23-03-2011, 12:46 PM
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1. INTRODUCTION
Pulse Width Modulation (PWM) technique is an effective way of controlling the speed of induction motor, and thus allowing the motor to be applied in the area requiring speed control. Available techniques to control the speed of induction motor are: varying the slip by changing rotor resistance or terminal voltage and varying synchronous speed by changing number of poles or supply frequency. Changing rotor resistance requires wound-rotor induction motor and any resistances inserted to the rotor circuit will reduce the efficiency of the machines. Changing terminal voltage has limited range of speed control. Changing the number of poles requires a motor with special stator windings. The best method is to change the electrical frequency because it is applicable to any types of induction motor. The speed of induction motor depends on the rate of rotation of its magnetic fields or the synchronous speed, which is directly proportional to any change of electrical frequency . PWM technique is used to control the electrical frequency of the 3-phase voltage supplied to the motor from the Insulated Gate Bipolar Transistors (IGBTs) inverter circuit, hence allowing the speed to be varied with respect to the frequency of the reference signal, input to the PWM signal generator. The Real time workshop from Math Works is an industrial PC for use in performing real-time analysis, simulation, and testing of control systems and digital signal processing (DSP) systems. The box works with Real time workshop software, a host-target environment that lets users connect models created in Math Works design tools such as Simulink, Matlab, and Real-Time Workshop, to physical systems, and execute them in real time . The Matlab/Real time workshop was used in some application other than induction motor speed control .This paper presents the development of the algorithm to perform the PWM operation using Matlab Simulink. Then the Matlab/Real time workshop was used to interface with the actual induction motor, finally through the I/O interface circuit and simulink slider gain, the speed of induction motor was controlled. The control algorithm also developed to switch the rotational direction of the motor, i.e. forward or reverse direction.
2. METHODOLGY AND PROCEDURE IDENTIFICATION
The first stage was to develop the algorithm to perform the PWM and to execute simulation in Matlab Simulink environment. The process started with developing the PWM signal generator. The set point value was represented by Signal block. The fundamental principle of AC machine operation is that if a balanced three-phase set of voltages, each with equal magnitude and shifted in phase by 0º, -120º and 120º respectively, the magnetic field produced inside the stator winding will rotate in the same direction of the phase rotation. If any two of the three-phase voltages are swapped, the magnetic field will reverse its rotational direction . If the phase input signal for three-phase PWM generator is in order of 0º, -120º, 120º, the rotational direction can be switched by changing the phase sequence order to 0º, 120º, and -120º. In the PWM generator block, the phase block was divided into two blocks, which are Forward and Reverse. The Forward block has the value of [0 -2*pi/3 2*pi/3], while the Reverse has the value of [0 2*pi/3 -2*pi/3], where the sequence for phase B and C was swapped Then, the PWM signal generator was connected to the IGBTs’ model and induction motor’s model in Matlab Simulink. The simulation proved that the developed algorithm could control the speed of induction motor. The second stage was to implement the PWM inverter model on actual IGBT module and actual induction motor. Using the control algorithm in the Matlab Simulink was linked through Real-Time Workshop to the hardware. After the connection was established, PWM signal was connected in real time to the IGBTs module. The IGBT module consists of six IGBTs. The module also contains electronics circuit that isolates the gate of IGBTs and protects the IGBTs against overheating, overvoltage, and over current. The controllable DC voltage was supplied to the IGBT module. There is a capacitor in the module, which is used to maintain a smooth DC voltage in spite of the current pulsation produce by the IGBTs .The Real time workshop was used to interface between the host PC and the IGBT inverter circuit. Communication page link uploaded the data from the host PC to the Real time workshop. I/O card at the target PC executed the control algorithm by providing the necessary signal to the IGBT inverter circuit. The switching sequence of the IGBTs followed the necessary signal of PWM technique that has been set in the control algorithm, thus converted the DC source to the PWM voltage output for the induction motor. There were two options to provide the input frequency to the control signal.. The input/output blocks were included into the Matlab Simulink model to enable the connection of the actual motor, IGBTs and keypad to the host PC.
3. INSULATED GATE BIPOLAR
TRANSISTOR (IGBT)
We are using Insulated Gate Bipolar Transistor (IGBT) for conversion of AC to controlled DC in our work. IGBT has been developed by combining into it the best qualities of both BJT and MOSFET. IGBT is also called Metal Oxide Insulated Gate Transistor (MOSIGT), Conductively Modulated Field Effect Transistor (COMFET) or Gain Modulated FET (GEMFET).
3.1 ADVANTAGES & DISADVANTAGES OF IGBT OVER OTHER SEMICONDUCTOR DEVICES
1.1.1 ADVANTAGES
♦IGBT possesses high input impedance like a MOSFET.
