How does a brushless motor work?  
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All Forums >> Electric Aircraft Universe >> Brushed/Brushless motors, speed controls, gear drives >> How does a brushless motor work?
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How does a brushless motor work? - 2/24/2004 2:03 AM   
scott88-RCU



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Status: offline 		Being the science related kid I am I was wondering how a brushless motor works. I took apart one of my GWS can motors and saw the brushes and the commutator shaft. I just can't figure out how a brushless motor would work. Is sensorless a different story also?

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RE: How does a brushless motor work? - 2/24/2004 1:32 PM   
Matt Kirsch



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Status: offline 		A brushless motor is "inside out" compared to your GWS motor. The windings are glued to the inside of the can, and the permanent magnets are bonded to the rotor. Since the wires don't move, you don't need brushes to transfer the electricity.

Take a look at the commutator on the GWS motor. It's cut into multiple sections. Depending on which two sections are in contact with the brushes at any given time, different sections of the windings are energized, creating the magnetic fields that push and/or pull against the fixed magnets in the can.

There are three wires coming from a brushless motor. Inside, all three wires are connected, and wound such that passing DC current through any two connections will create a magnetic field, making the rotor turn a partial revolution The computerized electronic speed control "commutates" a brushless motor by switching which two wires are being energized in a sequence.

Sensored and sensorless are two types of brushless motors. Sensored motors have a separate sensor, and an additional five wires, that tells the controller which direction and how fast the motor is turning. These are more complicated, more expensive, and difficult to reverse. They've pretty much gone the way of the dodo in R/C; only MaxCim still makes sensored brushless motors. SensorLESS motors use the fact that when a motor is coasting, it's generating electricity to see which direction and how fast the motor is turning. Knowing this information is crucial to making the motor turn in the correct direction, and knowing which two wires to pass current through at any given time to keep it turning in that direction.

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RE: How does a brushless motor work? - 2/25/2004 3:01 AM   
Bill Mixon


 

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Status: offline 		I was under the impression that a brushless motor actually runs on AC current after being converted from DC via the controller.

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RE: How does a brushless motor work? - 2/25/2004 1:08 PM   
Matt Kirsch



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Status: offline 		A popular myth that I fell victim to myself. Turns out that even though the ESC does switch the polarity back and forth on the various phases of the motor to make it turn, it's still putting out DC voltage. It's similar to AC, but the wave is a "square" type. Unlike AC, where the voltage gradually changes from maximum + to minimum -, producing a "sine" wave, this switched DC changes from full + to full - instantly.

The picture below kind of shows what I mean. This is what you'd see on the screen of an oscilliscope if you compared true AC to what the brushless ESC is doing, sort of, kinda, maybe

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< Message edited by Matt Kirsch -- 2/25/2004 8:10:07 AM >

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RE: How does a brushless motor work? - 2/25/2004 3:59 PM   
rogwabbit


 

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Status: offline 		Matt is correct on this. The ESC is a square wave generator. It produces 3 seperate square waves (one for each wire to the motor.) the speed of the motor has nothing to do with voltage or amps, but instead the timing of the current fed into it. By increasing and decreasing the wave length of the square wave on the 3 phases the ESC causes the motor to spin faster and slower. Amp draw is a direct result of the work load caused at the faster speed.

< Message edited by rogwabbit -- 2/25/2004 3:59:54 PM >

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RE: How does a brushless motor work? - 2/27/2004 1:21 AM   
scott88-RCU



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Status: offline 		Thanks a lot Matt! I've been very busy so I just checked back in. One question what part of the motor turns then? If the windings(inside of the motor) like you said doesn't turn then what does?

< Message edited by scott88-RCU -- 2/27/2004 1:23:09 AM >

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RE: How does a brushless motor work? - 2/27/2004 2:37 AM   
Bill Mixon


 

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Status: offline 		The rotor turns.
It has the magnets attached to it with a shaft running through. The construction of the rotor varies from one motor to another, but they all are basically put together in this manner. Even the outrunner type motors (Nippy, AXI, etc) are like this. Difference is the magnets are on a bell shaped part placing the magnet segments outside of the windings, vs. inner runners (Hacker, Aveox, etc) They have the windings placed inside the case surounding the rotor. Wish I had some pictures to post, it would make it simple to understand.

Thanks for the explanation on brushless current Matt, rogwabbit

Bill

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RE: How does a brushless motor work? - 2/29/2004 2:21 AM   
ZimboRaider



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Status: offline 		I am a beginner on this electric R/C thing and am totall mystified by what all the numbers mean. ie Meag 25/25-26(just off the top of my head, probably doesn't make sense)
thanks

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RE: How does a brushless motor work? - 3/1/2004 12:28 PM   
Matt Kirsch



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Status: offline 		The numbers are essentially meaningless. They tell you nothing about how the motor performs, nothing about what the motor would be useful for. In most cases, the numbers are the motor's dimensions. That is, rotor diameter, rotor length, and number of turns of wire on each pole of the motor.

