Brushless motor
Lightweight brushless motors are one of the defining features of the recent growth in popularity of electric aircraft. The brushed motor, which uses a friction 'brush' to switch polarity based on position, and requires electricity to run inside a rotating shaft, has been replaced by a set of non-rotating electromagnets paired with an array of rare-earth magnets, controlled by a Electronic Speed Controller.
Brushless motors are categorically far more efficient, and far more durable than brushed motors. With small props, they can also be operated without the gearbox often required of lower RPM brushed motors, saving weight and wear on several fragile mechanical linkages.
Brushless motors are constructed from precision-aligned arrays of magnets, and wound-coil electromagnets. Increasing the number of winds increases the inductance of the electromagnet, which allows it to provide torque at lower RPM for a given size and voltage. This may be desirable for a large prop, or undesirable for a small prop that needs to rotate quickly. The crucial measurement involved in tuning brushless motors is the RC term "kv", which indicates the number of thousands of revolutions per minute per marginal volt. Aside from this, gearboxes may sometimes be appropriate for adapting a motor.
[edit] Electronic Speed Controller
- Main article at ESC
"Brushless DC Motors" are made possibly by using coils in multiples of three. At all times, one of each set of coils is set to 'Positive' to pull the magnet, one is set to 'Sense' to electrically sample the position from passive inductance many times per second, and one is set to 'Negative' to pull the magnet. Switching these is done at a frequency based on the sensing circuit and the throttle position. The ESC is the brains that decides when to switch each electromagnet, and represents a relatively expensive part of the package.
It is important to set the voltage, prop, and motor combination so as not to exceed the current rating on the ESC - overheating is a concern. Current can be measured poorly using a clamp-type current meter, or a specialized RC tool known as a Power analyzer or 'watt meter', but not a classic digital multimeter - the impedence represented by the motor is too low, and the current is too high. Watt meters will give other numbers as well - from tachometer ratings to efficiency ratings.
[edit] Inrunner Type
Inrunners rotate a shaft affixed to a central core of rare magnets, which is surrounded by stationary electromagnets. These tend to be high RPM, low torque devices with a somewhat lower power to weight ratio. There is some overlap between high-KV outrunners and low-KV inrunners, but the generally higher-KV inrunners tend to be used to directly drive small, fast-rotating props, or to drive gearboxes with bigger props.
As an example, at the 200g-300g size, one store sells inrunners from 1500KV to 7500KV
[edit] Outrunner Type
Outrunners rotate a frame of permanent magnets, and the metal can encasing them, around a stationary shaft and a central core of electromagnets. These tend to be low RPM, low torque devices with a slightly higher power to weight ratio. There is some overlap between high-KV outrunners and low-KV inrunners, but the generally lower-KV outrunners tend to be used as direct-drive devices for large, slow-rotating props.
As an example, at the 200g-300g size, one store sells outrunners from 500KV to 2500KV
