Limitless Max RPM? The math doesn't work

[Jerold]

I'm trying to figure out how you would get 75.000 RPM out of the Castle 1412?

It's a 2S motor rated at 6400 KV. So that's 4.2*2*6400=53,700 rpm. Which is screaming but it's a far cry from the rated 75K rpm.

There is LiHV batteries that can be charged to 4.35V, but it's still not enough (55,600). To get to 75K rpm I would need 11.7V or (5.8V/cell @ 2S), which, eh doesn't exist?

So what's the secret?

 

[LibertyMKiii]

Its honestly a 3s or 4s motor, but it would get to hot. To avoid warranty claims they say 2s.

Lets say the motor is 90% efficient.
6,400*0.9 = 5,760 KV (loaded is likely even less)

Voltage sag on 3s under full throttle 10.7v ?

10.7v * 5760 = 61,632 rpms

 

[gotnitro]

The secret is.....

Have to run them on 4s and rely on your ears to know when you hit 75,000 RPM

 

[Hector_Fisher]

I've seen that frequently with the rocket motors, when the KV and "max rated voltage" don't quite add up to the max RPM. I'm sure it has something to do with the fact that they can't publish it as fully "3s" capable, since that would be almost 81,000 RPM with no load and a fully charged 3s pack, which exceeds the 75k rating. Since they have to cover their butts, and since they aren't going to rate the motor for "half charged" 3s lipos only, they limit it to 2s. I think it would be smarter to just give it a max voltage rating and require the buyer to use atleast some of their brain, but that's probably why i don't work at castle.

 

[Jerold]

I don't see a max voltage rating directly. It's implied through the 2S requirement. Is there a reason you couldn't run 3S at less voltage (3.9V/cell)?

 

[LibertyMKiii]

I don't see a max voltage rating directly. It's implied through the 2S requirement. Is there a reason you couldn't run 3S at less voltage (3.9V/cell)?

Usually, heat build-up in the motor and max RPMs are the only limiting factors.

I've seen that frequently with the rocket motors, when the KV and "max rated voltage" don't quite add up to the max RPM. I'm sure it has something to do with the fact that they can't publish it as fully "3s" capable, since that would be almost 81,000 RPM with no load and a fully charged 3s pack, which exceeds the 75k rating. Since they have to cover their butts, and since they aren't going to rate the motor for "half charged" 3s lipos only, they limit it to 2s. I think it would be smarter to just give it a max voltage rating and require the buyer to use atleast some of their brain, but that's probably why i don't work at castle.

Some of the stuff I see and hear in this hobby I understand why they have to underrate their stuff.
Everyone wants something for free when it was their fault too.

The warranty side of the business is why castle is who they are and hobbywing is who they are.

 

[Hector_Fisher]

You could certainly try to estimate the voltage drop on whatever 3s lipo you might be using (based on the load it will see and the quality of the lipo) and use that to determine what the max RPM would be at full load and max speed. (truck can't get to 81k rpm if the max voltage under load is around 3.9v per cell)

I know i've seen a few "thumb rules" thrown out there about how much voltage sag to expect, but I can't comment on which to use. It will differ based on using something like a Zeee versus using an SMC SRD pack. Odds are that if you don't do many full throttle no load spin ups at 3s, you will likely be around 75k or less.

Personally, I might consider doing some passes on 3s. Not sure what ESC you'll be using, but if it has logging, you could use that to validate that the voltage under load is within your acceptable risk limits.

 

[Johnny b]

I'm trying to figure out how you would get 75.000 RPM out of the Castle 1412?

It's a 2S motor rated at 6400 KV. So that's 4.2*2*6400=53,700 rpm. Which is screaming but it's a far cry from the rated 75K rpm.

There is LiHV batteries that can be charged to 4.35V, but it's still not enough (55,600). To get to 75K rpm I would need 11.7V or (5.8V/cell @ 2S), which, eh doesn't exist?

So what's the secret?

Maths not working for me either.

 

[Jerold]

I can say I'm still getting my head around the trade off of RPM and reasonable gearing vs more torque & higher gearing. I'm thinking that higher torque is going to get me a bit more power on the top end?