14-02-2013, 03:04 PM
VF Control of 3-Phase Induction Motors Using PIC16F7X7 Microcontrollers
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INTRODUCTION
An induction motor can run only at its rated speed when
it is connected directly to the main supply. However,
many applications need variable speed operations.
This is felt the most in applications where input power
is directly proportional to the cube of motor speed. In
applications like the induction motor-based centrifugal
pump, a speed reduction of 20% results in an energy
savings of approximately 50%.
Driving and controlling the induction motor efficiently
are prime concerns in today’s energy conscious world.
With the advancement in the semiconductor fabrication
technology, both the size and the price of semiconductors
have gone down drastically. This means that the
motor user can replace an energy inefficient mechanical
motor drive and control system with a Variable
Frequency Drive (VFD). The VFD not only controls the
motor speed, but can improve the motor’s dynamic and
steady state characteristics as well. In addition, the
VFD can reduce the system’s average energy
consumption.
VF CONTROL
A discussion of induction motor control theory is
beyond the scope of this document. We will mention
here only the salient points of VF control.
The base speed of the induction motor is directly
proportional to the supply frequency and the number of
poles of the motor. Since the number of poles is fixed
by design, the best way to vary the speed of the
induction motor is by varying the supply frequency.
The torque developed by the induction motor is directly
proportional to the ratio of the applied voltage and the
frequency of supply. By varying the voltage and the frequency,
but keeping their ratio constant, the torque
developed can be kept constant throughout the speed
range. This is exactly what VF control tries to achieve.
Figure 1 shows the typical torque-speed characteristics
of the induction motor, supplied directly from the main
supply. Figure 2 shows the torque-speed characteristics
of the induction motor with VF control.
MOTOR DRIVE
The 3-phase induction motor is connected to a 3-phase
inverter bridge as shown in Figure 3.The power inverter
has 6 switches that are controlled in order to generate
3-phase AC output from the DC bus. PWM signals,
generated from the microcontroller, control these 6
switches. Switches IGBTH1 through IGBTH3, which
are connected to DC+, are called upper switches.
Switches IGBTL1 through IGBTL3, connected to DC-,
are called lower switches.
The amplitude of phase voltage is determined by the
duty cycle of the PWM signals. While the motor is running,
three out of six switches will be on at any given
time; either one upper and two lower switches or one
lower and two upper switches.
Control
Members of the PIC16F7X7 family of microcontrollers
have three 10-bit PWMs implemented in hardware. The
duty cycle of each PWM can be varied individually to
generate a 3-phase AC waveform as shown in
Figure 4. The upper eight bits of the PWM’s duty cycle
is set using the register CCPRxL, while the lower two
bits are set in bits 4 and 5 of the CCPxCON register.
The PWM frequency is set using the Timer2 Period register
(PR2). Because all of the PWMs use Timer2 as
their time base for setting the switching frequency and
duty cycle, all will have the same switching frequency.
To derive a varying 3-phase AC voltage from the DC
bus, the PWM outputs are required to control the six
switches of the power inverter. This is done by connecting
the PWM outputs to three IGBT drivers (IR2109).
Each driver takes one PWM signal as input and
produces two PWM outputs, one being complementary
to the other. These two signals are used to drive one
half bridge of the inverter: one to the upper switch, the
other to the lower switch. The driver also adds a fixed
dead time between the two PWM signals.
3-Phase Sine Waveform Synthesis
Along with the three PWM modules, the 16-bit Timer1
hardware module of PIC16F7X7 is used to generate
the control signals to the 3-phase inverter.
This is done by using a sine table, stored in the
program memory with the application code and
transferred to the data memory upon initialization.
Loading the table this way minimizes access time
during the run time of the motor. Three registers are
used as the offset to the table. Each of these registers
will point to one of the values in the table, such that they
will always have a 120-degree phase shift relative to
each other (Figure 4). This forms three sine waves with
120 degrees phase shift to each other.
OVERVIEW OF SYSTEM HARDWARE
Figure 6 shows the overall block diagram of the power
and control circuit for the motor control demo board.
The main single phase supply is rectified by using a
diode bridge rectifier. The ripple on the DC bus is filtered
by using an electrolytic capacitor. The filtered DC
bus is connected to the IGBT-based 3-phase inverter,
which is controlled by the PIC16F7X7. The inverter
output is a 3-phase, variable frequency supply with a
constant voltage-to-frequency ratio.
A potentiometer connected to AN1 sets the motor
frequency. Push button keys are interfaced for issuing
commands, like Run/Stop and Fwd/Rev, to the
microcontroller. Acceleration and deceleration features
are implemented to change the motor frequency
smoothly. Time for both of these features are user
selectable and can be set during compile time. LEDs
are provided for Status/Fault indications like Run/Stop,
Forward/Reverse, Undervoltage, Overvoltage, etc.
Overcurrent Protection
A non-inductive resistor is onnected between the
common source point of the inverter and the power
ground. Voltage drop across this resistor is linearly proportional
to the current flowing through the motor. This
voltage drop is compared against the reference voltage
signal, through an optoisolator (linear optocoupler),
which represents overcurrent limit. There are three
possible ways to compare these voltage signals:
• Using an external comparator
• Using the PIC16F7X7 on-chip comparator
• In software, by reading the voltage drop across
the resistor through one of the ADC channels
The design discussed in this application note
implements an external comparator. It’s output drives the
shutdown signal of the driver through an optoisolator
(optocoupler). At the same time, this signal is provided
to RB4. By using the PORTB interrupt-on-change
feature, the microcontroller responds to Fault detection
and stops the motor.
Overvoltage and Undervoltage Protection
To implement voltage protection, the DC bus voltage is
attenuated by a potential divider. The resulting signal is
fed to AN2 through an optoisolator (linear optocoupler).
The application monitors the voltage via periodic A/D
conversions of the value on RA2; if the voltage falls
outside of a preset range, the motor is stopped.
CONCLUSION
VF control provides a simple and cost efficient method
for open-loop speed control of 3-phase induction
motors. A low-cost VF solution can be implemented
using the PIC16F7X7 family of devices. With three dedicated
PWM modules implemented in hardware, it is
ideal for controlling 3-phase induction motors. Additional
on-chip resources, like multiple timers and ADC,
allow users to easily implement safety and control
features, such as current and voltage protection and
configurable acceleration and deceleration time.