Motor theory? Size/rpm, wattage.

therobby3 · Jun 30, 2021

I've been getting into electric conversions for the past 8 months or so, with a few successful builds now! There are however, a few concepts about electric motors that I've been curious about and haven't been able to completely wrap my head around. And that is size and peak wattage of various motors.

So take for example, this large QS Motor that I've recently got for an ATV conversion: https://www.qsmotor.com/product/8000w-mid-drive-motor/
It is around 45lbs, is rated for around 32kw peak, and 8kw continuous. I own one myself and from my personal experience, it is a very powerful motor and I am very impressed with it.

I've also seen large RC motors that have a rather high power rating. I've stumbled across this animal the last couple days, and I am simply in shock. https://www.mgm-controllers.com/en/lmt-30100 It says this thing weighs around 5lbs and can put out 40kw!! That's over 50hp meant for an RC car!! I can't even fathom that. This motor is definitely at the top of its class, nonetheless there are still other RC motors rated at a very high wattage for their size as well.

So my question is, how on earth is this possible? How can a 5lb motor be rated for the same amount of power as a 45lb one?? This seems simply unbelievable. I realize that they are extremely different in terms of torque vs rpm. That small motor can probably rev to an insane amount. But you would think that the math (torque x rpm = power) would still work out in the favor of the larger motor, being as there is probably more copper inside the motor. I am very familiar with gas engines, and how high revving street bike motors can put out a ridiculous amount of power simply because they shift their power-band to high in the rpm range... But that's a combustion engine, and that involves a lot of different factors. So can anyone explain to me how such small motors can be rated for such high power?

To add to the questions and make things more confusing for myself, I also see very high powered RC speed controllers. Again, take for example this bad boy: https://www.mgm-controllers.com/en/tmm-80063-3-for-cars-x2-series-pro It says it's rated for 1000 amps peak, and it's sized for an RC car. My APT 96800 controller meant for an ATV (which puts out alot of power) has got to be like 8 times the size of that thing. How on earth is that possible? That's gotta be as much power as a mid-size Zero Motorcycle but placed in an RC car. I'm just blown away. Am I missing something here?

pwd · Jun 30, 2021

Some excellent explanations here: https://ebikes.ca/learn/power-ratings.html

mxlemming · Jul 1, 2021

That's an interesting link, but the reality of the rc car motor situation is that the number cited is probably just complete bullshit.

In any case, 50kW in a ~5kg RC car is going to accelerate it to 300mph in about 5 seconds, except that 99% of that power would go to vaporising the tires... So utterly useless and not verifiable.

If you put it on an ebike with gears, you'd probably find it behaved about the same as a similarly sized motor and controller.

larsb · Jul 2, 2021

For RC motors it's a special case where many of them are speced on the peak input power for a second or two. The load is normally a propeller so for a 40kW input to the motor for a second of super high RPM drive (50 000 rpm in this LMT30100 case) when you have 40kW of aircooling available. Put the same motor in the wheel of your moto and watch it burn in a minute.

spinningmagnets · Jul 2, 2021

RC planes and other vehicles can draw a very high peak power for a second or so, and then the RPMs are suddenly very high, and the amp-draw becomes very low.

This is not just theory, about ten years ago, there was a lot of experimentation in adapting tiny RC motors and controllers to ebikes. Many motors were overheated, and many controllers were fried from heat and inductance.

So...tiny RC motors can produce surprisingly high temporary peak power, but...you must also factor-in that the RC industry as a whole is rampant with gross exaggerations in their ads.

People with no electrical experience and a Visa gold card will spend thousands on one RC toy. Therefore...the ads are lies because that seems to keep working.

fatty · Jul 2, 2021

It's helpful to illustrate with a simplified mathematical model.

