DESIGN AND IMPLEMENTATION OF IGBT BASED SINGLE
PHASE AC DRIVE USING PIC 18F452
Electrical and Electronics Project by
Ravi Devani
ABSTRACT
This paper presents a design and implementation of single
phase AC Drive to drive and speed control of a 1-phase induction motor using
the PIC 18F452 Microcontroller. The use of this PIC microcontroller yields
enhanced operations, fewer system components, lower system cost and increased
efficiency. The control algorithm is based on the constant volts-per-hertz
principle because the exact speed control is needed. Reflective object sensors
which are mounted on concrete frame are used to detect accurate edge position
of revolving stage. The PWM control signals are generated by the PIC 18F452
controller to drive the IGBT based single phase AC Drive. The power section in
this includes rectifier (converter) and filter which provide dc output with less
distortion from single phase ac input supply. Then the main stage comes is
inverter where by controlling the firing of power devices (e.g. IGBT) by pulse
width modulation produced by the PIC controller using the control algorithm
based on the constant volts-per-hertz principle, the motor is rotated with the load
maintaining the volt-hertz ratio constant.
Keywords – PWM,1-Phase Induction Motor, VSI ,IGBT Driver.
INTRODUCTION
The word "drive" is used loosely in the
industry. It seems that people involved primarily in the world of gear boxes
and pulleys refer to any collection of mechanical and electro-mechanical components,
which when connected together will move a load, as a "drive". When
speaking to these people, an AC drive may be considered by them as the variable
frequency inverter and motor combination.
People in the electrical field and electrical suppliers
usually refer to a variable frequency inverter unit alone, or an SCR power
module alone (when discussing DC drives) as the "drive" and the motor
as the "motor". It is estimated that 50% of the electrical power produced
consumed by electrical motors. More than 50% motors can typically be found in a
house, and nearly as many in a car. To preserve the environment and to reduce
green-house effect gas emissions, governments around the world are introducing regulations
requiring white goods manufacturers and industrial factories to produce more
energy efficient appliances. Most often, this goal can be reached by an efficient
drive and control of the motor speed. This is the reason why appliance
designers and semiconductor suppliers are now interested by the design of
low-cost and energy-efficient variable speed drives. Because of their high
robustness, reliability, low cost and high efficiency ( 80%), AC induction
motors are used in many industrial applications . However, induction motors can
only run at their rated speed when they are connected to the main power supply.
This is the reason why variable frequency drives are needed to vary the rotor
speed of an induction motor. The most popular algorithm for the control of an
induction motor is the V/f control approach using a natural pulse-width modulation
(PWM) technique to drive a voltage source inverter (VSI). The aim of this
application note is to show how these techniques can be easily implemented on a
PIC based variable frequency drive dedicated to power control applications. The
requirements of electrical drive systems increases by the request of more
performance, higher integration, easier to program and lower cost.
SYSTEM DEVELOPEMENT
The basic circuit diagram of the PIC controlled single
phase inverter is shown in Figure 1.
Figure 1. PIC controlled single-phase PWM inverter
Hardware setup procedure can be divided into two sections: the control and
power circuit. Electrical isolation between control circuit and power circuit
is provided by the optically coupled devices.
CONTROL CIRCUIT DESIGN
The control circuit section is composed of three parts namely PC, PIC board
and IGBT driver. PIC 18F452 is programmed to produce gate pulses by comparing a
triangular carrier wave in 2-KHz and a sinusoidal reference wave in 50-Hz. The
reference wave amplitude adjusts the amplitude of the generated AC voltage and
the Reference wave frequency determines the frequency of the generated AC
voltage. 1MBH50D-060 coded IGBTs are used in PWM inverter circuit. IGBTs are
voltage controlled devices and it requires a minimum gate threshold voltage of
about 15-V for establishing the rated collector-to-emitter conduction. This
requirement makes it difficult to directly interface an IGBT to PIC. For proper
operation of IGBTs, correct power levels are required (Vge(th)=15V and Ic= 50
mA). Voltage and current levels of DSP output signals fail to operate the
IGBTs. IGBT driver circuit amplifies PIC output signal to the required level for
triggering the IGBTs and isolates the DSP from the power circuit. TLP250 ICs are
commonly used driver ICs for IGBTs. However, PIC output signals does not have
enough current level to activate the TLP250. 74LS40 NAND gate IC is used to
amplify the current value of signals from PIC at a gain of 50/2. In the buffer
circuit (74LS40) if two inverter gates are connected back to back, the two
inversion functions would "cancel" each other out so that there would
be no inversion from input to final output. DSP output signals are now ready to
activate TLP250. TLP250 consists of a light emitting diode and an integrated
photo detector. The output waveform will have the same waveform as the input
waveform except the magnitude and power.
