Variac electrical experiences anyone?

[leehljp]

On This thread titled "Variable speed conversion for delta midi..." I gave the suggestion of getting a Variac (variable transformer) for use as variable speed on the lathe.

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.

On a wood working forum, a fellow wanted to turn an old 10 in saw into a 10 disk sander and asked about VS to slow it down. I mentioned variacs. The one electrical engineer on the forum said that variacs would be the best but most people couldn't afford 20 amp variacs.

QUESTIONS:
1. What is your opinion of using a high amp variac as a VS (variable speed) controller?
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.

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".

Opinions from experienced people please.

 

[GouletPens]

I'm the guy who originally posted asking about VS conversions for my delta midi. I am currently turning out about 300-400 pens a year, and I'm looking to make it around 1000 this coming year. Changing out belts has worked okay but looking to the future I'm thinking turning a knob would be a lot faster. Hank turned me on to variacs (no pun intended and I'm intrugued especially since it would apparently give me more power (always better) and more reliability. I'm assuming variacs are what they put on the larger VS lathes, because I can't imagine having a 2 or 3hp DC motor is too practical...I'm an electrical novice so any info here is helpful!!

 

[leehljp]

I re-posted this on my woodworking forum where some electricians, motor rebuilders and few engineers hang out.

 

[SuperDave]

I built a custom bowl lathe capable of turning 96". I used a 5hp three phase AC motor coupled to a 5hp TB Woods VFD running 220 single phase input and three phase output to the motor. I have infinite variable speed, tons of torque, on the fly reversibility, etc. totally cool!

Motor: $80
TB Woods 5hp VFD: $199
AB Switches and potentiometer: $50
20 amp breaker: $10
Wiring: $30

I have converted all three of my lathes to VFD and would never go back to belts!

 

[marcruby]

I re-posted this on my woodworking forum where some electricians, motor rebuilders and few engineers hang out.

I think there are some power issues when you use voltage or cycles to control an AC motor, but I honestly don't know what they are. And I'm not exactly sure whether a penmaker would notice.

Marc

 

[leehljp]

I built a custom bowl lathe capable of turning 96". I used a 5hp three phase AC motor coupled to a 5hp TB Woods VFD running 220 single phase input and three phase output to the motor. I have infinite variable speed, tons of torque, on the fly reversibility, etc. totally cool!

Motor: $80
TB Woods 5hp VFD: $199
AB Switches and potentiometer: $50
20 amp breaker: $10
Wiring: $30

I have converted all three of my lathes to VFD and would never go back to belts!

This is EXACTLY the direction I want to go with my lathe in a couple of years when I retire!

 

[skiprat]

To alter the speed of an AC motor you need to change the frequency.
To alter the speed of a DC motor you need to change the voltage.

A Variac on it's own is normally used to lower the voltage output. As it's name implies ( Vari -AC ), it is an AC machine. If you have a suitable rectifier on the output of the variac you can use it on a DC motor.
Most Variacs are wound so that you can get about 10-15% more volts than you put in.

Just lowering the volts and attempting to use it on an AC motor is asking for trouble, even if you can sort of make it work. By lowering the volts by half, you will effectively double the current as the motor will still try to work ( until it stalls ). Increasing the current will make your motor run hotter and probably ruin it in the long run.

Modern controllers are the best way to go for either a AC or DC machine.

 

[SuperDave]

This is EXACTLY the direction I want to go with my lathe in a couple of years when I retire!

Hank,

It was a ton of fun to build. I didn't have a lot of funds but I had the time. Took a couple years. I drew all the plans for the housings, spindle, pulleys, etc., then spent time searching EBay for steals. The TB Woods VFD is a $1,500 item I got for $199... Brand spanking new!

I found a machine shop in Ohio that machined a 2" diameter x 18" 4130 steel spindle (ground and polished) 1 1/4 x 8 TPI on both ends and #2MT... delivered for $300, two 6160 aluminum poly v-groove 3 step pulleys (9" max diameter) for another $350, custom tail-stock with 1.5" diameter lead screw for a quill, drilled for #2MT, hand-wheel, etc and a second quill with a #3MT... $150... anyway, if you are not in a hurry, you can build a lathe you could never buy commercially, for less than a moderate priced lathe with nowhere near the quality or capacity.

If you ever get to that point, shout out... I still have the CAD files and contacts I made and will gladly help out!

D

 

[leehljp]

Hank,

It was a ton of fun to build. I didn't have a lot of funds but I had the time. Took a couple years. I drew all the plans for the housings, spindle, pulleys, etc., then spent time searching EBay for steals. The TB Woods VFD is a $1,500 item I got for $199... Brand spanking new!

I found a machine shop in Ohio that machined a 2" diameter x 18" 4130 steel spindle (ground and polished) 1 1/4 x 8 TPI on both ends and #2MT... delivered for $300, two 6160 aluminum poly v-groove 3 step pulleys (9" max diameter) for another $350, custom tail-stock with 1.5" diameter lead screw for a quill, drilled for #2MT, hand-wheel, etc and a second quill with a #3MT... $150... anyway, if you are not in a hurry, you can build a lathe you could never buy commercially, for less than a moderate priced lathe with nowhere near the quality or capacity.

If you ever get to that point, shout out... I still have the CAD files and contacts I made and will gladly help out!

D

This is being bookmarked! :biggrin:

 

[SuperDave]

I will try to post some pictures... but it will be a while, as the power company messed up a repair and somehow arced a line that blew power to our neighborhood... I was online and fried my hard drive from the surge... I only have access through work for a couple weeks while I repair the damage... I think I see a UPS in my future...