RC Electric Motors

1) Motor Identification

Ever wondered what all those numbers mean on the side of your electric motor power plant! Here’s a quick explanation.

Typical information for motors; details for two motors below.

D2836/8 1100KV SK3-4240-740KV

D SK3 Manufacturers reference
28 42 Overall diameter of the motor casing
36 40 Length of the motor not including spindle
/8 Number of turns of copper wire per stator
1100 740 This is the KV rating of the motor

The KV rating for a motor is defined as the number of revolutions per minute of the motor with 1 volt input and no load, therefore a motor with an 1100 KV rating running from a 3S LIPO battery would have a maximum theoretical RPM of (1100 x 11.1) = 12210 RPM under no load conditions (no propeller).

2) Motor Selection for RC Model

The most important thing to keep in mind before choosing a motor is its weight and dimensions. We would all agree that extra weight added to a model to achieve the correct centre of gravity is undesirable. I personally would prefer to have a larger, heavier and more powerful motor than a smaller less powerful motor and a lump of lead in my models. Sometimes there is no choice but to use lead, but just don’t forget about the relationship between the weight of your motor and the centre of gravity of your model. The dimensions of a motor are obviously important. Don’t buy it if it won’t fit in your model.
You will want a level of performance suitable for the type of model you are powering. A 3D model will need thrust greater than 1:1, and a scale WW1 biplane will need considerably less. Here is a table giving performance in Watts per pound. Remember that if you are running your motor above its maximum efficiency the Watts per pound rule won’t be accurate, as a higher percentage of the Watts going into the motor will be producing heat instead of power.

70-90 watts/lb. Trainer and slow flying aerobatic models.

90-110 watts/lb. Sport aerobatic and fast flying scale models.

110-130 watts/lb. advanced aerobatic and high speed models .

130-150 watts/lb. Lightly loaded 3D models and ducted fans.

150-200+ watts/lb. Unlimited performance 3D models.