Electric R/C Flight - Brushless Motors

Electric R/C Flight

By Heather Mardon

Part 5: Brushless Motors

Hi,

We’re back into the technical stuff this month on the subject of ‘Brushless Motors’. These are the ‘Holy Grail’ of electric motors, some say once you try one, you will never look at a brushed motor again!

The Theory

A brushless motor is like a brushed motor turned inside out. This time the coils (windings) stay put and it’s the magnets that do the turning. Each winding is electronically switched in succession by power Mosfet Transistors in the controller, causing the rotor to spin. This is exactly the same way a three-phase AC motor works, the only difference is it needs a controller circuit to generate the different phases.

There are two styles of brushless motor available for R/C aircraft, the conventional type with the coils around the outside, and the ‘outrunner’ style, which has the magnets & rotor on the outside

	*A cutaway of Kontronik Conventional Brushless motors* (This style motor can run at very high rpm’s, in some cases up to 80,000)
	*LRK TorqueMax Outrunner Design* (Notice the windings stay still and it’s the outercase that spins. These motors have higher torque due to having more poles, much like a multi cylinder engine. However they cannot spin as fast due to the greater mass of the outer case)

The controller needs to be able to tell the position of the rotor at any time so it knows when to energise the next coil. It does this either by means of Hall effect sensors installed in the back of the motor or by sensing a voltage coming back from the denergised windings. This is where the term’s sensored or sensorless motors & controllers come from. These days sensorless are more popular mainly due to a bit of electronics being cheaper to make than fitting sensors inside the motor.

What are the Advantages?

There are many:

Less radio interference due to lack of brush arcing
Less resistance in the windings due to lack of brush resistance and the fact that you can fit much more copper inside (thicker wires) than a brushed motor of equivalent size. Less resistance = higher efficiency and torque
No friction loss from spring pressure in the brushes
Due to the reasons above they are highly efficient over a much wider range of power. Efficiencies range from about 75% up to nearly 90% depending on style/brand etc.
Better cooling in the conventional style due to the windings being against the outer case.
No maintenance

If there so good why don’t we all have them?

There’s always a catch, and this is pretty simple one……MONEY! Yes they cost more than that $25 dollar can motor. They are of course a quality item not mass-produced in there millions so will always be dearer. Another reason is that unlike brushed motors there are no machines that can wind the coils, therefore making it a labour intensive process.

The other cost factor is that they require dedicated brushless speed controllers which have a lot more circuitry on them than there brushed counterparts. In some cases the controller can cost more than the motor.

Examples

This is a Model Motors AXI 2820/10, it is an "Outrunner style" motor. The weight is only 161grams but can handle about 300watts of power and drive a 12 x 8 prop on 7 cells without a gearbox. I plan to install this into a Global AT-6 ARF kit.

This is a Mega Motors Ac 22/30/4 conventional style brushless. It is presently installed in Ian’s ‘Beat’ Hotliner glider. It runs off 12 cells and drives a 12 x 8 prop direct drive with a current draw of about 40 amps. (~480watts)

What’s on my building board?

I’ve nearly finished my rebuild of the Electricub, just have to finish the cowl. I have flown it several times and I’ve been very pleased with the results. The power plant is an old Kyosho 360 motor that a had in a boat, now running in a 2.3:1 belt drive turning a 10x7 electric prop on 7 or 8 cells.

Flight times are around 8 minutes with very scale like performance even if the colour scheme is not.

Another project underway is an indoor aerobatic flyer, although I’m having doubts about its flimsy construction. I’ll be luck to get it finished without breaking it, let alone fly it!

That’s me for another month, keep your watts up!

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	This is a Model Motors AXI 2820/10, it is an "Outrunner style" motor. The weight is only 161grams but can handle about 300watts of power and drive a 12 x 8 prop on 7 cells without a gearbox. I plan to install this into a Global AT-6 ARF kit.
	This is a Mega Motors Ac 22/30/4 conventional style brushless. It is presently installed in Ian’s ‘Beat’ Hotliner glider. It runs off 12 cells and drives a 12 x 8 prop direct drive with a current draw of about 40 amps. (~480watts)