모터의 종류와 원리 2 - BLDC모터 - 이론

What is a BLDC Motor?

While conventional DC motors used mechanical components (such as brushes and commutators) to switch polarity, a
BLDC motor eliminates these mechanical elements and instead uses an ESC (Electronic Speed Controller) to perform the function of polarity switching.

Types of BLDC Motors

BLDC motors are broadly classified into two types based on the location of the rotating part.

Structure of a BLDC Motor

I will explain using the inrunner structure as an example. A
BLDC motor consists of three main parts.

• Rotor: The
  stationary part of the motor, wound with coils, which receives current from the ESC to generate a magnetic field.

• Stator: The stationary
  part of the motor, where permanent magnets are located; it rotates in response to the magnetic field generated by the coils.

• Hall Sensor: While conventional
  DC motors relied on mechanical means to automatically switch polarity,
  BLDC motors use Hall sensors to determine the motor’s current position.

Operating Principle of a BLDC Motor

The operating principle of a BLDC motor is fundamentally based on the Lorentz force (F = IL × B). It generates torque through the interaction between
the current flowing through the stator windings and the magnetic flux created by the permanent magnets in the rotor. Since
the coils are mechanically fixed, the reaction force of the Lorentz force acts on the rotor, resulting in rotational motion.

The three-phase windings (U, V, W) are arranged at 120° electrical angles, and the current flowing through each phase is switched according to a sequence defined by the rotor position θ_e.
For an ideal BLDC (trapezoidal back-EMF), 6-step (120° conduction) commutation is used, synthesizing the rotor magnetic field into six switching states per electrical cycle.

The rotor position is determined using three Hall sensors to achieve 60° resolution, or, in sensorless systems, by estimating the zero-crossing of the back-EMF during the non-conducting phase.

The generated torque is expressed as , where the torque constant is proportional to the product of the magnetic flux density, the number of turns, and the effective length of the conductor. For PMSMs
with sinusoidal back-EMF, the system is transformed into the d-q rotating coordinate system via the Clarke–Park transformation to obtain a
torque equation of the form . In surface-mounted
(SPM) configurations,the control is simplified to , whereas in integrated (IPM) configurations
, MTPA (Maximum Torque per Ampere) control utilizing the reluctance torque term is applied.

In terms of control, 6-step square-wave control (low-cost and simple) is generally standard for BLDC motors, while FOC (Field-Oriented Control) or DTC (Direct Torque Control) is standard for PMSM motors.
However, the distinction between "BLDC" and "PMSM" is ambiguous in the industry, and the classification may vary depending on which control method is applied to the same hardware.

Advantages and Disadvantages of BLDC Motors

Advantages

• Long service life and low maintenance costs.

• High efficiency (85–95%).

• Virtually no sparking or electromagnetic noise.

• Precise position control is possible (ESC).

• Compact and lightweight for its size.

Disadvantages

• High initial cost.

• An ESC is required.

• It has three wires (three-phase), which is more than a DC motor (two wires).

• The control algorithm is complex.

• Current control is unstable at low speeds, causing vibration issues.
  

Features and Applications of BLDC Motors