Adding potentiometer to bench grinder.

[DamoF]

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

 

[Dee J]

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

I'd be interested to know what 'potentiometer' you have in mind. Most motor speed controllers either work on chopping the waveform for universal /brushed motors or changing the frequency for induction motors. Simple potentiometers were huge heat generating devices last seen controlling the motors aboard the Ark (Noah, nor Royal).

 

[flh801978]

Reason no-one has done it before is because you cant alter the speed of a single phase induction motor easily..certainly not with a potentiometer..

 

[Sideways]

What the man said above. No offense intended but you don't understand this, it won't work, and it's dangerous.
There isn't an easy fix for this short of a £60+ box of electronics that you won't understand how to connect or set up.
Even if you did, it won't work well so there's no point.
Sorry.

 

[Dr Al]

What they said. You're probably thinking of brushed DC motors. With a brushed DC motor, it is possible (although extremely inefficient) to drop the speed of the motor by putting a resistor in line. The resistor drops some of the voltage (producing heat) and ignoring load, the DC motor's speed is (roughly) proportional to voltage.

For an induction motor like those fitted in a bench grinder, the speed is more closely related to the frequency of the supply (50 Hz in the UK), although the voltage is relevant too. Putting a resistor or potentiometer in line will drop some voltage across the resistor, which will move the motor out of its optimal operating conditions, but it will still want to run at the same speed. Most likely it'll just produce a bit less power or stall; it certainly won't give you a useful variable speed drive. For that, you need a variable speed supply, which (put simply) will typically allow you to vary the frequency while keeping V/f (voltage divided by frequency) constant.

 

[TheTiddles]

What problem are you intending to solve?
Aidan

 

[Rorschach]

It won't work.

 

[Inspector]

If lucky you will just fry the pot. If unlucky you'll need a new grinder.

Pete

 

[Spectric]

What you need is a Rheostat, basically a variable resistance designed to flow current, a potentiometer is designed to control mA in electronic circuits.

You need to remember that using a simple variable resistance that limits the current to reduce speed will also reduce torque, a phase controller is the better option.

 

[Dr Al]

What you need is a Rheostat, basically a variable resistance designed to flow current, a potentiometer is designed to control mA in electronic circuits.

You need to remember that using a simple variable resistance that limits the current to reduce speed will also reduce torque, a phase controller is the better option.

Not true. A rheostat is a variable resistor configured for current limiting; a potentiometer is one configured for voltage adjustment, but a potentiometer can be used as a rheostat.

Neither will make a single phase induction motor variable speed.

 

[flh801978]

What you need is a Rheostat, basically a variable resistance designed to flow current, a potentiometer is designed to control mA in electronic circuits.

You need to remember that using a simple variable resistance that limits the current to reduce speed will also reduce torque, a phase controller is the better option.

So what sort of phase controller are you suggesting? For a single phase induction motor with capacitor start?

 

[Jelly]

Neither will make a single phase induction motor variable speed.

It might a little bit... but not in a good (or useful) way.

The only way limiting voltage could reduce motor speed would be by making the induced force so weak relative to the rotors rotational momentum, that it couldn't actually get the stator and rotor windings to sync properly.

Which would both play havoc with anything sensitive to those kinds of phase imbalances on the same circuit, and run the risk of the motor suddenly slowing or even reversing when the wheel was touched.

 

[Spectric]

Eyup, I'm just about to order a potentiometer to give my 8" record power bench grinder variable speed

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.

 

[flh801978]

Spectric
do you have any idea how dangerous some of your ideas are?
a bench grinder is always an induction motor with a capacitor running synchronous with the ac frequency..
a sewing machine motor is usually a brushed motor in 95% of machines which can be speed controlled easily with a resistance in series..newer sewing machines have brushless motors and electronic control (brushless doesn’t mean a synchronous motor)

 

[Rorschach]

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.

I've never yet seen a bench grinder that doesn't use an induction motor, even my £15 lidl cheapo is induction.

 

[Dr Al]

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.

I've never yet seen a bench grinder that doesn't use an induction motor, even my £15 lidl cheapo is induction.

The "capacitor start motors" that @Spectric is referring to are single phase induction motors (which need one or more capacitor to generate the extra phase to make the motors go round, sometimes with a centrifugal switch to change the capacitance once the motor gets up to speed, although the switch is probably not necessary in a grinder). Controlling the speed of single phase induction motors (as opposed to 3 phase motors of any sort) is a pain in the proverbial posterior and certainly isn't as simple as a variable resistor.

As @flh801978 has said, the variable sewing machines are either brushed DC (sometimes in the form of a "universal motor", which is a DC motor with armature and field windings wired together so it keeps going the same direction regardless of current direction and can hence run off an AC supply) or they're brushless PM motors with a variable speed drive. I'd be very, very surprised if you can find an induction motor in any sewing machines.

Anyway, I think enough people have answered @DamoF's post to say "don't do that", so hopefully we've got the point across! @DamoF - happy to chat about other options if it helps (my PhD was in motor control, so I can probably claim little bit of knowledge about the subject, even if my PhD didn't get that involved with induction motors!)

 

[TheTiddles]

happy to chat about other options if it helps (my PhD was in motor control, so I can probably claim little bit of knowledge about the subject, even if my PhD didn't get that involved with induction motors!)

Yes Doc, but have you got a YouTube channel though?!

 

[Dr Al]

Yes Doc, but have you got a YouTube channel though?!

Not now, not ever...

 

[Jelly]

I'd be very, very surprised if you can find an induction motor in any sewing machines.

At the risk of being a pedant...

I have two sewing machine induction motors in the workshop, a 1kw and 1.5kw; both fitted with a flywheel and clutch pack with a mechanical linkage (which would go to the pedal if they were fitted to a sewing machine).

I think that's par for the course for freestanding industrial sewing machines and overlockers, but the motors themselves weigh more than a domestic sewing machine.

 

[Rorschach]

At the risk of being a pedant...

I have two sewing machine induction motors in the workshop, a 1kw and 1.5kw; both fitted with a flywheel and clutch pack with a mechanical linkage (which would go to the pedal if they were fitted to a sewing machine).

I think that's par for the course for freestanding industrial sewing machines and overlockers, but the motors themselves weigh more than a domestic sewing machine.

Show off!