Higher voltage and efficiency

[slimthelineman]

In my experience using the same kv motor on low and high voltages has negligable gains in anything but speed. To fully embrace it you must go down in kv and up in voltage at the same time. This is the only way I've found it to be worthwhile to use higer voltages. Not to beat a dead horse but the 4.5 ballistic is 5000kv and a amp hog I might add. The 6.5 ballistic is 3400kv and much better on three cells and makes the same rpm and power on the track but runs 30-40 degrees cooler iirc. That's the easiest comparison for now but my rc8 runs about the same temp margin cooler on 6s 1400kv vs. 4s 2050kv. Plus the 3&6s packs are smaller in capacity and weigh the same or less than a 2&4s pack of higher capacity. Hope this helps. Might be able to produce some data this weekend but we will see what I can come up with, I have a plan and a few rigs to try stuff with so maybe I can offer more than just my experiance.

Not to beat a dead horse but the 4.5 ballistic is 5000kv and a amp hog I might add.

They earn their nickname of spinning toaster of death for good reason BUT, Honestly, I must have a special one, it's the old version HV4.5 before the 5mm shaft, but has the good rotor and endbell. I can run it 45mph on 4s in my E-revo, no problem unless you go for 50 ;) , or drop it in my Slash 4x4, on 2,3 or 4s, and never have it even start to get hot, above 140. People are going to want to shoot me for this, but speed for speed , voltage to voltage, in my Slash 4x4, I think my Novak runs cooler then my Castle 2400. I have much love for my 4.5, it's been around along time, in alot of vehicles, pushed beyond what it's little size should have allowed, and my toaster still spins up a mean piece of toasted ground.

But let's face it, with Novak's latest system being 4s limited, they have no business being talked about in a high voltage thread.

But we are into details, we want to know how much more efficient. And we want to weigh the benefits of increased efficiency against the cost of hv setups. We want data and hard numbers!!

Yes, Yes, Yes. This is exactly what I am curious about.

I'm gonna have to agree with nuz, something looks funny in your math there Brian, Unless I'm misreading what you are saying. Cause that would let you go from a 5000mah 2s, to a 2000mah 4s, and still have the same runtime.

I found a couple different motors I want to test, found a 6s motor that will put the RPMs within a thousand of my 2s motor, otherwise identical. I think that will give some hard #'s to look at.

[Arct1k]

Physics 101

Power (J or watts with a constant) = V (volts) * I (amps)

V = I * R (resistance)

For constant power as volts increase amps decrease at same ratio

Losses W = I * I * R (just rearrange above )

Hence losses decrease with square of amp decrease

This is why power transmission lines are ultra hv.

Pretty simple conceptually.

[_paralyzed_]

I'll keep on running hv then

[slimthelineman]

He he yup. 500kv lines are pretty fun to build. Now if only I could run my rc8 on 500,000 v 250 mah packs. Lol. .00000001 amp spikes anyone? Sorry guys bored and hot right now. I am planning on ordering a v4 eagle tree and trying to contribute something more than reiterating ohms law and my experiences.

[Shonen]

the 'efficiency' claim for HV is not necessarily for the motors, but the batteries and wire which have more or less fixed resistance characteristics. Like everyone said, the heat losses goes up quadratically with current, and by going with higher voltage for a given wattage it reduces both current draw and heat.

To be truly scientific, the equivalent watt-hour batteries and different motors (with similar total RPM) should be put in the same chassis. Castle's CMS36-4600 on 3s 2650mAh and the CMS-6900 on 2s 4000mAH seems to be a good matchup for ground RC.

Nobody is saying that as your wattage needs increase, you shouldn't go up in voltage. But none of my systems average even 20 amps. Yes there are points that go much higher, but on average between 12-17 . So I'm sure someone can throw the math to it, but assuming a 15amp average draw in my E-revo on 4s, for 45mph, What would really be the gain going to 8s? Is it fair to go by the average amps?

[pinkpanda3310]

Originally Posted by asheck

Nobody is saying that as your wattage needs increase, you shouldn't go up in voltage. But none of my systems average even 20 amps. Yes there are points that go much higher, but on average between 12-17 . So I'm sure someone can throw the math to it, but assuming a 15amp average draw in my E-revo on 4s, for 45mph, What would really be the gain going to 8s? Is it fair to go by the average amps?

