Comparing published motor and engine power output specifications

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kubark42

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TLDR; before comparing the power ratings of motors and engines, first derate the motor by its efficiency.

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I realized that many people new to motors might not realize that the brushless motors we'd like to use in airplanes are traditionally spec'ed very differently from engines. The difference is crucially important when trying to find a motor equivalent to an engine.
  • An engine manufacturer specs mechanical power output at the shaft. So a 10kW engine can produce 10kW of mechanical power. It requires 200-300% more input chemical energy to make that mechanical power.
    • This is very likely because it would be somewhat challenging to measure input power effectively.s-l1000.jpg
  • A motor manufacturer specs electrical input power at the conductors. So a 10kW motor can consume 10kW of electrical energy. It outputs 2-25% less mechanical power on the shaft.
    • This is very likely because it's trivially easy to measure input power. It also is important to determining if the electrical system can deliver the demanded power.FATJAY-Underwater-thruster-IPX8-waterproof-2838-350KV-2-4KG-thrust-brushless-motor-with-60mm-p...jpg
The upshot is that if an aspiring designer wishes to swap out an engine for a motor, special care must be made to compare apples and apples. The motor output needs to be derated by the efficiency.

For instance, the T-motor V10L is a 9kW motor, but close attention to the Specifications tab on the product page shows that the motor is only ~77% efficient, so peak mechanical power,
\(\displaystyle \ \ \ \ \ \ \ \ P_{mech} = T \omega_{rpm} \frac{2\pi}{60}\)
is closer to 6-7kW.

This unfortunately does make motor specification a little harder, because it's important to calculate ballpark efficiency before determining if a motor can even produce the required power. Since efficiency of high-end motors can vary from 75-98%, this means a potential 30% power difference between motors which are published to be the "same power".

Of course, before anyone gets too mad at motor companies, as said above this is done for a good reason. It's not that they're pulling wool over our eyes, it's that traditionally you specify how big a battery pack and motor controller you need, and then give a torque and RPM spec for the downstream mechanical usage.

Plus it's not like our familiar aircraft engines produce their published power either, what with density altitude and all.

Hope this was helpful!
 
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BJC

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A motor manufacturer specs electrical input power at the conductors. So a 10kW motor can consume 10kW of electrical energy. It outputs 2-25% less mechanical power on the shaft.

... This is very likely because it's trivially easy to measure input power. It also is important to determining if the electrical system can deliver the demanded power.
Are you referring to a specific type / class of motor?

I’ve purchased lots of induction motors, and all had HP ratings, plus lots of other data that bounded the operating parameters.


BJC
 

kubark42

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Are you referring to a specific type / class of motor?

I’ve purchased lots of induction motors, and all had HP ratings, plus lots of other data that bounded the operating parameters.
My experience with industrial AC motors is limited, but if we assume 55-65% efficiency (not crazy for an induction machine) a random number plate I found online would hint that the hp rating of an industrial machine is the shaft output power. Unless the number plate current is referring to the inrush current. I don't have the AC experience to tell.

But I wouldn't typically expect to use a squirrel cage motor in a light-weight application. So I guess it's fair to say I'm referring to the class of motors which we're looking at adapting to aviation, in particular brushless. I cannot recall ever seeing a brushless motor which is not spec'ed for input power.

That being said, BLDCs usually operate at 92-95% efficiency so the difference is not typically dramatic. It's only when you get to some of the motors such as the T-motors V line, which sports very high heat tolerance but at the cost of significantly lower efficiency, that you could wind up missing the mark by an unexpectedly wide margin.
 

BJC

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... I found online would hint that the hp rating of an industrial machine is the shaft output power.
Yes.

That tiny motor typically would be described as a 3/4 HP motor, with the additional bounding criteria of voltage, current, service factor, phase, starting characteristics, temperature rise under rated conditions, etc.

Just a reminder that what is written here may be read by people with limited experience (who don’t understand the implied application or limitations) and may incorrectly extrapolate to inappropriate conclusions.

BJC
 

kubark42

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I agree with your point, but I still happily accept the consequences. In the event that someone is considering an industrial AC machine for an airplane (why?) the failure mode is spending a few seconds verifying they are indeed doing apples-to-apples comparisons. The alternative is someone designs around a motor which is 20-25% weaker than expected. I feel like this kind of unanticipated power reduction will force some ugly redesign choices.
 
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Map

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One also needs to consider a more practical issue. Electric motors offered for airplane use may be capable of a really high power output when compared to their weight, but this output can then only be used for a few minutes before the motor overheats.
Some motor manufacturers already take this into account and only list a continuous operation power, at which the temps are more likely going to stabilize.
 

lelievre12

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Many auto engine makers spec brake HP at the flywheel and don't include accessory loads.https://www.chevyhardcore.com/news/horsepower-ratings-gross-versus-net/

This measurement system would impact an engine with a PSRU such as a Yamaha Apex or Subaru. For these engines the losses of the PSRU and accessories need to be factored. However more importantly, losses from any changes to the induction and exhaust will have dramatic effects (as Viking owners discovered) and so these changes need to be accounted for as well.

Finally, the effects of altitude obviously affect naturally inducted engines and especially auto engines tuned for sea level.

None of this applies to certified aero engines which are specified at the prop and include a 'safety' margin which ensures the specified power is always available. Aero engines usually produce 5-10% more power than advertised because of this margin.

As one example here is a dyno test of a Continental TSIO520 by RAM which showed a measured prop HP of 335HP over the published 310HP. Admittedly this test was with aftermarket intercoolers fitted, however the engine is clearly making its rated power and then some.

1610036170784.png

Obviously the cure all for these differences is to carefully dyno test any motor before flight so that its characteristics are known. Given that dyno's are widely available I am not sure why it isnt done more often.

Electric motor manufacturers usually dyno test their motors as well. You just need to search for the data. Although as the OP says, the motors are spec'd for input power, the output power is usually easy to find. Here is a typical example you would look for:

1610036612538.png
 
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