POWER CIRCUIT DESIGN
The power circuit section is composed of four parts namely full bridge
inverter circuit, DC power supply, LC filter and load(induction motor). PWM inverters
include semiconductor devices with nonlinear characteristics and can generate dominant
harmonics in the system. The waveform quality of the sensitive load is improved
by putting an LC filter at the output of the PWM inverter. In order to design
an LC filter, there are many methods available. Optimum performance can be
obtained by using in simulation and experimental studies. A rule of thumb in
control theory is that the frequencies of such a configuration have to have at
least a factor of 10 between them to decouple the effects. According to this
rule, for 50-Hz fundamental frequency, resonance frequency has to be at least
500-Hz, pulse frequency of the inverter output has to be at least 5000-Hz.
Resonance frequency is determined by the product of L and C.
PERFORMANCE ANALYSIS
Principle of speed control the inverter unit controls the speed of a
single-phase induction motor by changing the frequency, f, of the voltage applied
to the motor. The inverter it changes the frequency, f, by changing the N/OFF
cycle of the six switching elements, and the rotational speed (N) of the motor changes
in proportion to the expression in formula (1).
N= 120·f ·(1-s)/P……………… (1)
N: Rotational speed [r/min]
F: Frequency 〔Hz〕
P: Number of poles of a motor
S: Slip
In addition, to make the voltage applied to the winding have a sine-wave
shape, the inverter controls the ON/OFF duty cycle as shown in Figure2. The ON/OFF
time is controlled so that the average voltage applied to the motor becomes a
sine-wave shape by comparing the triangular wave called a carrier signal with
the sine-wave shaped signal waveform.
The generated torque T of the motor is shown by the formula (2). From this
relation, it can be said that the torque will also be constant by making V/f,
the ratio of voltage V to frequency f, constant.
Ｔ＝Ｋ・Ｉ・Ｖ／ｆ …………（２）
Ｔ： Torque [N•m]
Ｖ： Power supply voltage [V]
Ｉ： Motor current [A]
ｆ： Frequency [Hz]
Ｋ： Constant
Figure 2. Duty control of ON/OFF
Electrical and Electronics Project by
Ravi Devani
However, the lower the speed is, the more difficult it is to keep constant
the input impedance of the induction motor with the change in f. Therefore, to
obtain a torque that is constant from low speed to high speed it is necessary
to adjust the V/f ratio at low speed in accordance to the characteristics of
the motor like the solid line shown in Figure 3.
Figure 3. V/f control
Figure 4. Block diagram of closed-loop control
Figure 4. shows the block diagram configuration
of the closed-loop control system used in our implementation. This method
detects the phase difference between the voltage of the inverter output block
and the primary current, which calculates the driving frequency corresponding
to the load using the characteristics data table (Figure 5) prepared beforehand,
and controls the inverter frequency without the need for a speed sensor on motor
Figure 5. Characteristics data table
With this characteristic table and the detected phase difference time t,
the inverter calculates an inverter output frequency that corresponds to the rotational
speed command Nset set by the speed potentiometer, and outputs it as the
inverter output frequency. After receiving the output frequency, the V/f
control block calculates the voltage applied to the motor corresponding to the
output frequency f, and performs the speed control by driving the PWM inverter.
In Figure 6, the 50-Hz fundamental frequency is compared with 3-KHz carrier
signal.
(A)
(B)
Figure 6. Simulation (a) and experimental (b) results
CONCLUSION
IGBT based AC Drive drives the single phase motor and control the speed of
it. By providing the PWM pulses to voltage source inverter, the 1 phase induction
motor rotates maintaining the volt-hertz ratio constant. By changing the
frequency the input signal which is given through the hardware knob. (e.g. potentiometer)
the period of the PWM pulses changes with respect to it and in correspondence
the speed of motor changes providing the volt-hertz principle constant .With
the application of the load on motor, the speed of motor decreases then by
closed loop the PIC Controller changes and try to bring it in actual applied speed.
REFERENCES
[1]. www.silicontecnolabs.in
[2]. V. M. Cardenas, S. Horta, R. Echavarria, "Elimination of dead
time effects in three phase inverters," in Proc. IEEE International Power
Electronics Congress, vol. , no. , Oct. 1996, p.258-262
[3]. Application Note, How to use IGBT Modules, Powerex Inc, 1999
[4]. Advanced PIC Microcontroller projects in C, DOGAN IBRAHIM
[5]. http://www.deccanrobots.com/
[6]. G. Rost, A. Sangswang and C.O. Nwankpa, “A Modular Simulink Based
Controlled Three Phase Switch Mode Inverter,” IEEE Power 4, 16-20 July 2000 pp.
2101 - 2106 vol. 4., 2000.
[7]. J. K. Steinke, “Use of an LC Filter to Achieve a Motor-Friendly
Performance of the PWM Voltage Source Inverter,” IEEE Transactions on Energy
Conversion, pp. 649 – 654, Volume 14, Issue 3, Sept. 1999. Engineering Society Summer
Meeting, Volume
Electrical and Electronics Project by
Ravi Devani
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