Let's calculate if torque is only determined by Kt (neither resistance/temperature- nor flux-limited):
torque * rpm = power

QS 8000W
112Nm peak torque, 7000rpm max
112Nm * 7000rpm / 9.5488 = 82kW

With real controller:
Kv = 60.4rpm/V
Let's use 120V controller max = 28s * 3.7V = 103.6V nominal
103.6V * 60.4rpm/V = 6259rpm

60.4rpm/V = 0.16Nm/A
Common 600A phase amp limit: 600A * 0.16Nm/A = 94Nm

103.6V * 600A = 62kW theoretical limit
94Nm * 6259rpm / 9.5488 = 62kW theoretical limit
62kW / 21.6kg = 2.9kW/kg

So 24-32kW rated peak from 21.6kg is realistic

LMT 30100
Same 28s controller
Kv = 50,000rpm / 103.6V = 483rpm/V; they have 467rpm/V
103.6V * 467rpm/V = 48,381rpm

9.5488 * 40kW / 48,381rpm = 7.9Nm
467rpm/V = 0.02Nm/A
7.9Nm / 0.02Nm/A = 386A

Sanity check:
7.9Nm * 48,381rpm / 9.5488 = 40kW
103.6V * 386A = 40kW
40kW / 5lbm = 17.6kW/kg -- 6x higher? Not realistic

How long will a 5lbm motor with 8mm connectors survive 386A?

pickworthi · Jul 3, 2021

pwd said:
Some excellent explanations here: https://ebikes.ca/learn/power-ratings.html

That article is very USA/Canada centric in my view. For the rest of us, there are regulations governing the power allowed in pedalecs, and they are fairly specific in my view:
- UK Regulation (in the road traffic act) references EU regulation No 168/2013
- EU No 168/2013 states "maximum continuous rated power means the maximum thirty minutes power at the output shaft of an electric engine as set out in UNECE regulation No 85"
- UNECE 85 has a full definition of how a peak and continuous power test is to be conducted
(copy here: https://op.europa.eu/en/publication-detail/-/publication/44471446-bc46-44f0-bc97-0de997b18106/language-en )

The point of motor power rating for us (non USA) types is that it either makes the motor legal or illegal. Only the manufacturer can get the rating approval according to the regulation.

A bit off topic for this I know, but the published information from Grin (i.e. "there is no standard at all for whether this is a continuous power rating, a peak output power rating, or a peak input power rating, or something stamped on the product for legal compliance") is as misleading as he claims the manufacturers to be, in my view at least.

larsb · Jul 3, 2021

Sure there are ratings, just that the small sized motor manufacturers don’t rate.

The reduction for a 50krpm motor that you need to drive at 50krpm to get that 40kW .. lets say 150kph 1.8m circumference wheel is 1390rpm and roughly a 1:36 gearing. Hard to do without adding a lot of weight and size.

stan.distortion · Jul 3, 2021

larsb said:
Sure there are ratings, just that the small sized motor manufacturers don’t rate.

The reduction for a 50krpm motor that you need to drive at 50krpm to get that 40kW .. lets say 150kph 1.8m circumference wheel is 1390rpm and roughly a 1:36 gearing. Hard to do without adding a lot of weight and size.

And maybe more importantly, friction. Big reduction adds a lot of drag and that takes up a bigger and bigger percentage as input rpm rises and torque correspondingly reduces, even if you could keep that little beastie cool at its peak output the gearing needed could easily reduce the output by 20% to get it to a usable rpm.

Inertia can be a problem too, something that feels like a lightweight rotor can have more rotational inertia at 100k rpm than a truck flywheel spinning at 2k rpm and feel sluggish where a slower motor feels responsive. It can also cause big problems with loads too, slam on the brakes on a drivetrain that's built to take 10mn and it can suddenly have to deal with 100nm as that inertia unloads.

DogDipstick · Jul 3, 2021

fatty said:
It's helpful to illustrate with a simplified mathematical model.

Let's calculate if torque is only determined by Kt (neither resistance/temperature- nor flux-limited):
torque * rpm = power

This is not correct maths. Units. Man. Units.

If you want to extrapolate Hp from Tq and RPM, I can do that for you.

H = T x rpm/5252, where H is horsepower, T is pound-feet, rpm is how fast the engine is spinning, and 5252 is a constant that makes the units

H = T x rpm/5252

it is not

"torque * rpm = power."

