Higher voltage and efficiency

[_paralyzed_]

Originally Posted by BIG-block

I don't get it guys? The higher voltage setups run cooler, right? So to me and my simple brain just the absence of heat means that setup is more efficient. After all isn't heat just wasted energy that isn't turned into motion by the motor????? Doesn't that dictate that cooler HV setups are more efficient?????

yes, and yes. 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!!

If cost is a factor than hv goes out the window for many. It cost me 2x as much for my mamba xl in my e-maxx compared to the previous MMM.

I feel cool when I tell people it runs on 30 volts!

[nuz69]

Originally Posted by BIG-block

I don't get it guys? The higher voltage setups run cooler, right? So to me and my simple brain just the absence of heat means that setup is more efficient. After all isn't heat just wasted energy that isn't turned into motion by the motor????? Doesn't that dictate that cooler HV setups are more efficient?????

You are taking the problem from the wrong side.
The point is that HV voltage runs not necessarily cooler... For the motor it's pretty much the same, although the ESC is a little cooler with HV setup. So the gain in efficiency can sometimes be negligible...

[BrianG]

Originally Posted by nuz69

You are taking the problem from the wrong side.
The point is that HV voltage runs not necessarily cooler... For the motor it's pretty much the same, although the ESC is a little cooler with HV setup. So the gain in efficiency can sometimes be negligible...

Even so, an efficiency difference of even a minuscule 0.5% on a 400w average draw is a difference of 2 watts worth of heat. That doesn't sound like a lot, but take a hold of a 5w resistor dissipating 2w and you can get the idea.

No matter what, ANY heat on the motor, ESC, battery, wires, etc is lost power. All those have a given resistance. Current^2 * Resistance=power.
Higher voltage means less current, so less voltage drop and power loss on those components. This means more of your battery is getting to the ground.

Here's some numbers for ya:

If your system is pulling say 300w average, running that on 2s means 40.5A. That 300w on a 4s setup is only 20.3A. 300w average power is pretty typical for a smaller 4WD vehicle.

So, if you have a total resistance of 0.05 ohms for the motor, esc, and wires, that's a loss of 82W on 2s for a system efficiency of 72.67%. But on 4s, that's a loss of 20.6w for a system efficiency of 93%. Big difference there!

So, I really don't understand the argument and confusion here. It seems pretty simple to me.

[nuz69]

In terms of heat, ok, a single watt loss is always too much. But in terms of runtime ? Worth it to add 200$ in order to gain 10 sec on 15 minutes racetime ?
Low Kv brings also smoothness for some people who drive brushless motors like nitro engines (trigger ON-OFF). Due to the higher inductance, a low Kv motor has higher response time, some kind of intrinsic punch control...

[nuz69]

Originally Posted by BrianG

So, if you have a total resistance of 0.05 ohms for the motor, esc, and wires, that's a loss of 82W on 2s for a system efficiency of 72.67%. But on 4s, that's a loss of 20.6w for a system efficiency of 93%. Big difference there!

I am sorry BrianG, your math is false ^^ This would be too easy to gain efficiency you don't think ?
The motor resistance gets higher as the Kv drops (more turns and smaller wires).
For example you need to double the number of turns to get the Kv divide by 2, that means for the same volume of copper (both motors have same weight), the wires section/surface is divided by 2, that means the resistance is multiplied by 4 :

R = resistivity * (lenth/surface)

So the R*I² loss is exactly the same for the two motor (I multiplied by 2 but R divided by 4). Someone explained this point above. And in reality it's not as simple as that. Sometimes it's easier to package thin copper wires in the can, sometimes not, so the resistance is also affected...

But there is a gain of efficiency in the ESC/Battery wires and connectors, and in the ESC. And this is this little gain you have to calculate, do you have some wires connector resistance values ? ESC is like 0.0003 Ohms...

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.

Originally Posted by nuz69

In terms of heat, ok, a single watt loss is always too much. But in terms of runtime ? Worth it to add 200$ in order to gain 10 sec on 15 minutes racetime ?
Low Kv brings also smoothness for some people who drive brushless motors like nitro engines (trigger ON-OFF). Due to the higher inductance, a low Kv motor has higher response time, some kind of intrinsic punch control...

First of all I am with Brian on this. Any heat is wasted power and lost efficiency. Might not make a big deal in run time but I do think that it has to make some. Is that worth the bigger $$$ outlay of a HV system??? Not for some and for me neither. Why I do like HV is the fact that my ESC runs cooler and has easier life for the same amount of power. This should mean that it should also have a longer life. Batteries too. So there has to be some money saved there to go some of the way of paying for the HV setup. Any of that make sense to anyone or just in my head?

Also I can swear that my motor runs cooler too. I remember my CC 1515 1Y getting rather toasty on 6S but my 1520 geared for some speed runs much cooler. Maybe because the can is bigger and can dissipate more heat????? Either way all of my MT's and Truggies will end up running 8S eventually with the buggies running 6S.

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 BIG-block

Also I can swear that my motor runs cooler too. I remember my CC 1515 1Y getting rather toasty on 6S but my 1520 geared for some speed runs much cooler. Maybe because the can is bigger and can dissipate more heat????? Either way all of my MT's and Truggies will end up running 8S eventually with the buggies running 6S.

Not really a fair comparison with those motors. One is smaller and higher kv the other large and low kv and running the same voltage.

[nuz69]

Originally Posted by BIG-block

Also I can swear that my motor runs cooler too. I remember my CC 1515 1Y getting rather toasty on 6S but my 1520 geared for some speed runs much cooler. Maybe because the can is bigger and can dissipate more heat?????

Totally normal, the longer motor has less magnetic losses because it spins at lower RPM and the can is longer so it dissipates more... This is not the compairison we talking about, here it is :

1515/1Y 2200Kv on 4S
1515/2Y 1100Kv on 8S

2 motors of same length and weight. I don't know the neu specs but I am pretty sure that the internal resistance of the 2Y will be ~4*more than the 1Y

You try the both setup, geared for the same speed, you will find ~ same temps for both motor, although the ESC will be cooler.
To keep the runtime equal, you need for example to run 4S 5000mAh and 8S 2500mAh, 2 batteries of same weight and price.
So the battery discharging rate is also divided by 2 ==> the 8S will operate at ~ same temp too.
That's why I said earlier that the efficiency gain in HV setup will be mainly in the wires and the ESC, not necessarily in battery and motor...
Just my 2$

[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?