Red B.
Posts: 1030
Joined: 8/29/2002
From: Jonkoping, SWEDEN
Status: offline
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Hi!
This question requires an answer of the type: "as simple as possible but not simpler".
As is usual the case a lot of contradicting information is spread and hence I will do my best to hopefully make things a little bit clearer ;-).
There are two kinds of brushless permanent magnet motors:
1) BRUSHLESS SYNCHRONOUS AC MOTORS
2) BRUSHLESS DC MOTORS
In principle either of the motor types can be used for modelling purposes but in practice type 2 motors are almost exclusively used nowadays.
First some basic information:
Brushless 3-phase DC motors have 3 windings evenly spaced at 120-degree intervals around the periphery of the motor. The rotor consist of permanent magnets. One end of each winding is connected in common and the other end is free to be driven externally. By placing a positive voltage on one coil and ground on another, a pair of coils forms a polarized magnetic field. Current are applied to the windings in pairs in such a way as to force the rotor to turn in the desired direction. However, since there is no commutator to time the current pulses applied to the windings, brushless DC motors must provide a way to sense the angle of the permanent magnet rotor, i.e. commutation must be performed by the speed controller.
And now for something slightly more complicated:
There are two reasons as for why type 2 motors are dominating the market for R/C aircraft motors:
a) Type 1 motors in general need to be powered by a three-phase sinusoidal current, requiring a fairly complex speed controller
Type 2 motors can be powered by by a set of currents having a quasisquare waveform (in principle direct current pulses that are switched on and off). This can be accomplished rather easily, resulting in a much simpler and possibly more efficient speed controller.
b) In order for the motor to produce torque, the current pulses applied to windings need to be timed so that they occur when the windings are in the correct positions relative to the permanent magnets of the motor.
For a type 1 motor this position must be accurately known and a constant supply of position information is necessary, i.e. a high resolution position sensor is needed.
For type 2 engines only the knowledge of six phase-commutation instants per electrical cycle is needed. What this means is that much simpler positioning detection techniques may be used.
Previously, Hall-effect sensor were used for this purpose but nowadays one usually does away with sensors altogether. This is accomplished in quite a clever way: Although there are usually three windings, hence the name "3 phase motors" only two of the three phase windings are conducting at a time and the third nonconducting winding carries the back EMF to give position information to the speed controller. Over a revolution the three different windings take turns to operate as either current carrying or feedback generating.
Speed control:
In order to alter the speed of the motor, the current pulses applied to the windings are usually pulse-width modulated by a set of power transistors as is the case for ordinary brushed DC-motors. Each winding of the motor is connected to a half bridge, each consisting of two power transistors connected to a common output point that can be driven either to ground or supply voltage. Thus, for a 3-phase motor the pulse width modulating circuit must have six controller outputs that are timed in relation to the position of the windings relative to the rotor permanent magnet. To cause the rotor to turn, two of the six PWM signals must be activated. The torque and thus the resulting speed of the motor is proportional to the duty cycle of the pulse width modulated currents.
/Red B.
< Message edited by Red B. -- 4/2/2004 3:30:36 PM >
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RE: How does a brushless motor work? - 