Torque multiplied by the RPM is not the power output figure.

QSMOTOR 8000w Motor
112Nm... 7000RPM... you will make

Ok.. 82.6 FtLb.... 7000 rpm....

62 Imperial horsepower. You will make Sixty Two with those numbers not looking at the nameplate label. You said 109 hp... ?

7K "EEERP EEE EEEMM" and 112 "Nootin Maders" does NOT (=/=) equal 110 "horziepower": aka 82 "kiwowhat".

I have no idea where you got "82 Kw.. ( 109 Mechanical horsepower.. Lol) ".. Absolutely incorrect. Well Yes I do know where. You got the Eq. wrong. Please redo the rest of the equations for our references, or else.. we all may need that "sanity check"..

As is right now, all your data above is null and pretty much made up numbers from incorrect eq.

It is 46.2334 Kw. That is the correct. Hp extrapolated from 112nM ( 82.6 foot pounds) and 7000RPM.

62 Imperial. Hp.

62.859 Metric Ps.

Gimmie some more math pls.

DogDipstick · Jul 3, 2021

mxlemming said:
In any case, 50kW in a ~5kg RC car is going to accelerate it to 300mph in about 5 seconds, except that 99% of that power would go to vaporising the tires... So utterly useless and not verifiable.

This is the REAL limit to ones power that they can put down... The TIRES on the RIG.

Overcome the tire and that s all she wrote on power application. Slip. Slip sliding away.

You guys are looking at distinctly labeled in formations that are relative ( thrust ratio, power to weight, Tractive effort).

Brushless motors have (some of the )the highest POWER to WEIGHT ratios in the world. Example: ( thats alotta hp for every lb..)

Emrax 268 brushless AC motor 19.9 kg , 44 lb 230 kW 310 hp 11.56 kW/kg 7.03 hp/lb Electric aircraft

ElectriFly GPMG4805 brushless DC 1.48 kg 3.3 lb 8.4 kW 11.3 hp 5.68 kW/kg 3.45 hp/lb Radio-controlled aircraft

Panasonic MSMA202S1G AC servo motor 6.5 kg 14 lb 2 kW 2.7 hp 0.31 kW/kg 0.19 hp/lb Conveyor belts, robotics

Other comparisons:

Top Fuel supercharged V-8 (nitromethane) engine 8203 kW 11,000 hp 36.46 kW/kg 22.2 hp/lb Top Fuel Dragster

Mazda 13B-MSP Renesis 1.3 L Wankel engine 184 kW 247 hp 1.5 kW/kg 0.92 hp/lb Mazda RX-8

GM Duramax LMM V-8 6.6 L turbo-diesel 246 kW 330 hp 0.65 kW/kg 0.40 hp/lb Chevrolet

GE LM2500+ marine turboshaft 30,200 kW 40,500 hp 1.31 kW/kg 0.80 hp/lb Cruiseship, Ocean liner

Now guess which one of those are the most Fuel ( energy, stored) efficient.

Mechanical advantage, also called force ratio, is the ratio of the output force (load) of a machine to the input force (effort). In the case of an ideal (frictionless and weightless) machine, mechanical advantage = velocity ratio.

Tractive effort : the force in pounds exerted by powered equipment (as a locomotive) as measured for statistical purposes at the rim of the driving wheels.

As used in mechanical engineering, the term tractive force can either refer to the total traction a vehicle exerts on a surface, or the amount of the total traction that is parallel to the direction of motion

A machine is a device that can change the magnitude and line of the action of force.

Newton's second law, which states that the force F acting on a body is equal to the mass m of the body multiplied by the acceleration a of its centre of mass, F = ma, is the basic equation of motion in classical mechanics.

The thrust-to-weight ratio can be calculated by dividing the thrust (in SI units – in newtons) by the weight (in newtons) of the engine or vehicle and is a dimensionless quantity. The thrust-to-weight ratio can be calculated by dividing the thrust (in SI units – in newtons) by the weight (in newtons) of the engine or vehicle and is a dimensionless quantity. Note that the thrust can also be measured in pound-force (lbf) provided the weight is measured in pounds (lb); the division of these two values still gives the numerically correct thrust-to-weight ratio. For valid comparison of the initial thrust-to-weight ratio of two or more engines or vehicles, thrust must be measured under controlled conditions.

