Author Topic: Hobbyist RC plane brushless DC motor  (Read 9083 times)

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Offline cyberfish

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Hobbyist RC plane brushless DC motor
« on: February 05, 2011, 09:27:49 am »
Does anyone have experience driving hobbyist RC plane use brushless DC motors?

I have this one: http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=7520

Seems like it has 12 solenoids, so 4 electrical revolutions per mechanical revolution (but that shouldn't matter, right?).

And I can't seem to drive it properly.

With power supply set to 0.5A limit, I tried giving it this pattern by hand -

UL, VH (U phase low, V phase high, W phase floating)
UL, WH
VL, WH
VL, UH
WL, UH
WL, VH

But it just doesn't want to turn. It would snap to first position, then when I apply subsequent steps, it would move a little, around the current position. Way less than how much they should move, and in seemingly random direction each time.

Anyone wants to take a guess as to what may be going on?

Thanks
 

Offline scrat

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Re: Hobbyist RC plane brushless DC motor
« Reply #1 on: February 05, 2011, 09:43:24 am »
The sequence seems right for making the motor rotate.

I think it's not so easy to apply the steps manually...
What is your setup? Switches? Wires and hands?
If you use switches, then a free-wheeling diode should be put there, but I think that, however, current has the time to go to zero while switching from one phase to the other, So rotor could place itself in a quite random position at every phase change.

If you really need this, try making a little 3-phase inverter. You can find one already done on CD players or computer fans. Computer fans have a controller that takes simply a DC voltage, and does the switching for you.
One machine can do the work of fifty ordinary men. No machine can do the work of one extraordinary man. - Elbert Hubbard
 

Offline Simon

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Re: Hobbyist RC plane brushless DC motor
« Reply #2 on: February 05, 2011, 09:47:28 am »
do you need such accuracy in an RC plane ? surely your adding extra weight as oposed to a brushed motor ?
 

Offline scrat

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Re: Hobbyist RC plane brushless DC motor
« Reply #3 on: February 05, 2011, 10:48:35 am »
Brushless DC aren't very accurate motors, in fact they are the ones used for low-cost high-reliability applications (e.g. PC fans) because of their lack of brushes (which, besides poor duration, produce sparks and so great noise).

Constructively they are little different from brushless AC motors (they can even be the same) but they are driven in a DC-like way (a DC current for one third of a period each phase).
One machine can do the work of fifty ordinary men. No machine can do the work of one extraordinary man. - Elbert Hubbard
 

Offline Simon

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Re: Hobbyist RC plane brushless DC motor
« Reply #4 on: February 05, 2011, 11:35:42 am »
Oh well whats the point then ? your adding extra weight to a plane
 

Offline DaveW

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Re: Hobbyist RC plane brushless DC motor
« Reply #5 on: February 05, 2011, 11:58:16 am »
Oh well whats the point then ? your adding extra weight to a plane

BLDC motors are far more efficient than brushed motors, you can hit 80-90% efficiency. They also develop a lot more torque in the lower end, and very usefully, the motor body can be enclosed, so there's less chance of getting foreign bodies inside the motor
 

Offline Simon

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Re: Hobbyist RC plane brushless DC motor
« Reply #6 on: February 05, 2011, 12:02:57 pm »
so in that case a lighter motor can be used and from what you say is has a more even power over varying speeds ?
 

Offline DaveW

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Re: Hobbyist RC plane brushless DC motor
« Reply #7 on: February 05, 2011, 12:18:26 pm »
Exactly, and being able to put lots of power into a small motor is particularly useful for model planes. There's a picture of a typical example at the top of the wikipedia page

http://en.wikipedia.org/wiki/Brushless_DC_electric_motor
 

Offline Zero999

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Re: Hobbyist RC plane brushless DC motor
« Reply #8 on: February 05, 2011, 12:19:41 pm »
As far as I'm aware DC brushless motors and AC motors are the same thing, it's just the latter is designed to work with a square wave, rather than a sine wave.

Do you know whether it's synchronous or asynchronous? If it's the latter, it certainly won't work when you apply the pulses at a low frequency because there won't be enough EMF generated in the rotor.

 

Offline scrat

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Re: Hobbyist RC plane brushless DC motor
« Reply #9 on: February 05, 2011, 01:12:28 pm »
BLDC motors should have trapezoidal Back-EMF, rather than sinusoidal, but this is not a general rule, since as I said above many sinusoidal synchronous motors are called BLDC because they are usually fed with a square wave current (and, also, you can drive a trapezoidal motor sinusoidally).
BLDCs make speed regulation easier, since (at steady-state) frequency means speed. The problem is they require proper driving, so a 3-phase inverter, and in the majority of cases a proper sensing to know rotor's position (usually hall effect sensors or sensor-less techniques, relying on voltage sensing on the phase winding which is in turn left open).

Small motors (except for some rare cases, such as mains connected fans) aren't asynchronous, since their efficiency and power density would be really poor.

Both brushed DC and asynchronous motors have rotor windings (and currents, of course), which makes them have inherently less efficiency and higher inertia. Synchronous motors (except for some exceptions, like car alternators) don't have rotor windings, since the field is given by permanent magnets.
One machine can do the work of fifty ordinary men. No machine can do the work of one extraordinary man. - Elbert Hubbard
 

Offline cyberfish

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Re: Hobbyist RC plane brushless DC motor
« Reply #10 on: February 06, 2011, 05:19:04 am »
Quote
I think it's not so easy to apply the steps manually...
What is your setup? Switches? Wires and hands?
If you use switches, then a free-wheeling diode should be put there, but I think that, however, current has the time to go to zero while switching from one phase to the other, So rotor could place itself in a quite random position at every phase change.
I started with my triple half bridges board with microcontroller, but that didn't work, so I am doing wires and hands now.

