# Discussing high voltage, kv, amps, and efficiency

I’ve been spending time in the esk8 force calculator, and some interesting things made themself more obvious as I played with parameters.

First, let’s take a look at basically a pretty standard high voltage board (HV) with 130kv motors. I have already bumped up the motor amps to 95a. This is a very powerful board, but how can we push it to extreme levels of power.

You might think, let’s slap a lil foccer in there and push the amps to 150a per motor, but take a look at our motor torque. We are already at 7Nm, which means we are at the max torque of many common motors (reacher 6485 for example). Increasing amps at this point will just saturate the magnetic field and turn the motors into space heaters, regardless of what the theoretical calculator spits out for force.

Next, let’s look at a basically equivalent setup with higher kv motors. If you increase the gear reduction to compensate for higher kv motors, you end up with the same top speed and power.

Going higher kv, we have less motor torque for the same amount of amps, but we have increased the gear reduction, so we end up with the same amount of torque (then force) at the wheel.

One interesting thing to think about here is, just changing motor kv, our motor is operating in a lower torque range on every ride we will ever go on. The motor in v2 below, will never have access to operate in the 4.4-7Nm torque range. This will have an effect on efficiency.

Most of our outrunners are very efficient at >0.7Nm torque. If we end up substantially increasing the amount of time our motor spends operating below 0.7Nm, I suspect we will negatively impact motor efficiency. Borrowing some real world data from @Tony_Stark , shared in the radium motors thread.

What if we are building a race board and what maximum power? Let’s increase amps to push those motors to the torque limit. This gives you an unreasonable amount of force at the wheels. Like, truly an amount 95% of us would never come close to touching. But, if you are building a hill climbing race board / BDE flex monster, this is the path.

In summary, if you want the highest performance available, high voltage, high kv, high amps is what you should pursue. Not ground breaking, but I think it is interesting to understand why this is the case.

If you are running high voltage and reasonable amps, kv probably doesn’t matter as much as you think it does, so long as you adjust with gear reduction. Depending on how you ride, you could see better efficiency on 130kv or 205kv.

I posit that you will get better efficiency, if you can gear reduce and choose a motor kv that gets you riding more often in the 0.7nM-2Nm “high efficiency range”.

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Couple of extra thoughts on that subject;

First is there’s often a practical limit to gear reductions that hits faster than expected so you get forced to lower KV, I’m toying with a caguama HV build ATM and 36T wheel pulley is already not a huge amount of road clearance. Belt tension is also a consideration, my current daily board has low Kv and high tooth counts on the pulleys all round so I can have much looser belts. It’s either 48/16 or 48/18 I can’t quite remember, running 120mm Clouds and with an idler, and on relatively long mounts so it’s engaging even more teeth

Outside of torque limits for motors, you’ll also hit RPM limits and accelerate bearing ageing a lot. Assuming you don’t hit an ERPM limit first, I was looking through a bunch of motors spec sheets yesterday and most of them don’t want to go above 7-8k rpm. So again higher Kv might be efficient but if you’re at HV you’re gonna hit upper limits or knee points very quickly, no matter how much you reduce the torque loading with proper gearing

Then there’s the black magic nonsense of iron losses that I’ve wiped from my brain, but some other more qualified nerd will probably explain why high rpm also produces something something eddy currents something something permeability

Also the units “7nM” are giving me an eye twitch, I know it’s pedantic but it hurts

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I fixed it.

Reacher has 8495 rated up to 15,300 rpm, but I believe they recommend not going over 12k.

Definitely something to work into a model when we have dyno data and real world testing. I didn’t get into copper losses and iron losses, because that is basically a whole other can of worms that you can argue all day and night over, but it isn’t particularly insightful if you don’t have data to support your claims (and I don’t have that).

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Iron losses aren’t complicated and they’re just hysteresis losses, which increase linearly with erpm, and Eddy currents, which are more so the problem as they increase exponentially.

If there were no iron losses then could get a huge increase in power possible from a motor while staying cool as the “speed” in the speed x torque equation would be lossless.

Ironless/coreless motors do very high speeds because they don’t have iron losses

If you wanted the most efficient motor youd compare your iron and copper losses and adjust gearing to end up with equal losses to both.

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We’ve definitely found 300kv @ 12S which is reaching almost 15k rpm no load to be above the heat dissipation limit for our 6485 motors when the ambient temperature is over 25C. Frustratingly through winter I could dump as much power as I wanted into them and they never got hot. So now I’m working on modifications to keep the same rpm but reduce the iron losses without reducing power too much so we still end up ahead compared with running lower kv.

I also really don’t think voltage matters if you can get the watts you want from a lower voltage ESC. Stooge boards on 8S will beat most HV vesc boards

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What are you going to possibly adjust to reduce iron losses? I’m guessing you can’t change the stator and instead change magnets. I think modeling such stuff only works so well and u have to try it.

