Brushless 36v electric motors?

[stevensfo]

I've seen more cordless DIY tools being advertised as having brushless electric motors and thus being faster and longer-lasting.

But why is this? I'm thinking of getting a 36v Makita chainsaw (We use mainly cordless tools these days) and there does indeed seem to be a huge difference in speed of chain rotation. But does this translate into increased power? The voltage is the same, so surely the torque is the same?

Steve

[Urbandreamer]

Ok, I'll have a do.

All motors energise windings in sequence to cause the motor to turn. What we call a standard DC motor achieves this by connecting the windings in the rotating part to segments in a ring at one end of the motor. Carbon brushes carry the power to the ring and hence to the winding. As the motor turns the brush connects to another segment and winding.

Carbon brushes wear, as does the commutator (the segmented ring).

Brushless motors usually replace a wound rotating part with a magnet and switch on and off windings around it. No comutator and brushes to wear means a longer life.

Semiconductors can switch faster reliably than a mechanical commutator too.

You need the semiconductors to control a motors speed anyway. Just fewer if using a standard DC motor.

Torque and power are a matter of motor design. However by having the magnet in the center of the motor, rather than on the outside, can make things a lot easier. Torque in particular is NOT a factor of voltage, but of current. Hence your car starter motor (a standard DC motor) has hugely more torque than your 36V electric drill, despite running on 12V. Of course it does draw 100's of amps.

Power is torque times speed by the way.

[servodude]

Torque in particular is NOT a factor of voltage, but of current.

That's it in a nutshell
- It's the current that produces the torque

For any given set of motor windings increasing the voltage will:
- allow more current - because the current is the voltage over the winding impedance
- allow more speed - because the voltage generated by the motor (the back EMF) is limited to the supply voltage

In addition to the power being torque x speed it's also V x I (or V^2 / winding impedance)
- i.e. it increases with the square of the voltage

- sd

[dspp]

I've seen more cordless DIY tools being advertised as having brushless electric motors and thus being faster and longer-lasting.

But why is this? I'm thinking of getting a 36v Makita chainsaw (We use mainly cordless tools these days) and there does indeed seem to be a huge difference in speed of chain rotation. But does this translate into increased power? The voltage is the same, so surely the torque is the same?

Steve

faster = yes, due to the electronic switching as opposed to the intrinsically mechanical switching of the brushes
longer-lasting = generally yes, as there is no brush to wear out; but when they fail they are generally uneconomic to repair whereas brushed motors are 50/50 repairable
huge difference in speed of chain rotation = yes, see faster
increased power? = depends on the design, but for a given motor size (i.e. o/a tool weight) the power tends to be about the same
so surely the torque is the same? = for a given speed and power. will be the same

regards, dspp

[servodude]

faster = yes, due to the electronic switching as opposed to the intrinsically mechanical switching of the brushes

shaft rotational speed is a function of the winding voltage
- if your commutation is mistimed it will be similar to reducing the winding voltage a bit (as your generated field will be not where it needs to be)
- but both brushed and electric switching are "almost" instantaneous with respect to the rotation

- sd

[Urbandreamer]

increased power? = depends on the design, but for a given motor size (i.e. o/a tool weight) the power tends to be about the same
so surely the torque is the same? = for a given speed and power. will be the same

regards, dspp

Well yes and no.

Torque relates to the magnetic design of the motor as well as current. Rare earth magnets are not cheap. Gaps between winding and magnet pole need to be as small as possible. Putting the magnet in the rotor means that you can use a lot less material and have higher flux densities. We have also agreed that top speed of the motor is less constrained with brushless, so the "given speed and power" is not likely to be the case.

NOTE I'm just talking DC V Brushless DC and small motors for cordless tools. Things can get really quite exotic even there. Dyson use an asymmetric two pole motor in their handheld vacuum. It runs as fast as it can without the rotor exploding due to centrifugal forces. Some motors run faster and wrap the magnet in carbon fibre belts.

