Advice please – Brushless motors

[Tim StevensParticipant @timstevens64731]

I like to know how stuff works, and I have a question about brushless motors, as used in cordless drills etc. I have just acquired a new 21V Lithium-battery drill, and the web pictures seem to show a ring of magnets around a set of coils. This makes sense to me. When the drill is operated, it emits a fairly musical steady tone as the trigger is pressed, changing suddenly to a higher tone in about six steps, as more pressure is applied. I think this means that instead of a gradually varying control knob producing a gradually variable frequency of rotation of the magnetic field – like I have on lathes with VFD controllers – this drill has steps in the output, rather like primitive battery-electric vehicles in the olden days, such as invalid carriages and milk floats. No doubt the step method is cheaper (it was a very cheap drill) but apart from that, what difference does it make, when used to drive things like lathes, milling machines, etc?

Or have I got the wrong end of several sticks (again) ?

Regards, Tim

[Tim StevensParticipant @timstevens64731]

[Ady1Participant @ady1]

The brushless drill I have has massive torque compared to conventional hand drills, including corded drills

I use it for reaming out holes to 12mm and drilling in steel

Have to hold it a certain way to stop it twisting my arms/wrists

[John OlsenParticipant @johnolsen79199]

There several different ways to implement a brushless motor. For instance, the combination of a VFD and an induction motor can be said to be a brushless motor. But more usually it refers to a motor with permanent magnets and a solid state circuit instead of a commutator. Typically these will have a fixed stator with three windings on it, and a moving rotor with permanent magnets By applying three phase AC to the windings, a rotating field is created, and the rotor will try to follow it. Varying the frequency of the AC will vary the speed. The simple units in model aircraft use the back EMF from the coils to determine what the motor is doing and so control the AC frequency being sent to it. More sophisticated units like those in disc drives use hall effect sensors to determine the actual rotor position and so can maintain better control. This can give better starting and probably higher torque. I haven't seen what is inside a drill one, but it would be good to have extra torque in this situation.

John

[Ian PParticipant @ianp]

When there is a motor controlled by electronics there is little difference on cost whether the speed is set by a potentiometer or a multiposition switch.

'Ring of magnets around a set of coils' hints of it being an external rotor motor (often with silly description of being an 'Outrunner&#39 similar to the type used in model aircraft. Motor speed is controlled by changing the commutation frequency.

What make and model is it, also the one Ady mentioned withe the very high tourque?

Ian P

[Tim StevensParticipant @timstevens64731]

Model: Ruyika –

Brushless 21V Cordless Drill Driver Set Electric Combi Screwdriver 2.5A Battery

No, I haven't heard of them, either. The seller is called Ironman, on ebay.

The maker's name is carried on a sticker (which is peeling) so my guess is that there are other 'makes' with the same spec but variations on colour.

So far, it seems to be OK – but time will tell.

Regards, Tim

[Ian PParticipant @ianp]

It certainly looks good for the money!

Hard to tell from the pictures what the motor is, I have seen hundreds of different brushless motors over the years but no examples of ones used in cordless power tools.

The listing first picture shows something that might be the motor (with what looks like a section picture of heavy duty cable! ) but its hard to reconcile that with the the other computer generated images that appear to show a fairly small diameter motor. Ballraces appear in one picture but maybe they are just for decoration. Does the charger come with a UK mains plug or if not is that the reason for the strategically placed bit of white cloth.

I'm still tempted to get one though!

Ian P

 

Edit, White cloth is gloves!

Edited By Ian P on 22/12/2020 22:35:12

[Anonymous]

Posted by Tim Stevens on 22/12/2020 18:53:23:

Or have I got the wrong end of several sticks (again) ?

I'm afraid so.

A VFD driving an induction motor effectively creates three sine waves spaced 2pi/3 apart electrically, and mechanically in the motor. These create a rotating magnetic field of, theoretically, constant amplitude.

On a brushless DC motor each set of coils is simply activated in turn and is either on or off. So the magnetic field does rotate and the rotor follows. But it's more like a stepper motor than an induction motor.

In either case there's nothing that says the rate at which the frequency changes needs to be linear or continuous. With regards to the drill it will be much cheaper (and reliable) to switch in steps than having to use a potentiometer. Practically speaking, changing speeds in a series of steps rather than linearly isn't going to make much difference. It's a piecewise linear approximation.

Andrew

[Tim StevensParticipant @timstevens64731]

Andrew J says: With regards to the drill it will be much cheaper (and reliable) to switch in steps than having to use a potentiometer.

Yes, that is what I was thinking. So that stick (at least) was properly held. Thanks.

Ian P asks: Does the charger come with a UK mains plug or if not is that the reason for the strategically placed bit of white cloth.?

Yes, a very small charger-plug but it works, and with lights to say charging (red) and finished (green). And yes, the white cloth is a pair of gloves – fit my smallish hands perfectly.

As could be expected, the tools supplied seem to be mickey-mouse, but even without them it seems good value so far. And when it finally fails I will enjoy peering inside to see what did what …

Seasonable wossnames to you all – Tim

[Tim StevensParticipant @timstevens64731]

Andrew J says: With regards to the drill it will be much cheaper (and reliable) to switch in steps than having to use a potentiometer.

Yes, that is what I was thinking. So that stick (at least) was properly held. Thanks.

Ian P asks: Does the charger come with a UK mains plug or if not is that the reason for the strategically placed bit of white cloth.?

Yes, a very small charger-plug but it works, and with lights to say charging (red) and finished (green). And yes, the white cloth is a pair of gloves – fit my smallish hands perfectly.

As could be expected, the tools supplied seem to be mickey-mouse, but even without them it seems good value so far. And when it finally fails I will enjoy peering inside to see what did what …

Seasonable wossnames to you all – Tim

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