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didier
12-17-2005, 09:24 PM
Hey,

I won't be the first one with this question, but can you use only 1 ESC with 2 brushless motors?
I mean, just connect the wires, and plug it in..
Thx,
Didier

qban_flyer
12-18-2005, 12:57 AM
Some have done so successfully, though I use individual ESCs and battery packs for each motor whether they happen to be brushed or B/L types. I also have disabled the BEC on both and use a small 250 mil pack to power the servos and RX.

roccobro
12-18-2005, 01:02 AM
Yes, but not always. Try it out and make sure the motors start up together every time. Some combos of ESc's and motors just don't play well together and adding another motor doesn't make it better.

Have fun!

Justin

cyclops2
12-19-2005, 12:36 AM
Q Ban. I like your seperate receiver and servo pack . Saves the plane everytime.
Turns out the big A-10 is a neutral balance. I was thinking of putting a seperate RC system in the rear for ELE. and RUD. And a system up front for everything else.

qban_flyer
12-19-2005, 01:22 AM
Q Ban. I like your seperate receiver and servo pack . Saves the plane everytime.
Turns out the big A-10 is a neutral balance. I was thinking of putting a seperate RC system in the rear for ELE. and RUD. And a system up front for everything else.

YUP!

You know it!:D

That's the way I fly my 76" Red Zephyr. A separate 4.8V 600 mil pack to drive the airborne, while the big Sanyos drive the ESC and motor (an old Astro FAI 15 geared swinging a 12X6E APC). Has never failed me! :)

Are you going to use two RXs? One complete RX & airborne on the back and another one on the front each with their separate battery packs?

Sounds cool and intriguing at the same time, though when I think about it, that's what the "big Boys" do on their large 40% gassers. They double up on everything just to be on the safe side! :)

cyclops2
12-19-2005, 04:54 AM
Thats what I am doing. I have a load of receivers and double crystals.
If I loose everything up front. I may failsafe to a glider.
If the rear fails, maybe power and airlerons will save the bacon.:)

qban_flyer
12-19-2005, 12:46 PM
Thats what I am doing. I have a load of receivers and double crystals.
If I loose everything up front. I may failsafe to a glider.
If the rear fails, maybe power and airlerons will save the bacon.:)

Or at least it will minimize the damages! :D

CorsairJock
12-19-2005, 01:46 PM
Thats what I am doing. I have a load of receivers and double crystals.
If I loose everything up front. I may failsafe to a glider.
If the rear fails, maybe power and airlerons will save the bacon.:)
Doesn't that double the chances of something going wrong? Kind of like the full scale twin: people perceive it as being safer because it has 2 engines, and if one fails, the other will keep it flying. In truth however, most twins are designed to fly using both engines, and become very difficult to fly when one engine is lost. And by having 2 engines, the chances of engine failure are doubled.

I also would like to power 2 brushess motors from one ESC, and would use seperate battery pack for radio power. I have a Hobby Lobby P-38 that has been setting around for awhile, and have been considering such a setup. I could even go with contra-rotating, IF I could finf appropriate props.

cyclops2
12-19-2005, 02:46 PM
The counter-rotating props available would make a GREAT thread and project. The small one on my China Clipper were a end of run years ago.
Replacements are probably not available.


I will put in a hour looking, if others pitch in to.
Lets do ALL sizes the first time around. :)

CorsairJock
12-19-2005, 03:38 PM
Here's a set of 10 x 4.5:
http://www.toddsmodels.com/Props/propsother.htm

Also, altho I recall that Hobby Lobby had some electric only pusher props available in sizes that matched tractor props, I could not find any on their web-site.

panzerd18
12-20-2005, 10:23 PM
Doesn't brushless ESCs require feedback pulses from the engine inorder for them to work? What I'm getting at is that if you have two engines the esc will not work due to feedback timing from both engines at once.

