VFDs & 3 Phase Motors
All-righty then. This may sound stupid, but when you run a 3 phase motor
off of a VFD what is your speed range relative to the original speed of the
Approximately of course. Mileage will vary I am sure.
Depends a bit on the motor and more on the VFD. Flux vector VFDs
(more expensive) can run down to zero RPM at full torque, if the motor
has adequate forced cooling. For general purpose motors and VFDs,
you can usually go to 50% RPM (watch out for motor overheating)
up to 200% rated RPM. You get constant torque below rated speed,
and constant HP above.
I get good results from 6Hz to 90Hz on my mill (3HP) and small lathe
(1 HP). They're both running open loop. so there's a significant drop
in torque at the very low end, but those speeds are still useful in
many cases, e.g., spot, drill, and power tap without changing belt
speeds. I've never had a problem with overheating, but I'll typically
switch to a lower belt or gear ratio rather than running at low motor
RPM for long periods of time.
I'm getting ready to install a VFD on my big (10HP) lathe, and will
probably implement a similar speed range, but expect the low Hz range
will be most useful for gear changes and jogging.
A sensorless flux vector drive will improve low end torque somewhat. A
closed loop (encoder feedback) vector drive can produce close to full
torque down to zero speed.
If you have the option of changing belts or gears when you really need
lots of torque at low spindle speeds, an open loop drive is probably
all you need.
Well, yeah... You can put a fan blowing over the motor case, but
unless you've got the inside fan stirring the air and eliminating hot
spots it's still going to release the Magic Smoke.
Works much better to externally ventilate an open motor.
I like Marathon, used a lot of them at work. I put a 1 hp. on my
Hardinge TM mill. It's inverter rated, TENV, so fan cooling doesn't
exist. It was about $230 new five years ago, no idea what they're