[Urbandreamer]

Oh, here's a link to an article in electronics weekly about the electric motor used in the Dyson hand held vac from 10 years ago.
https://www.electronicsweekly.com/marke ... y-2009-06/

104,000 rpm!

[Mike4]

And opening out the discussion, a higher battery voltage on a cordless tool strikes me as irrelevant.

All lithium batteries of a given size/weight store the same amount of energy, and the voltage of the battery is simply a function of how all the 3.2v lithium cells inside the battery are wired up. So higher voltage means lower current and power = voltage x current, so choosing tools according to battery voltage is pointless. You should choose according to battery weight, I reckon!

[servodude]

And opening out the discussion, a higher battery voltage on a cordless tool strikes me as irrelevant.

All lithium batteries of a given size/weight store the same amount of energy, and the voltage of the battery is simply a function of how all the 3.2v lithium cells inside the battery are wired up. So higher voltage means lower current and power = voltage x current, so choosing tools according to battery voltage is pointless. You should choose according to battery weight, I reckon!

...but to get the required voltage for the necessary current for the torque you need you would need a DCDC boosting regulator if you didn't do it via cells in series (and yes they're typically 18650s in a box)

- sd

[Urbandreamer]

The issue with high power low voltage is wires.

The heating effect (wasted energy) is I*I*R*t.
I'm going to claim that power is V*I. It's actualy V*I*Pf for DC motors.
Where V is voltage, I is current, R resistance and t time.

So for a given power, doubling your design voltage cuts the power loss in the wires by a factor of 4 for the same size wire, or allows you to reduce the amount of copper in the wires (size) by a similar factor.

Moving from 3.2V to 36V the factor is (36/3.2) squared, or 126 times.

It's not too much of an issue with low power hand held tools, but as the motor power increases the heating effect in the windings also becomes an issue. Putting the windings on the outside of the motor allows better thermal design and avoids the issue of needing to force coolant (ie air) around the rotating windings (meaning bigger gaps and less torque).

[dspp]

faster = yes, due to the electronic switching as opposed to the intrinsically mechanical switching of the brushes

shaft rotational speed is a function of the winding voltage
- if your commutation is mistimed it will be similar to reducing the winding voltage a bit (as your generated field will be not where it needs to be)
- but both brushed and electric switching are "almost" instantaneous with respect to the rotation

- sd

Mmmmm .... I think we are talking at cross purposes. I am noting that for the same voltage you can get different rotational speeds. It depends on how you wind the motor and arrange the poles, and manage them. Ultimately in a mechanical (brushed) motor the constraint will become the physical size of the brushes required to carry the current and simultaneously be in a 'break-before-make' geometry. In contrast a inverter drive is switching the power semiconductors which of course do not have a brush and so can, for the devices we are discussing, switch faster.

There are whole books written on the subject and it has more complexities than I care to think about, and it is these complexities that the more modern motors are exploiting. Especially those related to the power semiconductor chipsets, and the automated manufacturing equipment, that together make this economically viable. (I think you know this mind you, but the OP may not)

regards, dspp

[servodude]

Mmmmm .... I think we are talking at cross purposes. I am noting that for the same voltage you can get different rotational speeds. It depends on how you wind the motor and arrange the poles, and manage them.

Indeed, mea culpa
I was coming at it from the position of having a motor and changing the driving (as that's where I normally find myself )
- If you can change the motor to suit the voltage you have you got you can probably get it to spin at a speed you like

One of the great things about controlling BLDC motors is the curves all look the same (just the points they cross the axes change)
- and yeah as they're constrained by brush requirements you can get as fancy as you need for the control
- sd

[dspp]

Mmmmm .... I think we are talking at cross purposes. I am noting that for the same voltage you can get different rotational speeds. It depends on how you wind the motor and arrange the poles, and manage them.

