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Old Jun 12, 2012, 01:10 PM
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A40-10s 750kv on a protos 500?

Yep, I'm probably crazy for asking this, but will this work on a protos 500?

The stock scorpion 3026-880 comes down really hot and I'm only doing mild sport flying. I'm getting 6 minute flights with an ending voltage of 22.5v on 6s 2650mah lipos, so no problems there.

My concern is the maximum RPM rating for the a40-10s. It says 16,000 rpm. If I do the math 22.2v * 750kv = 16,650 rpm which is not bad, but 25.2v * 750kv = 18,900 rpm. Waaay over that 16k rpm mark.

So what to do? Use a bigger pinion? The protos has a 93 tooth main gear btw.

Off to find a head speed calc.
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Old Jun 18, 2012, 11:22 PM
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Its been a week and no reply from anyone. Not even aeromodel.

Is anyone out there?
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Old Jun 19, 2012, 01:27 AM
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The A40-10S 14 Pole (750 kV) would not be a good choice for a high rpm application. It would be very difficult for the ESC to commutate well.

In your RPM calculations, you would never be at 25.2 volts under any load, and you are also not taking motor efficiency into consideration - so the 16,000 number is likely fine.

750 kV x 22.2v (3.7 v/cell x 6s) x 85% (Loaded motor efficiency) = 14,153 RPM

You might consider the A40-16S 8 Pole (1000 kV) with a small pinion.
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Old Jun 19, 2012, 07:49 PM
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Can you explain what the ESC not being able to commutate well means?
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Old Jun 20, 2012, 11:17 PM
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Kavic,
Every ESC has a maximum RPM it can commutate, or "switch the motor pulses on and off" in order to drive the motor. (I am making this overly simplistic).

The higher the motor RPM, and the more motor poles, the faster the ESC must switch to spin the motor at whatever that speed is.

If the combination of motor RPM (+ motor pole count)and voltage is higher than the ESC can commutate, the result can be as simple as the motor not producing it's full power and RPM potential, to as drastic as a complete ESC failure and fire.

Usually, it results in the first issue, accompanied by excessive heat in the motor and ESC.

Using the regular 14 pole A40-10S at such a high RPM will likely not work well. If you do decide to try it, I can tell you that a Kontronik or Castle ESC are probably your best chance of it working okay. They have the highest switching speed capability in this range.

Mike
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Old Jun 21, 2012, 07:14 AM
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Thanks for the advice.

I think it will be ok though. I did some checking and found out that for a YGE 60a the maximum RPM is 240,000 for 2 pole motors. 120,000 RPM for 4 poles and so on.

According to this:

http://www.bavaria-direct.co.za/models/files/Max_Speed_of_Controllers.pdf

I've got plenty of overhead RPM wise. 34,286 RPM max at 14 poles.

I did switch back to the 15T pinion so one tooth bigger will help a bit.
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Old Jun 27, 2012, 02:58 PM
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Well, I flew it once and was not happy with the results. It did much better than the stock scorpion 880kv motor in terms of mah used during the flight. But I had to bump up my idle up curve to 80% to 100% to get a similar headspeed.

And the motor came down almost too hot to touch just like the scorpion even though the hacker motor is a little bit larger. I'm guessing it was 115F or so right after I landed. Keep in mind that the motor temps are a bit cooler in flight since the fan is running and when it stops all the heat from the winds is no longer getting the fan airflow so all the heat is radiated out to the magnet can.

Btw the hacker motor is now in the for sale section if anyone is interested.
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