More Voltage = Less Amp Draw?

[captain harlock]

They're rated up to something like 100,000rpm/s maxximum. Sounds crazy.

[glassdoctor]

Originally Posted by VintageMA

I think you guys are missing something in your calculations of high voltage = less amps. If you take the same motor with the same KV rating, higher volts will = higher amps.

I know I'm not missing it... :010: ...to increase the voltage you must reduce the kv accordingly. I assumed that would be assumed. :030: hee hee

[VintageMA]

Well it's just a point I wouldn't assume everybody reading these threads would know to make.

When I first got into this I would've just thought "Oooh, faster motor cool!!" Without first understanding that means higher current draw, lower starting torque, and generally a drop in source voltage as the resistance of the motor starts to drop to that of the internal resistance of the cells.

[glassdoctor]

True... I remember thinking that one time for a second, then saying to myself "doah!" ha ha

Originally Posted by Serum

If the KV of the motor stays the same (as well as the gearing), it would be obvious that the current will increase too.

But if you gear a vehicle at 40 mph on 2S with a certain motor and gear that same vehicle on 40 mph with 8S, the current will be significant lower with the 8S.

This is correct. Running high voltage involves two primary considerations - motor kv AND available gearing. If you can gear 25:1 then there're only two things that might stop you from running a 2500 Kv motor on 8S - iron losses and motor quality. On a setup like that the motor will be spinning at 75k RPM at full tap. The motor needs to have high quality bearings capable of running at that speed, and the motor itself needs to be made of large/strong enough quality parts so that it doesn't break itself. As far as iron losses go, brushless motors generally need to spin below 250k RPM per pole. For a 2 pole motor this means 120k RPM. A 4 pole 60k RPM. An 8 pole 30k RPM, etc. This is why Neu 1500 motors are rated to max 60k RPM (4 pole), and 1900 motors to 30k RPM (8 pole).

The sweet spot for a motor is just below its peak RPM. This is where you will get the most torque and power out of it before heat becomes an issue. This is why I want to run a Neu 1912 at 30k RPM in my 1/8 buggy - optimal motor RPM and gearing is (fairly) easily available. I also think a motor that can run optimally at a lower RPM will result in less energy wasted in the gearing.

The problem I see with BP's setup is that he wants to run HV in a small car. This means he'll need to find a motor physically small enough with a Kv low enough for the gearing available. If you can find a motor with the right specs you will have one kick ass little car. :)

[BP-Revo]

Problem solved - Motor: LMT 1530/20. 1428 KV loaded on delta configuration. Can be used in wye configuration for ~825KV (my backup if delta is too fast - doubt I will use it though ;) ).

The motor is ~30mm in diameter. So, larger than the 20-22mm Mini T and RC18T BL motors, but smaller than the standard ~36mm diameter 540/550 motor. Seems like the perfect size for this thing.

Plus, when ran on 8S, it should have PLENTY of power. Also, on 8S, considering about 3.5V per cell (due to load), the motor should spin like 2 or 3 RPM's shy of 40,000 (assuming the KV values are accurate, etc), which is perfect.

Let us know how it goes. I plan to run 10S in my 1:8 buggy and will be using a Kontronik Jazz controller to start with as it has two car modes and proportional braking which apparently should work. Failing that I'll have to try the new MGM 12032. :)

[BP-Revo]

I'll be sure to make a thread once I finish the project which will include build pics, write up, and hopefully a vid :)

I've got pretty big plans for this thing.

The ESC I will be using is the Castle Creations Phoenix HV85. Its rated 85 amps continuous (which is plenty for my application), is capable of 12S Lipo (!), and has tons of awesome programmability like hard or soft cutoff, adjustable overcurrent protection, motor timing, or throttle type (and a few more). It looks pretty awesome. Bad part is no brakes (since its an air controller) but I'm using the disc brakes anyway, so its no biggie. Plus, for just 180 bucks, I haven't found any 12S capable controller with comparable or better current ratings. In fact, I haven't really found any other 12S controllers for under 300 bucks (besides the other Phoenix HV contollers like the HV45 and HV110).

Bad news is that I have to wait until at least May or so to start the project.

[starscream]

Originally Posted by BP-Revo

captain - PHEW! :)

starscream -I don't know. Did you try messing with the programming?

There are so many throttle configurations and stuff on the Pheonix HV line its insane. You can adjust in hard cutoff, soft cutoff, current limiter (which cuts off when you draw too much for the ESC to support). You can set throttle type to remember your radio, automatically adjust endpoints each time you start it up, or have it "govern" where it translates throttle to a certain RPM and tries to hold that RPM regardless of load, etc. Then there is also start power, timing, etc. There is like 9 things you can program on that controller, vs like 5 on my little Mamba 25 (my only other CC controller).

