# Motor mass vs torque



## donrafa7 (Mar 4, 2019)

Is there any guidelines or relationships between a DC motors mass and Torque output?


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## Duncan (Dec 8, 2008)

The general relationship is that the Torque is directly proportional to the motor mass

Which is why the motor is geared to the wheels by a reduction gear


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## donrafa7 (Mar 4, 2019)

I guess i asked the wrong question. What kg/kw ratio is realistic for current motors?


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## MattsAwesomeStuff (Aug 10, 2017)

> I guess i asked the wrong question. What kg/kw ratio is realistic for current motors?


https://www.evwest.com/catalog/index.php?cPath=8&sort=2a&page=2

Warp 9: 32 HP and 65kg = 0.0027 kg/kw
Warp 11: 44hp and 106kg = 0.0032 kg/kw

Roughly, ballpark, any DC motor's going to be in that range.

https://www.evwest.com/catalog/product_info.php?cPath=8&products_id=476

Tesla Model S: 400kw and 295 lbs = 0.00034 kg/kw

That's an AC motor, but it includes the controller and gearbox, and if you missed it, it's 1/10th the kg/kw as the DC motors above.


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## Russco (Dec 23, 2008)

MattsAwesomeStuff said:


> https://www.evwest.com/catalog/index.php?cPath=8&sort=2a&page=2
> 
> Warp 9: 32 HP and 65kg = 0.0027 kg/kw
> Warp 11: 44hp and 106kg = 0.0032 kg/kw
> ...


I don't think so. The two DC motors you have rated by HP and weight.

The AC motor is rated by power and weight.

Can't compare apples and oranges.

The DC motors, rated at 32 and 44 HP sound like a one hour rate. 

The Tesla, at 400 KW, and at 800 watts per HP, works out to 500 HP!!!!

The Tesla motor weight, at 295 pounds equals 134 KG, similar to the DC motors.

Soooooooo, the Tesla weights just a little more than the DC motors, but produces 500 HP.

Maybe for five seconds. Poof!

If it sounds too good to be true, it probably is.

And motor weight is not important. Voltage, amps, RPM, number of field poles and number of commutator segments are what produce the desired RPM, Torque, and Horsepower.

Weight works out fine when the correct motor is selected by the parameters in the above paragraph.


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## brian_ (Feb 7, 2017)

MattsAwesomeStuff said:


> Warp 9: 32 HP and 65kg = 0.0027 kg/kw
> Warp 11: 44hp and 106kg = 0.0032 kg/kw


Those are mass per watt (not kilowatt) of rated power...
Warp 9: 32 HP and 65kg = 2.7 kg/kW
Warp 11: 44hp and 106kg = 3.2 kg/kW


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## Duncan (Dec 8, 2008)

The Tesla motor does NOT weigh 295 lbs - that is probably the weight of the transaxle - the motor is more like 70 kg

The heating effect is why I said TORQUE is related to WEIGHT

NOT POWER

Modern AC motors produce a lot more POWER by revving at a very high speed

The Torque is supplied by the magnetic effects - and is proportional (ROUGHLY) to weight
The maximum torque is also proportional to weight - due to the heating effect

But if you can spin faster you can develop more power

Big DC motors are limited to about 6000 rpm - AC motors?? - 15,000??


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## brian_ (Feb 7, 2017)

Russco said:


> I don't think so. The two DC motors you have rated by HP and weight.
> 
> The AC motor is rated by power and weight.
> 
> Can't compare apples and oranges.


"HP" means horsepower, which is simply a unit of power... and power is power.

The problem is that the DC motors in this example are rated by *output* power, and the Tesla motor data appears to be for *input* power. The difference between the two depends on efficiency.



Russco said:


> The Tesla, at 400 KW, and at 800 watts per HP, works out to 500 HP!!!!


A horsepower is 746 watts; I assume you're using 800 because you are throwing in an assumed 93.25% efficiency. That's about right for absolutely ideal conditions.



