Electronic speed reduction on conventional headstock?

[nogbert]

I'm wondering if anyone had ever tinkered with using an in-line potentiometer or other variable control element to reduce the base output velocity of the electric motor. Seems like someone surely could come up with something along these lines that would allow for very slow spindle speeds for large turning or giant drill bits etc.

[DLB]

Yes, people have tried that. I recall people writing to the forum wondering why a commercially available controller wouldn't work with their Shopsmith. The reason is that those controllers aren't compatible with induction motors, they primarily work with universal motors. Variable speed induction motors do work with Variable Frequency Drives (VFD) but as far as I know those are three-phase motors and are specifically designed for the purpose.

Quite a few people have gone with some form of DC motor and DC controller. Shopsmith solutions include PowerPro or a mechanical Speed Reducer.

- David

[nogbert]

I talked with an electrical engineer friend of mine and he said that there are several other ways to control the speed of a single phase AC induction motor without losing any performance such as a pulse width modulator and as you mentioned a single phase VFD. once i get my machine tuned up i may spring for a PWM nd see how that works

edit: removed bad info about using a variac to reduce speed

[BuckeyeDennis]

I talked with an electrical engineer friend of mine and he said that using a 20 amp Variac controller would be able to reduce the speed without impeding power delivery etc. There are several other ways to control the speed of a single phase AC induction motor without losing any performance such as a puls width modulator and as you mentioned a single phase VFD. once i get my machine tuned up i may spring for a variac and give it a try

Your EE friend is mistaken. And to be a good citizen, I can’t leave misinformation like this unanswered on the internet.

A Variac (a brand of variable-ratio autotransformer) allows you to adjust voltage, but not frequency. And the speed of induction motors is controlled primarily by frequency, not voltage. If you change either very far from the design input of a single-phase capacitor-start induction, you will very soon release the “magic smoke” from the motor.

My credentials: MSEE 1982, 40 years of electronic motion-control product development experience.

[RFGuy]

Fellow EE here. Dennis and David are correct. What your EE friend said about power delivery through a VARIAC transformer is technically correct, but that is a statement of the VARIAC and NOT of what happens to the induction motor load under reduced input voltage supply. The latter is more important here.

[nogbert]

Thanks for the input gents. i have edited my post to remove bad info about the Variac. Both of you seem very knowledgeable about this; surely there is a simple/plug-and-play way to electronically slow the motor on a SS right? VFDs are not exactly unobtrusive, ideally you'd be able to use something that could mount directly into/onto the casework.

[RFGuy]

The problem is that the motor used in Shopsmith equipment is an induction motor design...as such it is designed to run at a synchronous speed. This speed is set by the AC line frequency (commonly 60Hz in the USA) and the number of poles in the motor winding. Typically this works out to 3600RPM for most common 120VAC induction motors. This is why Shopsmith designed variable pulleys (motor sheaves) which effectively is a mechanical transmission to lower the 3600RPM to usually 700RPM at its lowest speed on the speed dial. A further mechanical reduction can be obtained by used the Shopsmith speed reducer (pic below), which if memory serves is a 7:1 reduction. Now induction motors do have slip, meaning under load the actual shaft RPM can be lower than the designed synchronous speed of 3600RPM. IF it slips too much then motor current can increase significantly even leading to meltdown. It would be great if there was a straightforward electrical way to lower the speed of a Shopsmith, i.e. one with an induction motor. The PowerPro is a completely different motor architecture, so not going to talk about it here. Unfortunately, there just isn't a way to safely, reliably and effectively lower the shaft speed on a Shopsmith induction motor using any kind of external variac (transformer or semiconductor solution). In principle, VFD's can and do work on induction motors. I don't have enough experience to comment here, but my understanding is it would have more to do with the frequency change of the line voltage input into the motor than the voltage change. On a single phase, 120VAC induction motor, whether a VFD could give you a wide enough speed range is in question, I think. However, VFD's can be very effective at speed control on 3 phase induction motors. Bottomline, a VFD solution is expensive and still may not get the job done. A router style speed control (or variac of any type) works well on universal motors like routers, but could damage your Shopsmith induction motor and NOT give you the speed change you want. For what you want to do, I think your best bet is to try to find a Shopsmith speed reducer (pic below of one). I think Shopsmith stopped selling them, but they do show up on the used market.

P.S. Your question was a good question and it comes up from time to time here on the forum. What I am saying is IF there was a better way, someone would have already figured it out, but there are hard technical reasons for why this can't/shouldn't be done with an electrical speed control on an induction motor. Having said this, there are other ways, perhaps cheaper ways, to accomplish what you want. This thread linked below may be of interest to you:
viewtopic.php?t=28165

ShopsmithSpeedReducer.jpg (189.31 KiB) Viewed 203 times

[nogbert]

thanks a lot for the thoughtful response. i've seen many homebrew pully speed reducers for these but I thought there must exist a more elegant solution!