Variac usage opinions wanted

[leehljp]

On the pen turning forum, I gave the suggestion to a person to get a Variac (variable transformer) to make his lathe as variable speed. He was talking about buying a new motor and VS switching set up - and changing his lathe to that. I pointed out that the setup, with its EVS motor had considerably less power (torq), but it would not matter from pens. (The VS motor was almost half the diameter of the original motor. Small!

MY questions to you.
1. What is your opinion of using a high amp variac as a VS (variable speed) controller on a lathe, or other motors.
2. Do Variacs have advantages over electronic VS controls or not? I never hear of a variac burning out but I do hear of the electronic controllers (even PSI's) going bad.

New 20+amp Variacs run $600 or more but can be had on Ebay for $100 - $150 for up to 30 amps.

I have been using mine for a while like this but before I recommend to him to go this route, I would like input from people with Variac experiences on motors or from electrical engineers.


Variacs was the way it was done in the "old" days. I have several because I have 120V shop machines in a 100V environment and use them basically as step up transformers. Only recently did I think to use them as "VARIACS" and varying the speed through voltage.

Opinions?

[Uncle Cracker]

Variacs work on the principle of reducing operating voltage, basically starving the motor for power. This is not an ideal way of slowing down a motor, even one designed for the purpose. Lower voltage without a corresponding increase in current results in lower power (less work).

Variable-frequency technology instead uses power in differing frequencies to affect changes in motor speed, simply by changing the rate at which the various fields in the motor transfer force from one to the other. Much more efficient, on several levels.

[LCHIEN]

A Variac is Not the suitable way for slowing an induction motor. The Induction motor is basically a synchronous machine with slip - it will try and stay synchronous with the power line frequency and thus will resist slowing unless loaded nd will rapidly lose torque when slipping in speed.

A Variac is a variable autotransformer and has a variable output voltage constant frequency output. When used with a indction motor, it will appear to slow the motor as you reduce the voltage but you will find it has no torque. At some point it can become possibly dangerous to the motor since it will try and make up for the loss of torque by drawing more current. This proble is offest byt the lower voltage. Still its a combination I would not recommend.

Variacs will work well on Universal motors, but router speed controls are usually smaller, lighter and cheaper.

AC induction motor speed controls are usually implemented as variable frequency AC drives. Basically a AC to DC power supply, connected to a DC to AC power supply (like an inverter) except with a knob for varying the line frequency output.

[leehljp]

Thanks fellows for the clarification on this. So, What I am reading is - basically a Variac is VARIABLE but meant to be set to the ideal voltage and left.

Thanks again. And yes, I noticed the quick decrease in torq way before voltage was turned down much.

[LCHIEN]

Thanks fellows for the clarification on this. So, What I am reading is - basically a Variac is VARIABLE but meant to be set to the ideal voltage and left.

Thanks again. And yes, I noticed the quick decrease in torq way before voltage was turned down much.

Well, its actually a pretty good variable AC voltage source given a FIxed AC source - as long as you want to vary the amplitude (voltage) and not the frequency.

There is no ideal voltage to set it at. It works pretty well at any voltage and variacs can be made to step up as well as down. When used for the appropriate loads it works well at any setting, and can be varied freely, is what I am saying.

[leehljp]

Well, its actually a pretty good variable AC voltage source given a FIxed AC source - as long as you want to vary the amplitude (voltage) and not the frequency.

There is no ideal voltage to set it at. It works pretty well at any voltage and variacs can be made to step up as well as down. When used for the appropriate loads it works well at any setting, and can be varied freely, is what I am saying.

Loring,

I appreciate your help with this, and this extra bit of explanation.