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Old Apr 21, 2012, 09:03 PM
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I rewound the c05 but need some help on the results. PiCS

I have been running a little c05 with the stock wind is 8t of 2 strand copper wire 34awg wye 11,000kv. Its 9 tooth stator with 6 magnets. My batteries are 3.7v 350mah 20c zippy bare cells and have two 25c batteries. I am running this motor in a Walkera Minicp heli. I get flight times of 4min with heavy use using a 9t pinion. I had a 7t pinion but the headspeed isnt good for 3d flight. I have an 8t in the mail.

I wanted to rewind my own c05 but found someone willing to ship me one. I rewound it with thicker wire then what was on there. I could only get 7 turns of this thicker wire. I wound it exactly like the stock wye .When I tested the motor it seemed to work just as good as the stock wind. I think the power is the same, it feels like it.

The problem is that runtime is now lower and the batteries get warm. They stay pretty cool with the stock wind. The motor gets warm too but so does the stock wind.

Did I miss a turn? or is it because I didnt have 8 turns like the stock? I dont understand why the batteries are warmer then on the stock wind and also dont understand why the runtime is lower by almost a minute. But it seems like all the power is there.

I wish I could tell you guys what size wire I used but it had come out of another motor. Hopefully I have given you guys enough information.
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Old Apr 21, 2012, 10:47 PM
Jack
USA, ME, Ellsworth
Joined May 2008
17,118 Posts
Using 7 turns instead of 8 will have raised your KV. The way to figure that is

Kv for winds_new = winds_old * Kv_old / Kv_new

7 turn Kv = 8 * 11000 / 7

7 turn Kv = 12,572

And a higher Kv means it is going to draw more current with the same load on it. In this case the Kv was 12.5% higher so maybe the current draw would be too. That that could explain the battery being warmer and the run time being shorter.

Everyone needs a watt meter... And a tach too... Or maybe an Eagle Tree eLogger which can be like a watt meter and tach and even more all rolled into one...

Jack
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Old Apr 22, 2012, 07:50 AM
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One more question and thank you for the reply!

Since the kv is higher, would a 8 or 7 tooth pinion give me more power with longer runtimes? I am using a 9t pinion in there now. Thats all I have.
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Old Apr 22, 2012, 11:07 AM
Jack
USA, ME, Ellsworth
Joined May 2008
17,118 Posts
Going to a smaller pinion gear would reduce the load and the propeller RPM and also reduce the current draw/battery load.

As guesstimates, your 3.7V batteries and the original and new Kv's would work out about as follows. I am estimating that the batteries will be about 4.1V charged and rested and 3.7-3.9V under load. I don't know if that is right or not but those are typical voltages. For the no load RPM the voltage drop will be small, I'll use 4.0V for that.

4.0 x 11,000 = 44,000 RPM no load RPM with old Kv

4.0 x 12,572 = 50,288 RPM no load RPM with new Kv

I don't know if it is right for your motor too or not but it is typical for brushless motors to run at about 75% of their no load RPM when they are loaded to their continuous ratings.

75% of 44,000 RPM = 33,000 under load

75% of 50,288 RPM = 37,716 under load

I don't know how many teeth are on the ring gear but ring gear / 9 (or 8 or 7) will give you the reduction ratio and tell you the rotor RPM.

If you had a 120 tooth ring gear (a pure guess on my part) and a 9 tooth pinion the reduction would be 13.3:1 with the rotor turning once for every 13.3 turns of the pinion.

Dividing the theoretical loaded RPM's by 13.3 gives you theoretical rotor speeds of 2481 and 2835 RPM.

If you can get the count on your ring gear and re-do the math you can see what the theoretical rotor RPMs will be for the three gears.

In the end it is what actually happens (performance, battery temp, etc.) in flying it that will decide it all for you. If the battery load is dropping more than the 3.7-3.9V I mentioned then the load is, by my personal standards for battery use, on the excessive side and that is what makes the battery get hot. And if a battery gets to 105F or so (a little more than warm, almost approaching hot) it is getting to the temperature where it's service life will soon be hampered by irreversible damage. It is OK to get a battery warm to the touch but not hot. The solution is a larger battery or a better quality (higher C rate) battery.

