Question using a Variac

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.

It depends on whether the motor is designed for high slip operation and on the torque curve of the load. Multispeed split-phase fan motors for instance are designed to operate safely with a high degree of slip. This is because they are generally built for 'air-over' operation wherein adequate forced cooling air is always present regardless of the motor's internal fan speed. Whereas a typical TEFC motor running E.G. a milling machine or lathe will not play even remotely nice with that form of speed control due to both cooling and torque curve considerations.

Your use case deviates from the normal design parameters of what a general purpose induction motor is intended for and delves deeply into the realm of reverse engineering. Your OP suggests that the torque curve of your load is suited to high slip operation so the biggest consideration on my mind is adequate cooling, as that motor of yours is liable to get hot. I would not operate it for extended periods of time without either fitting an auxiliary blower to it or embedding a temperature probe into it's windings arranged to protect the motor if it overheats.

As for the 120/140V rating on the variac, a 140V output range is typical in order to account for situations where low input voltage exists. For example, a 0-140V variac rated for an input voltage of 120V which is being supplied by a source voltage of 107V will still be able to output up to 124.8V.

Thanks for the explanation. Makes enough sense in my pea brain of electrical stuff to know I won't do it. Too much heat. I'll tell the customer he needs a dc drive or more pulleys for reduction.

Ray Behner said:

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.

Click to expand...

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.