Jim McPherson
12-21-2005, 12:41 AM
Exactly Panzerd18. Brushless ESC's use feedback from the motor to advance/retard the timing to properly run the motor. If you have 2 motors giving different feedback the ESC will not be able to drive either motor correctly. This causes excess heat and very low efficiency. Think about doubling at least the rating of the ESC past the actual draw of each motor if you do this.
Additionally, it ONLY works if you have two motors that are extremely well matched. If you have an old pair of hackers, Aveox, or Neu motors it MIGHT work. However, any of the Chinese motors are not so closely matched and you'll likely blow up your ESC if they run at all.
The reason this question won't go away is that it sometimes works and people say "It works for me so it will work for you". What they don't realize is that in most cases they are running very inefficiently and hurting their equipment.

-Jim

jskrebs
12-21-2005, 03:50 AM
www.m-a-e.com (http://www.m-a-e.com) CHeck out Jinx!

www.westport-design.com (http://www.westport-design.com)
Video here of dual CDR's running on a PH 10 with 10x4.5 counter rotating props from Todd's models.

CorsairJock
12-21-2005, 02:33 PM
Doesn't brushless ESCs require feedback pulses from the engine inorder for them to work? What I'm getting at is that if you have two engines the esc will not work due to feedback timing from both engines at once.
Unless I am mistaken, The 1st generation brushless motors do have a feedback ciicuit: they actually have sensors within the motor which provide feedback to the controller. The current generation does NOT have these sensors, and are sometimes reffered to as "sensorless".
1st generation motors include most of the AstroFlight, Aveox, and MaxCim motors. Axi, HiMaxx, and nearly all the rest are sensorless.
Nontheless, the ESCs DO get some type of feedback from the motor(s), to technical for me to explain, but I don't know if it means we cannot run 2 motors from 1 ESC.
What we really need here is someone who really knows what they are talking about, such as a motor designer or ESC designer. As for the motors I would use: of COURSE they would be the same make and model, and would probably be both brand new.

Jim McPherson
12-21-2005, 03:21 PM
Unless I am mistaken, The 1st generation brushless motors do have a feedback ciicuit: they actually have sensors within the motor which provide feedback to the controller. The current generation does NOT have these sensors, and are sometimes reffered to as "sensorless".
1st generation motors include most of the AstroFlight, Aveox, and MaxCim motors. Axi, HiMaxx, and nearly all the rest are sensorless.
Nontheless, the ESCs DO get some type of feedback from the motor(s), to technical for me to explain, but I don't know if it means we cannot run 2 motors from 1 ESC.
What we really need here is someone who really knows what they are talking about, such as a motor designer or ESC designer. As for the motors I would use: of COURSE they would be the same make and model, and would probably be both brand new.
I really do know what I'm talking about believe it or not. I've built motors and ESC's (although never a brushless sensorless, that is VERY difficult).
The old motors had internal sensors to detect the position of the rotor. New motors don't have sensors built in to the motor but do have sensors in the ESC. The ESC sensors detect back EMF and tell the ESC where the rotor is just like the old motors. So there isn't as much difference in sensored and sensor less as you'd think. Sensorless is of course much more complicated to detect and run and requires complex software to do the job.
When you hook more than one motor up to an ESC your confusing the ESC by sending 2 sets of back EMF that can add to each other or cancel each other out, or be out of phase... etc. When one motor says "slow down" and another motor says "speed up", what is the ESC supposed to do?
Thus the inefficient running.
When I say "matched" motors, I don't mean the same type of motors, I mean motors with windings in all 3 phases of the motors must be exactly the same electrically and mechanically so that they give the same, resistance, KV, inductance, back EMF, etc.
You'll find motors of the same model number with slightly different KV's, this confuses the daylights out of an ESC since one wants to run faster than the other. Take a 5400kv motor. If one motor is actually 5350kv and the other is 5450 surely you can see the problem with running these on the same voltage and controller.
If you want to run 2 motors, use 2 controllers!

-Jim

CorsairJock
12-21-2005, 04:02 PM
I really do know what I'm talking about believe it or not. I've built motors and ESC's (although never a brushless sensorless, that is VERY difficult).
If you want to run 2 motors, use 2 controllers!
-Jim

Jim, No offence intended, and I'll take your word for it. It's just that if you don't say so, how do we know who is an expert and who is not?

I am an indusrial electrician by trade, but make no claims about being an electronics technician. My schooling was many years ago, and my memory of most of the elctronics stuff has been long lost due to not needing/ using it.

