Have a Variac that I want to reduce RPM on a ¼ hp split phase motor. It does what I want, but have doubts.
The controller is 1.4 kva 10 amp, with 120/140 volts. Is it safe to use for hours at a time? Why would I use 120 or 140v. When it comes to electric, I get confused with a flashlight.
To be clear, the variac will not actually reduce the speed of the motor, if it is an induction motor. Not even if it is a series motor.
The induction motor wants to run such that it advances a pole pair per half cycle. So a 4 pole motor will run at near 1800 rpm even at lower voltage without a load.
The ONLY way reducing the voltage works to reduce motor speed is when the lower voltage does not supply enough power to run the load that fast. Then it runs as fast as it can with that load. If a single phase motor cannot run fast enough, it stalls, because single phase motors have discontinuous torque.
The problem is that it is a balance of the power and the load. It really only works well against loads that vary in sharply in power demand vs speed. A fan is perfect, because if has a power requirement that is not linear, it goes up as speed cubed. So the power needed can vary quite a bit in a small range of speed, and the fan can find a stable speed where power required and power supplied balance.
Other loads that vary more linearly will not be as easy to control. NO varying load will be stable in speed unless the voltage is varied to compensate.
This is why speed control of induction motors is done with VFDs, which vary the frequency. That allows the motor to operate normally, close to synchronous speed even though the rpm is varied. And VFDs are usually used with 3 phase motors, which have continuous torque.