What do KV ratings mean in a BL motor - and should I care?
11-11-2004, 12:16 PM
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What do KV ratings mean in a BL motor - and should I care?
I am looking to buy my first brushless motor.
Boy, I am confused ![sm=drowning.gif]
What do KV ratings mean for the motor? Is higher better? More torque / speed/ duration?
Thanks
Boy, I am confused ![sm=drowning.gif]
What do KV ratings mean for the motor? Is higher better? More torque / speed/ duration?
Thanks
11-11-2004, 03:51 PM
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RE: What do KV ratings mean in a BL motor - and should I care?
Kv is RPM/Volt (used for brushed motors, too). More Kv means more shaft RPM.
It doesn't *necessarily* have a lot do do with duration and torque, but they all come in the mix. Kv and throttle setting determine the rpm the motor wants to be at, and it will draw as much current as is necessary to produce the required torque to get there. (Or burn itself up trying). More current draw means less run time. The amount of current required to make that torque (efficiency) is a function of the motor design, but I don't know which part.
The required torque is determined by the prop and gearing.
I'm rather new to this as well, but AFAICT the rule of thumb is high Kv for geared and DF, lower KV for direct drive. Geard setups trade RPM for torque, and DFs need RPM rather than torque to work. Propellors (especially big ones) spin at relatively low RPM, so you want a slower motor for direct drive.
It doesn't *necessarily* have a lot do do with duration and torque, but they all come in the mix. Kv and throttle setting determine the rpm the motor wants to be at, and it will draw as much current as is necessary to produce the required torque to get there. (Or burn itself up trying). More current draw means less run time. The amount of current required to make that torque (efficiency) is a function of the motor design, but I don't know which part.
The required torque is determined by the prop and gearing.
I'm rather new to this as well, but AFAICT the rule of thumb is high Kv for geared and DF, lower KV for direct drive. Geard setups trade RPM for torque, and DFs need RPM rather than torque to work. Propellors (especially big ones) spin at relatively low RPM, so you want a slower motor for direct drive.
11-12-2004, 09:24 AM
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RE: What do KV ratings mean in a BL motor - and should I care?
When designing a power system from scratch, Kv really doesn't have much meaning because you can gear the motor down if necessary.
If you're repowering with a brushless, looking for a drop-in replacement for a brushed motor, Kv is VERY useful. When you go from brushed to brushless, you want to choose a brushless motor that has a similar, or slightly higher Kv rating than the brushed motor you're replacing. This way, you can be confident that you won't overload the battery, but you'll still get a boost in power and/or duration due to the increased efficiency of the motor.
If you're repowering with a brushless, looking for a drop-in replacement for a brushed motor, Kv is VERY useful. When you go from brushed to brushless, you want to choose a brushless motor that has a similar, or slightly higher Kv rating than the brushed motor you're replacing. This way, you can be confident that you won't overload the battery, but you'll still get a boost in power and/or duration due to the increased efficiency of the motor.
11-12-2004, 10:34 AM
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RE: What do KV ratings mean in a BL motor - and should I care?
Terry,
Best way to choose a BL power system is not to worry about anything like Kv at all! - that's only important if you are the first person trying to find a brushless combination for a new plane design (in which case www.motocalc.com is invaluable) but if you're just trying to put brushless on an established design don't worry about all the heavy math - just search here and Ezone for others who have already found a good combo for the same plane - takes all the brain work out of it for you.
For example if you wanted to make an Ikarus shockflyer brushless you could puzzle over motor specs, prop pitch/diameter, gear ratios, cell capacity, cell weight until the cows come home but equally you could just do a thread search and find that about 80% of people plump for an AXI 2208/34 turning a 9x4(ish) sized prop on 3S Lithiums of about 600-800mAh capacity - job done! Sure, the Kv of a 2208/34 may happen to be 1,140 RPM/V but it's not really imprtant for you to know that cos some clever person worked all this out for us months ago and tried loads of experiments until they came up with a "good fit" - we can just sit back and reap the rewards of their painstaking research.
Same goes for pretty much any plane you care to mention - on ~10-12oz planes, for example a Himax 2015-4100 in a 'D' ratio gearbox turning an 11x4.7 is a very popular combination - sure it's Kv is 4,100 but that needn't concern you.
Cliff
Best way to choose a BL power system is not to worry about anything like Kv at all! - that's only important if you are the first person trying to find a brushless combination for a new plane design (in which case www.motocalc.com is invaluable) but if you're just trying to put brushless on an established design don't worry about all the heavy math - just search here and Ezone for others who have already found a good combo for the same plane - takes all the brain work out of it for you.
For example if you wanted to make an Ikarus shockflyer brushless you could puzzle over motor specs, prop pitch/diameter, gear ratios, cell capacity, cell weight until the cows come home but equally you could just do a thread search and find that about 80% of people plump for an AXI 2208/34 turning a 9x4(ish) sized prop on 3S Lithiums of about 600-800mAh capacity - job done! Sure, the Kv of a 2208/34 may happen to be 1,140 RPM/V but it's not really imprtant for you to know that cos some clever person worked all this out for us months ago and tried loads of experiments until they came up with a "good fit" - we can just sit back and reap the rewards of their painstaking research.
Same goes for pretty much any plane you care to mention - on ~10-12oz planes, for example a Himax 2015-4100 in a 'D' ratio gearbox turning an 11x4.7 is a very popular combination - sure it's Kv is 4,100 but that needn't concern you.
Cliff
11-14-2004, 07:14 PM
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RE: What do KV ratings mean in a BL motor - and should I care?
Kv IS important!
You should care if the motor has a very high Kv and you try to run it at too high voltage - 'cos its rpm will go through the roof! For example, don't even think about running a tiny 8000Kv Feigao 1208456 motor or a 7800Kv Feigao 1308412 12T motor on 3s - they'll be trying to rev at 80-85,000rpm before you know it!
Most motors above about 5000Kv should be limited to 2s.
Cheers, Phil
You should care if the motor has a very high Kv and you try to run it at too high voltage - 'cos its rpm will go through the roof! For example, don't even think about running a tiny 8000Kv Feigao 1208456 motor or a 7800Kv Feigao 1308412 12T motor on 3s - they'll be trying to rev at 80-85,000rpm before you know it!
Most motors above about 5000Kv should be limited to 2s.
Cheers, Phil