Power-to-weight ratio (PWR) (also called specific power, or power-to-mass ratio) is a calculation commonly applied to engines and mobile power sources to enable the comparison of one unit or design to another. Power-to-weight ratio is a measurement of actual performance of any engine or power source. It is also used as a measurement of performance of a vehicle as a whole, with the engine's power output being divided by the weight (or mass) of the vehicle, to give a metric that is independent of the vehicle's size. Power-to-weight is often quoted by manufacturers at the peak value, but the actual value may vary in use and variations will affect performance.

The inverse of power-to-weight, weight-to-power ratio (power loading) is a calculation commonly applied to aircraft, cars, and vehicles in general, to enable the comparison of one vehicle's performance to another. Power-to-weight ratio is equal to thrust per unit mass multiplied by the velocity of any vehicle.

Starting tractive effort: Starting tractive effort is the tractive force that can be generated at a standstill. This figure is important on railways because it determines the maximum train weight that a locomotive can set into motion.

Maximum tractive effort: Maximum tractive effort is defined as the highest tractive force that can be generated under any condition that is not injurious to the vehicle or machine. In most cases, maximum tractive effort is developed at low speed and may be the same as the starting tractive effort.

Continuous tractive effort: Continuous tractive effort is the tractive force that can be maintained indefinitely, as distinct from the higher tractive effort that can be maintained for a limited period of time before the power transmission system overheats. Due to the relationship between power (P), velocity (v) and force (F), described as:

P = v F or P v = F

Tractive effort inversely varies with speed at any given level of available power. Continuous tractive effort is often shown in graph form at a range of speeds as part of a tractive effort curve.

DogDipstick · Jul 3, 2021

stan.distortion said:
Inertia can be a problem too, something that feels like a lightweight rotor can have more rotational inertia at 100k rpm than a truck flywheel spinning at 2k rpm and feel sluggish where a slower motor feels responsive. It can also cause big problems with loads too, slam on the brakes on a drivetrain that's built to take 10mn and it can suddenly have to deal with 100nm as that inertia unloads.

The maximal specific energy of a flywheel rotor is mainly dependent on two factors: the first being the rotor's geometry, and the second being the properties of the material being used. For single-material, isotropic rotors this relationship can be expressed as:

E m = K ( σ ρ )

where

E= E is kinetic energy of the rotor [J],
m= m is the rotor's mass [kg],
K= K is the rotor's geometric shape factor [dimensionless], ( SHAPE .... important in flywheels) ( Dimensionless constant)
σ= sigma is the tensile strength of the material [Pa],
ρ = rho is the material's density [kg/m3].

FUN FACT....:

Flywheel power storage systems in production have Energy storage capacities comparable to batteries, and faster discharge rates.

"Costs of a fully installed flywheel UPS (including power conditioning) are (in 2009) about $330 per kilowatt (for 15 seconds full-load capacity)"

Ok End trivial trivia.

A Prony Brake used.

The Prony Brake is a simple device invented by Gaspard de Prony in 1821 to measure the torque produced by an engine. The term "brake horsepower" is one measurement of power derived from this method of measuring torque.

Power output in SI units may be calculated as follows:

Rotary power (in newton-meters per second, N·m/s) = 2π × the distance from the center-line of the drum (the friction device) to the point of measurement (in meters, m) × rotational speed (in revolutions per second) × measured force (in newtons, N).

Or in English units:

Rotary power (in pound-feet per second, lbf·ft/s) = 2π × distance from center-line of the drum (the friction device) to the point of measurement (in feet, ft) × rotational speed (in revolutions per second) × measured force (in pounds, lbf).

If a force is allowed to act through a distance, it is doing mechanical work. Similarly, if torque is allowed to act through a rotational distance, it is doing work. Mathematically, for rotation about a fixed axis through the center of mass, the work W can be expressed.

