On Mon, 31 Aug 2009 21:48:17 -0400, GeoLane at PTD dot NET
GeoLane at PTD dot NET wrote:


In addition to the old gearbox speed changer, speed can also be varied
with a pot (0-60 Hz).


I have a TECO FM-50 for a 2HP Bridgeport. A pot and on-off switches
in a separate box would be a lot more convenient. What are typical
specs for a potentiometer for one of these? Something I could pick up
at Radio Shack or do I need to order on line? Under what section of
the manual would you go looking for specs if it's something that
varies by manufacturer and model? I only read enough of the manual to
get my mill up and running so I could play with it. Now that I've had
it for several months, it's time to make improvements.

RWL

VFDs supply the speed pot from a low impedance DC
source typically 10V or less and the pot slider is almost
completely unloaded. This makes the choice of pot value very non
critical - any pot resistance between 3 and 10kohm is typically
recommended as suitable and an even wider range would make little
difference.

Power dissipation is also very low. Even in the worst
case scenario, with the lowest value pot, dissipation is much
less than 1/4W so even small radio type pots are perfectly
usable. However you may prefer the mechanical ruggedness of a pot
with higher power rating.

What is seldom mentioned, but important in practice,
is the resistance law. Most wirewound and many film pots are
linear law which is not very suitable for a machine tool working
over a wide range of work/cutter diameter and speeds. The useful
low speed end is limited to narrow region near anticlockwise.
What is preferable is is a log law pot as this opens up the low
speed end and gives a close approximation to equal % speed change
for equal pot angle change over the whole speed range.

Log law radio type pots are easy to find but heavy duty
types are much less common. A solution that I find preferable is
switched speed setting. The 12 step series resistor sequence

100 Ohm
120
150
180
270
330
390
470
560
680
820
Gives a constant 20% speed change per step which is about
as close as is ever needed in typical machine tool use
For more VFD information try "Electric Motors"- second
edition- Jim Cox
Pentagrid