I am very familiar with common industrial 3 phase AC motors tho, and know that they are designed to run at a somewhat constant speed, dictated by number of poles and supply frequency. In the past 20 years or so, variable frequency drives (VFDs) have become popular, and allow these motors to be driven at different speeds (even in excess of design speed).

That said, I have a question: are these hobby motors basically the same thing (3 phase AC motors)? And if so, are the ESCs basically VFDs?

Also, I thought the KV/RPM ratings were a function of the number of winding turns (among other things, such as number of poles). If so, and we assume that 2 motors made by same company will same number of turns, how can there be a detectable difference?

TRASHBUG
12-22-2005, 12:36 PM
What effect does a separate power supply for the receiver have on the BEC circuit in the ESC? If I use a 4.8v nicad pack with my recevier as well as an ESC with BEC will I be creating a problem for my ESC or battery pack?

Jim

qban_flyer
12-22-2005, 01:01 PM
What effect does a separate power supply for the receiver have on the BEC circuit in the ESC? If I use a 4.8v nicad pack with my recevier as well as an ESC with BEC will I be creating a problem for my ESC or battery pack?

Jim

Jim.

You will be creating a conflict if both were to be connected to the RX at the same time. Every ESC manufacturer states in their instructions that in order to use a separate power supply for the airborne system, one must "always" disengage the BEC in their ESCs first.

To do so you must either remove the red wire connector from the plug coming from the ESC to the RX and either cover it with tape or heat shrinking tube, or just "cut" the wire so that no voltage from the ESC is entering the RX. I do mine with heat shrink tubing.

I always remove the red wire connector from the plug as it allows me to use it later on on another aircraft if I so choose. :)

CorsairJock
12-22-2005, 01:23 PM
Jim.
You will be creating a conflict if both were to be connected to the RX at the same time.
To do so you must either remove the red wire connector from the plug coming from the ESC to the RX ..............
:)

OR, you could just "Disable BEC" if your ESC has that option (MOST brushless ESCs do have that option).

Probably biggest issue: your ESC's BEC probably has higher voltage output than your battery. Thus, it would constantly trying to charge your battery, and most like over charge it. That is just one of the issues with simultaneous BEC & battery operation. In attempting to charge the battery, it could also overload the BEC.

NOW, that I have helped answer that question, anybody knowledgeable enough to answer my questions in a previous post?

TRASHBUG
12-22-2005, 01:28 PM
Purrrrrrrrfect!

Thanks guys:)

qban_flyer
12-22-2005, 01:53 PM
Purrrrrrrrfect!

Thanks guys:)

No problem.

That's what Watt Flyer is all about. Helping each other out within the scope of our "limited" knowledge of this hobby and of electricity and its properties in general.

Take care.:)

Jim McPherson
12-22-2005, 02:39 PM
That said, I have a question: are these hobby motors basically the same thing (3 phase AC motors)? And if so, are the ESCs basically VFDs?

I have no experience with VFD's. So I can't answer this question.

Also, I thought the KV/RPM ratings were a function of the number of winding turns (among other things, such as number of poles). If so, and we assume that 2 motors made by same company will same number of turns, how can there be a detectable difference?

There are other factors that play, magnet strength comes to mind. The example given above was used to illustrate the point, a difference of 1% in KV can have significant effect on the timing advance of the ESC.
I challenge you to measure two identical motors with the same prop, voltage (probably want to use a power supply for this), and ESC. In most cases (unless the motors are well matched) you will see variations in RPM.

I understand that it works in some cases, many seem to hold the opnion that since it works in one case then it's a good idea to do. The simple fact is that ESC's are not designed to run 2 motors. The BEMF from combined motors gives velocity cues that are incorrect causing the ESC to run the motors inefficiently.

2 Motors, 2 Controllers.


-Jim

CorsairJock
12-22-2005, 03:28 PM
Jim,
My question remains un-answered: surely you must know of common 3 phase motors, my question is: are these hobby motors the same?
Or, in simpler terms: is the speed of these motors being controlled by variations in frequency genetated from the controllers? OR is speed control strictly a function of applied voltage from the controller. That is what a VFD does: it converts 'normal' AC into DC, then back to a variable rate of AC (Hz is variable), THUS allowing a standard 1800 RPM 3 phase AC motor to have variable speeds. This is all basic electric motor theory.

