AndrewV wrote:
I have a 7.5 hp vfd currently used to run a 4500 cfm blower fan. Since
the use is intermittent I want to power my drill press as well. The DP is an
old delta soon to have a new to me 3ph 2hp 230 1740 rpm motor transplant
(more questions on that project later). What I was thinking for the wiring
goes like this--- place a 240v dpdt shut off on wall next to DP. wire it to
shut off power(240 sp) to vfd. next wire a twist lock to the 3 ph output of
the vfd and twist lock plugs to the DP and Blower fan. Plug in what I want
to use and get to work. Also keep my eye out for another vfd for the future
when the dp is farther away from the fan unit.


If the voltages of both motors are the same, then the VFD shouldn't care
which motor it drives as long as its within its capacity, which yours
seem to be.

About the motor change on the DP, I drill mostly mild steel holes to about
3/4" a few to 1" I'd like to set this up with annular cutters for the larger
holes. I'm not sure what my target rpms should be. That is rpm's before I
use the vfd to adjust the Hz. The driven pulley is 8" so I figure a 3" (I
have a 3" & a 7") drive pulley @ 1740 rpm gives me 652.5 rpm @ the spindle
before I lower it with the vfd, that's faster then it runs now but probably
a good top end for me. I'm also not sure how the torque drops off when you
lower the motor speed or what Hz = what rpm. Anybody do this before? I'd
like to size the drive pulley so there's plenty of torque @ the low end.

The motor speed is directly proportional to the frequency displayed by
the VFD. If the motor runs at 1750rpm at 60Hz then it will run at
3500rpm at 120Hz and 875rpm at 30Hz.
Normally, at speeds below the base frequency (the frequency the motor
was designed for) the torque remains constant and, since power is rpm x
torque, this means that at 30Hz, for example, the power is half that at
60Hz. Above base frequency the torque drops so that the power remains
constant, so you get half the torque at 120Hz. So its constant torque up
to and including base frequency, and constant power above base frequency.

I hope this helps.

--
Regards, Gary Wooding
(To reply by email, change feet to foot in my address)