Hi DeDog,
Yes, you can control smaller motors with a variable resistor,
its normal to put this in line with the field winding.
On a larger motor such as you describe, over 2kw, i don't think
using a variable resistor would be practical.
I assume the motor has the field winding in parallel with the
armature winding, yes ... ?
Could you describe the motor, such as does it have brushes?
does it have a centrifugal switch?
Does it have to start under load, or can load be applied once
its running?
Any numbers or model indication or makers name on it?
I was wondering if you could use a dimmer-switch on the field
dunno, i'll have to have a think ...
Is it expected to be running continuously or will it be
switching on and off with temperature?
And as Stevez says, the air-con won't work properly if
the motor speed drops very far, if you want to control air
con by reducing motor speed, running too slow will give other
problems, it may be ok for reducing by a small amount.
Regards, John