- Joined
- Jan 2, 2011
- Messages
- 331
For reference, the reason that you can't regulate the speed of an induction motor by just turning down the voltage (that's what a variac does) is how the motor controls the current draw under normal conditions. A motor is also a generator. When a motor starts turning, it starts generating a voltage (counter EMF or CEMF) that opposes the voltage that's making it turn (EMF) . When the CEMF balances the EMF, the speed and current stabilize.
If you put more load on the motor, the speed is reduced and the CEMF is reduced. Without the counter effect, the current increases, adding enough torque to the motor to hold the new, lower speed and set a new balance. If too much load is added, the rotor will stall and maximum current starts to flow. If the control system (or operator) doesn't shut it down, the high current will generate high heat in the windings, burning out the motor. Running at reduced speed and high current, even though still turning, will over-heat the motor and eventually damage it.
VFDs control speed by changing the frequency and adjusting voltage and current for optimum performance. This is quite easy to do on a 3-phase motor, not so easy on single-phase. I'm using a jack-shaft on my bandsaw conversion. Much easier and cheaper.
Thanks for that explaination.
I guess if I use the base from the AMT, I won't have enough room under there to mechnically regulate the rpms. (?)