More Voltage = Less Amp Draw?
 

Is this true? I know the guy that got the 160+mph run in RCCA was running only a CC Pheonix HV85, which is rated at 85amps continuous, in his car running 8S on a Neu.

The motor I plan to use is a 1530, and BK-electronics.com says its rated like 55amps continuous. If I run something like 6S or more, is ok to run an CC Pheonix HV45 (45 amps continuous?).

And damn...my third thread in like 2 days...I think I'm starting get annoyed LOL

With the same power output, a higher voltage setup with the appropriate motor will have a current draw that is less than half of a lower voltage setup with an appropriate motor. With higher voltage comes higher efficiency, so for the same power input, a slower motor with a higher voltage will put out more power than a faster motor/lower voltage setup.

It's annoying when you visit the forum without posting new threads and share information.

???? That makes no sense whatsoever. I've been on the forum quite a while and do contribute as much as I can whenever I can, but compared to a majority of guys I know very little about brushless.

I said that as a joke...not to get flamed. :028:

I think he was talking about people in general, not you.

Hi,

From my dim memory :

P=IV where P is power & I is current & V is Voltage

where heat is wasted power probably using this formula P=IsquaredR (R is internal resistance of the system)

Of course efficiency comes in somewhere too.

From this, one can say that higher voltage X lower current gives a similar result to higher current X lower voltage. However, in the process, less energy is wasted using a higher voltage.

Does this make sense ? I hope I'm right here. Feel free to correct me if I'm leading everyone down the garden path.:o

I'm in the process working on something based on the above thinking. I am seriously thinking of using a higher voltage esc like the BK36120 or BK3695 with 8S & a 12XL motor instead of a 4S with 7XL motor.

Regards,
Joe Ling

I hope so...cause if that was directed to me... :012:

joeling - that was exactly what I was looking for - thanks. I think 8S should be plenty to overcome the 10 amp difference in continous draw, though I may just get the Pheonix HV85 and forget about it. Also depends on the packs...dropping ~200 bucks for 1 set of packs is pretty hard for a smaller truck lol... Your explanation also seems quite logical and reasonable. It must be pretty accurate.

Seeing the top pics of the CRT .5, I really have no idea how you'd be able to fit a 6S pack in there unless they're small capacity. This thing ain't exactly got an abundance of real estate:
http://www.jamminproducts.com/CRT5images/Large3Qtr.jpg

Oh, I don't know. I just looked at that pic and the chassis is at least 14" long.

Of course, that was on my 21" monitor. :005: ;)

Power is watts and watts is power.... pure horsepower. :)

No doubt the best way to build power is with high voltage/low current.

The key is to resist the urge to strap in more motor than the formula requires. That will only lead back to higher amps and the heat that comes with it.

Nothing's free... it's too bad we can't just take the same motor that works well on 4s... and run it on 6s or 8s and get less current. ;) But it doesn't work that way unfortunately.

More voltage equals higher amps (and heat) if everything else remains the same. So we have to change everything....

Wheelbase is ~220mm according to Tower, which means that the chassis plate area itself must be at LEAST 150mm, probably more. The only thing the lipo's must be short enough to clear is the steering servo, which can't be more than 25mm or so.

However, I am using smaller packs which are only 35mm wide and 98mm long. These will be used in 8S configuration. The higher voltage should reduce the current requirements. I did some simple calculations and even factoring ~25% more watts than the motor's continuous rating is, I only came out to 35 amps continuous. The cells are capable of 42amps, and my ESC will be capable of 45 (and has a programmable cutoff which protects against over-drawing). Again, this is all preliminary, but as of now, I am assuming it should work pretty well. My planned motor is a LMT1530 by the way.

My system is pulling 60A at 15V (4s) on a typical full throttle accleration... 60 x 15 = 900 watts.

So to get the same 1.2 or whatever, horsepower (900 watts) using an 8s setup would only need to pull 30 amps. Which also means less heat, more efficiency. Also, because of the lower amps... the battery voltage will not drop as much, so you need even less motor to make that 900 watts... or you get a little bonus... maybe the battery holds at 31.5V instead of the 30v... that's 945watts.

Hey that's 45 watts for free... so I was wrong about the nothing for free thing. :)

I think you're a little off on your servo length. Most standard steering servos are around 1.6" long (40.64mm). Even a mini servo like a Hitec HS-81MG is 30mm long.

The packs sound like they should fit ok. They sound like the same size as I use in my 18th scale vehicles.

Yay! Free power! Thats gotta be the best thing I heard all day.

Hmm, so 60A at full throttle on 15V. I'm running 30V (to be safe) and my motor is rated 770 (say 800 to be safe). 800/30 = 26.7 - which is alot less than the battery and speed controls rating. Sweet! This is gonna be a fun project (but does anyone think this may be too much motor?)

