Speed control for a brushless motor

Thread Starter

Jonlate

Joined Dec 21, 2017
110
Just a quick question today guys,
I want to buy a cheap bench grinder that has a variable speed controller on it, but they don’t seem to around, and only expensive ones seem to be variable speed controlled.

Now I do know you can’t easily put a speed controller on an induction motor and this is what most bench grinders are made with today.
However a lot of the cheaper ones have brushless motors now.

So my question….
Is it as simple as putting a potentiometer into the circuit, or a dimmer switch?
Or won’t this setup work on a brushless motor?

Thanks again all you wonderful people!
 

MaxHeadRoom

Joined Jul 18, 2013
25,469
However a lot of the cheaper ones have brushless motors now.

So my question….
Is it as simple as putting a potentiometer into the circuit, or a dimmer switch?
Or won’t this setup work on a brushless motor?
I have never come across a grinder with a brushless motor? Do you have a model example?
Normally brushless motors require some kind of sophistication for speed control.
Again, the specific motor would need to be known.
 

MisterBill2

Joined Jan 23, 2018
12,442
I have a couple of bench grinders and all of the motors are brushless because they are induction motors. Two of them do have a capacitor, though.
and right now I am trying to imagine why you want to put a speed control on a bench grinder.
 

MaxHeadRoom

Joined Jul 18, 2013
25,469
easily put a speed controller on an induction motor and this is what most bench grinders are made with today.
However a lot of the cheaper ones have brushless motors now.
I have a couple of bench grinders and all of the motors are brushless because they are induction motors. Two of them do have a capacitor, though.
and right now I am trying to imagine why you want to put a speed control on a bench grinder.
I was responding to the OP's misleading statement by mentioning induction motors and brushless in the same sentence as two alternatives!! :confused:
 

DC_Kid

Joined Feb 25, 2008
924
All depends on actual motor type.
A VFD can control speed of some AC motors.
As example, Pentair pool pumps, can run different speeds using it's built-in VFD.
 

DC_Kid

Joined Feb 25, 2008
924
It isstill very much a puzzle as to why there is any desire to reduce the speed of a bench grinder.
Depends on what the motor with threaded shaft ends is being used for. Grinding wheel to perhaps buffer wheels, I would want a VFD for speed control. This is better than having 2-3 different machines for hobby stuff. "all-in-one", etc.
 
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Thread Starter

Jonlate

Joined Dec 21, 2017
110
Sorry for the misleading statement!
I read on the Screwfix website it had a brushless motor, and thought that this was the same as a brush motor, but without the brushes.
I have just noticed thought, it does say it’s an induction motor.
I did think the two were different things(brushless and induction), but in this case it seems they are not!
So sorry for that confusion, my fault entirely.

My small bench grinder with speed control can be fitted with a dremel type extension thing, so I can use small cut off wheels and small polishing pads, hence needing lower speeds.
So I was looking for a new one I could still use this on, and thus wanting the speed controller bit.
Also a slower speed is good for grinding smaller items as they don’t get so hot!
 

MisterBill2

Joined Jan 23, 2018
12,442
Sorry for the misleading statement!
I read on the Screwfix website it had a brushless motor, and thought that this was the same as a brush motor, but without the brushes.
I have just noticed thought, it does say it’s an induction motor.
I did think the two were different things(brushless and induction), but in this case it seems they are not!
So sorry for that confusion, my fault entirely.

My small bench grinder with speed control can be fitted with a dremel type extension thing, so I can use small cut off wheels and small polishing pads, hence needing lower speeds.
So I was looking for a new one I could still use this on, and thus wanting the speed controller bit.
Also a slower speed is good for grinding smaller items as they don’t get so hot!
OK, most of the grinders, including the small one with the attachable flexible shaft, are just plain "induction motors" Probably that small one is a shaded pole induction motor, even.
The term "brushless" currently is primarily a shortening of "D.C. Brushless motor." That is a rather different animal entirely.
The DC BRUSHLESS motor is closer to an inverter driven synchronous motor. Most of those motors use a rotor position sensor to determine which windings get power in which sequence. For these motors the speed is often set by some electronic signal, so the motor speed can often be adjusted easily. There are also a lot of pre-set speed BLDC motors that do not have an adjustable speed. Computer fans are one example.
 

MaxHeadRoom

Joined Jul 18, 2013
25,469
There is BLDC (Brushless DC) and 3ph AC ECM motors which are essentially identical in construction having three stator windings and in that the rotor is made up of Permanent magnet.
The difference is in the commutation of each.
With the BLDC they have the same 3 windings but only two are energized at any one time and replicate a wound rotor DC motor but turned inside-out.
With the 3ph AC version the commutation is a 3phase sine wave.
 

MaxHeadRoom

Joined Jul 18, 2013
25,469
If brushless induction motor, then it is not BLDC or PM field AC
If BLDC or AC version, shorting the three leads together and try and spin the motor, if much resistance is felt, then it is the PM variety.
In many cases the AC PM variety has internal commutation electronics, but hard to tell the difference between the two without some examination.
.
 

MaxHeadRoom

Joined Jul 18, 2013
25,469
The only other brushed induction motor that I can think of is a wound rotor ac induction motor where the rotary winding exit via slip rings in order to control the current through the rotor to control RPM. as in lift cranes etc, they are not that common any more, In new applications, that is.
 

MisterBill2

Joined Jan 23, 2018
12,442
For speed control of the motor, assuming that adjustment over the top half of the speed range will be adequate, you can try a triac light dimmer as a cheap way to go. There are many published speed control circuits, including quite a few on the "Schematics for free" site.
I would not trust anything on yootoob or similar cartoon sites.
 

Thread Starter

Jonlate

Joined Dec 21, 2017
110
I have read many reports that say you can’t put a speed controller on an induction motor as they over heat, hence the question in the first place.
But there seems to be different sorts of motors advertised as all sorts of thing, some if which does make complete sense.
So not sure if to risk buying one and adding it, or buying one with it built in from new So I know it will work.
 

Alec_t

Joined Sep 17, 2013
12,906
I think any induction motor could have only a limited reduction of speed without a drastic loss of torque. If you want a large reduction (e.g. for sharpening tool blades with the grinder) then a DC motor would be more appropriate.
 

MisterBill2

Joined Jan 23, 2018
12,442
I have read many reports that say you can’t put a speed controller on an induction motor as they over heat, hence the question in the first place.
But there seems to be different sorts of motors advertised as all sorts of thing, some if which does make complete sense.
So not sure if to risk buying one and adding it, or buying one with it built in from new So I know it will work.
This is why I stated " For speed control of the motor, assuming that adjustment over the top half of the speed range will be adequate. Certainly at some point the torque will drop quite a bit. But quite often the required torque drops along with the speed. So for many applications, such as the small grinder with the flexible shaft attachment, that is acceptable. For some other applications, no it is not.
But trying it with an external light dimmer is cheap and easy and does not modify the motor at all.
 
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