Operation of a Brushless DC Motor (BLDC Motor)

Fig.1: Low Cost BLDC motor: Circuit Diagram of Stator

1. Overview: The motor consists of three stator windings, energized by three power transistors (T1, T2, and T3). These transistors are supplied from a DC source, denoted as $V_{d}$. Each stator winding—labeled Phase 1, Phase 2, and Phase 3—is driven by a corresponding transistor. Additionally, each phase is connected in parallel with a freewheeling diode to allow current to continue flowing when the transistor is turned off.
2. Rotor and Sensor: The rotor is a permanent magnet with distinct North and South poles. Since this is a closed-loop drive, a position sensor is required to detect rotor position and trigger the transistors appropriately. This sensor is typically an encoder attached to the rotor shaft.

Encoder and Rotor Structure:

Fig.2: Structure of Rotor and Encoder

The encoder consists of a disk mounted on the rotor. The disk has 120 degrees transparent and 240 degrees opaque segments. The rotor magnet and disk rotate together. Positioned around the disk are three photo-couplers: Pc1, Pc2, and Pc3. Each photo-coupler contains a light-emitting diode (LED) on one side and a phototransistor on the other.

• When a transparent segment aligns with a photo-coupler, light passes through the disk and activates the phototransistor, resulting in a logic ‘1’.
• When an opaque segment obstructs the light, the output is ‘0’.

Each stator phase (Phase 1, 2, and 3) is aligned with one of the photo-couplers. When a photo-coupler is activated (logic ‘1’), it drives the base of its corresponding transistor, energizing that phase.

Motor Operation – Clockwise Rotation:

From Fig. 2,

• Pc3 output = 1 (transparent region),
• Pc2 and Pc1 outputs = 0 (opaque regions).

Under this condition:

• Pc3 activates transistor T1,
• T1 energizes Phase 1,
• This produces a South Pole in Phase 1.

The rotor’s North Pole is attracted toward this South Pole, causing the rotor to rotate clockwise.

As the rotor moves, the encoder disk also rotates. The opaque/transparent pattern changes, and: