AN1910 Freescale Semiconductor / Motorola, AN1910 Datasheet - Page 12

• Manufacturer Part Number: AN1910
• Description: 3-Phase AC Motor Control with V/Hz Speed Closed Loop Using the DSP56F80X
• Manufacturer: Freescale Semiconductor / Motorola

Software Design
6.1.1 Acceleration/Deceleration Ramp
6.1.2 Speed Measurement
6.1.3 PI Controller
6.1.4 V/Hz Ramp
12
The process calculates the new actual speed command based on the required speed according to the
acceleration/deceleration ramp. The desired speed is determined either by push buttons or by the PC
Master.
During deceleration the motor can work as a generator. In the generator state the DC-bus capacitor is
charged and its voltage can easily exceed its maximal voltage. Therefore, the voltage level in the
DC-bus link is controlled by a resistive brake, operating in case of overvoltage.
The process input parameter is Omega_desired, the desired speed.
The process output parameter is Omega_required, used as an input parameter of the PWM generation
process.
The speed measurement process uses the on-chip quadrature decoder. The process output is
MeasuredSpeed, and is only used as an information value in PC Master.
The PI controller process takes the input parameters, actual speed command Omega_required, and
actual motor speed, measured by a incremental encoder Omega_actual. The PI controller calculates a
speed error and performs the speed PI control algorithm. The output of the PI controller is a frequency
of the first harmonic sine wave to be generated by the inverter: Omega_command.
The drive is designed as a volt per hertz drive. It means, the control algorithm keeps the constant
motor’s magnetizing current (flux) by varying the stator voltage with frequency. The commonly used
volt per hertz ramp of a 3-phase AC induction motor is illustrated in
V
V
V
V (%)
boost
base
start
3-Phase AC Motor Control with V/Hz Speed Closed Loop
Freescale Semiconductor, Inc.
Point
Start
For More Information On This Product,
Figure 6-2. Volt per Hertz Ramp
f
boost
Boost
Point
Go to: www.freescale.com
Base
Point
f
base
f (Hz)
Figure
6-2.