Russco said:


> The DC motors, rated at 32 and 44 HP sound like a one hour rate.
> 
> The Tesla, at 400 KW, and at 800 watts per HP, works out to 500 HP!!!!
> 
> ...


Performance information published for DC motors sold to EV enthusiasts is almost random, and certainly not subject to any oversight or standards. A DC motor value could be a one-hour rate, a one-second rate, or just fiction.

The production automotive world is different; although Tesla often doesn't follow typical practice (due to incompetence, arrogance, or just spite), their advertised values should be reasonably plausible. The large Model S drive unit (which is the one shown by EV West) was rated by Tesla at 382 hp or 285 kW in regular models, and 469 hp / 350 kW... and vehicle performance confirms that these values are plausible. It's true that in practice the car only needs to run at this power for about 5 seconds to reach highway speed, but if you go longer it just gets too hot and gets reduced in power by the controller - there's no reason to expect it to go "poof".

For a perhaps more useful comparison point, consider the motor of the Nissan Leaf. It was rated at 80 kW from its introduction to 2017, and 110 kW now, and I don't see any reason to doubt that these are continuous ratings as the ratings are for conventional engines in cars. It weighs 58 kg... a bit lighter than the example 9" DC motor.


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## brian_ (Feb 7, 2017)

Duncan said:


> The Tesla motor does NOT weigh 295 lbs - that is probably the weight of the transaxle - the motor is more like 70 kg


I agree. The 295 pound value is the complete drive unit (which is what is being sold in this listing), consisting of motor, inverter, and transaxle. For comparison, someone who disassembled a Leaf weighed the motor + transaxle at 180 pounds (82 kg), but the motor itself is only 58 kg.



Duncan said:


> ... TORQUE is related to WEIGHT
> 
> NOT POWER
> 
> ...


Right - the question about what is related to weight was about torque, not power, and it is torque which is roughly related to the "amount" of motor (radius to the flux gap, length of the rotor, windings producing flux).

Power = torque X speed

In very rough terms, torque requires current and speed requires voltage, so if you have enough voltage available you can increase power by spinning faster... but heat quickly becomes the limitation. Typical AC motors in production EVs hit their maximum rated power at relatively low speed (e.g. 2700 rpm in a Leaf), but can maintain it to much higher speeds by tapering off current with increasing speed, to some extent because of insufficient voltage, but in practice limiting the power to limit heat load.

The Nissan Leaf motor spins to 10,500 rpm, and would likely go faster without mechanical issues and while still developing the rated power if more battery voltage were available. I think I remember 14,000 rpm as the Tesla maximum speed, but yes, 15,000 is entirely reasonable.


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## MattsAwesomeStuff (Aug 10, 2017)

brian_ said:


> Those are mass per watt (not kilowatt)


Erm, oops. Correct.

Regardless, they were consistent among themselves at least.



> It's true that in practice the car only needs to run at this power for about 5 seconds to reach highway speed


I mean, at best it's a 12 minute rating, as, in ~12 minutes the batteries are going to be empty at 400kw draw, even if you could sustain that as voltage sags with drain.

And, it's water-cooled. So, same as an engine wouldn't last long at max power without a radiator, neither would a Tesla. Doesn't seem fishy to me.


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## donrafa7 (Mar 4, 2019)

Thanks all for the replies, the reason for wanting a torque/kg or kw/kg was i was doing a some calcs for a quad copter project and wanted to estimate the motor mass for the motor power i needed. the #'s above are a good starting point and i'll look at a few other vendors as well.


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## MattsAwesomeStuff (Aug 10, 2017)

> i was doing a some calcs for a quad copter project and wanted to estimate the motor mass for the motor power i needed.


All the info we've given will be nearly completely useless to you.

You should've given some context earlier.

Brushless DC motors used in quadcopters are designed to be as low weight as possible. Our answers based on vehicular motors aren't going to be very helpful.


