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#### donrafa7

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

#### Duncan

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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

#### donrafa7

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

#### MattsAwesomeStuff

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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.

#### Russco

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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.
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.

#### brian_

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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

#### Duncan

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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??

#### brian_

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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.

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.

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.
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.

#### brian_

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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.

... 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??
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.

#### MattsAwesomeStuff

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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.

#### donrafa7

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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.

#### MattsAwesomeStuff

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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.

#### donrafa7

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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.

#### brian_

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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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#### brian_

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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...

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.

#### MattsAwesomeStuff

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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.

#### brian_

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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.

... 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.

#### MattsAwesomeStuff

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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.

#### brian_

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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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