I'm thinking, with the current still flowing, the rotor should go to the right position.

Quote
Do you know whether it's synchronous or asynchronous? If it's the latter, it certainly won't work when you apply the pulses at a low frequency because there won't be enough EMF generated in the rotor.
This is a synchronous motor, assuming I understood the Wikipedia page correctly.
« Last Edit: February 06, 2011, 05:38:11 am by cyberfish »
 

Offline cyberfish

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Re: Hobbyist RC plane brushless DC motor
« Reply #11 on: February 06, 2011, 05:39:10 am »
Problem solved!!

Turned out I just fed it too little current. Magnetic field wasn't strong enough to pull rotor into place. Cranked it up to 3A and it worked perfectly (the motor is rated for 9A).

Next stage - back emf commutation.

Thanks
 

Offline Simon

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Re: Hobbyist RC plane brushless DC motor
« Reply #12 on: February 06, 2011, 02:13:25 pm »
sounds like a powerful motor, you going flying ?

How feaseable are electric model aroplanes ? can they compete with petrol yet ?
 

Offline cyberfish

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Re: Hobbyist RC plane brushless DC motor
« Reply #13 on: February 06, 2011, 08:01:33 pm »
Yeap! This is for a quadrocopter I'm building.

I have never played with petrol, so no idea about that. My guess is probably not, though.
 

Offline Simon

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Re: Hobbyist RC plane brushless DC motor
« Reply #14 on: February 06, 2011, 08:18:55 pm »
ooh I'd love to have one of those but I'm not good enough to build one and certainly haven't got the money
 

Offline Jon Chandler

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Re: Hobbyist RC plane brushless DC motor
« Reply #15 on: February 07, 2011, 06:17:48 pm »
Some of these motors for R/C vehicles are amazing!  Imagine a third of a horsepower in a package 28mm long x 27 mm diameter.  Some of these motors draw more than 20 amps under load.

HobbyPartz shows many different types. Ohhh....shiny :)
 

Offline Simon

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Re: Hobbyist RC plane brushless DC motor
« Reply #16 on: February 07, 2011, 09:50:30 pm »
well sounds like electric motors have caught up with the power to weight ratio of petrol engines, just need to condense the enery storage
 

Offline Zero999

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Re: Hobbyist RC plane brushless DC motor
« Reply #17 on: February 07, 2011, 09:56:43 pm »
It's always been possible to build smaller and more powerful motors than petrol engines, the advantage petrol has over batteries is energy storage density.
 

Offline monpjc

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Re: Hobbyist RC plane brushless DC motor
« Reply #18 on: February 08, 2011, 07:41:19 pm »
As someone who work for a BLDC motor manufacture I like to comment on the cheep PC fans - They are rubbish cos they are made for $0.20, have plastic bearings and very poorly built.

I'm not stating who I work for but all our fans have high quality bearings are as stated else were, 80-90% Eff and give massive power output for size and weight - but not cheap!

Check out some of the new RC BLDC cars that are around - these things are just amazing the power they put down. So they are nothing like a AC motor which has massive losses.
 

Offline sonicj

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Re: Hobbyist RC plane brushless DC motor
« Reply #19 on: February 09, 2011, 02:11:05 am »
As someone who work for a BLDC motor manufacture I like to comment on the cheep PC fans - They are rubbish cos they are made for $0.20, have plastic bearings and very poorly built.

I'm not stating who I work for but all our fans have high quality bearings are as stated else were, 80-90% Eff and give massive power output for size and weight - but not cheap!
do you build computer fans? ducted fans?

i agree that most cheap computer fans are complete rubbish. they'll move air, but they won't do it quietly or efficiently.

Check out some of the new RC BLDC cars that are around - these things are just amazing the power they put down. So they are nothing like a AC motor which has massive losses.
i concur! heres a good example. i think he makes just shy of 10kW at one point in this flight.  :o


« Last Edit: February 09, 2011, 01:35:51 pm by sonicj »
 

Offline kaptain_zero

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Re: Hobbyist RC plane brushless DC motor
« Reply #20 on: February 09, 2011, 02:12:41 am »
Regarding energy storage, the current Lithium Polymer packs are quite amazing..... This perky 4000mAh battery pack is 37V and can sustain a 45C constant discharge rate (that's 45 x 4 =180A) and can supply/survive brief periods of 90C aka 360A current draw. Naturally, this pack is only 4000mAh in total capacity, but things are moving quickly in the Lithium Polymer storage arena. I've seen some amazing demonstrations of electric powered models and most are capable of 15 to 20 minutes of flying before needing to be recharged.

These packs are not cheap but it seems more and more modelers are switching to electric flight, just because it's clean and effective.

Regards

Christian

 

Offline kaptain_zero

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Re: Hobbyist RC plane brushless DC motor
« Reply #21 on: February 09, 2011, 02:19:23 am »
Stepping up in size, the Killacycle is just amazing..... you may view the current world record run of 169MPH here.

Regards

Christian
 

Offline monpjc

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Re: Hobbyist RC plane brushless DC motor
« Reply #22 on: February 09, 2011, 10:00:41 pm »
do you build computer fans? ducted fans?

Yes :0)
 

Offline firehopper

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Re: Hobbyist RC plane brushless DC motor
« Reply #23 on: February 09, 2011, 10:48:16 pm »
I have a quad built that uses 4 30 amp esc's to drive 4 motors spinning 12 inch props.. weighs about 2 lbs, no idea on thrust or carrying capability.
 


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