If you cannot answer these, you are not convincing me that you know what you are talking about about, and you are not convincing me of your "2 Motors, 2 Controllers." idea no matter how many times you state it.

qban_flyer
12-22-2005, 08:31 PM
2 Motors, 2 Controllers.

-Jim

Thank you Jim.

My feelings exactly.

Since my knowledge of things electrical is very limited, I can afford an extra ESC and battery pack to be on the safe side. After all the money "invested" in the model, to me the added nominal cost is worth every penny of it. Peace Of Mind is called, I believe.:)

PS: In my case I am convinced that the way to go is individual ESCs and battery packs for multi motor electric flying R/C models. When I was flying a gasser DC-3 powered by two Saito 65s, I used two independent gast tanks. One per engine.:)

Again, in my case it is not my province to try and convince anyobody else as to my beliefs, whether they are R/C related or not. I just express my opinions based on my experiences, then let the chips fall where they may.:cool:

slipstick
12-22-2005, 09:42 PM
Jim,
My question remains un-answered: surely you must know of common 3 phase motors, my question is: are these hobby motors the same?

No they are not the same. Brushless (electronically commutated) motors are different in many ways from conventional AC induction or synchronous motors.

If you cannot answer these, you are not convincing me that you know what you are talking about about, and you are not convincing me of your "2 Motors, 2 Controllers." idea no matter how many times you state it.
I don't know about Jim but I personally could not care less about convincing anyone with that sort of attitude. Information on brushless motors is readily available for those with the intelligence to find it.

Steve

Jim McPherson
12-22-2005, 09:55 PM
Ditto Steve,
Most 3 Phase AC motors use an iron core and are inductance based motors. The RPM is determined by the frequency of the AC going in. They can do this becuase of the iron core. The rpm is a result of the frequency.
Hobby 3 Phase AC motors use a magnetic core with poles. They use pulsed voltage to determine the RPM and BEMF sensors to detect the RPM and change phase at the appropriate time to keep the rotor spining. The frequency is a result of RPM.
Two fundamentally different modes of operation. Hope that helps.

-Jim

ragbag
12-22-2005, 10:13 PM
Sent double,
By George

ragbag
12-22-2005, 10:14 PM
With an attitude like that he isn't realy wanting help or opinions, just an argument.:rolleyes:

All it takes is one variable in the system to throw it off. One magnet, one winding, one motor wire not the same length.:(

Some manufaturers ask that you not change the length of the motor wire, it changes the timing.:D

It might still work, but if you throw a brick in the air, some would call it flying.:eek:

$0.02:rolleyes:

By George

qban_flyer
12-23-2005, 01:31 AM
I don't know about Jim but I personally could not care less about convincing anyone with that sort of attitude. Information on brushless motors is readily available for those with the intelligence to find it.

Steve

Thank you Steve,

Just what I felt like saying, but confrontation is something I would avoid if at all possible. The key word in your reply is ATTITUDE, it says it all in a nutshell. :)

CorsairJock
12-23-2005, 02:27 PM
Fellow modelers and electric power enthusiests:
This thread began as a simple question.
The question was given a direct answer.
I, however, do not believe something just because someone gives a simple 'yes or no' answer, especially if it is someone I do not know.
Call it a character flaw if you will.
Thus, I questioned (challenged, confronted, bullied, whatever) the person with the answer, asking his qualifications for providing such answer, and to provide some explanation.
If this is all too harsh for you guys, I apologize.
I didn't realize that protocal here dictates that we all accept replies from total strangers as unquestionable fact.
Merry Christmas to All.

ragbag
12-24-2005, 10:50 AM
This does not answer to the question, but could help understand the ESC.

Fly RC magazine.
January 2006
page 62: Electro-Active column by Tom Hunt

Power system Selection
part 6

The electronic speed controler.

