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  #41  
Old 08-12-2013
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I don't feel I can properly answer all of those questions, but here is my take on it.

1. The outrunner principle moves the force application point significantly away from the rotating axis. While I haven't calculated the relative effect of this against the increased pole-count, I suspect this is just as strong a contributing factor.

2. These motors have 12 stator poles and 14 rotor poles. 540-size car motors generally have 2 rotor poles and 3 stator poles. The average spacing of a live stator pole to a permanent magnet is FAR closer in the outrunner design.

3. In order to make an inrunner with the same pole count, it would have to be physically much larger than an outrunner of the same power, so I don't think they are really directly comparable.

4. Every sensorless ESC I have tried with outrunner motors runs them. Currently I am using a Hobbywing Extreme Stock and the starting smoothness and speed control is just as good as any sensor-based system I have used on normal car motors. There is likely to be a stronger back-EMF signal at low rotating speeds than you get from a conventional system, leading to improved sensorless performance.
As an idea of the starting performance, I can grip one of these motrors quite tightly in my fist and smoothly throttle up, and it just starts rotating without any trouble.

5. Switching frequency does need to be higher for outrunner motors, but it is still way within the capability of all modern ESCs.
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  #42  
Old 08-12-2013
morpheus2010 morpheus2010 is offline
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Cool Keep those wheels turning

I would have thought the out runner motor would have High Torque because of the multiple coil energising coupled with the effectively large rotor diameter.

That being said the rotor mass is also quite large albeit rotating at a far lower speed that your typical 540 rotor. Wonder if gyroscope type forces come in to play? Could be a stabilising force helping keeping the wheels pinned to the track.

Crude check could be spin up the motor on the axle set up and try to rotate the chassis (hand held) at 90 degrees to the axle line, you could feel any torsion/twisting effect if its significant.

Just food for thought
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  #43  
Old 08-12-2013
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Interesting idea about the gyro forces - I'll spin it up and give it a test. One thing that is different is the torque reaction force. This will have an anti-squat effect on the pod.
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  #44  
Old 09-12-2013
mr. ed mr. ed is offline
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The more I read about them the more I'm inclined to get one for experimenting.
For m purpose though I'd like to have one with a 5mm mm axle, and I did find some... but alll 3S or higher.
One had a chart showing performance for different voltages and showed far from optimal for the voltage range of a 2S pack.
Is there anything you could recommend with a 5mm diameter axle?
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  #45  
Old 10-12-2013
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The way I specified the motors and decided what to get:

1. I wanted RPM to be similar to a geared 21.5T car motor. This is roughly 2:1 on a 1800kv motor. So that puts the kv requirement at 900kv.

2. I decided what power I thought would be needed, and looked for motors sized to be able to develop that power - that also had the required kv.

Most small outrunner motors suitable for these cars will be designed to run on 3S. In aircraft, that would give the best compromise of weight and efficiency while still giving reasonably high voltage - always the best route to efficiency and therefore performance. That doesn't mean that you can't run them on lower voltage, but to get the required power you will need to punch a bit more current through it. This wouldn't be a problem for most motors as the higher currents only flow in short spikes in cars.

The motor I am using is the Turnigy 3536/9. This has a 5mm shaft. For this application I actually turned the shaft down to 4mm and with the motor diff unit, don't use the original shaft at all. Why not give this motor a try? You're not exactly going to be much out of pocket if you don't like it!
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  #46  
Old 14-12-2013
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I've completed the motor-diff unit. Now with more than enough rotor support >

Here's the new stator arrangement. I'm using a 35x42x16 needle roller bearing to keep everything spinning in-line. It's the thinnest section bearing I could find, I'm just hoping it isn't going to be affected by carpet dust too much as it is totally unsealed. It isn't running any lubrication so it might not attract too much.



Here is the MDU completed. It comes in at about 1.5mm smaller at the largest point than the diameter of the most worn tyre I have used... should just about fit.

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  #47  
Old 16-12-2013
morpheus2010 morpheus2010 is offline
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Default The saga continues

Looking good LongRat. You could always try graphite powder as a dry lubricant. I use it wherever dust or grit may be a problem

£3.64 free postage - CPC - microfine-graphite-powder
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  #48  
Old 17-12-2013
mr. ed mr. ed is offline
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Graphite powder? I'd be afraid of causing short-circuit with that.

