Author Topic: Target RPM...what percentage of max RPM? (Read 1115 times)

Crist Rigotti · « **on:** April 11, 2011, 06:18:59 PM »

Up for discussion is the target RPM we shoot for in relationship to the maximum RPM. In the past I always shoot for 75% as my target RPM of max. Give or take 300 or so RPM. Here's the situation, I have a 710Kv Scorpion motor that I was going to use with a 5 cell battery and run it at about 10,400 rpm which is 79% max rpm. I want to use the same motor with a 6 cell battery and at 10,400 rpm I would be at 66% max rpm. If I had a motor of 625Kv then with a 6 cell battery at 10,400 would be right at the 75% max rpm mark.

So what are your thoughts on this subject? Do you think the 710Kv and the 6 cell battery would give me good service with the target rpm at 66% of max? Would I be wasting efficiency? The motor would have plenty of "overhead". Would I'd be drawing too much current with a 710Kv motor? etc.

I know I can always try it and see what happens but I thought I get some of you smart guys to weigh in.

Here is my set up:

Scorpion SII-3026-710
Turnigy 6S 2200mah 40C battery
APCE 13x4.5P cut down to 12 inches to start with. Going up in diameter if the battery can handle it.
CC ICE Lite 50
Plane should come in at 58 ounces
5.2 lap speed
62 foot line length eye to eye .018 dia
motor run time 5.5 minutes

Dean Pappas · « **Reply #1 on:** April 11, 2011, 08:50:05 PM »

Hi Crist,
sure enough, you'll be throwing away some efficiency. The lowest I've messed with is 70% but it will work.
The 5-cell would put you at 79%, a nice compromise between voltage overhead and efficiency.
Are you looking to use the 3.3 A-h battery rather than a 2500 or 2600, as will work with a 5S?

There are a lot of very good 2500/2600 cells that are light for the capacity.

just my two cents,
Dean

Crist Rigotti · « **Reply #2 on:** April 11, 2011, 09:06:06 PM »

Dean,
I'm using some 6 cell batteries because they are short and the 5 cell batteries were a lot longer and the plane would be very nose heavy. As it is with the shorter 6 cell batteries, I'm still a bit nose heavy.

So you think around 80% of max rpm would be a happy place? I can get a motor custom wound to give me the Kv that I need for the 6 cell batteries.

WhittleN · « **Reply #3 on:** April 12, 2011, 12:05:49 PM »

Crist

I am using an Orbit 20-16 at 710 Kv with a 5S pack. Works real well; I'm sure it would work just as well with a 6S pack. After you told me that the Hyperion motors were made by Scorpion I looked into them. I'm thinking of using a 3026 710 Kv on my next Sultan-e with the 5S packs. It is a little larger and heavier than the AXI 2826-12 but the same specs as the Plettenberg just about $130 cheaper. I guess with the 6S pack you could use a smaller capacity pack?
Norm

Crist Rigotti · « **Reply #4 on:** April 12, 2011, 01:09:07 PM »

Quote from: WhittleN on April 12, 2011, 12:05:49 PM

Crist

I am using an Orbit 20-16 at 710 Kv with a 5S pack. Works real well; I'm sure it would work just as well with a 6S pack. After you told me that the Hyperion motors were made by Scorpion I looked into them. I'm thinking of using a 3026 710 Kv on my next Sultan-e with the 5S packs. It is a little larger and heavier than the AXI 2826-12 but the same specs as the Plettenberg just about $130 cheaper. I guess with the 6S pack you could use a smaller capacity pack?
Norm

Norm,
Yeah with a 6S battery I'll only need a 2200mah battery. With a 5S battery, I'll need a 2650mah battery with everything else being the same.
I'll most likely get a SII-3026 kit and have it wound for 600Kv by Dan at Go Brushless.

Dean Pappas · « **Reply #5 on:** April 12, 2011, 03:15:56 PM »

Custom-wound! My, my, my

I love it: we are no longer limiting ourselves to thinking like squatters: making use of whatever we can get; and instead getting made whatever we want.
It's another stage in the evolution of thinking electric.

Regards,
Dean

Tim Wescott · « **Reply #6 on:** April 12, 2011, 04:05:25 PM »

Quote from: Dean Pappas on April 11, 2011, 08:50:05 PM

Sure enough, you'll be throwing away some efficiency.

Do you happen to know how much? Speaking as a circuit designer, I know that there are a lot of different ways that one can approach building a PWM drive for a motor, with varying impact on the part-throttle efficiency. But all that means vis-a-vis this issue is that it's going to vary by ESC, and the only real way to know is to test.

So -- do you happen to know if anyone's done the tests on any of the popular ESC's for stunt?

Crist Rigotti · « **Reply #7 on:** April 12, 2011, 04:08:56 PM »

This is in response to Dean Pappas post #5

Yeah, and talking to Dan, he'll wind the motor to the Kv I want using larger diameter wire instead of the many strands of smaller wire, balance the rotor, realign the bearings, bascially "blueprint" the motor and I'll end up with a very smooth running very efficient motor! All for a cost though. Still compared to a, let's say T&L ST 51 and muffler package, the motor won't cost as much.

Igor Burger · « **Reply #8 on:** April 12, 2011, 04:27:08 PM »

Quote from: Tim Wescott on April 12, 2011, 04:05:25 PM

Do you happen to know how much? Speaking as a circuit designer, I know that there are a lot of different ways that one can approach building a PWM drive for a motor, with varying impact on the part-throttle efficiency. But all that means vis-a-vis this issue is that it's going to vary by ESC, and the only real way to know is to test.