Function

The main function of the ESC is to smoothly and eficiently change the rpm of the electric motor(s).This, in turn, changes the rpmof the prop, which changes the speed of the model. An auxiliary funtion in some ESCs can include providing poer to the reeiver, (RX) in lieu of a standard 4.8-6 volt Rx battery. This is called an ESC with a BEC (battery eleiminator circuit). Eliminating the wight of a 4-cell battery pack (even a small, lightweight one) to provide power to the RX is all but essential in the smaller class electric models. Mdern ESCs adjust motor speed by rapidly switching the motor on and off in proportion to the transmitter stick position. At half throttle, the ESCsends the motor equal pulses of full and zero power. Because this switching happens so rapidly, hte net effect is that the motor to be running ata 50% voltage setting. Depending on the pilot inputs, the average voltage varies from zero volts to the packs maximum of the pack minus some small electrical losses in the ESC and wiring. The motor responds to this varying power by changing the rpm, just as the fuel/air mixture of an internal combution engines varies the output rpm.

End paragraph.

The rest of the article is very interesting, but doesn't answer the original question.

It does help to understand the fuction of the ESC and might help to come to your own conclusion on the original question.

Two motors, one controler?

It is early in the AM, so please excuse the typos.


Have a Merry Christmas.

By George

Jim McPherson
12-24-2005, 11:27 AM
Merry Christmas,
Nice find, that article describes PWM very well. However it appears to be talking about brushed motors. The exact same principles apply to brushless, except that the brushless controller has to use the BEMF of the motors to determine when to switch coils. When it changes the PWM duty cycle the velocity of the rotor changes and it has to keep the rotor spinning by detecting those velocity changes and switch coils at exactly the right time to keep the motor running efficiently.
Think about how little time it has with even a 2 pole inrunner, if that little rotor is going 40000 rpm and the ESC has to get every 1/3rd of a rotation perfectly timed such that 2 coils fire and one turns off. If the ESC is off by even an extremely small amount, your motor will run hot as the magnetic fields will repel and slow the motor down. The BEMF tells the ESC when to make it's critical timing changes, running 2 motors changes and corrupts the BEMF if the motors are not 100% identical in every way.

-Jim

ragbag
12-24-2005, 11:48 AM
All it takes is one variable in the system to throw it off. One magnet, one winding, one motor wire not the same length.:(

Some manufaturers ask that you not change the length of the motor wire, it changes the timing.:D

By George

I did send this thread to Mr. Hunt, maybe he will jump in.

As you an see from one of my previous posts, I am aware of the timing part of the controler, just couldn't explain it as well as you did.

Thank you,
By George

After rereading your explanation, I don't think anyone could explian it any better. To go in to any more in depth would be above the average modelers needs and head.
BG

CorsairJock
12-25-2005, 02:22 PM
Ditto Steve,
.............Hobby 3 Phase AC motors use a magnetic core with poles. They use pulsed voltage to determine the RPM and BEMF sensors to detect the RPM and change phase at the appropriate time to keep the rotor spining. The frequency is a result of RPM.
-Jim

Then these ARE essentially 3 phase AC motors.
At the risk of sounding confrontational, isn't RPM then a function of frequency (along with appropriate voltage changes) generated from the controller? I can understand that the generated frequency is 'adjusted' according to feedback, but which comes 1st? And isn't the bottom line that these (ESCs) are in fact variable frequency controllers?

BTW, I realize these questions, and some of the previous replies are too technical for the average modeler. I do hope however, that the average modeler understands the difference between AC and DC, as that appears to be the major difference between brushless motors and brushled motors. And, as Jim McPherson suggested: the artical about ESC operation pertains to BRUSHED motors and controllers. The 3rd wire should be a clue to some of you that these motors are very different from conventional brushed DC motors.

ragbag
12-25-2005, 03:12 PM
I do hope however, that the average modeler understands the difference between AC and DC, as that appears to be the major difference between brushless motors and brushled motors. And, as Jim McPherson suggested: the artical about ESC operation pertains to BRUSHED motors and controllers. The 3rd wire should be a clue to some of you that these motors are very different from conventional brushed DC motors.

The artical covers both types of ESCs.
AC is alternating current.
DC is direct current.

It was when I went to school in '55.

I am getting the idea that this is a pulsed dc, the emf controling the timing.
If this is so then any difference in the basic construction of the system would affect the operation of the motor.

Does the ESC create AC?

Does the capacitor in the AC 3 phase remove one leg of the AC thus making it a DC motor?

