AN1915D Freescale Semiconductor / Motorola, AN1915D Datasheet - Page 7

• Manufacturer Part Number: AN1915D
• Description: 3-Phase BLDC Motor Control with Quadrature Encoder using DSP56F80x
• Manufacturer: Freescale Semiconductor / Motorola

Freescale Semiconductor, Inc.
Target Motor Theory
3.1.3 Commutation
Commutation creates a rotation field. As was explained, for the proper operation of a BLDC motor, it
is necessary to keep the angle between stator and rotor flux close to 90°. With six-step control we get a
total of six possible stator flux vectors. The stator flux vector must be changed at a certain rotor
position.
The rotor position is usually sensed by Hall Sensors. The Hall Sensors directly detect the commutation
moment. The presented application uses the Quadrature Encoder to sense rotor position. The rotor
position is usually sensed by Hall Sensors, which directly detected the commutation moment. The
presented application uses the Quadrature Encoder to sense rotor position. Therefore the rotor position
must be translated to determine the commutation moment.
The electrical revolution can be divided into six sectors. Each sector corresponds to a certain stator
flux vector as illustrated in
Figure
3-6. The commutation sequence is illustrated in tables
Table 3-1
and
Table
3-2.
Figure 3-6. Stator Flux Vectors at Six-Step Control
The next two figures depict the commutation process. The actual rotor position in
Figure 3-7
corresponds to the sector ABC[110] (see
Figure
3-6). The actual voltage pattern can be derived from
the
Table
3-2. Phase A is connected to the positive DC-bus voltage by the transistor Q1, Phase C is
connected to the ground by transistor Q6 and Phase B is unpowered.
As soon as the rotor reaches a certain position (see
Figure
3-7) the sector is changed from ABC[110]
to ABC[100]. From
Table 3-2
a new voltage pattern is selected and applied to the BLDC motor.
As can be seen, using a six-step control technique there is no possibility of keeping the angle between
the rotor flux and the stator flux precisely at 90°. The actual angle varies from 60° to 120°.
The commutation is repeated per each 60 electrical degrees. The commutation event is critical for its
angular (time) accuracy. Any deviation causes the torque ripples and hence speed variation.
MOTOROLA
3-Phase BLDC Motor Control with Quadrature Encoder
7
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