AN1917D Freescale Semiconductor / Motorola, AN1917D Datasheet - Page 8

• Manufacturer Part Number: AN1917D
• Description: 3-Phase PM Synchronous Motor Control with Quadrature Encoder Using DSP56F80x
• Manufacturer: Freescale Semiconductor / Motorola

8
Target Motor Theory
3.1.2 Position Sensing
3.1.3 Position Alignment
To avoid the current measurement there is one of possible solution which aligns the vector of the
applied voltage to be orthogonal to the rotor position (see
between the stator and rotor flux is not exactly 90 electric degrees because the voltage drop on the
stator inductance is not compensated. The real angle is lower than 90 electric degrees and depends on
the load.
For low-cost applications, such a solution is fully sufficient. For high-end applications, the current
measurement and the stator flux need to be evaluated. (see [16])
The rotor position is obtained from a Quadrature Encoder mounted on the rotor shaft. The encoder
transfers the rotational movement into signal pulses corresponding to the position. The Quadrature
Encoder output signals are connected to the on-chip Quadrature Decoder input. The signals go through
a digital filter to the Quad Timer. The Quad Timer is set to count in quadrature mode. During
alignment, the Quad Timer is preset to a value which represents a shift of the rotor position by 90
electrical degrees. Thus, the applied voltage is aligned with Back-EMF according to
Since the Quadrature Encoder doesn’t give the absolute position, we need to know exactly the rotor
position before the motor is started. One possible, and very easy implementable, method is the rotor
alignment to a predefined position. The motor is powered by a selected static voltage pattern (usually
the zero position in the sinewave table) and the rotor aligns to the predefined position. The alignment is
done only once during first motor start.
alignment the position counter is set to 90 electric degrees from alignment position in order to preset
the angle between stator and rotor flux.
u
s
j L
s
i
s
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R
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s
Figure 3-6. PM Synchronous Motor Phasor Diagram
a)
3-Phase PMSM Control with Quadrature Encoder
Freescale Semiconductor, Inc.
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Go to: www.freescale.com
Figure 3-7
shows the position of the aligned rotor. After
u
Figure 3-6
s
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b)). As can be seen, the angle
b)
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Figure 3-6
MOTOROLA
b).