Higher voltage and efficiency

[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.

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?

[slimthelineman]

Must have been the super ma packs you were running! It's contradictory to some of the above posts but my hv low kv setups have always had way more snap and power through the throttle band over my lv high kv setups.

[pinkpanda3310]

Thanks Shonen, my brain didn't hurt on that one

[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.

[BrianG]

Panda: FYI, the setup guide algorithm takes vehicle weight and target speed and produces a "required power" figure. Then, calculating the current draws (average and burst) is a matter of joule's law based on the voltage used. As such, this is not exact, so keep that in mind.

[pinkpanda3310]

I realize that real world testing will have different results. I was just showing heat loss as a calculation. I could've added further losses from a nominal 200mm of wire but I forgot and today I don't have time.

BTW the sliders used in the car setup guide is a handy tool

[BrianG]

Thanks.

BTW: There is also a tool to figure wire losses: http://www.scriptasylum.com/rc_speed...esistance.html

[pinkpanda3310]

Yes, of course, I saw that too. There seems to be no end of growing uses on your calcs page, from newbs to pedantics. But I won't look at it tonight, it's 8:15pm and I'm still in the office. The last post was 4:00am this morning before work

[Thomas]

Originally Posted by slimthelineman

It's contradictory to some of the above posts but my hv low kv setups have always had way more snap and power through the throttle band over my lv high kv setups.

You are comparing two setups. Do the batteries have the same C rating and the same amount of energy content (say, same peak power)?