Well, off or not with the servo lengths, the packs should fit with room to spare.

all i have to say is, bp ur a psyco!!!!!!!!!! 8s in a flippin crt.5!!!!!!! DAM!!!!!!!! LOL

But with a 1530 - smaller motor than what Brian plans to run - he's running a full 19 series!

Anyway, reason behind 8S is because I was going to run 4, but then I decided they are small cells, and more voltage will require less current and I get more efficiency. On top of that, I need to pump a little extra out of the somewhat small 15 series motor.

So then I though of 6S. When I was looking for a controller to handle it - I saw the Pheonix HV series (Plane controller is fine since I'm using mechanical brakes) - and then figured, if I'm going to buy an ESC capable of 12S, might as well go higher. So I went to 8S, since my charger does 4S max, I do 4S per side - perfect!

Power should be PLENTY (if its too much, I can always go and wire the motor in Wye connection - which I just learned today! - which will reduce KV and thus reduce power).

So, there's a reason to the madness :)

Sounds perfectly healthy to me!

LMAO!!!!!! as long as theres vids i cant complain :D

My thoughts exactly.

Don't worry, I won't hold out on you guys. Full build up with pictures and video's are planned.

Now, for the bad side...project will not start until May at the soonest :035:

lol now "THE WAIT" LOL

Yea...no kidding

But only 8 days until GorillaMaxx re-opening...and then I get to have a bunch of ideas go through my head on my G2R.

thats true... i hope i dont buy something new from them lol, new stuff like that is always soo tempting lol

Oops! You almost got me wrong my friend. I've just tried to say that posting three threads in one day does not mean you have to get annoyed nor annoying the others around you.

It is annoying to you when you just come over here and do nothing but visit the forums without sharing info or seeking them.

You're doing just fine, really. No hard feelings whatsoever..:)

Did I miss something?
The last time I tried a HV110 on 8S, it was too buggy to use consistantly. (Unless you like having your truck take off WOT after applying the brakes)
I eventually unloaded the HV110 on the bay because it could not be tamed :007:
Hmm, wish they would release the MMM already...

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 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.

If you take the equation V = R * I, if volts goes up for across the same amount of resistance then current has to go up proportionately.

To take an example, if you have a motor of 1000KV and a battery of 10V the max speed of the motor will be 10,000RPM. If you then double the source voltage the max speed of the motor will be 20,000RPM. Remember the concept that a BL motor will take as much power as it can to rev up to the RPMS it should hit for any given voltage. So if you hit the throttle to 50% on 10V, the motor will draw whatever amps it needs to get to 5,000 RPM. If you do the same with 20V it will try to rev up to 10,000RPM, doing this as fast as it can it will draw more current following along with Ohms law above.

I have seen this in action with a LMT 1950/7t connection up through a MGM 24160 in my LST2. I spec'ed out the system to run with 6S when I bought it, and it does so beautifully with very reasonable heat. I tried to put 8S in it (max S for the MGM controller) and in less than a few minutes the controller was up to 180 deg. and the motor hit around 140 deg.

In order to gain the benefit of higher voltage = less current, you need to take that into consideration when you are spec'ing out your project before you buy your motor. If you want to double the voltage of the source in order to get a lower current draw, you need to take this into account with the turns and KV of the motor you buy. For example, if you have spec'ed out a motor that is 2,000KV for a 4S setup, you can just go to 8S and say you'll get less amp draw because you're actually doubling your motor speed. If you want to go to 8S, you will have to lower your motor to a 1,000KV motor to stay in spec with your project and get the benefit of the lower amperage draw.

Yes, but with a higher voltage you still aim for the 'sweet spot of a BL motor. Meaning it's not screaming at 65000 RPM. but rather at 25-30000 rpm.

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.

It takes a certain amount of power to accelerate a vehicle, and at a higher voltage with the same speed the P=U.I becomes relevant

The higher the voltage, the more efficient a setup runs, and the lower the voltage the more current is needed to get the same power (watts)

I think the general assumption is that if you use high voltage, you also have to use a high turn motor to reduce amperage.

People using HV setups are trying to do one of two things: 1) Have a VERY high power setup without having a ridiculous amperage, or 2) be able to reduce current draw on a more typically-powered setup so that losses due to high currents are reduced.

I'm still aiming for ~40K. Is this ok with a LMT 1530? This already requires a 1530/19 or so. Do I need to go with an even higher turn?

Yeah, these tiny motors seem to do nice on a higher RPM. 40k would be nice imo.

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

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

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.

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

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. :)

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. :)

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.

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:

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?