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## donrafa7 (Mar 4, 2019)

Not useless. Calcs are for full size copter. I want to aee what is commonly available. I know siemens / remy / emrax have some ultralight 5kw/kg motors but those arent really an option yet.


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## brian_ (Feb 7, 2017)

donrafa7 said:


> Not useless. Calcs are for full size copter. I want to aee what is commonly available. I know siemens / remy / emrax have some ultralight 5kw/kg motors but those arent really an option yet.


The problem is that for even a "full-size" (person-carrying?) aircraft the brushed DC motors will be inadequate and modern automotive motors will be far too large (even if their power to mass ratio is adequate).

Also, unless you need to directly drive the rotors (with reduction gearing), it is power (not torque) which is the concern.


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## MalcolmB (Jun 10, 2008)

I don't remember which motor this guy is using, but four of them should be more than adequate: https://m.youtube.com/watch?v=eNSN6qet1kE


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## brian_ (Feb 7, 2017)

MalcolmB said:


> I don't remember which motor this guy is using, but four of them should be more than adequate: https://m.youtube.com/watch?v=eNSN6qet1kE


It doesn't seem reasonable to me to define a suitable motor without knowing anything about the target application, beyond the very vague "full size quadcopter". No weight, no payload or performance requirements from which to guess a weight...

That YouTube page links to a motor:
Turnigy RotoMax 150cc Size Brushless Outrunner Motor
It has a 9.8 kW power spec, which I'm guessing is only momentary and wildly optimistic at that... since that would assume max current at the same time as max voltage, and perfect efficiency. It is probably power-dense, for as long as it can run without melting down.


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## MattsAwesomeStuff (Aug 10, 2017)

brian_ said:


> It has a 9.8 kW power spec, which I'm guessing is only momentary and wildly optimistic at that


From the E-bike world, where RC motors are commonly used as mid-drives on E-bikes, they can actually sustain that kind of load just fine.

IIRC, some 7 years ago someone tested a 5kW motor and, yeah, no problem, sat there and pushed out 5kW.

Then the conversation came up about power density in motors, and someone doing some destructive testing on that versus a Series DC or maybe Permag motor the same power rating. What they concluded was that the RC motors are on the knife's edge. 5kW max is 5kW max. Where, other motors can easily be pushed well beyond their rated spec. Also, thermal mass plays a role in both directions. More thermal mass means brushed DC motors can soak up some abuse, but, brushless motors air cool themselves and, if they can handle the power at all, they can handle it steadily.

I might have the details fuzzy but I think that's correct for the relevant context.


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## brian_ (Feb 7, 2017)

MattsAwesomeStuff said:


> From the E-bike world, where RC motors are commonly used as mid-drives on E-bikes, they can actually sustain that kind of load just fine.


E-bikes don't run multiple kilowatts.

Whatever the power level, they are not perfectly efficient! The specs are apparently only for electrical *input*: no *output* power or torque specifications. We just don't know anything about what the motor might produce.



MattsAwesomeStuff said:


> ... Then the conversation came up about power density in motors, and someone doing some destructive testing on that versus a Series DC or maybe Permag motor the same power rating. What they concluded was that the RC motors are on the knife's edge. 5kW max is 5kW max. Where, other motors can easily be pushed well beyond their rated spec. Also, thermal mass plays a role in both directions. More thermal mass means brushed DC motors can soak up some abuse, but, brushless motors air cool themselves and, if they can handle the power at all, they can handle it steadily.


That makes sense to me. Cooling air will definitely be critical; that seems okay in an aircraft which is always moving at significant speed, but in a rotorcraft one would need to be careful.

Plus the reliability issue: I wouldn't go up in a "quadcopter" without redundant motors (or entire rotors, i.e. an octocopter with reserve lifting capability)... especially using motors designed and sold for unmanned vehicle use, and especially in an aircraft type with no gliding ability. Even a helicopter than autorotate - not a quadcopter.