Power is the work per unit time, given by
P = tau *omega

where P is power, τ is torque, ω is the angular velocity, and represents the scalar product

If the force is perpendicular to the displacement vector r, the moment arm will be equal to the distance to the centre, and torque will be a maximum for the given force. The equation for the magnitude of a torque, arising from a perpendicular force:

tau = τ = ( distance to centre )( force ) = tau =( moment arm) (force).

Traction. Tractive effort, thrust to weight ration.

I have an RC heli. It does 160mph with one of those little motors. Pushes 12kw from a 655 gram motor. 12lb craft, 300Wh 14s batter, Four min flight time ( sometimes more if I fly gentle).

You can really gain traction with blades in air. Hella fast. Something called "Propeller efficiency". 13 Hp/Lb is what it peaks at. It cannot sustain this tractive effort. it can only sustain 6.5Hp/Lb continuous tractive effort. 7Kw. It will destroy itself quickly at 12Kw.. but.. it only needs that for accelerations.. Just flying around I average 4 Kw.... Takes power to spin up, does not take power to keep it spinning.

It can handle a max of ~7kw continuous. 4541,stator, 4T+4T, YY winding. KDE 700XF.

As long as the blades dont stall. AoA.. Lost that traction.

It is always mentioned how much power (Y) it takes to go (X) fast.. but..

Noone ever mentions that it takes power to slow down too. Takes alotta power to slow down. Drag. Thrust to weight. Inertial moments. All that jazz.

Energy in a system, balanced, means energy in, energy out. Takes power to slow down as much as it takes to speed up. Another fun fact often ignored ( probably irrelevant to the actual applications of power delivery, ... right? How fast was that wheel spun up on that drag strip? Inertia? Whats mass anyway. ).

A 'brake' dynamometer applies variable load on the prime mover (PM) and measures the PM's ability to move or hold the RPM as related to the "braking force" applied. It is usually connected to a computer that records applied braking torque and calculates engine power output based on information from a "load cell" or "strain gauge" and a speed sensor.

An 'inertia' dynamometer provides a fixed inertial mass load, calculates the power required to accelerate that fixed and known mass, and uses a computer to record RPM and acceleration rate to calculate torque. The engine is generally tested from somewhat above idle to its maximum RPM and the output is measured and plotted on a graph.

A 'motoring' dynamometer provides the features of a brake dyno system, but in addition, can "power" (usually with an AC or DC motor) the PM and allow testing of very small power outputs (for example, duplicating speeds and loads that are experienced when operating a vehicle traveling downhill or during on/off throttle operations).

therobby3 · Jul 6, 2021

Thanks for the information everyone! So my main take away here is pretty simple: The manufacturer's are over estimating, and that there is no baseline measurement for a motor's "peak power output". In other words, the said 40kw RC motor would probably only output that for a short amount of time.

As far as my understanding about electric motor theory goes, it seems the main limiting factor for an electric motor is basically the heat dissipation. Is this correct?

larsb · Jul 7, 2021

i would rather say that efficiency is the limiting factor. It's popular here to use a small motor, overdrive it and focus on cooling. "Hey, this small motor will produce 2kW!" when actually it barely can take 2kW input at a terrible efficiency.

A motor has it's sweet spot for use, no point using a large 5kW motor with its high noload losses at 500W since it will be poor compared to a motor designed for 500W, the same goes for the opposite, don't run a small motor at too high load even if it will survive it with cooling.

fatty · Jul 19, 2021

DogDipstick said:
This is not correct maths. Units. Man. Units.

If you want to extrapolate Hp from Tq and RPM, I can do that for you.

Gimmie some more math pls.

So you converted to non-native units and think your math is better? That's not at all a refutation/correction.
Learn how to do math in real units first and then show the correction.

DogDipstick said:
it is not
"torque * rpm = power."
Torque multiplied by the RPM is not the power output figure.

"torque * rpm = power" is an explanation of relationship, not an actual formula. We both included the conversion constants in our respective formulas, so it may be that you haven't derived the formulas you're copying.

therobby3 · Sep 3, 2021

I'm bringing this topic back up, as I'm still not 100% understanding everything.