Yep changing magnets. I’ve considered changing the stator but it’ll increase the cost on an already expensive motor. I’ll be wrapping up the development of the 6485 soon to focus more on our new motor which has a much better stator anyway.

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There’s a way to calculate it for a specific ride from a VESC log, the phase resistance which is measured during detection, and one more motor specific measurement: the idle current (and the RPM at which it is measured). Only issue is that the VESC sucks at measuring the idle current, so a separate current measurement tool is required.

It’s quite easy to end up in the more iron loss than copper loss range in my experience, unless you actually put down your 500N+ force during most of the ride (chances are you are not). I’ll make a thread on it some day. I already wrote up a good chunk of it, but got sidetracked with uni and I didn’t make my mind up completely over how the calculator should look like and how it can be made user friendly-ish. Right now I can however make the calculations manually to compare iron losses to copper losses in Excel.

I want to add one more feature to the calculator though before posting the thread: ideal KV estimate for overall efficiency (in this case the KV is considered ideal if the maximum motor efficiency is reached over a ride, thus the copper losses and iron losses are equal). This is still not perfect in order to minimize peak temperatures, but I doubt I’ll be able to figure that out anytime soon.

I’ll try to get to finishing up the thread and the calculator once the university exam period is done

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I would love to see this. I noticed in vesc tool, for rt data, it reports you average watt usage over the ride. I was thinking that could be used to calculate average iron and copper losses, but haven’t gotten any farther with with.

The part I can’t wrap my head around is how you would determine an ideal kv from that. Wouldn’t you need to make assumptions about resistance and no load current for a range of different kv motors?

Hope you have some time soon to put the calculator together. Sounds awesome

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About resistance yeah I would have to, but as long as copper infill stays the same (which it ideally does) it’s calculatable. Technically there’s two way to change resistance, one is by changing the windings from star to delta arrangement, and the other is to just more or less turns around each stator teeth. I can only calculate with the more or less turns. So if there’s a change from star to delta or vice versa there will be some error. But afaik most motors use a star config, and it’s uncommon to use delta, and afaik also uncommon to change the KV with that but motor manufacturers feel free to correct me

About the idle current, actually using the same stator and magnets just different windings would mean that the idle current is not affected. As having multiple stator and rotor variants cost extra money, in most of not all cases only the windings are different between different KV models of the same motor. Also thanks to some measurements from @jaykup the idle current actually stays the same at all motor RPMs, which basically means that the iron losses actually scale linearly, not quadratically for esk8 sized motors and typical esk8 RPMs. The formula for iron losses is P_ironloss=I_noload*BEMF, so basically the no load current which is constant multiplied by the voltage on the windings which is battery voltage * duty cycle

Shoutout to @Jaykup for these measurements and the graph

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This gap is closing quick

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I’ve been considering this, from what I’ve gathered it does end up with the watts at the motor being the main thing we’re all chasing after, but with HV and outrunners you have more heat at the ESC while with high current and inrunners you get more heat at the motor, right?

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Same heat at motor.

Arguably same heat at the ESC too as the HV mosfets I’ve looked at seem to double in resistance with a double in voltage but I guess that may not always be the case

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Not sure if this is the right thread to ask for opinions but any opinions on my set up pairing?

Here’s what I have in mind, 18s6p on a Ramm deck, possibly a DV100x, motors… 145kv reachers?
Drivetrain is a 11/44 4GS with 7 inch wheels, may do 8.

That’s a fine setup, but personally, I’d probably bump the kv up to 170 or 190 and lower the gearing to 1:5 or 1:5.5.

You’ll end up with an overall more power board if you do that, but your setup is good too if that’s your preference.

How does that work? I’ve been doing 10s and 12s so high voltage is new territory for me. I assume this is so that the motor doesn’t spin as fast or?

higher kv motors can handle more amps before saturating so if you gear down the higher kv motors to the same speed and you’ll have more torque. @Shadowfax has The Ultimate Esk8 Calculator where you can put in all the info and see actual values.
if you had a board on 18s and a board on 12s, if they’re both running the same everything, only difference is the battery, the 18s board will be faster

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Ash explained it above and like he shared, it won’t really be intuitive unless you play with the calculator to understand why.

The upper limit to this path is how fast you want to spin those motors. 170kv on 18s = 12,200 RPM, which IMO is high but still reasonable for reacher motors. You will start to see pretty high iron losses if you are pushing high kv motors at high voltages for sustained periods, but 170kv is not in that territory at 18s.

I would do 170kv motors, or as close as you can get to that in the motor size of your choice.

All that said, 145kv with 1:4 on 18s will be ungodly powerful compared to any of your previous builds, so there’s no wrong choice here.

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This sounds like the ballpark of where i am at.
Planing on 18s with 173kv motors from SKP…

Its likley going to be a race focused “do it all” kinda board. Should be able to hit 45 plus.

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