I'm assuming it should work fine (hoping it will work fine - because there aren't many 8S controllers available for just 130 bucks!)

I wish I could say it was as easy as changing esc settings but it wasn't. The HV line of controllers are configured with a safety feature which requires the user to arm the esc every time it's powered up. In order to arm the esc you must advance the throttle to full brake. I was able to "trick" the esc with my JR XS3 by basically letting the initial programming timeout during the full brake setting thus setting full brake to the nuetral position of the trigger. This worked fine for me as I disabled braking and used mech brakes however, the esc had a nasty little caviate to this "trick"...
Randomly, during a run, the esc would "reset" when the throttle was advanced past nuetral toward reverse (while I was applying the brakes). This "reset" resulted in setting the throttle nuetral position to half throttle and viola my MT was out of control. The only way to stop the truck was to apply full brakes and even full brakes was not always enough to fully stop the motor...
The nuetral position trick only worked with my JR radio. I could never get my Nomadio Sensor to perform this same "trick"
In my opinion, the HV line of esc's are more than adequate to handle a car but because of the esc arming functionality, the esc becomes more of a hassle than its worth.
If you decide to go this route and figure out how to tame this arming feature, PLEASE share with us your secret :017:

[BP-Revo]

Oh, thats actually pretty simple for me, I have it all worked out. I will program the ESC as normal on the throttle channel. Full brake will actually be "full brake" on my transmitter (with no tricks). Neutral be neutral, throttle be throttle.

Then, channel 3 will be mixed in (since my CRX has channel mixing) to operate the servo for the brakes. The Servo's EPA will be set so it does not move (or moves very little) while throttle is applied.

Thus, when I power up, I hold full brakes (arming the ESC and applying the actual brakes at the same time), and then drive as normal.

If the ESC somehow has a problem seeing full brake every so often (when I am braking), I will simply make two profiles. 1 that has the EPA for full brake on the ESC channel set to normal, so I can arm it, and the other one set to 0, so the ESC never actually receives any brake level signals - just neutral to throttle. (The CRX's have 20 model profiles, so I don't care wasting 2, even 5 on one truck...since I'm only using like 2 between the two trucks I am using it on right now, and one of these will be sold to help fund this project).

Edit: Which HV are you using star? The 85 or 110?

[starscream]

I had the HV110.

I hate to be the bearer of bad news but there is no "nuetral" position on an airplane controller. Just so we're on the same page, If you don't "trick" the esc into setting full brake to nuetral, your throttle will act just like an airplane stick meaning nuetral is half throttle. If you plan to use it the way you described, then as soon as you begin to let off the brake you will be engaging forward throttle ( (50% reverse trigger = 25% throttle, Nuetral = 50% throttle, 50% forward trigger = 75% throttle etc) and if you can handle that, then you are expotentially more coordinated than I. I have a hard enough time driving my truck as it is :005:
I hope this makes sense.
My experience has been that half throttle may as well be WOT.
Trust me, I fought with this for a week or two and never experienced a consistant setup because of that damn'd "reset" glitch.
I would have escalated this issue to Castle but I'm sure they would have told me that they don't support these esc's for cars so I gave up. Maybe you can sweet talk them into some support :dft012:

BTW
I have to do this same "trick" with my MGM 12032 "hydro" version. The MGM must also be armed and this arming functionality works very much like the CC HV's but does NOT have this "reset" glitch when I apply brakes so its a trust worthy setup. I'm just waiting for my 1515 2Y to show up so I can run 8S on my G2R :017:

[captain harlock]

Hey, Aragon, which Jazz are you gonna use? the 10-32-55( I doublt that) or the PowerJazz( it has no car programs)?

StarScream, how did you find your 12032?
How much do you like it?

[BP-Revo]

Thats ok then. If I set "full brake" as neutral, but my EPA is set to 0, then the controller will still not receive anything further than its percieved maximum full brake, and all should be ok.

Captain, yes, the 55-10-32. Why the doubt?

[captain harlock]

I have it. It's good, but it thermals alot, although I used it in a 2wd 10th scale buggy( very light) with a 1920/8 and 10 GP3300mah. I even used a 12v fan and even that did not help alot.
IF, you are gonna use it with a 540 sized motor, then you probably need to feed the motor with lots of volts to cut down the amp draw. Even Mike Franz, an official of Kontronik's technical department, said that it isn't really made for use in cars, despite the fact that it has two types of car programs.
I found that it thermals alot when you apply brake more often. Probably if you use mechanical brakes, that'll keep the controller much cooler. Either than that, this thing is smoooooooooooooth.