Thanks again for the info longrat
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  #49  
Old 17-12-2013
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Quote:
Originally Posted by LongRat View Post
Love to see some pics dodgydiy.
But a Keda 3200 on a Mardave? That would be out of control. I doubt many speed controllers that could actually fit in a Mardave could handle those start up currents!
keda 450h is only a 400 class heli motor, very small and light, rated at 300w on 3 cells. i made up a carbon chassis mardave based car using that motor, a 35A ezrun and 1800 2 cell lipo's, 13 tooth 32dp pinion in those days!. intended to make it into an f1 style car but ended up just putting a lexan shell on it, was just under half the weight of a 4 cell brushed alloy chassis mardave. still have the chassis plate somewhere......
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  #50  
Old 18-12-2013
morpheus2010 morpheus2010 is offline
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Default Dry lubrication

Quote:
Originally Posted by mr. ed View Post
Graphite powder? I'd be afraid of causing short-circuit with that.

Thanks again for the info longrat


Only applying powder to bearing should not be a problem there are no exposed/uninsulated electrics or electronic circuitry in the motor/stator.
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  #51  
Old 18-12-2013
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Quote:
Originally Posted by dodgydiy View Post
keda 450h is only a 400 class heli motor, very small and light, rated at 300w on 3 cells. i made up a carbon chassis mardave based car using that motor, a 35A ezrun and 1800 2 cell lipo's, 13 tooth 32dp pinion in those days!. intended to make it into an f1 style car but ended up just putting a lexan shell on it, was just under half the weight of a 4 cell brushed alloy chassis mardave. still have the chassis plate somewhere......
I thought you were talking about running the Keda direct drive, not with gearing. A 3200kv motor direct would be mad in a Mardave.

I'm not worried about graphite powder in the motor, it wouldn't cause an electrical issue.
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  #52  
Old 18-12-2013
morpheus2010 morpheus2010 is offline
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Quote:
Originally Posted by LongRat View Post
I'm not worried about graphite powder in the motor, it wouldn't cause an electrical issue.
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  #53  
Old 07-01-2014
cunawarit cunawarit is offline
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That's very cool! The idea of the car silently gliding by on the carpet is cool, not to mention seems supper efficient! I'm very impressed, brilliant work.

Anyway, I have a question regarding the motors, these are meant for planes and are sensorless. Correct? How is the throttle response? Nice and progressive or peaky?
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  #54  
Old 07-01-2014
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The throttle response is much smoother than a conventional 2-magnet pole system. It's like driving double cream. Not at all peaky. I run a lot of sensorless systems anyway, and the only problem I have with them is the sometimes slightly rough start up. This set up doesn't suffer from that, you would never know it was sensorless. I think because there are so many more poles and at any time the magnets are closer to a sensing pole, the induced EMF is stronger at a given rotating speed. This seems to give great feedback to the ESC and as a result, low uncertainty on rotor position. As I said before, you can enclose the motor in your fist and smoothly throttle up, and it will just start smoothly turning.
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  #55  
Old 07-01-2014
morpheus2010 morpheus2010 is offline
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Lightbulb Nice technical tool

Spotted this (ordered one) Also thought it may be of use for your motor development, cheap enough £7.85 Happy New Year :-)
Handheld Digital Laser Photo Tachometer (e-bay)
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  #56  
Old 28-01-2014
mr. ed mr. ed is offline
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So... any more news from the race front?
I've ordered the 740KV one for a trial project, but had some troubles with the checkout from the hobbyking site.
I hope it turns up one of these days: paypal went through about 2 weeks ago...
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  #57  
Old 29-01-2014
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I've almost finished my new chassis and pod to accommodate the motor-diff unit, now it's a bigger diameter with the needle roller bearing. Other things have got in the way recently but I might get a chance to race this Friday.
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  #58  
Old 30-01-2014
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What kv outrunner for say 13.5bl 2S
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  #59  
Old 31-01-2014
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Typical 13.5T RC car inrunners have kv parameters of 3000-3500 depending on the make etc. To spec a direct drive equivalent, multiply the kv of the 13.5T motor by the gear ratio you run and that will get you in the ballpark. For example if you run 25:75 that is a 1:3 ratio, so if your 13.5 motor was a 3000kv then your direct drive motor would need about 1000kv to be roughly RPM-equivalent. That's only part of the equation though, you need to ensure it produces enough torque and can dissipate enough power.
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  #60  
Old 03-02-2014
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I've finished my chassis and pod for the motor diff unit. All ready for testing on the track this Friday.







The chassis is 2mm carbon plate, the main difference is the cut out to accommodate the extra diameter of the needle roller housing. The pod is 14mm wider than a standard Mardave pod which makes the axle extensions shorter and less vulnerable (the wheelbase and track are standard V12 dimensions).
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