Motor Size and Power

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Jan 032016
 

I have just been looking at some 1804 motors; just an example.  The theory applies to larger motors as well, probably more so as there is more variety.  Thought I’d make a few notes.  Please comment if you see any major errors.

Hobbyking 1804 motor1804 refers to the size; 18mm diameter and 4mm long, usually of the stator or internal windings, but can occasionally refer to the overall motor size.  This tells us the size of the motor and provides only a rough idea of what it is capable of.  The KV and power rating consisting of Volts, Amps and Watts is probably more important.

Hobbyking list two 1804 motors, the “DYS 1804-23 2000KV BX” and the “DYS 1804-2300KV BX“.

  • RPM/V: 2000kv or 2300kv
  • Dimensions: 23 × 13mm
  • Shaft: 2mm main shaft and 5mm prop shaft
  • Voltage: 2S~3S (7.4v to 11.1v)
  • Weight: 16g
  • Watts: 55.5W (2000kv) and 89W (2300kv)
  • Max Current: 7A (2000kv) and 8A (2300kv)
  • ESC: 7amp to 12amp
  • Suggested Prop: 5×3~6×2 and 5×3 (3S 11.1v) ~ 6×2 (2S 7.4v)
  • Motor Mount Holes: M2 x 12mm

Performance Figures for the 2000kv motor:

  • 7.4V with 5×3 prop: 14,600rpm/2.9amps/21.5watts
  • 7.4V with 6×2 prop: 14,600rpm/2.8amps/20.7watts
  • 11.1V with 5×3 prop: 19,450rpm/5amps/55.5watts
  • 11.1V with 6×2 prop: 19,450rpm/5amps/55.5watts

Notice that the 2300 is rated 1A more than the 2000, but 44 watts more. That’s a lot more power. I suspect there may be a typo in there somewhere 🙂

Notice also in the performance figures, that on 2-cell they only show 21 watts from a motor that is supposed to deliver 55 watts. A larger diameter and/or larger pitch prop would increase the load and power delivered. This would have to be tested as there may be greater efficiency losses.

Ratings

The KV rating refers to RPM per volt applied with no load. So 2000KV on a 11V supply will theoretically want to spin at 22000 RPM. The speed controller etc. will affect this.

The power rating should be considered  a maximum with good ventilation. all that power is going to heat the motor and needs to be removed by air-flow or the motor will overheat and fail; probably burning out the windings.

The maximum current rating tells you how much current the motor windings should be able to handle.

These all come together: Volts * Amps = Watts. But not always without a catch.

So a 7A maximum current and 55 watt rating means you should not load the motor to the full 7A on 3-cell because it would draw 77 watts.  On a 3-cell it should be limited to about 5 Amps. On a 2-cell supply you can run to the full 7A and the motor will draw about 50 watts.

The motor will try to deliver as much power as it can based on the load applied. The propeller is the load on the motor.  A larger prop (either in diameter or pitch) will apply more load to the motor and draw more current and therefore more power (watts).

Propeller

Propeller size and power is also complex. A higher KV motor will spin the prop faster. More pitch and/or more diameter means more load for a given RPM. There are on-line calculators that help with prop selection and suggest maximum possible speed etc.  Generally, a larger diameter and more pitch spinning at a lower speed is more efficient, providing longer flight times.

Power in watts relates to “get up and go”, but prop diameter, pitch and speed determine how the “get up and go” works. Whether it’s a glider that flies out of your hand and goes vertical, or a fast sport model that you have to catapult into the air.