If you don't need to go up in voltage why consider it? Is that revo for racing? Sounds like you have a smooth finger so the gains will be less than if I were to race that setup and move to hv.

Hv setups usually have more grunt right? Is it possible some people learned to be lighter on the trigger as a result?

[nuz69]

Hum... No, HV and low Kv have less grunt, because the motor has higher inductance, so it limits amp spikes too.
A LV setup need a smooth finger to spare some mAh and limit amp spikes.

[pinkpanda3310]

I had a look at Brian's car set up guide and compared a stock e-revo geared for 45mph on 2s and 6s.

6s result was 15a, 333w average draw and 98a, 2175w burst
OMGWTFBBQ result was 45a, 378w average draw and 292a, 2453w burst

I went a step further and made my own hyperthetical using Asheck's revo as target power. Trying to assume the same driving style etc.. Average 15a on 4s = 252w average guessing 1680w burst (100a burst * 16.8)

HV setup-

Motor- Neu 1515 3D, 1360kv, 0.012ohms, 30200rpm (on 6s)
Battery- Hyperion 6s, 2200mah, 35c, 0.014ohms
Esc- MMM, 0.0003ohms

Power-

22.2v * 11.4a = 253w average
22.2v * 76a = 1687w burst

Losses-

11.4a * 11.4a * 0.0263ohms (combined resistance) = 3.4w average
76a * 76a * 0.0263ohms = 152w burst

3.4/253 = 1.34% power loss to heat average
152/1680 = 9% burst

OMGWTFBBQ set up-

Motor- Neu 1515 1D, 4100kv, 0.003ohms, 30340rpm (on 2s)
Battery- Hyperion 2s, 6500mah, 35c, 0.0021ohms
Esc- MMM, 0.0003ohms

Power-

7.4v * 34a = 252w average
7.4v * 227a = 1680w burst

Losses-

34a * 34a * 0.0054ohms (combined resistance)= 6w average
227a * 227a * 0.0054ohms = 278w burst

6w/252w = 2.4% power loss average
278w/1680w = 16.5% burst


Those 2 setups do not take into account losses from wires or voltage drop which would affect the LV setup more than HV. On top of that, the rate of temperature increase will be more on the LV setup due to higher amps which of course affect all components leading to more resistance, heat, voltage drop.

I did this for my own curiosity but figured I would post it since I took the time to work it out.

If you don't need to go up in voltage why consider it?

Honestly, I'm mostly after knowledge. I have seen many a great thing claimed from higher voltage systems, but have never seen data logs, or anything else, showing it.

[pinkpanda3310]

My brain hurts when I try to understand inductance. From what you're saying Nuz I understand higher inductance means less magnetic pull?

That's not how i understand wiki's explaination -

Inductance=magnetic flux/amps
When a conductor is coiled upon itself N number of times around the same axis (forming a solenoid), the current required to produce a given amount of flux is reduced by a factor of N compared to a single turn of wire. Thus, the inductance of a coil of wire of N turns is given by:


The formula didn't paste ^ ??? Anyway, more turns means more magnetic flux per amp. Higher resistance in the winding wire but less current being drawn through it. The voltage is just to reach a desired rpm.

Sorry if this seems basic but just trying to understand. So a LV setup produces heat from amps to reach the same inductance and a HV setup produces heat from higher resistance.

[Shonen]

A higher value of inductance means the more it will resist changes in current flow in either direction. Under the current impulse of sudden WOT (ie. full voltage applied to motor) it will initially not have much power due to the coils building up their magnetic fields and resisting a change of current flow.

Howstuffworks has a great analogy of how inductors react with changes in current. http://electronics.howstuffworks.com/inductor1.htm

edit: pinkpanda3310, Wikipedia uses images for the functions to keep it looking the same without having to bother with IE/Firefox/Chrome/etc. specific formatting issues.

[lincpimp]

My experince with HV vs LV was mainly with the feigao 2 pole xl motors.

An 8xl motor on 4s and a 14xl on 8s spin about the same rpm, and thus would run the same gearing in the same rc to get the same overall speed. The 14xl ran cooler (it actually did not overheat in a 30min bash) vs the 8xl (20 mins was about all it could handle).

Also, going from 4s to 6s made a big difference too. a 1515 sized motor on 4s always seemed to have issues (like a 1515 1y) with a heavy truck, but dropping to a lower kv motor and going to 6s worked fine with the 1515 size.

Any reason why I saw this?