AN1911 Freescale Semiconductor / Motorola, AN1911 Datasheet - Page 4

• Manufacturer Part Number: AN1911
• Description: 3-Phase AC Motor Control with V/Hz Speed Open Loop Using DSP56F80X
• Manufacturer: Freescale Semiconductor / Motorola

Target Motor Theory
4
In adjustable speed applications the AC motors are powered by inverters. The inverter converts a DC
power to AC power at required frequency and amplitude. The typical 3-phase inverter is illustrated in
Figure
The inverter consists of three half-bridge units where the upper and lower switch are controlled
complementarily -- meaning when the upper one is turned on, the lower one is turned off and vice
versa. As the power device’s turn-off time is longer than its turn-on, dead-time must be inserted
between the turn off of one transistor of the half-bridge and turn on of its complementary device. The
output voltage is mostly created by a Pulse Width Modulation (PWM) technique where an isosceles
triangle carrier wave is compared with a fundamental-frequency sine modulating wave, and the natural
Figure 3-1. Torque-Speed Characteristic at Constant Voltage and Frequency
3-2.
0
1
0
Load Torque
Indirect Power Factor Correction Using DSP56F80X
Freescale Semiconductor, Inc.
+ DC-Bus
- DC-Bus
For More Information On This Product,
Figure 3-2. Three Phase Inverter
Motor Torque
Motor
C
0.5
+
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T
T
1
2
s
n
r
r
T
T
Ph. A
3
4
0
Working
n
Point
s
AC Motor
3-Phase
Generator
T
T
Ph. B
5
6
Slip
Speed
Ph. C