Listed below are two types of motors which differ based on the implementation of the stator and rotor:

• In-runner Motor: The stator is external and the rotor is internal. In-runner motors are more lightweight and can achieve greater rotational speeds due to their smaller rotating diameter.

• Out-runner Motor: The stator is internal and the rotor is external. Out-runner motors can achieve greater torque due to their larger rotating diameter and generated BEMF.

Stator

The stator of a BLDC motor consists of stacked steel laminations with windings placed in slots that are axially cut along its inner periphery. The windings are created with many interconnected coils placed in the slots and are distributed evenly along the structure of the stator.

There are three classifications of the BLDC motor:

• Single-phase

• Two-phase

• Three-phase

For our discussion, the numbers for each configuration correspond to the number of windings the stator has. The single-phase and three-phase motors are the most used out of all BLDC motors.

Single-Phase Motor

The figure below shows a diagram of a single-phase BLDC motor.

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Single-phase BLDC Motor Diagram

A single-phase motor has one stator winding, wound either clockwise or counter-clockwise along each arm of the stator, to create four magnetic poles as shown above.

Three-Phase Motor

The figure below shows a diagram of a three-phase BLDC motor.

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Three-phase BLDC Motor Diagram

A three-phase motor has three windings wound either in a star or Delta shape.

Stator Winding Types

There are two types of stator winding variants. The differentiation between the two variants is made based on the interconnection of coils in the stator windings to give different types of back electromotive force (BEMF) and phase currents.

• Trapezoidal: BEMF and phase current is generated in a trapezoidal fashion.

• Sinusoidal: BEMF and phase current is generated in a sinusoidal fashion. The torque output of a sinusoidal motor is smoother at the expense of greater copper usage for extra winding interconnections.

Shown below are figures demonstrating trapezoidal and sinusoidal BEMF respectively.

Rotor

The rotor of the BLDC motor consists of a shaft and a hub with permanent magnets that uniformly alternate in pole polarity (north and south). The number of pole pairs in a rotor typically range between two and eight. Three different rotor magnet arrangements are shown in the figure below:

Rotor Material

The magnetic field density of the rotor is governed by the type of magnetic material chosen to create it. Listed below are the two common types of materials used in rotors:

• Ferrite magnets: Inexpensive and have low magnetic density per volume

• Rare earth alloy magnets: Expensive and have high magnetic density per volume (Nd, SmCo, NdFeB, etc.)

The high magnetic density per volume characteristic of rare earth alloy magnets enables the rotor to compress further while achieving the same desired torque. This ultimately improves the size-to-weight ratio and allows for higher torque to be achieved for the same sized motor.