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09-01-2009, 11:57 PM | #2 |
RCC Supreme Contributor
I am: Chang P
Join Date: Sep 2007
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Re: What is KV
kv is just short for rpms/volt. So take the kv rating of the motor, multiply by the voltage of the battery and you'll have the number of rpms output by the motor.
A higher kv is not always better, depends on many factors but basically you need to take the gearing, battery and motor into account to get the prop speed (head speed for helis) you want. |
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09-02-2009, 12:11 AM | #4 |
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I am: Michael H
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Re: What is KV
Kv rating
The Kv rating of a design of brushless motor is the constant relating the motors unloaded RPM to the (peak, not RMS) voltage on the wires connected to the coils (the "back-EMF"). For example, a 5700 Kv motor, supplied with 11.1 volts, will run at a nominal 63270 rpm. By Lenz's law, a running motor will create a back-EMF proportional to the RPM. Most ESCs do not boost the battery voltage. Once a motor is spinning so fast that the back-EMF is at or above the battery voltage, it is impossible for those ESCs to "speed up" that motor, even with no load. Kv is the voltage constant (capital-K, subscript v), not to be confused with the kilovolt, whose symbol is kV (lower-case k, capital V). Not that one motor is necessarily better than the other .... it really depends on voltage, amperage and watts used. The 1360KV motor will have a higher rpm than the 1080KV motor.
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09-02-2009, 01:21 AM | #5 |
RCC Senior Contributor
I am: Marc P
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Re: What is KV
Also keep in mind that the kv rating is established with no load (ie a flat piece of wood as the "prop"). Given that assembly and design standards vary from one manufacturer to another, 2 similar sized motors with the same kv rating will not give you the same RPM spinning identical props.
Generally speaking, comparing 2 motors of identical kv and similar dimensions, the more expensive motors will spin the same prop faster for the same amount of input current. Going with a cheaper motor sometimes requires getting a higher kv rating and expecting a higher current consumption. Recently, I've been comparing cheap (hobbycity) motors versus mid-price motors (Scorpion). To obtain similar thrust, the Scorpion requires a 710kv motor whereas the hobbycity (Turnigy) requires an 800-900kv motor and consumes more current to spin the same prop at the same speed. So the best way to shop is with the manufacturer's specs for a specific motor, at a specific voltage, with a specific prop. Ex: http://innov8tivedesigns.com/Scorpio...30%20Specs.htm http://www.bungymania.com/imageshome...erf1.php?id=73 There are websites dedicated to help us ballpark electric power systems. This is one I use often, although it takes a little getting used to: http://www.rcadvisor.com/ Last edited by marcham; 09-02-2009 at 01:25 AM. |
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09-02-2009, 08:25 AM | #6 |
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Re: What is KV
Kv (and by the way its thousands of revs/min per volt, hence the 'K') - you'd choose depending on the number of cells you'd prefer using and the size of prop you'd like to use. Generally lower Kv for fewer cells; higher for more cells, basically.
I told you - it's more a b--dy black art than a science!! BTW Didn't quite follow you line of thought, Micheal. |
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09-02-2009, 08:28 AM | #7 |
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Re: What is KV
And Kv says nothing about torque, quality, power.
Other e-flight calculators http://www.rcgroups.com/forums/showthread.php?t=606703 For a rainy day ... * Presentation: de-mystifying Electric Flight http://www.rcaircrafters.org/The%20Electric%20Show.pdf * http://www.ampaviators.com -> Beginner Guide * E-book: Everything You Wanted To Know About Electric Powered Flight http://homepage.mac.com/kmyersefo/ev...ng-e-power.pdf Corresponding discussion: http://www.wattflyer.com/forums/showthread.php?t=31071 * Several Wiki's http://www.wattflyer.com/forums/vbglossar.php * Get a current/Watt-meter, it will save you money and will more than pay for itself! 3$ diy Watt-meter: http://www.rcgroups.com/forums/showthread.php?t=949923 * http://www.ezonemag.com -> Faq * System wiring diagrams: http://scriptasylum.com/rc_speed/_wiring.html * LiPo pack and balancer cable wiring: http://scriptasylum.com/rc_speed/_lipo.html * Monthly Ampeer newsletter, on paper and online available. Excellent articles in the archives by e-flight pioneers Ken Meyers and Keith Shaw a.o. http://homepage.mac.com/kmyersefo Get a current/Watt-meter * Brushless motor animations and simulations. Mostly outrunners but inrunners are just outrunners turned inside out, nothing fancy: http://www.rcgroups.com/forums/showthread.php?t=216928 Get a current/Watt-meter * Choosing a power setup http://www.rcgroups.com/forums/showthread.php?t=739069 http://www.wattflyer.com/forums/showthread.php?t=27019 Get a current/Watt-meter * E-flight calculators, a compilation http://www.rcgroups.com/forums/showthread.php?t=606703 Get a current/Watt-meter * Current, voltage, Watt, battery-types and -C-rating explained http://www.rcgroups.com/forums/showthread.php?t=417868 Get a current/Watt-meter * 'Dry' testing brushless motors after a mishap: http://www.wattflyer.com/forums/showthread.php?t=35216 http://www.rcgroups.com/forums/showthread.php?t=240993 Get a current/Watt-meter * How to repair a brushless motor http://www.rcgroups.com/forums/showthread.php?t=1079423 Get a current/Watt-meter * Motor_rewinding_101, about poles, winds, delta, star http://www.gobrushless.com -> knowledge base -> basic overview (1-5) * And I almost forgot, get a current/Watt-meter |
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09-02-2009, 08:31 AM | #8 |
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Re: What is KV
The 'K' in Kv stands for the German 'Konstant'. Kv is the 'generator-constant'.
Motorconstants explained: http://www.rcgroups.com/forums/showt...ight=bergmeyer Vriendelijke groeten Ron |
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09-02-2009, 08:40 AM | #9 |
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I am: Cedric V.
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Re: What is KV
Vriendelijke groeten aan u -
I never realized that - hey, always assumed it meant 1 000s! But you're correct. Lots of good reading there in your earlier post, thanks - but it's still a verdomde black art, isn't it?!! |
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09-02-2009, 08:52 AM | #10 | |
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I am: Marc P
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Re: What is KV
Quote:
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