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## MattsAwesomeStuff (Aug 10, 2017)

brian_ said:


> E-bikes don't run multiple kilowatts.


Yes, they sure do.

Enforcement knowledge hasn't really kept up with the DIY community, so, almost no one respects the 500 or 750w legal limits. 1500 is about the minimum most DIYers bother with, plenty of guys running 5kW or twin 5kWs.

They're functionally hill climbing mopeds that still look like bicycles.


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## brian_ (Feb 7, 2017)

MattsAwesomeStuff said:


> Enforcement knowledge hasn't really kept up with the DIY community, so, almost no one respects the 500 or 750w legal limits. 1500 is about the minimum most DIYers bother with, plenty of guys running 5kW or twin 5kWs.
> 
> They're functionally hill climbing mopeds that still look like bicycles.


Okay, good to know - I was thinking of legal, commercially-available electrically-assisted bikes, not motorcycles.


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## aeroscott (Jan 5, 2008)

Just saw today on youtube a silicon valley co. has a 8 motor ultralight (250 lbs.) legal ,1 person copter.
He was being interviewed in front of 1 motor on test stand and as he was saying how much hp it developed someone off screen turned it on . a digital 

readout said 109 . The sound and prop blast was huge.
Sub 20 lbs. including batteries , controller , prop,would needed to meet the 250 lb ultralight rule.
If he used a srm motor it could be 95% eff. (controller and motor combined) leaving 5kwhr/hr.. of heat to get rid of.
very high voltage ,6 to 12 phases, helium or hydrogen interface to liquid cooling (used by utilities ) and speed up to 60,000 rpm for srm motors is possible
That leaves the batteries , say 10 lbs each
ps: 250 lbs. without pilot ,add another 10+ lbs/motor/controller/battery


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## aeroscott (Jan 5, 2008)

I looked on Youtube and the closest thing was the Boeing ,same configuration but larger.
It must have been in a 10 of the most interesting type vids.


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## remy_martian (Feb 4, 2019)

"Just saw on Youtube"

What about posting the links?


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## electronjen (Mar 13, 2011)

I assume you are talking about the "BlackFly" by opener. I saw this aircraft in person actually. The thrust is around 110 LBS. Motor power is around 40hp. I assume the readout you saw was measured in lbs of thrust not power.


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## brian_ (Feb 7, 2017)

aeroscott said:


> Just saw today on youtube a silicon valley co. has a 8 motor ultralight (250 lbs.) legal ,1 person copter.
> He was being interviewed in front of 1 motor on test stand and as he was saying how much hp it developed someone off screen turned it on . a digital
> 
> readout said 109 . The sound and prop blast was huge.
> Sub 20 lbs. including batteries , controller , prop,would needed to meet the 250 lb ultralight rule....


If this is BlackFly by Opener, then Opener says 313 pounds, for the version with 8 kWh battery. As Opener explains in their FAQ:


> BlackFly is an amphibious ultralight vehicle. This subcategory has an increased weight allowance.


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## brian_ (Feb 7, 2017)

electronjen said:


> I assume you are talking about the "BlackFly" by opener. I saw this aircraft in person actually. The thrust is around 110 LBS. Motor power is around 40hp. I assume the readout you saw was measured in lbs of thrust not power.


That would make sense. Opener says that total thrust of the 8 props is 1000+ pounds. Unfortunately, static thrust doesn't imply any power value, and a power spec is not provided by Opener... and no other motor information is provided, either.


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## remy_martian (Feb 4, 2019)

That 255 lb weight limit appears to be an engineer's attempt to classify the machine as an ultralight under FAA AC103-7 . But 12(e) says: "(4) Has a power-off stall speed which does not exceed 24 knots calibrated airspeed."

That means to be an ultralight it has to have a fixed wing because you have to maintain the ability to DEMONSTRATE compliance with Part 103 at all times of ultralight operation. In a quadcopter, you can't (unless it has a wing on it) show the stall speed. FAIL.

