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Old Mar 03, 2013, 11:37 PM
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Lnagel's Avatar
Moab, Utah, USA
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Quote:
Originally Posted by jpatton4 View Post
What is the relationship between KV and the motor design? By definition, Kv is the rpm/volts. If the speed of the motor goes by the pulse frequency, then what does voltage have to do with speed. ... I thing many of us need a better understanding of the subject.
You definitely need a better understanding. The speed of the motor does not go by the pulse frequency, it goes by the pulse duty cycle. If you do not know the difference then it's time to hit the books.

Larry
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Old Mar 04, 2013, 06:42 AM
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USA, GA, Cochran
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A larger motor will usually run cooler if it is a good designed motor.=less heat so less power loss to heat=yes,more power at the prop.Will this power be majorly felt? maybe,maybe not. More likely to squeeze out a little more flight time or have a little more battery left at the end of a flight with the same timer settings.Generally KV or voltage and/or amperage supply needs to be changed to get a viable power difference in the form of watts produced,or rpms or torque output to be different.
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Old Mar 04, 2013, 07:41 AM
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But a larger motor is also going to have more residual friction... ie. higher I0. (There should be a subscript on the '0'.)
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Old Mar 04, 2013, 08:02 AM
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In my book ... bigger motor if all else is same should be able to spin up the prop better. This could in effect slightly reduce the amps draw as motor is not having to work quite as hard to achieve what the smaller motor does.

But if you gun it and use the full potential of the motor - you could draw more .... for very litttle gain.

Nigel
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Old Mar 04, 2013, 08:19 AM
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An example from my own experience:

Two Motrolfly motors:
  • DM 2820 / 750Kv - 150g
  • DM 3615 / 750Kv - 190g
I ran both in the same plane using the same prop and bettery. General observations were that both pulled about the same Amps and Watts, however the heavier motor produced higher RPM (by a few hundred revs) and increased thrust. In flight the difference was clearly noticable.

So basically the input power measured on a wattmeter was near enough the same but the heavier motor was more efficient and turned more of that electrical input power into mechanical power to spin the prop.

It should be born in mind that these motors were getting pushed very hard, in the 900-1000W range, too much for the smaller motor. The difference would likely be much less if the motors were not being used so hard.
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Old Mar 04, 2013, 09:25 AM
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United States, AK, Anchorage
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Jet, did you verify the kV on both? And which prop did you use?
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Old Mar 04, 2013, 11:50 AM
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Quote:
Originally Posted by bryansifsof44 View Post
Jet, did you verify the kV on both? And which prop did you use?
He used motrolfly motors, you can bank on the kv within a very close margin.
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Old Mar 04, 2013, 11:58 AM
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I know but there was talk at one time that jet may have got one of the 720kV models. Not sure though. I am getting in the 900-1000W range with the 750kV 2820 using a 14x7 prop.
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Old Mar 04, 2013, 12:00 PM
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Quote:
Originally Posted by bryansifsof44 View Post
I know but there was talk at one time that jet may have got one of the 720kV models. Not sure though. I am getting in the 900-1000W range with the 750kV 2820 using a 14x7 prop.
Smokin!!!
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Old Mar 04, 2013, 01:01 PM
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Quote:
Originally Posted by bryansifsof44 View Post
I know but there was talk at one time that jet may have got one of the 720kV models. Not sure though. I am getting in the 900-1000W range with the 750kV 2820 using a 14x7 prop.
The motor was purchased as a factory standard 685Kv (no standard 750Kv version existed at the time) but soon as I powered it up I know it was 'hotter' than that. I measured the Kv at a little over 740.
Long story but it turns out that Motrolfly had changed the Kv of the motor without Ken knowing. There was some debate about what the Kv actually was with Ken at first thinking they were around 720Kv but in the long run the motors like mine were listed as 750Kv, so that's the exact same motor as you are running.

So to cut a long story short.. mine is the one now being sold as 750Kv and I measured it at a bit over 740Kv.

The 3615 motor is hand wound by Ken so I'd assume the Kv on that one is pretty close, I've not measured it.
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