Variable speed conversion for delta midi...

[GouletPens]

I'm sick of changing belts a hundred times a day. Has anyone out there tried converting a delta midi lathe to variable speed? I see that Penn State Industries has a 1/2hp VS motor conversion kit for $109.99 and I'm real tempted. I just want to see if anyone else has done it and if it works as well as I'd hope. I appreciate any advice!!

 

[joeatact]

I used this kit on my mini jet no problems and no more belt swapping! Should be ok for yours also. Simple installation on the mini jet

 

[leehljp]

I posted a note concerning that PSI VS kit here.

The question on that post dealt with it fitting it onto a Rikon lathe, which is what I have.

More than likely, there will be a few minor modifications or adjustments that will need to be done. The power is about 25% less than the original motor, but since I focus on pens, it is not noticeable.

I am a very big dummy from one point of view (and maybe more). I have several variacs and I use them as step up transformers for my saw and drill press - Japan has 100 volts and most of my tools are 120V. I never thought about using my variacs as "VARIABLE" transformer until AFTER I installed the PSI VS on the Rikon. For several years, I have kept the Variacs out of sight and set on 120V. I pulled one out and tried it as a VS and then I went "duh" I could have had a total VS on the original Rikon motor.

Mine are old but they work well. If you can find one on ebay or craigslist, look for at least a 15 amp. All four of mine are 20 amps. New ones go for $700 or so. I saw some on Ebay for $100 - $120 and these do faaar better than the electronic VS - IMO. My Variacs are 20 to 30 years old and are still as good as they were when new. Search Ebay with "variac".

 

[GouletPens]

I'm intrigued by this variac. I've found a couple on ebay, and they are interesting. I'm going to have to do a little more research, but it seems like it might be a viable alternative.

 

[jskeen]

A note of caution about trying to use a variac as a speed control on tools. Most capacitor start induction motors (IE most single speed lathe motors) operate at a set speed determined by the hardwired number of "poles" or separate current loops in the stator windings and in the rotor assy. These operate together with the frequency or "cycles per second" of the line current coming into the motor to determine it's operating speed.

A variac changes the input VOLTAGE to a tool, but does not change the frequency of that voltage.

Trying to slow down a single speed motor by reducing the voltage is a very good way to burn out the windings in the stator, and wreak havoc with other internal components. Motors do tend to make good anchors for trotlines after this happens though.

Variable speed motors come in two distinct flavors, DC or "permanent magnet" motors which are speed controlled by varying the input voltage. But it is Direct Current voltage, not alternating current voltage that is changed. These are commonly seen on treadmills. The second type of variable speed motor is a three phase alternating current motor, and these are controlled using a VFD or variable frequency drive, which does not change the input voltage, but rather changes the cyclic rate or "frequency" of the input power to the motor.

There is a third catagory of motors called "free running" or "max speed" motors. These are commonly seen in routers. They can be slowed down by reducing the input voltage using a variac. I don't know the internal structure of these, but they run at very high rpm's and are usually relatively low torque motors. I do not know of any lathe's using this type of motor.

It's been a long time since electrical motor theory in college, but that's what I remember. Feel free to correct, amend or modify if you're memory is sharper than mine. Bottom line is that you should do a lot of research before trying to vary the speed of a single speed tool by any means.

 

[leehljp]

James (JSkeen) gave some good information. I guess I have been very fortunate. Variacs are intended to deliver a precise voltage and are variable in order to set the voltage needed at a specific point and leave it there - NOT use as a speed changer.


Below are two comments on the Variac. When the engineer recommended to me to use a Variac some months ago as a speed controler, he was referring to 1. dropping the voltage from 120 to 100 as a steady output or 2. with a universal motor if I didn't want to fool with a VS switch on the universal motor. Anyway, here are the quotes, which say basically the same as James did in the above post.



1. 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.

2. 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.
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Red emphasis above is mine (Hank Lee). I don't think the Jet has a universal motor.

 

[GouletPens]

What I'm kinda getting from all of this is that I'd just be best off getting the VS conversion motor from PSI...

Now that I think about it, the variac is really a big router controller, and since the motors on the lathes don't work with router controllers it would make sense that variacs aren't really meant for that purpose either. I mean, the PSI motor VS kit is only $110, and then I'd still always have the old delta motor I could throw back on if the PSI gives out.

I think I'll try the PSI motor, and I'll let you all know how it works out. It might be a little while before I get it, but I'll keep in touch. Thanks for all your help!!!