AN1857 Freescale Semiconductor / Motorola, AN1857 Datasheet - Page 36

• Manufacturer Part Number: AN1857
• Description: A 3-Phase ac Induction Motor Control System Based on the MC68HC908MR32
• Manufacturer: Freescale Semiconductor / Motorola

Application Note
36
The general principle behind a PI control loop is shown in
The speed closed-loop control is characterized by the measurement of
the actual motor speed. This information is compared with the reference
set point, and the error signal is generated. The magnitude and polarity
of the error signal corresponds to the difference between the actual and
the required speed. Based on the speed error, the PI controller
generates the corrected motor frequency to compensate the error and
accomplish the required motor speed.
This PI process takes the two input parameters: actual speed command
V_com_actual and actual motor speed measured by a tachogenerator
V_tacho. The PI controller then calculates the speed error and
performs the speed PI control algorithm.
The output of the PI controller is a frequency of the fundamental sine
wave to be generated by the inverter, V_out.
The drive is designed as a constant volts-per-hertz drive. This means the
control algorithm keeps the magnetizing current (flux) of the motor
constant by varying the stator voltage with frequency. The ratio of voltage
divided by frequency is constant during the linear portion of the profile.
The commonly used volts-per hertz profile of a 3-phase ac induction
motor is illustrated in
Freescale Semiconductor, Inc.
REFERENCE SPEED
(V_com_actual)
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SPEED ERROR
Figure 22. Closed Loop Control
Figure
PI CONTROLLER
23.
CORRECTED
(V_pi_out)
SPEED
CONTROLLED
SYSTEM
ACTUAL MOTOR SPEED
Figure
(V_tacho)
MOTOROLA
AN1857
22.