AN1911 Freescale Semiconductor / Motorola, AN1911 Datasheet - Page 5

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

3.2 Volts per Hertz Control
points of intersection determine the switching points of the power devices of a half bridge inverter.
This technique is shown in
and thus a 3-phase motor can be supplied.
The most popular devices for motor control applications are Power MOSFETs and IGBTs.
A Power MOSFET is a voltage controlled transistor designed for high frequency operation. It has low
voltage drop and thus low power losses. However, the saturation and temperature sensitivity limit the
MOSFET’s application in high power circuits.
An Insulated Gate Bipolar Transistor (IGBT) is a bipolar transistor controlled by a MOSFET on its
base. The IGBT requires low drive current, has fast switching time and is suitable for high switching
frequencies. Its disadvantage is the higher voltage drop of the bipolar transistor that causes higher
conduction losses.
The Volt per Hertz control method is the most popular method of Scalar Control which controls the
magnitude of the variable like frequency, voltage or current. The command and feedback signals are
DC quantities which are proportional to the respective variables.
This scheme is defined as a Volts per Hertz control because the voltage applied command is calculated
directly from the applied frequency in order to maintain the air-gap flux of the machine constant. In
steady state operation the machine air-gap flux is approximately proportional to the ratio Vs/fs, where
Vs is the amplitude of motor phase voltage and fs is the synchronous electrical frequency applied to
the motor. The control system is illustrated in
point of the motor. Below the base point the motor operates at optimum excitation, called constant
torque operation, because of the constant V
under-excited, called constant power operation, because of the rated voltage limit.
PWM Output T
PWM Output T
(Lower Switch)
(Upper Switch)
Indirect Power Factor Correction Using DSP56F80X
Freescale Semiconductor, Inc.
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Figure 3-3. Pulse Width Modulation
Figure
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3-3. The three phase voltage waves are shifted 120
Generated
Sine Wave
Figure
s
/f
s
ratio. Above this point the motor operates
3-4. The characteristic is defined by the base
PWM Carrier
Wave
t
t
t
Target Motor Theory
o
to each other
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