Motor Construction
Motor Construction “In the BLDC motor, the armature is in the stator and the field is in the rotor. The rotor is a permanent magnet. In high-end brushless dc motors, a rare earth magnet is used to provide a strong magnetic field.” (El-Sharkawi, 127) The motors used for drones are almost always of the outrunner configuration, that is, the rotor spins outside the stator, which is opposite the configuration of a typical brushed dc motor. This allows the motor to contain more magnetic poles of a greater strength than would be possible if the magnets had to be arranged on the smaller diameter inner space. In order to improve the electromagnetic characteristics of the motor stator, the iron core is made from a laminated stack.
The same effect is enhanced by using the appropriate iron alloy to produce the laminations of the core.
Another aspect of motor construction which greatly affects performance is motor windings. While maintaining the same connection topology, a single type of motor may be wound with different number of turns, different conductor thickness, and stranded or solid conductors. One of the most common motor specifications used for drones is the motor velocity constant KV.
“The Kv rating of a brushless motor is the ratio of the motor's unloaded rotational speed (measured in RPM) to the peak (not RMS) voltage on the wires connected to the coils (the back EMF). For example, an unloaded motor of Kv 5,700 rpm/V supplied with 11.1 V will run at a nominal speed of 63,270 rpm (5,700 rpm/V × 11.1 V).” (Wikipedia: Motor constants)