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Comparing published motor and engine power output specifications

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kubark42

Well-Known Member
Joined
Oct 19, 2020
Messages
90
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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