Higher voltage and efficiency

[Thomas]

Originally Posted by asheck

... IOW a 5000mah 2s would have to be compared to a 2500mah 4s, ...

I totally agree. Further, to be completely fair, you would have to use different ESCs for the different voltages. The setup with lower voltage could do with FETs for lower voltage, but needs more of them (or higher current rating). However, it's not very practical. But I would argue that a 1/8 buggy on 3s probably needs an MMM, while it works with a smaller and lighter MMP at 6s.

I have not tested or measured different setups, but try to look at it from a scientific point of view:
Batteries: When going from 3s2p to 6s1p with the same cells, the amount of cells and the required power are constant. So the losses in the cells should be the same, both setups should see the same battery temperature. But in the wires and connectors, losses are higher because of P = R * I^2
Please tell me if you find a mistake in my calculations

ESC: More current through the FETs, wires etc. equals more losses. HV setup -> cooler ESC.

Motor: Double the voltage -> half the kV. Let's compare a 1509/2Y (1820 kV, 0.018 Ohm) to a 1509/1Y (3600 kV, 0.004 Ohm). From the specs, they should have roughly the same ohmic losses. This is because they have the same amount of copper, just different winding configuration (very much similar to the battery example above).

When theoretically comparing two setups, I assume both carry the same amount of energy (Wh), motor RPM is the same and the performance (W) is the same.

I'm afraid with this simple math, I can only explain the difference concerning wires, connectors and ESCs, not motors and batteries. I'm pretty sure the simplification holds true for the batteries, but not the motors (impedance etc. not taken into account).

[nuz69]

Totally true Thomas, a HV setup makes the ESC cooler for sure, but theoritically it's the same for the motor and the battery (with the same runtime). For a low Kv motor, the windings wires are thinner (more turns in the same space). But it's maybe easier to package thinner wires (low Kv) in the motor than larger wires (high Kv)...
Some people noticed the some change on motor temp using low and high voltage setup, I didn't (Castle 2200Kv 4S versus leopard 1400Kv 6S with heat sink). But my ESC now operates at cooler temp.

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?????

[_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...

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.