CHAPTER 2 VSI FED INDUCTION MOTOR DRIVE

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1 CHAPTER 2 VI FE INUCTION MOTOR RIVE 2.1 INTROUCTION C motors have been used during the last century in industries for variable speed applications, because its flux and torque can be controlled easily by means of changing the field and armature currents respectively. Furthermore, operation in the four quadrants of the torque speed plane including temporary standstill was achieved. Almost for a century, induction motor has been the workhorse of industry due to its robustness, low cost high efficiency and less maintenance. The induction motors were mainly used for essentially constant speed applications because of the unavailability of the variable- frequency voltage supply. The advancement of power electronics has made it possible to vary the frequency of the voltage supplies relatively easy, thus extending the use of the induction motor in variable speed drive applications. But due to the inherent coupling of flux and torque components in induction motor, it could not provide the torque performance as good as the C motor. In AC grid connected motor drives, a rectifier, usually a common diode bridge providing a pulsed C voltage from the mains is required. Although the basic circuit for an inverter may seem simple, accurately switching these devices provides a number of challenges. The most common switching technique is called Pulse Width Modulation (PWM). PWM is a powerful technique for controlling analog circuits with a processor s digital outputs. PWM is employed in a wide variety of applications like UP, electric drives, HVC reactive power compensators in power systems, ranging from measurement and communications to power control and conversion. In AC motor drives, PWM inverters make it possible to control both frequency and magnitude of the voltage and current applied to a motor. As a result, PWM inverter-powered motor drives are more variable and offer in a wide range better efficiency and higher performance when compared to fixed frequency motor drives. The energy, which is delivered by the PWM inverter to the AC motor, is controlled by PWM signals applied to the gates of the power switches at different times for varying durations to produce the desired output waveform. To improve the quality of the product, variable speed is required, for that step less speed control is required. epending on the type of load and the type of speed different methods are adopted for speed control of 31

2 motors. For step less speed control below and above the rated speed with high torque and to avoid the harmonics, the PWM inverter fed induction motor control is the best suitable one. The block diagram of VI fed Induction Motor is shown in Fig 2.1. AC supply ingle phase Rectifier Three phase INVERTER Induction Motor drive Fig2.1 Block diagram of VI fed Induction Motor 2.2 THREE PHAE PWM INVERTER The PWM inverter has to generate nearly sinusoidal current, which can control the voltage and current with 120 degrees difference in each phase. The controlling signals of three-phase PWM inverters have many pattern controls. The operation of three-phase inverter can be defined in eight modes, which shows the status of each switch in each operations mode. In inverter operation, the necessary phase-leg-short is naturally realized through anti-parallel diodes in the three-phase bridge. Accordingly, the same gate pulses as in the conventional VI can be applied. On the other hand, the switch on the C link must actively operate. The recent advancement in power electronics has initiated to improve the level of the inverter instead of increasing the size of the filter. In multilevel inverter, design involves parallel connection of the inverter. For these redundant switching a space vector modulation is needed which is based on vector selection in dq stationary reference frame. For a multi level system either space vector modulation or sinusoidal triangle modulation may be taken. However space vector modulation is having more advantages due to low harmonic production. The performance of the multi level inverter is better than the classical inverter. The total harmonic distortion of the classical inverter is very high. The diode clamped inverter provides multiple voltage levels from a series capacitor bank. The voltage across the switches is only half of the C bus voltage. These features effectively double the power rating of 32

3 g g voltage source inverter to the given semiconductor device. The total harmonic distortion is analyzed between multilevel inverter and other classical inverters. Field oriented control (FOC) of induction motor was introduced which has opened a new horizon to the induction motor applications. The method, which uses frame, has transformed the performance of induction motor similar to that of the C motor. The implementation of this system however is complicated and furthermore FOC, in particularly indirect method which is widely used, is known to be highly sensitive to parameter variations due to the feed-forward structure of its control system. In the TC drive, flux linkage and electromagnetic torque are controlled directly and independently by the selection of optimum inverter switching modes. The required optimal switching voltage vectors can be selected by using a so called optimum switching voltage vector look up table. In the present work an attempt is made to simulate TC system. 2.3 IMULATION REULT The circuit of six switch three phase inverter system is shown in Figure 2.2. In threephase inverter fed drive system, AC is converted into C using uncontrolled rectifier. C is converted into variable voltage variable frequency AC using three-phase PWM inverter. The variable voltage variable frequency supply is applied to the motor Gate pulses for 3 phase inverter are shown in Fig 2.3. Phase voltages are shown in Figure2.4. The phase currents are in Figure 2.5. Rotor speed is shown in Fig 2.6. FFT analysis is done for the output voltage and the spectrum is shown in Figure 2.7. The TH is 13.9%. iscrete, Ts = 5e-006 s. pow ergui Out1 Out1 6 Out1 9 M3 4 A B C a b c <tator current cope1 is_a (pu)> <Rotor speed (wm)> -K- Out1 7 Out1 10 M7 Out1 5 M6 g g M4 g g 1 M Tm AC A B C m 2 3 M5 Thre Asynchronous Machine pu Units Gain cope5 Vabc From Iabc From1 cope2 cope3 Fig 2.2 ix switch three phase inverter fed drive 33

