Speed control for disk sander using variac

[dzarren]

Hello. I just got this partial sander. It needs a motor.



I have a few beginner questions.
I want to set this up with variable speed, and to run off the 120V of my mains power.
I have a few variacs, my biggest one can do 5 amperes, but I have several that do 1.5 or 3 amperes.

What type of motor am I looking for, that will allow me to have variable speed, by giving it 0-120 VAC (maybe a bit more because of overwindings)
I don't need it to run in reverse.
I would be sanding both wood and metal on the disk sander, so I would like to not sacrifice the torque i might need.

Am I looking for trouble, given my limited knowledge?

What other things may I need asides from the motor, and the variac? and some wire?

Thanks for reading!

[hermit]

You can control AC motors a little bit with frequency variation but may be limited.  Variac will cut voltage and that will shorten the motor live considerably.  If you really need speed control, look at DC.

[Ian.M]

AC synchronous motors cant be speed controlled by reducing the voltage, all that does is reduce the torque till they stall.  AC induction motors have a very limited speed control range by voltage reduction, unless the load torque drops far faster than the speed, e.g. driving a fan.  Brushed DC and universal motors can be speed controlled by reducing the voltage, but don't expect a lot of torque at low speeds.  They normally run at high speeds and usually need a reduction gearbox.

The traditional option for tools like your sander would be a capacitor start single phase induction motor and a stepped pulley to choose a speed.  The up-market option would be a three phase motor and a VFD.    It looks like you need a 3/4 HP motor if you go for the stepped pulley (as it boosts torque at low speeds) or somewhere around 1.5 to 2 HP if using a VFD to have enough reserve torque at low speeds.

You might be able to do something with a big enough treadmill motor, and a really heavy duty bridge rectifier but 3/4 HP would be pushing the limits of your 5A Variac so you'd better build/buy a PWM speed controller. N.B. treadmill motors are usually overrated for HP. The actual continuous HP they can handle without burnout is typically less than half the marketing HP for the treadmill they came out of.

[jpanhalt]

1) Get a 3-phase motor and VFD to drive it.
2) Get a DC motor and PWM controller to drive it.

I have done both with my power tools and have not been disappointed.  Controlling the speed of a single-phase motor is a losing proposition.  Off hand, I would go the 3-phase route with the sander (more HP) and DC route with a smaller FHP device like a hobby lathe.

John

[Benta]

Mains-supplied hand tools and smaller table-top machines usually have a universal motor. This can speed controlled with a simple Triac-type dimmer.

[jpanhalt]

Just FYI:  Homecraft is a very old American brand and was the predecessor to Delta power tools. This sander is belt driven and choice of motor is up to the user.  I think an FHP universal motor would be a poor choice.  Sanders require good torque.  For example, my 12" disk sander has a 2 HP motor.

[Benta]

I think an FHP universal motor would be a poor choice.  Sanders require good torque.

I didn't say anything about fractional HP. And torque is something universal motors really have. There's a reason they're used for car starter motors.

[Ian.M]

Series wound universal motors have the high torque, and a car starter is typically several HP.  However a series wound motor without active speed control would be totally unsuitable and may even be dangerous driving a disk sander as it will tend to overspeed when unloaded.

The other issue when choosing a motor is the motor needs to be rated for a side load on the shaft bearing if its going to be used with a belt drive direct from the motor shaft.

[Benta]

However a series wound motor without active speed control would be totally unsuitable and may even be dangerous driving a disk sander as it will tend to overspeed when unloaded.

Well, that's not my proposal. I was just pointing out, that all the mains-powered hand tools I've seen use universal motors, though with compound elements for speed limiting. The speed control in a hand drill "trigger" is a simple Triac circuit.
Lightweight, lots of torque, easy to control. Downside: noisy as h... and normally need a reduction gear.

[oldway]

Indeed, why not use a cheap  (20 bucks) hand drill ?

[Ian.M]

Because a cheap drill motor will be fractional HP, probably doesn't have a low enough speed reduction gearbox, and the output shaft bearings cant take a sustained side load, so it would need to be mounted inline with the sander shaft, preferably with a flexible coupling, which means you have to accept the drill's output shaft speed as you cant get the further speed reduction you could with a belt drive.

If you have an old heavy duty >1000W fixed speed drill with a two speed reduction gearbox, e.g. 2400RPM and 900RPM, and add an external speed controller it may be suitable for intermittent use, using the 900 RPM gear whenever a low range speed is required, but its still a clunky solution, and a good condition old drill like that is worth far more as a drill than as a sander.  I've got such a drill + a bench stand and can use it as a bench disk sander with the disk holder directly chucked.  The only thing its missing is a nice adjustable table, but that could easily be added.

[rstofer]

Is the motor and speed control going to cost more than a brand new Grizzly 12" Disk Sander ($179)
http://www.grizzly.com/products/12-Disc-Sander/G7297

I have never heard of a speed control on a disk sander and I can't imagine why one would be useful.  Yes, it can be done but why?

New 1 HP motors on eBay tend to run around $100 and at Grainger for $600 (Baldor)

[Ian.M]

You don't need a lot of speed control, just a selection of speeds.  A stepped pully + a sprung idler on the slack side of the belt would be fine, then use a motor salvaged from a junk compressor (bad tank and bad pump).  If your scrounging skills are good enough it should be doable for under $30.

[Kleinstein]

The power needed depends a lot on what you want to grind and how much pressure and speed is used / needed. For grinding things like plastics - something like 50 W might be all it takes, as more power would only cause the part to get too hot. For use with wood or metal works, something like 0.5 - 1.5 HP might be more useful. Much more gets tricky with single phase 120 V.

The variac would be only useful for an universal motor - usually rather high RPM and noise and made for limited time usage. In the AC version the no load RPM is limited - it is the DC operation that is tricky. There are hardly standard motors of this type to buy - so it would be more like a full tool (like a hand drill) to use anyway.

Usually not to many different speeds are needed. My choice would be more like moving the belt to different pullies.  If one uses an 3 phase AC motor, one could get one with 2 speeds (like 3400 and 1700 U/min).