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Jan 01, 2011, 03:57 AM
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Another power rating/prop selection observation

I've found a very common thread in question in power systems, so I though I'd persist with blogging some of my relies, if for no other reason than being able to find em myself!

Motor nomenclature seems pretty much all over the place; however within that mess you can usually find the Kv for the motor and how much power the seller claims it can safely handle.

Power ratings: Note 'handle' is deliberate, not create or generate as the motor doesn't have an inherent power rating like the horsepower of an IC engine - it just converts the electrical energy from the battery with varying degrees of efficiency and delivers it to the prop. So think of power ratings as the max you can jam through a motor without it getting so hot it kills itself. Also, don't feel you have to run a motor at its max rating - most 300W 'rated' motors will be very happy handling say 200W. In other words, the power a motor has to handle is not determined by its claimed rating, but rather by the combination of voltage supplied to it, its Kv, and the load from the prop it is trying to turn. The ability of the battery to deliver the demanded current is also a limitation.

Typically also running significantly below the rated power probably means running closer to the motor's peak efficiency power handling point (which varies depending on voltage btw). That hypothetical 300W motor handling 200W delivered by the battery might be converting 75% to the prop, but up around 300W, maybe only 50%. When you realise that at 300W/50% that's 150W heating up the motor, compared to 200W/75% only 50W waste heat, it's worth trying to optimise rather than just running at a so called 'power rating'.

The main job then job is to choose that combination to met your needs without killing the components. To help with that, I recommend this series of blog posts as an invaluable reference for choosing power systems:

There is an excellent tool, referred to in the blog and available here:, that I think can answer your question, ie relating KV to model weight, wingspan and type. It's not a chart, but with basic model parameters and how you want to fly, you can quickly figure out those answers.

There's also a sticky in the electric beginner's section:
Last edited by scirocco; Jan 01, 2011 at 06:26 AM.
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