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Abstract
In induction motor control CSI and VSI techniques were used earlier which produces undesirable harmonics. This harmonic results in reduction in efficiency of induction motor speed control. In order to overcome this drawbacks Z-Source inverter is been used.
By using this inverter the harmonics are reduced which helps in giving better variable speed for speed control of induction motor. In this Z-source inverter consists of two inductors and two capacitors which acts as filter that reduces harmonics.
INTRODUCTION
Z-Source inverter can be used as buck or boost converter i.e. as a CSI (Current Source
Inverter) and also VSI (Voltage Source Inverter). This is the greater advantage compared to CSI or VSI. Because earlier either CSI or VSI were used to control/vary speed of the induction motor. Z-source inverter consists of six switches i.e. MOSFET (Metal Oxide Semiconductor Field Effect Transistor). The voltage obtained by using VSI and CSI is minimum that is not sufficient for proper control/vary speed of induction motor. In other words Z-source inverter used here is acting as a driver circuit and as a filtering circuit as well.
It also allows two switches of the same leg to be gated in the circuit, thus it is eliminating the shoot through fault occurring in traditional converters.
TRADITIONAL METHODS USED FOR SPEED CONTROL OF INDUCTION MOTOR
1. VOLTAGE SOURCE INVERTER (VSI):
Voltage source inverter provides variable frequency supply from the dc supply. Mostly MOSFET is used in low voltage and low power inverter, IGBT (Insulated Gate Bipolar Transistor) and IGTC (Insulated Gate Commuted Thyristor) are used for high power levels.
VSI can be used as stepped wave inverter or a pulse-width modulated (PWM) inverter. When it is operated as a stepped wave inverter, transistor are switched in the sequence of their numbers with difference of T/6 and each transistor is kept on for the duration T/2, where T is the time period for one cycle. VSI employs capacitor in shunt that acts as a buck converter.
At the end of this paper the simulation result on VSI is shown. It indicates greater amount of harmonics compared to Z-Source Inverter.
2. CURRENT SOURCE INVERTER (CSI):
A thyristorcurrent source inverter (CSI) consist of Diodes D1-D6and capacitors C1- C6 provide commutation of thyristors T1-T6, Which are fired with a phase difference of 60 degree in sequence of their numbers. It also shows the nature of output current waveforms. Inverter behaves as a current source due to the presence of large inductance in dc link. For given speed torque is controlled by varying dc link current by changing the values of source voltage.
Therefore, when supply is ac, a controlled rectifier is connected between the supply and inverter and when supply is dc, a chopper is interposed between the supply and inverter. The major advantage of CSI is its reliability. In case of VSI, a commutation failure will cause two device in same leg to conduct. This connects conducting devices directly across the source.
Consequently, through device suddenly rise to dangerous values. Expensive high speed semiconductor fuses are required to protect the devices. In case of CSI, conduction of two devices in same leg does not lead to sudden rise of current through them due to the presence of a large inductance. This allows time for commutation to take place and normal operation to get restored in subsequent cycles. Further, less expensive HRC fuses are good enough for
protection of thyristors.
Block Diagram description:
Input supply: 1 phase 230 V ac.
Rectifier: Used for converting AC TO DC Supply.
Z Source Inverter: It is a combination of two inductors and two capacitors.
Driver circuit: It consist of
Buffer IC 4050
Small ICs – MCT2E
Transistors
Darlington transistors
Capacitors
Resistors
Connections to the 3phase invertor
It has two functions,
a) Amplification b) Isolation.
It can be used to amplify the 5V pulses to 12V for using transistor technology and provide Isolation for using opto-coupler.
Pulse generator: It consist of
Bridge rectifier (1mA)
Electrolytic Capacitors
Crystal oscillator
Resistor – 33 ohm , 5Vdc
IC PIC165877A
Voltage Regulator
Reset , speed varying Buttons
Here we have used PIC microcontroller (PIC 16F877A) to make a switching signal.
MULTILEVEL INVERTER: It can output ac voltage with the same dc power supply, which has a wider modulation index range than a traditional inverter.
Load: AC load.
IMPEDENCE SOURCE INVERTER
The Impedance Source Inverter is used to overcome the problems in the traditional source inverters. Fig. 2 shows the main circuit of the proposed Z-source inverter based induction drive system. A voltage-type Zsource inverter is utilized, instead of the traditional voltage source inverter (VSI) or current source inverter (CSI), to feed electric energy from the dc source to the squirrel cage induction motor. To obtain the buck/boost ability, the pulse width modulation (PWM) technique should be used to control the Z-source inverter to generate shoot-through states. Unlike the Z-source inverter based ASD system with induction machines, the output currents of the Z-source inverter in the proposed induction drive system are composed of square waveforms of 120° electrical degree. Consequently, the operation principle, the modeling method and the control are all different from the Z-source inverter based ASD system with induction machines.
Fig.3 shows some equivalent circuits when the phase a and b windings are conducted, with the current flows from phase-a winding to phase-b winding. The shoot-through states can be generated via shorting either any one arm or both arms in the bridge For ease of illustration, assume that the upper switches of the bridge operate in chopping modes, while the lower are used to short the bridge arms. The broad-brush lines and arrows indicate the path and direction of the currents, respectively.
CONCLUSION
This paper has proposed a Z – source inverter based induction motor drive. This drive system has the advantages of both PMBDCM and Z-source inverter. The system configuration, operation principle and control method have been analyzed in detail. And based on the equivalent circuits, the mathematical model has been established by state-space averaging method. Simulation results have validated the preferred features as well as the possibility of the proposed drive system. Additionally, the shortcoming of switching loss has been discussed, and a possible improvement method has been presented.