kv rating of electric motor question

[truckerzero]

i know what the kv rating is its howe many rpms per volt on a motor with zero load (shaft spinning nothing on it)so lets say you have a motor with a 100kv rating witch will spin at 7200 rpm at 72 volts what should the min and max rpm that motor be run at once its under load to properly utilize its power band

 

[safe]

...what should the min and max rpm that motor be run at once its under load to properly utilize its power band

Have you been lurking at RC brushless motor websites?

Me too.

After Recumpence showed up with that fancy geared down RC brushless motor build I've been checking them out.

From what I can tell they normally anticipate about 10,000 rpms as the normal maximum for their motors. However, I've also read that people have successfully taken that well above that level without problems.

Gearbox

This is kind of interesting... they have gearboxes that can gear down the motors about 8:1. My only question about that is if they can handle much in the way of torque.

 

[TylerDurden]

kv alone is insufficient info. You need the full spec sheet with load ratings.

:?

 

[Miles]

From what I can tell they normally anticipate about 10,000 rpms as the normal maximum for their motors. However, I've also read that people have successfully taken that well above that level without problems.

What's "their motors", safe?

 

[lazarus2405]

kv ratings are only part of the story. As TD stated, you need a full spec sheet to properly gear the motor for maximum power or maximum efficiency at a given load. If the motor is $20 chinese outrunner, 'gear' it to 80% of the unloaded speed by choosing the appropriate propeller. (if it doesn't have a spec sheet, it isn't suitable for ebikes).

As for torque on propeller reduction gears, in most cases the answer is 'no'. Even the really high power setups are still low torque high RPM machines. Some might work, but I doubt it. Of course, one test is worth... :wink:

 

[Drunkskunk]

I fly a 275 watt 3D plane that runs a 8:1 gear reduction. it can hover on its prop at low throttle, and weighs about 2 pounds.
The acto of slamming it to full power to pull out of a hover will strip the gears in fewer than 10 cycles.

The gears are very light weight, 10-20 grams to handle 250-500 watts.

 

[truckerzero]

the kind of motor i was talking about is the big neo motor nicknamed bam(big ass motor) enginers at castle creatins have pushed them to 15,000 watts before they blew and they can put out 4,000 to 8,000 all day long but thier kv rating is like 150kv and would still require signifikant downgearing i belive. a setup i think would be great would be a bottom bracket drive like the one on ebay because it allows you to use your own bikes gears but has a freewheel desing so you dont have to pedel if you dont want to when the motor is running but you can gear down for big hills

 

[Dr. Shock]

I spotted this one about a year ago:
http://www.modelmotors.cz/index.php?page=61&product=5360&serie=20&line=GOLD

It is probably obsolete for or purposes by now. There seems to be a trend for RC motors to get bigger, take higher voltage and amps, and go slower. I think they need these features to fly bigger planes. We, of course, need them for bikes. I'm pretty sure I saw some rated below 100 rpm per volt.

 

[truckerzero]

on 10 a 10 cell lipo it good for 2000 watts witch on a ebike is good for 45mph and the math says on a 26 inch bike the motor would have to be geared down 7.3 times that would be a 8 tooth pinon with a 56 tooth spur sounds doable any body else want to chime in?

 

[Miles]

I posted this list elsewhere. Might as well have it here, too.

http://www.progressiverc.com/Brushless_Motor.html

 

[truckerzero]

just went through the list and rs motoren has a 82 kv motor rated at 10000 watts heafty price at 1600 british pounds i think thats what that symble ment to give you an idea i think the 5303 hub motor is around 13 kv roughly and thats based on 72 mph unloaded on a 26 inch bike kv is probably much higer with no load at all(no wheel)

 

[Miles]

Don't forget that these things are rated for use in RC planes (mostly) where this is a significant airflow along the axis of the motor. Outrunners need to be vented to take advantage of this. You'll need to de-rate the motors somewhat, for ebike use, despite what some people seem to think.... A few of the motors in the list are inrunners but most of the ones used in RC applications have even higher kV than their outrunner equivalents.

 

[truckerzero]

true but these high numbers will be for a short distance or accelaration its not like any bodys going 60 mph for any distance or you just be better with the emotorcycle and lots more motor choices in that game i think a lot of this is if we can make it faster lets do it even if we may never use it i guess all the power we really need is enough to maintain our normal speed when we hit lage backroad hills arond were i live can be as high as about 22% there are alot of them non more than 1/4 mile long though and most a few hundred to a thousand feet

 

[Miles]

Yes, but don't forget - the smaller it is the quicker it heats up.... So, you'll need an effective thermally-activated current reducer, if you want to push it...

 

[truckerzero]

how about some big heat sinks on the shell casing room permitting

 

[Miles]

That doesn't help much with outrunners. What you need is to add a fan, as Matt did.

 

[truckerzero]

like a integrated one thats on power drills?

 

[Miles]

Yes.