I have yet to see a numeric factor but it’s coming down to the efficiency of series motor windings (delta) vs parallel (wye)
Sharts and hummie would always claim higher kv motors are better at heat disappation because of joules law (copper loss proportional to I^2 * R)
But then while I was separately looking up motor can materials earlier I read this:
" There are two common electrical winding configurations; the delta configuration connects three windings to each other (series circuits) in a triangle-like circuit, and power is applied at each of the connections. The Wye ( Y -shaped) configuration, sometimes called a star winding, connects all of the windings to a central point (parallel circuits) and power is applied to the remaining end of each winding.
A motor with windings in delta configuration gives low torque at low speed, but can give higher top speed. Wye configuration gives high torque at low speed, but not as high top speed.
Although efficiency is greatly affected by the motor’s construction, the Wye winding is normally more efficient. In delta-connected windings, half voltage is applied across the windings adjacent to the driven lead (compared to the winding directly between the driven leads), increasing resistive losses. In addition, windings can allow high-frequency parasitic electrical currents to circulate entirely within the motor. A Wye-connected winding does not contain a closed loop in which parasitic currents can flow, preventing such losses.
From a controller standpoint, the two styles of windings are treated exactly the same."
And it seemed counter intuitive.
Diving even deeper, it comes down to the overall copper used in a motor. High kv motors have less windings but thicker windings. Low kv has more windings of thinner copper (thanks for the correction @ducktaperules)
Interesting enough, they come out almost identically in terms of copper weight and Kv has nothing to do with efficiency in reality.
This doesn’t account for heat saturation and other real world factors I can’t calculate but was very interesting to watch.
The follow up video helped as well.
Nice vids. Thanks for sharing. So are we basically saying that all that time we’ve been wrong thinking that gearing down is more efficient?
This was mentioned in the video I linked and dismissed as overall copper weight being more or less equal between different kvs of motor. They were claiming more windings of thin wire was equal to less windings of thick in terms of copper loss.
No idea if it’s true, just what was told.
Yeah, good point. The more so if we’re discovering that “higher kv => more efficiency” may actually be a myth. From what I gathered so far, it’s the overall motor construction that makes the difference in efficiency, not the kv.
Highly agree! Neo about to turn our world upside down with curved magnets and 6 phases
That’s what I’m getting from my reading today.
Air gap, larger and stronger magnets, larger motor cans to hold aforementioned upgrades… all help the most.
that just seems counterintuitive to me. a longer run of thinner wire would have a higher resistance than a shorter run of thicker wire. and that intuition bears out when measuring the resistance between phases.
the fact that it’s equal volume of copper doesn’t seem relevant to this comparison.
maybe it would be relevant if I had asked “6s 200kv vs 12s 100kv with the same gearbox and wheel”, but that’s a different issue
Do Teslas run with mechanical reduction? I’m sure whatever they use is better
Right but if you have MORE windings of thin wire equal in mass to the LESS windings of thick wire is what they were trying to claim.
The same volume of copper would experience the same loss in energy. It’s a constant.
i agree that copper mass effect should be similar but as current in the motor is AC so the difference may be caused by the copper “skin effect”.
skin effect means “the current density is largest near the surface of the conductor”. “The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor”
I would guess that many thinner wires means larger surface area and so skin effect becomes more significant, increasing resistance and causing more energy to get lost as heat.
Yes. 9.734:1 or 9.325:1 reduction. https://en.wikipedia.org/wiki/Tesla_Model_S
Shall we email Elon and ask him why they haven’t simply used a lower kv motor instead?
Edit: fixed the link
if you have 1 wire vs 2 thin wires then they have the same mass and same resistance . . . if the 2 small wires are in parallel. this is not true if the thin wires are in series.
with the formula in your picture, 2 thin wires in series means “L” doubled and “A” is half so R is four times higher.
Doubling length would double resistance, halving its cross-sectional area would double its resistance… Yeah that makes sense.
Is this just accounted for in the winding configuration and that is what is being referenced when the parallel wye winding is used “more efficiently”?
So theoretically there must exist a power throughput equation that has the inverse relationship to account for series winding? Or why would we bother winding in that fashion at all?
@BillGordon at your leisure could you spin the motor efficiency related parts of this into a new topic? Really interested in figuring shit out in logical discussion for a change.
If you half your Kv, you double your Kt, but if you adjust your current, your losses on the windings per unit of torque stays constant
Direct drives heat up because people take a motor that has a way lower Kv and thus higher resistance with the same current they would run a higher Kv
And regarding comparing a high Kv motor with a high gearing and a lower Kv with lower gearing, if the currents and gearing are adjusted so they have the same speed and same torque, they should perform the same apart from a bit higher core loses on the higher Kv one
Interesting information. I keep hearing more that kv doesn’t affect efficiency. The second video I linked above directly compares motor kv to efficiency and claims that they aren’t directly correlated at all.
He claims when you change kv, you have to maintain slot fill percentage and the differences in efficiency in varying motor kvs comes down to the diameter of wire used and the larger air gaps created by using thicker wires run side by side. He then mentions how you can run into the issue of using too thin wire as well.
The guy is dead set that motor kv doesn’t affect efficiency one bit. I JUST NEED ANSWERS.
I think I’m finally getting it. @Pedrodemio thank you for bringing up the torque constant (Kt), it connected a lot of dots I didn’t know existed.
Therefore, increasing or decreasing motor Kv wouldn’t have an affect on motor constant.
The bottom paragraph indicates that running 2 motors is more efficient than 1 by a factor of √2
Yeah but only if you ignore all other loses
First ignoring them. If you run two at half the current, you have the same torque but the copper loses goes down by a factor of 4 due to the I^2 dependency
Now, if you take all other loses into account things get more complicated, bearings drag, belt/gear, friction from the can with the air and most important, core loses. This last one is roughly proportional to between rpm^2 and rpm^3 depending on a bunch of factors
The tricky part is weighing everything, if analyzing your riding style you can say that most of the time you have low currents going into the motor, two motors will probably be worse, but if you are riding at high speed or with mostly hills that required the motor to stays at the upper levels of current, it’s almost certainly that two will be better for efficiency
It’s ridiculously how much loses happen due to the motor resistance, specially in direct drives, I think both Hummie and the R-Spec have around 40 miliohms phase resistance, that running at 60 A is 216 W of loses on EACH motor
That’s exactly what I will test as soon as the weather gets better in Germany.
I think in theory it’s true, but in RW it’s not.
I will compare the range of our 2WD 12S8P 21700 Interceptor with the AWD 12S8P 21700 Interceptor at a constant speed. Both are hub motors and I thing at a constant speed the range will be similar. But since we like to accelerate the RW range is about 30% lower with the AWD.
If I’m reading that correctly, then basically Benjamin says:
- if you’re running 12S, always use 190 kv motor
- if you’re running 10S, always use 225 kv motor
Adjust your gearing as appropriate for the desired max speed bearing in mind that your max motor RPM will be 8600. (All of this assumes 14 pole motors.)