So many cats, so few recipes.

By George

CorsairJock
12-25-2005, 03:39 PM
The artical covers both types of ESCs.
By George

If so, then the article is incomplete. As posted, it only covers basic DC/ brushed motor operation.

I am getting the idea that this is a pulsed dc, the emf controling the timing.
By George

Voltage (electro motive force) is controlling the timing? The 2 are likely coupled/ related, as higher frequency (and corresponding RPM) means more voltage is needed by the motor to maintain torque for additional load (power/ thrust). In a 3 phase AC motor tho, frequency is the main determining factor of motor speed, but speed is limited by load, which is when more voltage is needed.


Does the ESC create AC?
By George

If you are talking brushless ESC only, that has been one of my questions all along. It most certainly is possible, and probable IMO. If not sine wave AC, then it must be square wave DC, which is very simular.


Does the capacitor in the AC 3 phase remove one leg of the AC thus making it a DC motor?
By George

I haven't heard of using capacitors in AC 3 phase motors, except for (can't think of proper term) phase shifting to sync current and voltage cycles. Don't know why one would want to make it a DC motor. Removing a legs results in single phasing, which will cause it to just hum (at Hz) if it was not turning already, and will be overloaded if it was turning.

qban_flyer
12-25-2005, 03:50 PM
Now I know why didier has never chimed in again. His thread has gone way off tangent, taken over by something else. :eek:

Does anyone remember how this thread began? :confused:

http://www.wattflyer.com/forums/showpost.php?p=29653&postcount=1

CorsairJock
12-25-2005, 05:05 PM
In case anyone forgot:

..............but can you use only 1 ESC with 2 brushless motors?
Didier

What followed was a simple 'yes/ no' answer.
I for 1 (and possibly/ probably the ONLY 1) sought ( and still seek) a better/ more technical answer.
As of this time, I fail to see where the subject has changed/ gone off into a different tangent.
In any event, subsequent posts have kept this thread alive, allowing more time for others to participate/ learn/ contribute.

TRASHBUG
12-25-2005, 10:40 PM
2 brushless,1 ESC?

In an effort to be as direct as possible.

Yes you can............probably.:)

I have two GB brushless running flawlessly on a CC 35. If the wind ever quits I expect the 150 watts produced by this setup to fly a GWS C-47 with no problem.

I know some have had problems with one esc and two motors, so.............your motors will probably work if they are a closely matches pair. I don't think they have to be 100% exact since I doub't if the ones I built are that precise. Of course they are the same wind, diameter, magnets etc.

Jim

hul
12-26-2005, 01:02 AM
Then these ARE essentially 3 phase AC motors.
At the risk of sounding confrontational, isn't RPM then a function of frequency (along with appropriate voltage changes) generated from the controller? I can understand that the generated frequency is 'adjusted' according to feedback, but which comes 1st? And isn't the bottom line that these (ESCs) are in fact variable frequency controllers? our brushless motors are synchronous 3 phase AC motors.

RPM is a function of power going into the motor and the load on the motor (propeller's power demand goes up with the cube of speed). Power into the motor depends on duty cycle (PWM, pulse width modulation) and speed of the motor for a given battery.

The brushless ESC basically does two things:
- PWM to adjust power (just like the ESC of a brushed motor)
- commutation (just like commutator and brushes in brushed motors)
Commutation depends on rotor position only, which is measured by EMF (electromagnetic feedback, for sensorless ESCs) or sensors in the motor (for sensored controllers).

I ran 2 sensorless brushless motors on one ESC (2 x Axi 2212/26 and Phoenix 35) with no problems. Starting the two was the only issue, successful in 1 out of 3 tries, which I think is because 2 cogging motors have a 1 out of 3 chance to be in the same relative position (rotor to stator). There were absolutely no problems once they were running, starting in the air was no problem either if the motors were windmilling (brake disabled). The motors run at exactly the same speed at all times (because that's what synchronous motors do). This takes away from a twins charm though, no more resonances. How much power each motor takes depends on its Kv and load, both may be slightly different for each.

