Electric Flight in Australia

 

What is a brushless motor?

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Peter Pine
PO Box 324
Pottsville Beach NSW 2489
Phone (02) 6676 1437
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E-mail: ppine@northnet.com.au  
 
 
  
 
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First you need to have a basic understanding of a conventional brushed electric motor. A conventional electric motor has an armature that runs on two bearings inside a motor case which has permanent magnets mounted around the inside. The armature is made up of windings of copper wire that are wound around an iron core. This armature is in segments – each segment is connected to a section of the commutator. The commutator is the copper part at the back of the armature which is also broken into segments.

All this works when an electric current is applied to the commutator, which electrifies one section of the armature at a time, which creates a magnetic field in the core, which reacts with the permanent magnets mounted on the case. This magnetic repulsion of two magnets (one electrical, one permanent) causes the armature to rotate on its bearings. As the motor rotates, the commutator “switches” the current from one segment to another to maintain motion.

Now, the current is applied to the commutator by brushes, which are usually made from carbon which is conductive. Herein lies the problem. The carbon running on the rotating commutator causes friction, and creates sparking which can interfere with radio reception. Also, the brushes wear away, eventually need replacing, and foul up the motor with carbon dust that needs to be cleaned from time to time. All this action creates heat, which builds up in the motor and reduces efficiency (and can demagnetise the permanent magnets).

A brushless motor overcomes many of these disadvantages. The magnets are reversed – the permanent magnets are mounted on the shaft which still runs in two bearings. The windings are mounted in segments around the inside of the case. Current is applied to these windings to produce a magnetic field which reacts with the permanent magnets on the shaft to create movement.

Such a brushless motor is similar to an AC self-synchronous motor. All self-synchronising devices need a commutator to “switch” the current. The brushless motor has a commutator, but it is an electronic commutator rather than the traditional mechanical commutator of the brushed motor. This commutator function is now built in to the electronic speed controller, which also converts the DC current from the battery into AC. That is why you can pay so much more for a good brushless speed controller!

The advantages of the brushless motor are that there is no mechanical contact between the windings and the rotating shaft, so there is less friction and less wear. Also, because the windings are energised by the speed controller (rather than spark producing brushes), there is less heat build up, and the heat that does build up is removed more rapidly.

The result for us as electric fliers is that motors can be smaller, lighter, and more efficient. The power that can be obtained from a brushless motor far exceeds that available from a brushed motor.

However there are still valid applications for brushed motors in electric flight. For example, it is easier to organise multiple motors (which are popular at the moment). There are also applications where we do not need the maximum power available, and brushed motors will continue to be used in sport applications, especially where cost is a consideration. Brushless motors are becoming less expensive, but the speed controller cost will probably remain an issue.

I have tried to explain brushless motors in simple terms and may have become imprecise in my attempt to simplify. Please let me know if you find any anomalies in this explanation, or anything difficult to understand! The comments here refer to an inner-runner brushless motor. Outer-runners are now very popular and need to be the subject of another page.