So my take away from this post was that:

A. Manufacturers are most likely inflating their power numbers for better sales or they are rating them at very short "max power" times, like for a few seconds. In conclusions, their lying or misleading.

B. Amps is the primary creator of heat in an electric motor (just like it is for wires), so high torque motors generate more heat, making smaller high spinning RC motors more easily capable of higher power ratings for their size.

However, if those are correct then things are still fuzzy to me. So I have a couple successful electric conversions under my belt now, but I have been tinkering with trying to build an electric surfboard for around 2 years now. I've learned some things, but I'm still far from a fully successful surfboard. During my Googling I've come across this motor that is said to be specifically designed for this purpose: https://www.mhz-watercraft.com/shop...orpion-hk-7455-320kv-22-4kw-for-jet-64-jet-80

It is quite an expensive motor at just over $1000. It's rated for a MAX of 22.5kw and a CONTINOUS of 14kw. So that throws the idea that these high power RC motors are only rated for a few seconds out the window for this thing. I realize it is water cooled, so that allows it to be pushed a bit harder, but those still just seem like absolutely HUGE power numbers for a motor that isn't even a full 5 pounds. And surely for that price tag, they can't be lying?? If they were lying, I'd imagine you'd fry that thing in an electric board in seconds, which I don't think is the case since I've seen at least a few test videos on Youtube.

So my question still stands, what am I missing here? How can such a small motor produce such an incredible amount of power? And as such, why can that said motor make 22.kw peak, but some other brand of similar sized motor and RPM only be rated for say 5kw? What's the difference? Electric motors seem fairly simple: some magnets, some copper coil, bearings, etc. So what's going on here?

To further complicate things, according to standard copper wire charts I found online, a continuous current of 250 amps (at which that motor is rated) should be around 0000 gauge, which obviously the wires to that motor (or the recommended ESC) are no where even close to. So again, what??? How, why? I am confused.

spinningmagnets · Sep 3, 2021

Its not complicated, the advertised power ratings are gross exaggerations.

How much torque do you think you'll need for this project? Horsepower ratings for electric motors are near useless, even when they are telling the truth...

Also, you might not think the application affects the motor selection, but it can have some impact on which motor would be the best choice. What will you be using this for?

therobby3 · Sep 3, 2021

But I've seen Youtube videos with that specified motor, in a surfboard, travelling at around 30 mph. So it has to be some degree true. Simply put: in order to propel yourself through the water at that speed, it would require that kind of power. Here is one example of said video: https://www.youtube.com/watch?v=N9LSXC0B61s

So even though it doesn't make sense to me, I'm thinking there has to be some degree of truth to that motor's claims (as well as others). Additionally, there are several other surfboard manufacturers out there, with motors in them that are similar to the one I just posted, that go a similar speed and claim to put out a similar amount of power. While they don't reveal the weight or specs on their motors, I know for a fact they are not 20+ pounds motors, maybe not even 10 pounds. I can't say for certain because I don't have one, but I heavily doubt those surfboards have massive 2 or less gauge wire in them.

I'm not really worried about talking about the details or torque needed for the project in this thread. I'm just simply curious about motor theory and how such small motors can claim to make such a large amount of power. And seeing as there are some videos, I can't see it all being fake claims.

spinningmagnets · Sep 3, 2021

I suspect that powered surfboards specify motors that have the option of using water-cooling. As you stated earlier, that could be a big benefit, allowing a smaller-diameter motor to be fitted in a space where a larger motor would typically be used for a given power level.

They are claiming 22.5-kW (22,500W), so...divided by 50V that would be 450A, which is highly unlikely, as those sizes of coils would likely be magnetically saturated. 100V would be 225A.

I agree, the numbers don't make sense. Does look liquid-cooled in the pic (the clear hoses).

74/55, 320-kV

edit. OK, this set-up is using 44V nominal, 12S LiPO, which is a hair over 50V when fully charged. Therefore, he is claiming this motor draws 450A peaks. You may not know this, but a few years ago, there was a LOT of experimentation with 80mm RC outrunner motors just like this (this one is 77mm diameter).