Hans

CorsairJock
12-26-2005, 01:40 AM
A Google search for "synchronous 3 phase AC motors" turned up this site 1st:
http://en.wikipedia.org/wiki/Electric_motor#Three-phase_AC_synchronous_motors

Briefly, it states that:
"Three-phase AC synchronous motors............will rotate in synchronism with the rotating magnetic field produced by the polyphase electrical supply".

The rotating magnetic field is from the power source, and rotation is a function of Hz.

In another paragraph, it states:
"The speed of the AC motor is determined primarily by the frequency of the AC supply and the number of poles in the stator winding, according to the relation:
Ns = 120F / p
where:
Ns = Synchronous speed, in revolutions per minute
F = AC power frequency
p = Number of poles, usually an even number but always a multiple of the number of phases"

Or, to put it another way: (120 x Hz)/Number of Poles = RPM

Matt Kirsch
12-26-2005, 01:41 AM
Right there is the main issue: Starting. It rarely works because the motors are rarely in the same phase and rarely react identically to the starting pulses. Some ESCs are built to handle this, but most are not. The exact ones escape me, but I believe they're one of the fancy Eastern European brands.

Notice that a brushless motor has three wires. The ESC works by picking two of those wires to make a circuit, then sending a pulse of electricity through that circuit. That causes a magnet and a pole of the motor to attract and move a part of a revolution. This movement also moves another magnet past another pole, generating a small amount of electricity. It's that "back EMF" that the ESC uses to sense when to send the next pulse.

My understanding of the situation is that the back EMF is "sent" down the third wire of the motor and sensed by the ESC. One wire is the hot wire from the ESC, one is a common ground, and the third becomes a sort of sensor wire.

CorsairJock
12-26-2005, 01:46 AM
More yet:
"Brushless DC motors

Midway between ordinary DC (http://en.wikipedia.org/wiki/DC) motors and stepper motors lies the realm of the brushless DC motor (http://en.wikipedia.org/wiki/Brushless_DC_electric_motor). Built in a fashion very similar to stepper motors, these often use a permanent magnet external rotor, three phases of driving coils, one or more Hall effect (http://en.wikipedia.org/wiki/Hall_effect) devices to sense the position of the rotor, and the associated drive electronics. The coils are activated, one phase after the other, by the drive electronics as cued by the signals from the Hall effect sensors. In effect, they act as three-phase synchronous motors containing their own variable frequency drive (http://en.wikipedia.org/wiki/Variable_Frequency_Drive) electronics."

That said, it would seem unlikely that 2 identical motors (same make and model) would be at different RPM (driven from the same controller), UNLESS one of them was significantly loaded down more than the other.

Jim McPherson
12-26-2005, 03:12 AM
Ok, this is my last contribution to this thread.
The above description of brushless motors has nothing to do with the motors that we use in hobby applications today.
I also thank the other knowledgeable posters who have attempted to explain the hobby brushless motor/esc.
The meat: I've already posted this but it didn't seem to be understood by some. Brushless ESC's used in aircraft are NOT the same as VFD's used for other types of 3 phase AC motors.
Our hobby motors have fixed magnets, "normal" 3 phase magnets have no fixed magnets and the rotors are turned through induction. These two types of motors require completely different drive systems.
VFD's use frequency to determine the RPM of motor/s. Hobby ESC's use voltage to determine the RPM of the motors. Hobby motors do have variable frequency, but that is a byproduct of the voltage and has nothing to do with setting the speed of the motor.
The consequenses of this are already posted above.

Hans has an excellent description of how this setup works. He also notes one of the problems/natural consequenses. His two motors run at exactly the same RPM. This of course is correct. But what if one of the motors wants to run a little slower becuase of its design? Perhaps its magnet is slighty more powerful, or one of its coils is not exactly the same as the others? The motor is forced to run at an RPM that causes magnetic conflicts between the coils and the fixed magnet rotor. Thus inefficient running is possible. Again, if his motors are identical in everyway then this is not an issue, however his starting problems suggest this is not the case. Sure it works, is it optimal... no way.

And on a final note: brushless ESC's are DC (square wave) devices. However becuase of the electrical properties (inductance mainly) of the motor and the switching of poles in the coils, the delivered voltage ends up being AC in the motor. So is it DC or AC? It's all semantics, call it whatever you want.

And with that, I've said all I can. The choice is as it always was: up to you.