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RE: How does a brushless motor work? - 3/13/2004 2:01 PM   
GeraldO


 

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quote:

ORIGINAL: Matt Kirsch

A popular myth that I fell victim to myself. Turns out that even though the ESC does switch the polarity back and forth on the various phases of the motor to make it turn, it's still putting out DC voltage. It's similar to AC, but the wave is a "square" type. Unlike AC, where the voltage gradually changes from maximum + to minimum -, producing a "sine" wave, this switched DC changes from full + to full - instantly.

The picture below kind of shows what I mean. This is what you'd see on the screen of an oscilliscope if you compared true AC to what the brushless ESC is doing, sort of, kinda, maybe
Matt, your description is incorrect, and illogical. DC means Direct Current, AC means Alternating Current. The terms have nothing to do with the shape of the waveform whether squarish or rounded.

As you stated, the ESC switches the polarity of the phases. This means that the current through any given winding flows 'Alternately' one direction then the other. Also the voltage waveform is more trapezoidal, not square. And the current waveform is more sinusoidal due to the inductance of the motor, not square. The brushless motor is a true 3-phase AC motor and is described as such by the motor manufacturers themselves.

Now the ESC, on the other hand, that IS a DC device. But the current in the ESC's motor leads is AC

See my post here for actual scope photos of the current and voltage waveforms and further discussion: Why three phase AC?

< Message edited by GeraldO -- 3/13/2004 2:10:25 PM >

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RE: How does a brushless motor work? - 3/15/2004 12:48 PM   
Matt Kirsch



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Status: offline 		That is how it was explained to me by a person that builds brushless ESCs for a living. I made the mistake of calling it "three phase AC" one too many times, and was sternly reprimanded...

Anyway, we may be looking at this in different contexts. The voltage flowing through the motor may be "alternating," switching direction, but it's not AC as you'd see out of your power lines. There's no gradual ramp-up to full voltage; it's ON or it's OFF...

What you seem to be describing is the behaviour of the electricity at the motor. I'm describing the "mechanics" of the ESC.

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RE: How does a brushless motor work? - 3/15/2004 8:27 PM   
GeraldO


 

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Status: offline 		Matt, the ESC is indeed a DC device. There is no AC current in it. I have designed 3-phase brushless motor controls too so I understand quite well how they work. You are correct that the ESC turns the switches either on or off, not in-between. Because of the dynamics of the motor being attached though, the current doesn't actually start and stop instantly when the switch (mosfets) turns on or off. This is why the ESC can smoothly regulate the speed when it is in part throttle mode. The ESC is 'chopping' the voltage rapidly on and off at part throttle but the motor reactance smoothes it out somewhat so if you monitor the current waveform the slopes are ramped and the corners are rounded. Also the mosfets have parasitic diodes within them that permit some 'freewheeling' of the current even when the switch is shut off.

My comment about the innacuracy of your prior post was in regard to your response to Bill Mixon's statement when he said he thought the motor runs on AC current. You said "Turns out that even though the ESC does switch the polarity back and forth on the various phases of the motor to make it turn, it's still putting out DC voltage. " This is wrong and self-contradictory, because it implies that the output voltage polarity alternates and yet is constant and consequently that the current will be Direct Current and constant also. I'm not trying to pick on you but rather just trying to provide an accurate explanation for everyone's benefit.

The motor DOES run on AC current. In fact it would be very happy to run on a 3-phase AC current with a sinusoidal waveform like that of industrial 3-phase AC (household AC is single-phase), so long as the phases were synchronized to the rotor position. Some high-end brushless motor controllers used in industrial applications actually do synthesize a virtually sinusoidal drive waveform using DC electronics essentially the same as our ESCs, by modulating the timing of the voltage chopping. The sinusoidal current drive is the most efficient way to drive a brushless motor but it is more complicated to design the controller so they don't bother for our ESCs. It could be done in the ESC software though so don't be surprised if you start seeing sine-modulated ESCs soon.

Unlike a brushed type DC motor, if you were to spin the shaft of an unconnected brushless motor and display the output voltage waveforms on an oscilloscope, you would observe nearly perfect AC sine waves. You could use it as an alternator. To get DC out you'd have to rectify the current with diodes.

It is a bit confusing to try to understand that even though the ESC is DC and trying to chop the current into nice square waves, the actual motor current that results becomes not-so-square AC.

