RE: How does a brushless motor work?   
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All Forums >> Electric Aircraft Universe >> Brushed/Brushless motors, speed controls, gear drives >> RE: How does a brushless motor work?
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RE: How does a brushless motor work? - 4/2/2004 8:23 AM   
john 8750



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Status: offline 		Matt,
I just reread all post's and am very confused.


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RE: How does a brushless motor work? - 4/2/2004 3:21 PM   
Red B.


 

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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? - 4/2/2004 5:09 PM   
Ron v. Sommeren



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quote:

... Brushless 3-phase DC motors have 3 windings ...
Or 6, 9, 12, 15, 18, ... etcetera. The number of coils has to be a multiple of 3. See the simulations above for example

quote:

... One end of each winding is connected in common and the other end is free to be driven externally.
That's a 'star/Y' configuration, there's also the 'triangle/delta' configuration. See the drawings almost at the bottom of this cd-rom motor conversion page:
http://members.jcom.home.ne.jp/4223215501/CD_Brushless.htm

If you want to know more about this subject, have a look at my signature at the bottom of my messages.

A 24-pole direkt drive helicopter motor made from a floppy disk drive


< Message edited by Ron v. Sommeren -- 4/2/2004 6:11:26 PM >

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RE: How does a brushless motor work? - 4/16/2004 3:02 AM   
electricfan



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Status: offline 		maybe i missed this, but how do esc's brake motors (brushless or brushed)? is it a mild reversed polarity?

ron, can a floppy disk drive really fly a heli?

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RE: How does a brushless motor work? - 4/16/2004 8:16 AM   
Ron v. Sommeren



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Status: offline 		To use a FDD motor as a direct-drive motor in a helicopter, it has to be rewound and remagneted with stronger magnets:
http://www.rcgroups.com/forums/showthread.php?s=&threadid=197862
http://alexrc.com/voytek_pl.html (German and Polish)

Have a look at the foour cd-rom conversion homepages mentioned in the first message of this two page thread:
http://www.rc-unionen.dk/nytforum/topic.asp?TOPIC_ID=368&whichpage=1
http://www.rc-unionen.dk/nytforum/topic.asp?TOPIC_ID=368&whichpage=2




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RE: How does a brushless motor work? - 4/18/2004 5:12 PM   
Tipover


 

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quote:

ORIGINAL: Red B.
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.

/Red B.


I'm having trouble visualizing how three windings can share a common, and still have a free end to be driven externally, with only three wire comming out of the motor. What am I missing??

Kevin

< Message edited by Tipover -- 4/18/2004 12:15:46 PM >

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RE: How does a brushless motor work? - 4/18/2004 7:31 PM   
Ron v. Sommeren



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Status: offline 		In a star configuration, the starpoint is floating.

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RE: How does a brushless motor work? - 4/19/2004 5:09 AM   
john 8750



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RE: How does a brushless motor work? - 4/19/2004 10:55 AM   
Ron v. Sommeren



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Status: offline 		star = wye = y
triangle = delta

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RE: How does a brushless motor work? - 4/19/2004 5:03 PM   
Tipover


 

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Kevin

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RE: How does a brushless motor work? - 4/30/2004 1:37 PM   
expat flyer


 

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From: La Salvetat St Gilles, FRANCE
Status: offline 		Sorry to be so ignorant, but what determines the direction of rotation?
Is it set by the timing in the brushless speed controller?
Will a brushless speed controller/motor run in either direction with equal efficiency once it is set up?

I am thinking of perhaps moving a motor from a normal tractor aircraft to a pusher model using the original prop.

thanks in anticipation

Jerry

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RE: How does a brushless motor work? - 4/30/2004 6:32 PM   
Matt Kirsch



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Status: online 		Yes, the timing of the electrical pulses that are sent to the motor determines the direction of rotation. Most airplane brushless speed controls can only drive a motor in one direction. You reverse rotation by swapping any two of the three wires running to the motor.

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RE: How does a brushless motor work? - 6/3/2004 7:38 PM   
Flypaper 2



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Status: offline 		Does it matter which direction the wires are wound on each armature looking at them from the end, as long as they are all in the same direction?

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RE: How does a brushless motor work? - 6/3/2004 8:22 PM   
Ron v. Sommeren



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Status: offline 		Yes Flypaper, it matters a great deal. Wind it the wrong way and coils will be working against eachother. Motor will not start or may get hot. Winding direction/diagram depends on the configuration, #statorteeth versus #magnets. A windingdiagram table:
http://www.kcflyingcircuits.com/BLmotors/windingdiagram.html

(uppercase) stands for clockwise winding
(lowercase) stands for counterclockwise winding
(-) stands for an empty tooth.

Eg. AaBbCc means:
phase/wire 1: wind tooth 1 CW, continue to tooth 2 and wind it CCW
phase/wire 2: wind tooth 3 CW, continue to tooth 4 and wind it CCW
phase/wire 3: wind tooth 5 CW, continue to tooth 6 and wind it CCW

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RE: How does a brushless motor work? - 6/4/2004 3:31 AM   
Flypaper 2



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Status: offline 		Thank you Ron, exactly what I was looking for. Now I can wind to my hearts content.