-Jim McPherson E.E.

qban_flyer
12-26-2005, 04:22 AM
Thank you Matt, thank you Jim.:)

I also am unsubscribing from this thread.:o

EOM:p

hul
12-26-2005, 05:01 AM
His two motors run at exactly the same RPM. This of course is correct. But what if one of the motors wants to run a little slower becuase of its design? Perhaps its magnet is slighty more powerful, or one of its coils is not exactly the same as the others? The motor is forced to run at an RPM that causes magnetic conflicts between the coils and the fixed magnet rotor. Thus inefficient running is possible. Again, if his motors are identical in everyway then this is not an issue, however his starting problems suggest this is not the case. Sure it works, is it optimal... no way. if the motors aren't the same for whatever reason, the one with the higher load (i.e. slightly different prop, different magents) draws more current but it will stay synchronized (at exactly the same speed as the other motor). Even with differing Kv's; the ESC puts out a frequency, both motors spin exactly at the corresponding rpm.
I don't think inefficiency as described is a factor.

CorsairJock quoted Wikipedia: "The speed of the AC motor is determined primarily by the frequency of the AC supply and the number of poles in the stator winding."
In our case it's more like this: Motor speed depends on motor load. AC frequency corresponds to motor speed.

The fancy European brand ESC was Kontronik. It worked well because Kontronik motors are non-cogging. The first few pulses of current got the motors spinning, as soon as they are spinning they synchronize and stay synchronized until they stop again.
Kontronik's start up sequence may be better suited to this than other ESCs, but my CC didn't have a problem with this either.

I don't use that twin any more, but it would have been interesting to manually phase the motors (put them on the same cog) before starting. Never tried this but I'm reasonably sure that would improve the odds for a successful start considerably.

Hans

ragbag
12-26-2005, 10:34 AM
So many cats, so few recipes.

My last thougts also.

So much verbage to say yes it might work, so try it if you want to.

HAPPY NEW YEAR TO ALL!!!!!

By George

TRASHBUG
12-26-2005, 02:26 PM
if the motors aren't the same for whatever reason, the one with the higher load (i.e. slightly different prop, different magents) draws more current but it will stay synchronized (at exactly the same speed as the other motor). Even with differing Kv's; the ESC puts out a frequency, both motors spin exactly at the corresponding rpm.
I don't think inefficiency as described is a factor.

CorsairJock quoted Wikipedia: "The speed of the AC motor is determined primarily by the frequency of the AC supply and the number of poles in the stator winding."
In our case it's more like this: Motor speed depends on motor load. AC frequency corresponds to motor speed.

The fancy European brand ESC was Kontronik. It worked well because Kontronik motors are non-cogging. The first few pulses of current got the motors spinning, as soon as they are spinning they synchronize and stay synchronized until they stop again.
Kontronik's start up sequence may be better suited to this than other ESCs, but my CC didn't have a problem with this either.

I don't use that twin any more, but it would have been interesting to manually phase the motors (put them on the same cog) before starting. Never tried this but I'm reasonably sure that would improve the odds for a successful start considerably.

Hans

My experience with GWS C-47 with GB cdrom motors has been excellent. Occasionally one of the motors will squeal for a fraction of a second on a fast start up as it tries to sync. However, they have never failed to start. I can see a potential problem if the motors you use have strong cogging and are not on the same cog at start up. One will start right up and provide feedback to the esc and the other motor will have a problem trying to overcome its cogging and come up to speed. So.......MY logic tells me that motors without strong cogging will be better suited for the two motors one/esc purpose.

Comments?
No deep theory please.
I subscribe the KISS principal. I keep my head in the sand for one reason.....I like the view:)

CorsairJock
01-20-2006, 02:39 PM
The counter-rotating props available would make a GREAT thread and project. The small one on my China Clipper were a end of run years ago.
Replacements are probably not available.


I will put in a hour looking, if others pitch in to.
Lets do ALL sizes the first time around. :)

Just browsing Hobby Lobby amd found some more props available with both configurations (pusher & puller/ normal & reverse), altho they are EXPENSIVE:
http://www.hobby-lobby.com/freuden.htm

Note: the reverse pitch blades are at the bottom of the page.