What that pretty much says is the builder has to get an airworthiness certificate before the craft can leave the ground. Oops., it's now a helicopter to the FAA inspector...one that cannot safely come down with power off. FAIL.

Things got expensive all of a sudden and you need a pilot's license, even if an Arduino is doing all the flying. So, the next engineer response is "change the regs". F public safety and risk of damage to property -- two fundamental tenets for airworthiness. So now you gotta add a parachute to the airframe...

Meanwhile, things translate very poorly from terrestrial to aerial vehicles as far as design goes.

Off the cuff stuff like cranking the voltage to a bazillion or the current up to 11 as you would in a car to make it more efficient, or failing to realize (for cooling and bearing sizing) a car runs high speed at 10% or so power, while an aircraft takes off at 100% for minutes and cruises at 75% all the time (I used to cruise at 100% at 10,000ft all the time in a turbo fixed wing), is epic fail at high altitude. You can't even use an off the shelf car inverter (creep is not robust enough). Even thrust is rated at sea level...which is what the Bay Area's at.

Back on topic: the basic question of torque vs weight doesn't work from car to aircraft. At all. You need a specific motor designed for the application.

For everyone's info (it was missed in discussion) the two Wrightspeed garbage truck motors each are a 75lb, 25,000 RPM motor (WANT). Which will not work well (reliability factor is huge in this application) with a prop. 

An EDF (baby Harrier) might be interesting, though. And a 250HP EDF on a car (reciprocity) would be ridiculous.


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## electronjen (Mar 13, 2011)

FYI My power data was provided via https://en.wikipedia.org/wiki/Opener_BlackFly

FAA AC103-7 ,12(e) is quite interesting. Being a fixed wing pilot myself I wonder how the ultralight helicopter crowd deals with this. I assume a demonstration of autorotation. Of course that is hard/impossible in a quadcopter design.

Looking at the blackfly it looks as if it has a power off stall speed of 120 knots! 

Also, it is easier to make a high speed light weight motor than a low speed. So the Wrightspeed motors with the appropriate gear reduction set (like a turboshaft has) would work well for most aircraft. 

I think in aircraft it is the battery weight that kills the concept. Liquid fuel is just so much more energy dense. Plus you get lighter as you fly with fuel, in the electric aircraft you have dead weight when the batteries die! And with batteries your useful load destroyed. Plus cruise at 75% power means you need more batteries than a car. It really does not work out very well. 

Torque in a propeller is also a function of pitch angle. So absolutely remy_martian, there are a bunch of other things to think about when choosing a motor for aviation.


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## brian_ (Feb 7, 2017)

remy_martian said:


> For everyone's info (it was missed in discussion) the two Wrightspeed garbage truck motors each are a 75lb, 25,000 RPM motor (WANT).


Wrightspeed doesn't publicize a lot of information about their motors (which are not just for garbage trucks); the only feature of their "Geared Traction Drive" units which they mention is the four-speed transmission. Although the importance of power (rather than torque) has been discussed in this thread, "75lb, 25,000 RPM" doesn't tell us about either power or torque, so I don't know how that's useful.


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## brian_ (Feb 7, 2017)

electronjen said:


> FYI My power data was provided via https://en.wikipedia.org/wiki/Opener_BlackFly


The only problem with that is that there is no source listed for the power value (one listed source for the specifications section is not available, and the other is a BlackFly page which doesn't mention power), so it could be a random value plucked from the imagination of anyone who wants to edit the Wikipedia page. The line in the specifications section


> Powerplant: 8 × 112lbs thrust/engine electric motors, 42.0 hp (31.3 kW) each


conflicts with the statement in the text


> Each motor weighs 4 lb (2 kg) and produces 130 lb (59 kg) of thrust.


and the conversion of a thrust in pounds to a mass (in kilograms) instead of a force (in newtons) suggests an author who doesn't understand much.


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