The type of ESC's that control these often have data-logging. They may be claiming it's capable of 22-kW, but they don't show what it actually drew when running. Spinning a jet-propeller through water would be a fairly continuous amp-draw, unlike RC planes where there is a peak of acceleration and then a much lower continuous draw.

It's possible the factory is using 200V when it measured 22-kW peaks. The batteries are 10-Ah and rated as 25C, so 250A? I don't believe the system is drawing 450A, and I doubt it is data-logging 250A

50V at 250A is 12-kW

ES member Thud raced these style of motors, and then he re-wound them to see how much they could be improved.

"Re-wind of a Turnigy 80/100 (Now-tutorial w/Video)"
https://endless-sphere.com/forums/viewtopic.php?f=30&t=20618

So far, I haven't built a better motor & controller set up than the 6-turn, 2-in hand, 14G, dLRK/wye terminated motor...run the thing with a 12 or 18 FET XieChang with a 6-Kw limit on 20 cells & you will not find a better RC power combination...The motor finds balance right there, more rpm's (voltage) brings iron losses to un-acceptable levels, more current & the copper insulation is going to fry.

The description above is 6-kW without water cooling. 20 cells is 72V nominal, and 6,000W at 72V is 83A. Once you add water-cooling, you might be able to double that to 160A, but anything above the magnetic saturation level is completely wasted battery amps being converted to waste-heat.

DogDipstick · Sep 3, 2021

Load, lag, lope, power factors, apparent power, reactive power, true power, speed, voltage, current between poles, directions, pulse width modulation, the pwm cycle rate, the duty cycles and time at that rate, the processor cycles of operation, how fast it processes its code, the bridge(s) resistance(s), cleaning up the question, the battery horsepower, equations, the mechanical horsepower equations, the magnetic strength, the field vectors, the driving algorithms, there is much we are not clarifying here ect. Without ever getting into voltage harmonics or CEMF (counter electromotive force)... ridiculousness like that...... You can get lost in teh info.. we have to point it somewhere ( the question)...

We would have to narrow it down a little. Tune our discussion for pointed results. Fundamentally.. electric motors are simple beasts.

There is much to learn.

mxlemming · Sep 7, 2021

Water cooling makes an enormous difference.

And the trick on this motor mhz is too use crazy high epm. 15000rpm.

This means more power since power is torque x speed.

The losses will be heavily in the iron at this speed, but that's ok because it's got water pumping through it. The curves in that page show 80% efficiency... So like 4kW of hot water coming out. That's completely impossible to do with air but fine with water. A lot of water.

320kV motor means 450A won't saturated it. I know my 80mm 70kv motor saturates at about 80A. This one is slightly bigger and 5x the kV ergo... Probably won't saturate.

The Neumotor 80xx series (100mm diameter) makes bold claims similar to this with air... They're debunked as impossible but not completely ridiculous.

2kg is actually quite a big motor once there's water involved.

This might not be bullshit marketing this time, merely optimism and ignorance of the wasted heat.

therobby3 · Sep 8, 2021

Thanks for the posts and information!

It being water cooled, does make sense that it'd be able to take quite a bit more power before overheating. I suppose the more answers I get, sometimes the more questions I have. @mxlemming I realize rpm X torque equals power, but to me it seems like even though it spins very high, it's probably capable of less torque vs a lower KV motor. So to me, it seems like the math would still balance itself out. I guess I don't understand why a higher kv would be less prone to saturation either. Although good to hear some think it may actually be capable of putting out the claimed power.

Although comparatively it still isn't making full sense to me. As if you compare that motor to the ME1616 motor here https://www.electricmotorsport.com/me1616-brushless-65hp-liquid-cooled-ipm-motor-24-120v.html it is rated at 20kw continuous and 55kw peak. It iscalso liquid cooled, and weighs 48 pounds. To me, those numbers just don't scale right at all, and that's where my confusion lies largely. The MHZ motor weights 5lbs and can do 12 kw, as where the ME1616 is 48 and only can do 8kw more continuous. That just seems really weird. I realize the MHZ can spin a lot faster, but the ME1616 should put out a tremendous amount more torque, and I would think the math would still add up to way more.

