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Adding potentiometer to bench grinder.

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PhilTilson

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I'm still wondering why the OP wants to do this. As I recall from my workshop practice sessions decades ago, grinding wheels, like cutting tools, have a preferred cutting speed to produce clean cuts (which is what carborundum is doing on a microscopic scale) and avoid clogging etc. If his aim is to produce a more 'gentle' action, then he should be achieving this by using less pressure surely?
 

evildrome

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Ebay's full of these things:


Which definitely work for fan motors, I can tell you that.

I've got a massive vacuum cleaner motor that trips the board if I just switch it on at full skelp but if I put one of these between it & the supply, I can start at a low speed and turn the speed up to whatever I want.

Obviously, the torque will not be the same at 100rpm as at 10,000 rpm but it does very effectively control the speed.

And the price is right.
 

Dr Al

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Ebay's full of these things:


Which definitely work for fan motors, I can tell you that.

I've got a massive vacuum cleaner motor that trips the board if I just switch it on at full skelp but if I put one of these between it & the supply, I can start at a low speed and turn the speed up to whatever I want.

Obviously, the torque will not be the same at 100rpm as at 10,000 rpm but it does very effectively control the speed.

And the price is right.
That SCR module is delaying the turn-on of each cycle, which reduces the average voltage and will increase slip making the efficiency get worse and worse as the speed reduces. Fans are a bit of special case when it comes to motor control. The torque required for a fan is (ignoring losses) negligible at low speed and ramps up with speed. As a result, you can drop the voltage and accept the loss of torque and that's fine. With a bench grinder this isn't true.

Torque follows a square law with respect to voltage; drop the voltage in half and the torque goes down to a quarter. You'll get away with this for a fan or pump, but not for much else.
 

TheTiddles

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I'm still wondering why the OP wants to do this. As I recall from my workshop practice sessions decades ago, grinding wheels, like cutting tools, have a preferred cutting speed to produce clean cuts (which is what carborundum is doing on a microscopic scale) and avoid clogging etc. If his aim is to produce a more 'gentle' action, then he should be achieving this by using less pressure surely?
This is the pertinent question, slow running grinders are a thing, but they cost more for a reason

Aidan
 

Lefley

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There is a reason a variable speed bench grinder that id
s a dc motor is worth 1,500 dollars.
 

Yorkieguy

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Bench grinders and the like do not tend to use capacitor start motors, so a simple rheostat will reduce the speed and torque but you will probably only have a limited usable reduction in speed. An example of this type of speed control is the sewing machines foot pedal, a form of variable resistance.
You are mistaken. The OP said it's a Record Power 8" Bench grinder. It will be the 'RSBG8' which is a capacitor start. You can download the manual from Record Power if you like, but there's no need to bother - I've just had to replace the capacitor in mine as it stopped working, and a failed capacitor as often as not is the cause. It's located in the base of the grinder. Induction motors on grinders run at a fixed speed and you can't compare them to sewing machine motors.

If you think a potentiometer (wired as a rheostat) will work on a bench grinder, by all means give it a try, but ride at your own risk. Generally when people get up to those sort of antics, all too often they end up doing is proving Darwin's theory of the non-survival of the stupidest. ('Red to red, black to black, throw the switch and stand well back').

If it was as simple as just popping a rheostat on a bench grinder to vary the speed, do you not think that by now most grinders would have one? That's not to say that purpose designed grinders with variable speed controls don't exist - clearly they do - (well not really grinders as such, but especially for sharpening woodworking and woodturning tools.) Tormek for instance or the Record Power WG250 Wet-stone grinder system which is adjustable from 90RPM - 150 RPM, but the motor and the controller are designed for that purpose:

WG250-PK/A 10" Wet Stone Sharpening System Package Deal (recordpower.co.uk)
 

jcassidy

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That New England guy on YouTube with the cap and mustache has a hand-driven wheel in his studio. Workshop, I mean. There's a variable-speed grinder right there. Down in Sicily we still get old guys going door to door to sharpen knives, scissors, whatnot, all with a hand grinder on the back of their Ape.
 

andy48

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1. As Yorkiguy has said, it's an induction motor.
2. The only way of varying the speed is by varying the frequency. This requires an inverter, but these only work on three phase motors.
3. Even if you could fit an inverter, it would be extremely dangerous unless you found a way of limiting the output frequency to 50 hz or less. More than 50 hz and the motor will run faster than rated, with a high risk of bursting the grinding wheel.
 

Sideways

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Yes, the record 8" is a single phase induction motor, probably a permanent split capacitor rather than a cap start as there is no sound of a centrifugal switch engaging and disengaging (that I've noticed on mine anyway).
For Andy's info, there is a drive sold by Invertek specifically for controlling this type of motor. No problem, no drama. They're the only people I know who make one. It works properly by varying output frequency and voltage together, but it isn't a gearbox - dropping the speed drops the power and doesn't increase the torque. On a grinder it seems pointless as you'll tend to overload or stall the motor when you shove something into the wheel at low speed.
I have all the bits in my garage and could assemble this tomorrow. Pointless. Won't.
 

Lard

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Bet you wished you’d never asked now?😳 🙂
Eyup, I'm just about to order a potentiometer to give my 8" record power bench grinder variable speed. Should be a fairly simple job, has anyone done this before? Is there something I'm missing or is it really as simple as wiring the pot in front of the motor?

Cheers

Damo
 

John Brown

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You are mistaken. The OP said it's a Record Power 8" Bench grinder. It will be the 'RSBG8' which is a capacitor start. You can download the manual from Record Power if you like, but there's no need to bother - I've just had to replace the capacitor in mine as it stopped working, and a failed capacitor as often as not is the cause. It's located in the base of the grinder. Induction motors on grinders run at a fixed speed and you can't compare them to sewing machine motors.

If you think a potentiometer (wired as a rheostat) will work on a bench grinder, by all means give it a try, but ride at your own risk. Generally when people get up to those sort of antics, all too often they end up doing is proving Darwin's theory of the non-survival of the stupidest. ('Red to red, black to black, throw the switch and stand well back').

If it was as simple as just popping a rheostat on a bench grinder to vary the speed, do you not think that by now most grinders would have one? That's not to say that purpose designed grinders with variable speed controls don't exist - clearly they do - (well not really grinders as such, but especially for sharpening woodworking and woodturning tools.) Tormek for instance or the Record Power WG250 Wet-stone grinder system which is adjustable from 90RPM - 150 RPM, but the motor and the controller are designed for that purpose:

WG250-PK/A 10" Wet Stone Sharpening System Package Deal (recordpower.co.uk)
You obviously know more about electric motors than you do about Darwinian evolution.
 

DamoF

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Aha! What a response! Luckily, it would seem, just after I posted this thread my teething 7 month old boy decided to go utterly mental, thus making me forget (like so many other things) about both my idea and also this thread. Silver linings eh?

This thread has, however, been quite the wealth of information and I'm certainly glad I asked.

Thanks very much everyone :)

Damo
 

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