♦IGBT possesses low on-state power loss as in a BJT.
♦IGBT is free from the secondary breakdown problem that is present in BJT.
♦IGBT possesses lower gate drive requirements.
♦IGBT has smaller snubber circuit requirements.
♦IGBT converters are more efficient with less size as well as cost, as compared to
converters based on BJTs.
♦Switching losses in IGBTs are lesser.
♦Device rise and fall time switching capability is 5-10 times faster, resulting in
lower device switching loss and a more efficient drive.
♦The IGBT being a voltage rather than current controlled gate device has a lower
base drive circuit cost that also results in lower drive package cost.
♦Higher switching frequencies of IGBT drives produce less peak current ripple,
thus producing less current harmonic motor heating and allowing rated motor
torque with lower peak current than BJT drives.
1.1.2 DISADVANTAGES
♦For a similar motor cable length as the BJT drive, the faster output voltage rise
time of the IGBT drive may increase the dielectric voltage stress on the motor and
cable due to a phenomenon called reflected wave.
♦Faster output dV/dt transitions of IGBT drives also increase the possibility for
phenomenon such as increased common mode electrical noise.
♦Electromagnetic interference (EMI) problems and increased capacitive cable
charging current problems.
♦Any pulse width modulated (PWM) drive with a steep fronted output voltage
wave form may increase motor shaft voltage and lead to a bearing current
phenomenon known as fluting.
An IGBT is constructed in basically the same manner as a power MOSFET. There is however a major difference in the substrate. In IGBT there is a p power MOSFET, an IGBT has also thousands of basic structure cells connected a single chip of silicon
1.2 WORKING PRINCIPLE
When collector is made positive with respect to emitter, IGBT gets forward biased. With no voltage between gate and emitter, the two junctions J2 between n- and p is reverse biased. So noncurrent flows from collector to emitter. When gate is made positive with respect to emitter by a voltage more than the threshold voltage of IGBT, an n-channel or inversion layer is formed in upper part of p-region just below the gate. This n-channel short-circuits the n- and n+ regions. So electrons from n+ region flow into n- region. p+ is already injecting holes into n- region. So the conductivity of n- region increases considerably. IGBT gets turned on and starts conducting forward current IC .
1.3 LATCH-UP IN IGBT
When IGBT is on, hole current flows through transistor p+n-p and p-body resistance Rby. If load current is large, then drop through resistance Rby will be large. This drop will forward bias npn+ transistor. This further facilitates the turn on of p+n-p transistor. This can be a regenerative process. With parasitic thyristor on, IGBT latches up and after this collector current is no longer under the control of gate terminal. The only way now to turn off the latched-up IGBT is by forced commutation of current. If this latch-up isn’t aborted quickly, excessive power dissipation may destroy the IGBT. Hence to avoid this latch-up, collector current mustn’t exceed a certain critical value which must be specified by the manufacturer.
1.4 IGBT CHARACTERISTICS
3.5.1 STATIC I-V CHARACTERISTICS
Static I-V or output characteristics of an IGBT (n-channel type) show the plot of collector current IC versus collector-emitter voltage VCE for various values of gate-emitter voltages VGE1, VGE2, etc. In the forward direction, the shape of the output characteristics is similar to that of BJT. But here the controlling parameter is VGE as IGBT is a voltage-controlled device. When the device is off, junction J2 blocks forward voltage and in case reverse voltage appears across collector and emitter, J1 blocks it
3.5.2 TRANSFER CHARACTERISTICS
The transfer characteristics of an IGBT is a plot of collector current VC versus gate-emitter voltage VGE. This characteristic is identical to that of power MOSFET. When VGE is less than threshold voltage (VGET), the IGBT is in off-state
3.5.3 SWITCHING CHARACTERISTICS
The turn-on time is defined as the time between the instants of forward blocking to forward on-state. Turn-on time is composed of delay time tdn and rise time tr, i.e. ton=tdn+tr. The delay time is defined as the time for VCE to fall from VCE to 90% of initial VCE. Time tdn may also be defined as the time for IC to rise from its initial leakage current to 10% of final value of collector current. The rise time tr is the time during which VCE falls from 0.9 VCE to 0.1 VCE. It is also defined as the time for IC to rise from 0.1 IC to its final value IC. After time ton, collector current is IC, and VCE falls to small value called conduction drop=VCES, where subscript S denotes saturated value.
The turn-off time comprises three intervals:
♦Delay time: Time during which gate voltage falls from VGE to threshold voltage
VGET. The collector current falls from IC to 0.9 IC. At the end of delay time, VCE
begins to rise.
♦Initial fall time: Time during which collector current falls from 0.9 to 0.2 of its
initial value IC.
♦Final fall time: Time during which collector current falls from 0.2 to 0.1 of IC.
