AN1910 Freescale Semiconductor / Motorola, AN1910 Datasheet - Page 5

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

3.2 Volts per Hertz Control
the natural points of intersection determine the switching points of the power devices of a half bridge
inverter. This technique is shown in
other and thus a 3-phase motor can be supplied.
The most popular power devices for motor control applications are Power MOSFETs and IGBTs.
A Power MOSFET is a voltage controlled transistor. It is designed for high frequency operation and it
has a low voltage drop, thus it has low power losses. However, the saturation temperature sensitivity
limits the MOSFET application in high power applications.
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. The disadvantage is its higher voltage drop of the bipolar transistor, causing higher
conduction losses.
Volt per Hertz control methods is the most popular method of Scalar Control, controls the magnitude
of the variable like frequency, voltage or current. The command and feedback signals are DC
quantities, and are proportional to the respective variables.
The purpose of the volt per hertz control scheme is to maintain the air-gap flux of AC Induction motor
in constant in order to achieve higher run-time efficiency. In steady state operation the machine air-gap
flux is approximately related to the ratio V
is the synchronous electrical frequency applied to the motor. The control system is illustrated in
Figure
motor operates at optimum excitation because of the constant V
operates under-excited because of the DC-Bus voltage limit.
3-4. The characteristic is defined by the base point of the motor. Below the base point the
PWM Output T
PWM Output T
(Lower Switch)
(Upper Switch)
3-Phase AC Motor Control with V/Hz Speech Closed Loop
Freescale Semiconductor, Inc.
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1
0
2
1
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Figure 3-3. Pulse Width Modulation
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Figure
s
/f
s
3-3. The 3-phase voltage waves are shifted 120
Generated
Sine Wave
, where V
s
is the amplitude of motor phase voltage and f
s
/f
s
ratio. Above this point the motor
PWM Carrier
Wave
t
t
t
Target Motor Theory
o
to each
5
s