One thing is the MHZ is meant to be cooled by continuous river water, which the ME1616 is probably meant for a close looped system I'm going to assume. The river water would be much cooler and would therefore cool it alot better. But aside from that, it still seems a bit off to me for there to be that huge of a difference.

spinningmagnets · Sep 8, 2021

Something that hasn't been discussed yet is...reaching for max power with any given motor puts the system in a very inefficient part of the graph.

By that I mean...adding active cooling can double the power a small motor puts out without damage. However, doubling the power can take three times the battery.

Fine if you are drag racing, and you don't care about range. However, sometimes its better to have a slightly larger motor to avoid over saturation, and dramatically extend range.

In a powered surfboard, the size and weight of the battery are also a consideration.

Thats why Zero motorcycles have a large motor with no liquid-cooling. They tried it.

therobby3 · Sep 8, 2021

Makes sense. Weight in an electric surfboard is definitely super critical. So is it feasible that the ME1616 motor I linked could likely be pushed to much higher KW if cooled with cold river water like the MHZ, just that it's efficiency would drop?

mxlemming · Sep 8, 2021

therobby3 said:
Thanks for the posts and information!

It being water cooled, does make sense that it'd be able to take quite a bit more power before overheating. I suppose the more answers I get, sometimes the more questions I have. @mxlemming I realize rpm X torque equals power, but to me it seems like even though it spins very high, it's probably capable of less torque vs a lower KV motor. So to me, it seems like the math would still balance itself out. I guess I don't understand why a higher kv would be less prone to saturation either. Although good to hear some think it may actually be capable of putting out the claimed power.

Although comparatively it still isn't making full sense to me. As if you compare that motor to the ME1616 motor here https://www.electricmotorsport.com/me1616-brushless-65hp-liquid-cooled-ipm-motor-24-120v.html it is rated at 20kw continuous and 55kw peak. It iscalso liquid cooled, and weighs 48 pounds. To me, those numbers just don't scale right at all, and that's where my confusion lies largely. The MHZ motor weights 5lbs and can do 12 kw, as where the ME1616 is 48 and only can do 8kw more continuous. That just seems really weird. I realize the MHZ can spin a lot faster, but the ME1616 should put out a tremendous amount more torque, and I would think the math would still add up to way more.

One thing is the MHZ is meant to be cooled by continuous river water, which the ME1616 is probably meant for a close looped system I'm going to assume. The river water would be much cooler and would therefore cool it alot better. But aside from that, it still seems a bit off to me for there to be that huge of a difference.

Higher kV means fewer turns off copper. Magnetic flux is given by amps x turns not just amps.

A motors torque capability is defined by the magnetic flux (probably technically rate of change of flux linkage w.r.t. angle) in the air gap and the radius squared and the length. The kV doesn't affect the ultimate torque capability, just the number of amps it takes to reach the limit and the speed that a given voltage creates.

Given the above, you might observe that the little one is an outrunner and the big one is an inrunner. Despite huge difference in weight, the air gap diameter might be much less different.

So why aren't all motors outrunners?

Well several reasons... 1) outrunners require proportionally more magnets, 2) they're harder to cool 3) they don't respond well to more advanced techniques like field weakening or mtpa since they have low inductance.

Finally, the big 22kg me1616 is probably quite conservatively rated, the outrunner esurf is probably quite the opposite - barely just capable of what they claim.

Double finally, consider the target audience. The big motor is likely targeting companies designing 1000 of them into some e-vehicle, milk float, electric motorbike etc... The little one is trying to flog one or two per esurf diyer. Any company making 1000+ vehicles is going to employ someone who knows at least as much about this as I do and won't be sitting there wondering if maybe the little esurf motor might be a good substitute for the me1616 in whatever minirocket they're making.

Motor theory? Size/rpm, wattage.

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