Did I mention, everyone needs a watt meter?

Jack
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Old Apr 22, 2012, 11:16 AM
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The main gear is 64 teeth. I have a 61 too but prefer the 64 for lower head speed and longer flight times.

So does this motor have more power then stock in any configuration of gear and batteries? Or the only way to get more power was to rewind thicker with the same number of turns as stock?
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Old Apr 22, 2012, 01:17 PM
Jack
USA, ME, Ellsworth
Joined May 2008
17,118 Posts
So you can play with variables on gearing. And part of the fun would be to static test it for heating and then then flying it to see what you prefer.

That is what the watt meter would tell you.

You can run the motor up with a load on it and see the Volts and Amps (Volts x Amps = Watts). Run it for progressively longer periods of time and at higher throttle setting while, at the same time checking the ESC and battery for heat occasionally. And stop the motor and put a finger tip on the windings or base (the rotating part is the last part to get very hot).

I can use an eLogger V3 with two temp sensors (one against the windings, the other taped to the battery) and watch those temps on the Power Panel display as the test proceeds. And I can use that static or fly it. You helo will probably not fly the extra weight though.

Fingers will work for approximate temperatures. Warm to the touch is about 100F, too hot to keep a finger on it is about 130F and those are safe stopping points for batteries (100F) and motor windings (130F).

Or you can make yourself the $3 Watt Meter...

If you measure the size of the two strands that were there and what you have now you can look up the surface area of what was there and what is there now (look at the nearest smaller size and use the mm2 figures for comparisons) on the wiki AWG page.

If you increased the surface area of the wind it will handle more current, more current is more Watts at the same voltage. I would say as a rough estimate, when I rewind cheap Chinese motors, and use the where it gets too hot point as the cutoff, I would expect to see the input power to the motor increase by 25% or more and have doubled the power on some motors. That is on larger motors, I've never done anything smaller than a 22mm stator on a 50 gram or so motor.

We generally expect to see an increase of power and also find rewound motors to run more smoothly because we take more trouble to get the strands arranged more neatly. We most often replace the multiple strands with a single strand of larger size and that increases the power. I'm not sure if the C05 can be done that way or not.

Looking at the stats here:

http://www.aeorc.com/english/?product-146.html

The C05 11,000 Kv motor is a 4.6 gram motor rated for 30W max. So that is about 6.5W of power for each gram of motor weight. That would be a momentary peak rating for maybe 15 to 30 seconds or so. I would expect to be able to use it at 75% of that power level or about 23W for continuous power as would be used in a helo.

So if you have a watt meter and do some static testing you'll have your answer on the power increase. It would be hard to compare it to the data at the Aero page because the wire is different and the Kv is higher and that means it will have different capabilities.

I searched this forum and found a little more info on the motor in previous discussions and a review:

Review - http://www.rcgroups.com/forums/showthread.php?t=1616585 (look at page 5 for details on the motor's specs and dimensions)

http://www.rcgroups.com/forums/showthread.php?t=1608836

Jack
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Old Apr 26, 2012, 11:36 AM
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Canada
Joined Nov 2000
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Not too bad of a wind job actually Surprisingly so for a First? time. That's a TINY stator.
As above you have raised the KV significantly.
No big deal... you could fit the 61 tooth main but it would be only a tiny improvement (gear ratio)
Find an 8 tooth and fit it,
as Your rewound motor now needs to rev higher
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Old Apr 26, 2012, 05:53 PM
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Its going back to its owner before I got to try the 61tooth. I also dont have my 35c batteries yet. I have a minicp which is different from a mcpx which the owner of the motor has. I plan on getting more stators to wind and will update when I can.

mattbockman.com is selling c05 motor prototypes which is 6turns of 28awg wire. Someone had mentioned that there was room left over from winding the stator with 8turns of 30g which leads me to believe my wire was 28awg. I had no room left after 7 turns.
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