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Old Mar 06, 2006, 11:13 AM
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Question running high V on low Kv motors (himax, 2025 series)

I have learned that it is pretty beneficial to run high voltages on low kv motors to acheive higher efficiency and use less C on batteries.

I have a lot of 3s packs, but I was thinking of using them as 6s packs.
I use them (3s packs) on a himax 2025-5300 but I was thinking of going with a himax 2025-3200 and using a 6s pack of tp 1320 prolites.

That would give an rpm of around 71k compared to the 5300 on 3s of 59k. We regularly run these motors at around 300watts, so I'm sure the motor can handle it. But I'd like to know if it will work, I'm using a cc 25, but I figured I would never go above 17A on the 1320prolites and I'm obviously going to have to use a seperate bec...

I also forgot to mention, I am planning on using the 2025-3200 on alfa fans, microfans and 3x3 props...

Let me know your thoughts on this...
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Old Mar 06, 2006, 11:36 AM
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Just a note... People have used a himax 2025-4200 with 4s in the alfa fan and it ran at 18amps max. Compared to a himax 2025-5300 with 3s running at around 25 amps. They also got more thrust, around 18oz...
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Old Mar 06, 2006, 03:32 PM
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Maybe the thread is mistitled but a 5300Kv and a 3200Kv motor is not considered low Kv.

The max wattage rating for the Himax motor is 175W so you are running them way beyond their ratings. If you wanted to use these motors in a DD application like fans then they won't last long on 6S.

6S which run at about 6* 4.2 = 25V which underload really gets you about 21-22V.
67000-70400 rpm

3S which runs at about 3S * 4.2 = 12.6V which underload really gets you about 10-11V.
53000-58300 rpm

It really depends on your batteries ability to hold voltage under load and power losses due to IR.
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Old Mar 06, 2006, 03:59 PM
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The published rpm limit on the 2025 is 60,000. That would keep me worried, too.
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Old Mar 06, 2006, 06:00 PM
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Well people are using the 2025-4200 on 4s and 5s, both of which bring the rpm to over 60,000. I've read that these himax motors are limited to 150,000 rpm as they are 2 pole motors... They are getting better efficiency running the 4200 on 4s rather than the 5300 on 3s.
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Old Mar 06, 2006, 06:47 PM
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It might be true they get better efficiencies out of a 4S and a 4200 but they need to keep it under 12A!!!

Or cool it down very well!!!
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Old Mar 06, 2006, 07:45 PM
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Umm, they run it at 17-18amps, which is 266watts. They also run it on 5s at 20A which is 370watts. I run my 2025-5300 on 3s at 25A which is 277watts with a 3x3 prop (everyone who has a concrusher uses this setup), it doesn't have a heatsink and gets warm/hot but I can hold it comfortably. I just want to make sure that 6s isn't too much voltage for the motor, controller or whatever... I know there are planes that run on 10s even 12s, just making sure that the himax 2025-3200 can handle it..

I figured it would be running at around 300-320watts, 22.2v at around 14A. Am I close?
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Old Mar 06, 2006, 10:39 PM
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I'm sure you could run it like that but I'd like to see a flight log of how many flights before the motor demagged and blew itself apart!!!

Maybe it's possible and himax severely underrates their motors!!!
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Old Mar 07, 2006, 03:16 AM
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I thought about combining my 3s packs also... but the cc25 is only rated for 5s.. If you try it I'd get a separate BEC.
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Old Mar 07, 2006, 01:10 PM
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ESC's can tollerate over amping far more easily than over voltage. They will take several seconds of way too many amps, but they generally cannot be pushed beyond the max voltage rateing without letting out the magic smoke very quickly.

Throttle management does not help in the case of over voltage. As soon as you plug in the pack the smoke comes out.

The BEC wil not be usable on 4S or more in any case.

Larry
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Old Mar 07, 2006, 01:28 PM
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You can calculate the heat losses at different voltages and see how the motor will behave.

There are two basic losses that MotoCalc and most people use that are fairly simple and more or less accurate. They work very well in the middle range of rpm and power. At hi power loadings (near current saturation) and hi rpms they are on the low side.

V = voltage applied
Io = no load current (published Io is usually measured at 6 - 9 volts - so you need to check it at your actual used voltage. It can varry a lot sometimes)
Rm - motor resistance
I = opperating current

All loses are in watts

Iron losses are = V X Io

Copper losses are = I^2 X Rm

In the case of the Hymax 2025 3200 at 300 watts on 6S (3.5 volts per cell assumed)

iron loss at = 21volts X 1amps (guess) = 21 watts
Copper loss = 14.2 X 14.2 X .144 = 29 watts

Total loses = 50 watts

Increase the current to 20 amps, drop the voltage to 5S and total heat generated by losses goes up to around 75 watts.

One factor these formulas DONT take into account is that most all brushless motors have an additional drops in efficiency that starts to come into play at very hi rpms and at near thier max power ratings. 5S and 6S are pushing them to the very upper limits and they will have even more heat than these numbers show.

Think how hot your 25watt soldering iron getts

Larry
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Old Mar 07, 2006, 02:32 PM
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Ok, so lets do the 2025-5300 at 3s.

Iron - 11 x 1.8 amps(what I got on the whattmeter) = 20watts
Copper - 25 x 25 x .044 = 27.5watts

So we are already producing 50watts of heat with a 3s 5300...
And the motor is able to stay within its temp limit...

So lets do the 3200 on 6s.

Iron - 21 x 1.2(probably a better guess) = 25watts
Copper - 14.2 x 14.2 x .144 = 29watts

Thats only 5 watts more.
See what I'm saying... and thats for about 12k rpm more.

I'm gonna see what the mega 16/15/3 can do on 6s
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Old Mar 07, 2006, 02:37 PM
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Ohh, and I'm going to have to use a jeti advance 77 opto plus esc. its rated up to 10s at 77A, so I'm good , its only 1.4oz a little over twice the weight of the cc25, but who cares, if i'm going to get 20oz of thrust on a ~14oz plane with a crazy pitch speed. (going on a mini speedwing, hehe, and a concrusher, and a flying styro f16 with microfan)
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Old Mar 07, 2006, 11:54 PM
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Your forgetting this point that I made above...

Quote:
One factor these formulas DONT take into account is that most all brushless motors have an additional drops in efficiency that starts to come into play at very hi rpms and at near thier max power ratings. 5S and 6S are pushing them to the very upper limits and they will have even more heat than these numbers show.
That 3200 on 6S is going to be spinning at 67000 rpm. At that hi an rpm the iron and magnetic losses get extreem. Not to mention that you are exceeding the max rating and are likely to throw a magnet

Larry
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Old Mar 07, 2006, 11:58 PM
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Don't Hacker and Neu inrunners start seeing serious efficiency drops over 50k RPM?
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