Switch Configuration and PWM

BLDC motors use electric switches to obtain current commutation in order to continuously rotate the motor. These switches are typically connected in an H-bridge structure for single-phase BLDC motors, and a three-phase bridge structure for three-phase BLDC motors. The figure shown below demonstrates the two circuit structures:

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H-Bridge and Three-Phase Bridge

High-side switches are typically controlled with PWM, which converts a DC voltage into a modulated voltage that easily and efficiently limits the startup current, control speed, and torque. Raising the switching frequency typically increases PWM losses. Lowering the switching frequency limits the system’s bandwidth and can increase ripple current to destructive extents.

Commutation Sequence

Single-Phase BLDC Motor

The figure below shows the commutation sequence of a single-phase BLDC motor driver circuit.

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Single-phase BLDC Motor Commutation Example

Following the diagram carefully will provide the intuition necessary to grasp the commutation sequencing of single-phase BLDC motors.

The figure below shows an example of Hall sensor signals with respect to switch drive signals and armature current.

Three-Phase BLDC Motor

A three-phase BLDC motor requires three Hall sensors to detect the rotor’s position. Based on the physical position of the Hall sensors, there are two types of output: a 60° phase shift and a 120° phase shift. Combining these three Hall sensor signals can determine the exact commutation sequence.

The figure below shows the commutation sequence of a three-phase BLDC motor driver circuit for counter-clockwise rotation.

Three Hall effect sensors a, b, and c are mounted on the stator at 120° intervals, while the three phase windings are in a star formation. For every 60° rotation, a Hall effect sensor will change state. It takes six steps to complete an entire electrical cycle. In synchronous mode, the phase current switching updates every 60°.

Recall that the number of signal cycles needed to complete a mechanical rotation is equal to the number of rotor pole pairs. In the example, there are 2 rotor pole pairs. Thus, 2 signal cycles are required to complete a single mechanical revolution.

The figure below shows the timing diagrams where the phase windings U, V, and W are either energized or floated based on the Hall effect sensor signals. Remember that this is an example where the Hall effect sensor signals have a 120° phase shift with respect to each other where the motor rotates counter-clockwise.

Provided below is another example of three-phase BLDC motor commutation.