AN1910 Freescale Semiconductor / Motorola, AN1910 Datasheet - Page 4

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

Target Motor Theory
4
In adjustable speed applications the AC motors are powered by inverters. The inverter converts 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 is controlled
complementarily - meaning when the upper one is turned-on, the lower one must be turned-off and
vice versa. As the power device’s turn-off time is longer than its turn-on time, some dead-time must be
inserted between the turn-off of one transistor of the half-bridge and turn-on of it's 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
Figure 3-1. Torque-Speed Characteristic at Constant Voltage and Frequency
3-2.
0
1
0
Load Torque
3-Phase AC Motor Control with V/Hz Speed Closed Loop
Freescale Semiconductor, Inc.
+ DC-Bus
- DC-Bus
For More Information On This Product,
Motor Torque
Motor
C
Figure 3-2. 3- Phase Inverter
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