Indeed, mea culpa
I was coming at it from the position of having a motor and changing the driving (as that's where I normally find myself )
- If you can change the motor to suit the voltage you have you got you can probably get it to spin at a speed you like

One of the great things about controlling BLDC motors is the curves all look the same (just the points they cross the axes change)
- and yeah as they're constrained by brush requirements you can get as fancy as you need for the control
- sd

No worries, I did guess as such from your TLF-user-name To be fair I think you are probably far more hands-on with this stuff than I ordinarily am.

regards, dspp

[88V8]

Well, if* I use my cordless drill at high torque for too long it burns the tracks on the variable-speed control switch, so then I'm just left with Go and Stop. It's a Ryobi, not exactly top-drawer.

I think the key to buying tools is to buy the best, rather then worry about the minutae of the design, interesting though it is.

The trend that bugs me with cordless, is the tendency to build-in the battery, which means one can't swap in a charged spare - I have four - and the tool will eventually self-obsolesce when the battery dies.

V8 *or perhaps that should be 'when', some of us never learn.

[servodude]

The trend that bugs me with cordless, is the tendency to build-in the battery, which means one can't swap in a charged spare - I have four - and the tool will eventually self-obsolesce when the battery dies

That's a trend that's fading
- most manufacturers now have a range with swappable battery packs for just the reasons you've given
- even the cheap Ozito PowerX ones from Homebase are better than I expected (but probably not up to daily use)
-sd

[jfgw]

So for a given power, doubling your design voltage cuts the power loss in the wires by a factor of 4 for the same size wire, or allows you to reduce the amount of copper in the wires (size) by a similar factor.

This doesn't make sense to me.

If you double the voltage, you halve the current. You will need to halve the size (CSA) of the wire (or it won't fit) but you need double the number of turns. This will quadruple the resistance and I*I*R will remain the same.

Looking at it differently, let's say you have a 10V 2A motor with a 1 ohm resistance. The voltage drop due to resistance will be 2V which, at 2A, gives 4W.
Now rewind the motor for 20V at 1A. It will have a resistance of 4 ohms. The voltage drop due to resistance will be 4V which, at 1A, also gives 4W.

If the motor windings are switched by a semiconductor device, there will be a voltage drop (two voltage drops if the motor is driven by an H-bridge). This becomes more significant with lower voltages.

In addition to the power being torque x speed it's also V x I (or V^2 / winding impedance)
- i.e. it increases with the square of the voltage

A valid point, although it should be noted that torque x speed is output power whereas V x I is input power. Efficiency is also a factor which is, I believe, is one area where brushless motors are better than brushed ones.


Julian F. G. W.

[Urbandreamer]

So for a given power, doubling your design voltage cuts the power loss in the wires by a factor of 4 for the same size wire, or allows you to reduce the amount of copper in the wires (size) by a similar factor.

This doesn't make sense to me.

If you double the voltage, you halve the current. You will need to halve the size (CSA) of the wire (or it won't fit) but you need double the number of turns. This will quadruple the resistance and I*I*R will remain the same.

Looking at it differently, let's say you have a 10V 2A motor with a 1 ohm resistance. The voltage drop due to resistance will be 2V which, at 2A, gives 4W.
Now rewind the motor for 20V at 1A. It will have a resistance of 4 ohms. The voltage drop due to resistance will be 4V which, at 1A, also gives 4W.

Julian F. G. W.

You are right in what you are saying, but wrong in what you thought I was saying. I was mostly talking about the wires to the motor. Though I did also mention heating in the windings which could have caused the confusion.

[jfgw]

You are right in what you are saying, but wrong in what you thought I was saying. I was mostly talking about the wires to the motor. Though I did also mention heating in the windings which could have caused the confusion.

I would expect this to be minimal with a drill (I see you made that point).

Battery losses would be a factor, however, given that different battery packs tend to be made up of the same size of cell. Judging by these graphs, differences in current can make quite big differences to voltage,
https://batteryuniversity.com/learn/art ... ristics_li .


Julian F. G. W.