So -- do you happen to know if anyone's done the tests on any of the popular ESC's for stunt?

http://stunthanger.com/smf/index.php?topic=8664.0

Dennis Adamisin · « **Reply #9 on:** April 12, 2011, 05:22:42 PM »

I'll be using a custom wound Arrowind 2832-600kv motor on 6Sx2200 on the new bird at a tick over over 70% throttle - NICE match and will run coooooool!

Crist Rigotti · « **Reply #10 on:** April 12, 2011, 05:33:14 PM »

Quote from: Dennis Adamisin on April 12, 2011, 05:22:42 PM

I'll be using a custom wound Arrowind 2832-600kv motor on 6Sx2200 on the new bird at a tick over over 70% throttle - NICE match and will run coooooool!

Keep us posted on on well it works and your numbers. I'm sure it'll be potent!

Tim Wescott · « **Reply #11 on:** April 12, 2011, 07:16:31 PM »

Quote from: Crist Rigotti on April 12, 2011, 04:08:56 PM

Yeah, and talking to Dan, he'll wind the motor to the Kv I want using larger diameter wire instead of the many strands of smaller wire, balance the rotor, realign the bearings, bascially "blueprint" the motor and I'll end up with a very smooth running very efficient motor! All for a cost though. Still compared to a, let's say T&L ST 51 and muffler package, the motor won't cost as much.

There's a supposed advantage to the "many strands of smaller wire". Because each phase of the motor is seeing alternating current, there's a phenomenon called "skin effect" that starts happening with one big wire, where the part of the wire that effectively carries the current is just the outer 'skin'. Using multiple strands is supposed to mitigate that effect. Old-time radios used to use "litz" wire in their coils to defeat skin effect.

On the other hand, using lots of strands of little wire means that you're using up precious space that you could be filling with copper for holding insulation -- so you may not be able to pack as much copper into the motor. Since it's copper that's doing the pulling, and the insulation is just along for the ride (well, that and keeping the motor from going up in smoke) there's a downside to the 'lots of little wire'.

I'm not sure which effect outweighs the other. More opportunity for tests...

Crist Rigotti · « **Reply #12 on:** April 12, 2011, 08:02:20 PM »

Quote from: Tim Wescott on April 12, 2011, 07:16:31 PM

There's a supposed advantage to the "many strands of smaller wire". Because each phase of the motor is seeing alternating current, there's a phenomenon called "skin effect" that starts happening with one big wire, where the part of the wire that effectively carries the current is just the outer 'skin'. Using multiple strands is supposed to mitigate that effect. Old-time radios used to use "litz" wire in their coils to defeat skin effect.

On the other hand, using lots of strands of little wire means that you're using up precious space that you could be filling with copper for holding insulation -- so you may not be able to pack as much copper into the motor. Since it's copper that's doing the pulling, and the insulation is just along for the ride (well, that and keeping the motor from going up in smoke) there's a downside to the 'lots of little wire'.

I'm not sure which effect outweighs the other. More opportunity for tests...

For me, I'll go with somebody who is highly regarded in winding motors and his experience. If he says that larger wire is more efficient, then I have no reason to believe otherwise.

It would be interesting to have one motor wound to 710Kv, the same as a stock motor, and then do a comparison.

Dean Pappas · « **Reply #13 on:** April 12, 2011, 08:13:22 PM »

Hi All,
Now you're juggling skin effect against the percentage of winding area taken up by more little layers of insulating varnish/epoxy and then the whole question of what kind and how much commutation current-profiling (if any!) and the next thing we know ... we know next to nothing unless we work for or with Castle/Schultze/Jeti.
Sure you can estimate the relative efficiency based on constant power and the Idle current and WindingResistance of different motors.
At 8 kHz or 13kHz chopping rates, the skin effect is quite small.
Sorry that doesn't answer the question, really, but cut and try works wonders.
Dean

Dennis Adamisin · « **Reply #14 on:** April 12, 2011, 09:10:47 PM »

Just an FYI for anyone interested:

The "standard" Arrowind 2832 is a 960 kv. I got a custom version at 600 kv. The 600 weighs 0.3 oz more than the 960.

Author Topic: Target RPM...what percentage of max RPM? (Read 1115 times)

Crist Rigotti

Target RPM...what percentage of max RPM?

Dean Pappas

Re: Target RPM...what percentage of max RPM?

Crist Rigotti

Re: Target RPM...what percentage of max RPM?

WhittleN

Re: Target RPM...what percentage of max RPM?

Crist Rigotti

Re: Target RPM...what percentage of max RPM?

Dean Pappas

Re: Target RPM...what percentage of max RPM?

Tim Wescott

Re: Target RPM...what percentage of max RPM?

Crist Rigotti

Re: Target RPM...what percentage of max RPM?

Igor Burger

Re: Target RPM...what percentage of max RPM?

Dennis Adamisin

Re: Target RPM...what percentage of max RPM?

Crist Rigotti

Re: Target RPM...what percentage of max RPM?

Tim Wescott

Re: Target RPM...what percentage of max RPM?

Crist Rigotti

Re: Target RPM...what percentage of max RPM?

Dean Pappas

Re: Target RPM...what percentage of max RPM?

Dennis Adamisin

Re: Target RPM...what percentage of max RPM?