< Message edited by GeraldO -- 3/15/2004 8:31:01 PM >

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RE: How does a brushless motor work? - 3/18/2004 10:09 PM   
Wildstyle



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Status: offline 		Hi
I kind of understand that the controller outputs a voltage that follows a trapezoidal waveform so the current also follows a trapezoidal waveform but with rounded corners caused by inductance, Is this true.
What alters the motors RPM? Is it the frequency of the waveform.

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RE: How does a brushless motor work? - 3/18/2004 11:15 PM   
GeraldO


 

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Status: offline 		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.

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RE: How does a brushless motor work? - 3/22/2004 7:58 AM   
parabellum



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Status: offline 		woo! choppers! Thyristor! VVVF!

Am I going to see the variable voltage variable frequency in the RC world?

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RE: How does a brushless motor work? - 3/23/2004 9:36 PM   
john 8750



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Status: offline 		So is the ESC a VFD or a chopper?


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RE: How does a brushless motor work? - 3/24/2004 3:59 AM   
Tucson01



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Status: offline 		I have a bit of experience with AC VFD's.

It is amazing to see a 60hp AC motor spin a 1600 lb load to 800rpm then use the VFD for Braking to slow the load down.

It is commonly used in Industrial laundry equipment.

Gerald0 is quite correct!

Read a bit on "Tesla" ( father of AC current) , it will do wonders for your perception of electricity.

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RE: How does a brushless motor work? - 3/24/2004 4:45 AM   
Tucson01



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Status: offline 		Here is a good link for research for DC to AC conversion


http://www.eere.energy.gov/consumerinfo/factsheets/bb8.html?print

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RE: How does a brushless motor work? - 3/26/2004 1:13 AM   
GeraldO


 

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Status: offline 		Most of today's ESCs, brushless and brushed type, already have the capability to vary the chopping frequency. When you power up the ESC you can usually hear tones generated in the motor which are used to indicate the status. Often multiple tones are used to indicate different modes. It would actually be possible to play music through the motor this way (not very good though )

I can briefly describe the way in which sine drive can be accomplished in a brushless ESC. Basically it is equivalent to a 1-bit digital-to-analog conversion with the motor itself acting as the low pass filter at the output stage.

In a standard brushless ESC the phase currents are chopped at a fixed frequency for part throttle operation with the on/off duty cycle varying as a function of the throttle position. At full throttle the current is not chopped at all and the phase switches remain on constantly for the duration of each particular phase.

In a sine drive the current is chopped in a manner where the chopping frequency is modulated so as to approximate a sine wave in the time domain. In the sine drive motor controller there would be a digital representation of a sine wave encoded in its firmware to use as the 'pattern'. This is similar to the way in which a digital audio system can create analog music from a binary (ones & zeros) data source.

I should warn you though that although this is the most efficient drive from the motor's perspective; that is is results in a cooler running motor, there are increased switching losses in the controller at full throttle. So the overall net system efficiency might not necessarily be better.

-gerald

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RE: How does a brushless motor work? - 3/29/2004 3:05 PM   
Ron v. Sommeren



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Status: offline 		Brushless motor animations and simulations.

Waveforms in a brushless controller:
http://www.torcman.de/peterslrk/SPEEDY-BL_eng.html#Anker1591256

Two better animations, you can step through the sequence or maximise your window to get the speed down:
http://www.servomag.com/flash/2-pole/2pole-bldc-motor.html
http://www.servomag.com/flash/4-pole/smi-motor007.htm

CD-rom outrunner motor simulation
http://www.meinpapi.de/dieter/JAMO_0-33Wi10Abig.gif

LRK outrunner simulation, different colours for different magnetic field strength:
http://www.mfly.de/animation_b.gif

< Message edited by Ron v. Sommeren -- 4/18/2004 8:30:25 PM >

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RE: How does a brushless motor work? - 3/31/2004 6:58 PM   
KLRico


 

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RE: How does a brushless motor work? - 4/1/2004 8:45 AM   
john 8750



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Status: offline 		So, the brushless motor is not an AC motor?


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RE: How does a brushless motor work? - 4/1/2004 9:17 AM   
Ron v. Sommeren



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quote:

ORIGINAL: KLRico
Very impressive graphics!

The large cd-rom motor simulation explains the different colours. The cd-rom simulation was done with a free motor simulation package:
http://femm.foster-miller.net


quote:

ORIGINAL: john 8750
So, the brushless motor is not an AC motor?

Please, don't open that can of worms, I've seen lots of discussions on the AC versus DC subject

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RE: How does a brushless motor work? - 4/1/2004 9:24 AM   
john 8750



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Status: offline 		I agree and will let it rest.



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RE: How does a brushless motor work? - 4/1/2004 12:53 PM   
Matt Kirsch



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Status: offline 		It's an ELECTRIC motor

BTW, the AC vs. DC argument took place earlier in this thread.

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