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RE: How does a brushless motor work? - 6/25/2004 7:29 PM   
adam_one


 

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quote:

ORIGINAL: GeraldO

The speed of the motor is regulated by 'chopping' the current (turning it on-off rapidly) at part throttle. With the current being chopped the average torque is reduced so it spins the load slower. The ESC will detect the loaded RPM and adjust the phase switching timing to match.


That's true, but I guess that the max rpm is limited by the 3-phase's frequency and the number of poles.
The max rpm = 2 x frequency x 60/number of poles

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RE: How does a brushless motor work? - 9/3/2004 4:36 AM   
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I'm absolutely new to all this, as not much of it was covered in my Intro to Electronics course, years and years ago. Much of the information in this thread has been extremely educational, and I thank everyone for their posts. Now I have a couple of questions to add.

Question #1: How does the number of poles affect the performance of a given motor. If, for example I have two similarly-sized motors, and one has n poles (where n is the minimum number required (3?)), and the second has 12 poles. According to Adam_One's post, the number of poles dictates the maximum rpm, but are there other factors that depend on the number of poles?

Question #2: I was told that for a given airplane model, I should buy a four-turn or five-turn motor. Can someone discuss the difference between with different "turn numbers", and explain the benefits and/or disadvantages of the different number of turns a motor has? If I'm buying a motor, is there a general level of specifications that I should look for, as a minimum, or does it completely vary by airplane type?

Thanks in advance for your answers. I look forward to continuing my education.

Mitchell Schaff
Williston, ND

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RE: How does a brushless motor work? - 9/3/2004 6:50 AM   
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quote:

Question #1: How does the number of poles affect the performance of a given motor.

Increasing the number of poles will decrease the max rpm but will increase the torque.

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RE: How does a brushless motor work? - 1/4/2005 12:36 AM   
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Status: offline 		This artilce goes over the types of electric motors nice and simple.http://www.avere.org/working/en/electric.html

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RE: How does a brushless motor work? - 1/11/2005 2:20 PM   
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RE: How does a brushless motor work? - 1/12/2005 3:11 PM   
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Great explanation, thanks for the info. Cleared up a lot for me. I play with unipolar stepper motors and controllers for CNC gadgets, but a 3 phase brushless motor is a new critter for me. Sounds like I have to get in to this side of things too. Fascinating.

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RE: How does a brushless motor work? - 2/4/2005 4:28 AM   
SBK


 

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Status: offline 		In search for some answers, I have found this great forum and hoping to get my questions answered by experts here. Be prepared for stupid questions since I am a total idiot in electrical things.

I am trying to understand the factors in choosing a brushless motor for RC helicopter application. I often encounter statements like this:"In a Logo 10 helicopter, Kontronik Fun 600-15 is much too weak for 3D flying, you would need Fun 600-18". But both motors can be driven to achieve the same motor rpm, and through gearing, same headspeed (main rotor rpm). So, my question #1: what make Funn 600-15 weaker than 600-18 when both can be driven (with input voltage) to the same headspeed?

In general, a Logo 10 helicopter may require around 40watts/lb to lift it and hover. A Logo 10 is about 5.5 pounds, so it needs about 220 watts total to hover. Then, it will require more, sometimes much more, to do heavier loading maneuvers.

My question #2: When input voltage is the same during flight, is the current draw the determinant for increasing torque to maintain the same headspeed for heavy maneuvers? and wouldn't incrased current draw heat up the motor?

Question #3: What would destroy a motor? -- too high of a voltage? too high of current draw? too high of a rpm? --- what are the physical limitations in a motor's performance and survival?

I try to organize my questions, I hope these are not too stupid questions.

Thanks ahead for comments.

SBK

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RE: How does a brushless motor work? - 2/9/2005 4:54 PM   
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Status: offline 		I always thought that the brushless esc contained a proximity sensor that relies on the electricity generated by the motor going back throught the third wire to tell it the position of the rotor in relation to the magnets, thereby telling the speedcontrol the corect time to pulse + or - .

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RE: How does a brushless motor work? - 2/28/2005 1:46 PM   
Ron v. Sommeren



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Status: offline 		The earlier brushless motors had Hall sensors. Nowadays, the voltage induced in the non-powered phase (only two of the phases are powered at any time) tis used to determine rotor position and speed.

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RE: How does a brushless motor work? - 2/28/2005 1:51 PM   
Ron v. Sommeren



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quote:

ORIGINAL: adam_one
That's true, but I guess that the max rpm is limited by the 3-phase's frequency and the number of poles.
The max rpm = 2 x frequency x 60/number of poles
Say, your controller can handle a two pole motor at 140krpm. This means it can handle a four magnetpole motor up to 70krpm, a 8pole motor up to 35krpm, a 10pole motor up to 28krpm and a 14pole motor up to 20krpm.

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