and constant surface speed, ect. The spindle head does have a two
speed gear change. HI and LO. Even when I shift into low the low end doesn't
seem like its all there.
Any ideas or past experiences would be appreciated
Thanks
Shawn

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