4 Fig. 2.3 Gate pulses for 3 phase inverter Fig 2.4 Phase voltages Va, Vb, Vc 34

5 Fig 2.5 Current waveforms i a, i b, i c Fig 2.6 Rotor speed (rad /sec) 35

6 Fig 2.7 FFT Analysis for stator current 2.4 HARWARE IMPLEMENTATION This hardware consists of the following sections: 1. POWER UPPLY 2. MICROCONTROLLER 3. RIVER CIRCUIT 36

7 2.4.1 POWER UPPLY Figure 2.8 Power circuit A step- down transformer (230/15)V is used to give input supply to the power circuit. The 15V AC input is rectified into 15V pulsating C with the help of full bridge rectifier circuit. The ripples in the pulsating C are removed and pure C is obtained by using a capacitor filter. The positive terminal of the capacitor is connected to the input pin of the 7812 regulator for voltage regulation. An output voltage of 12V is obtained from the output pin of 7805 which is fed as the supply to the micro controller. From the same output pin of the 7805, a LE is connected in series with the resistor to indicate that the power is ON AT89C2051 MICROCONTROLLER The AT89C2051 is a low voltage, high performance CMO 8-bit microcomputer with 2K bytes of flash, programmable and erasable read only memory. The device is manufactured using ATMEL high-density nonvolatile memory technology and is compatible with the industry standard MC-51 instruction set. By combining a versatile 8-bit CPU with flash on a monolithic chip, the Atmel AT89C2051 is a powerful microcomputer, which provides a highly flexible and cost effective solution to many embedded control applications. The AT89C2051 provides the standard following features: 2K bytes of flash, 128 bytes of RAM, 15 I/O lines, two 16-bit timers/counters, a five vector two level; interrupt architecture, a full duplex serial port, a precision analog comparator, on-chip oscillator and clock circuitry. In addition, the AT89C2051 is designed with static logic operation down to zero frequency and supports two software selectable powers saving down to zero frequency and supports two software selectable power saving modes. The idle mode stops the CPU while allowing 37

8 the RAM contents but freezes the oscillator disabling all other chip functions until their next hardware reset. 2.9 Block diagram of 8 bit microcontroller89c2051 Features Compatible with MC -51 products. 2K bytes of reprogrammable flash memory endurance: 1000 write/erase cycles. 2.7 to 6V operating range. Fully static operation: 0Hz to 24MHz. Two level program memory lock. 38

9 128χ8-bit internal RAM. 15 programmable I/O lines. Two 16-bit timers/counters. ix interrupt sources. Programmable serial UART channel. irect LE drive outputs. On-chip analog comparator. Low-power idle and power-down modes. Green (Pb/Halide-free) packaging option PIN CONFIGURATION Figure 2.10 Pin diagram of AT89C

10 OPERATING ECRIPTION The detailed description of AT89C51 includes Memory map and register. Timer/counter Other information Flash memory The AT89C51 has separate address paces for program and data memory. The program and data memory can be up to 64KB long. The lower 4K program memory can reside on-chip. The AT89C51 has 129 bytes of on-chip RAM plus number of special function registers. The lower 128 bytes can be accessed either by direct addressing or by indirect addressing. The lower 128 bytes of RAM can be divided into three segments as Register banks 0-3 Bit addressable area cratch pad area RIVER UNIT The IR2110 is a high voltage, high speed power MOFET driver with independent high and low side referenced output channels. It is fully operational to +500V or +600V and tolerant to negative transient voltage dv/dt immune. Logic inputs are compatible with standard CMO or LTTL output, down to 3.3 V logic. The output drivers feature a high pulse current buffer stage designed for minimum driver cross-conduction. Propagation delays are matched to simplify the use in high frequency applications. The floating channel can be used to drive an N-channel power MOFET or IGBT in the high side configuration which operates up to 500 to 600 volts. 40

11 Figure 2.11 Pin diagram of IR211 Applications: 1) Power supply regulator 2) igital logic inputs 3) Microprocessor inputs 2.5 EXPERIMENTAL REULT Laboratory model of hardware is fabricated and tested. The pulses required by the motor are generated using microcontroller. The hardware is shown in figure The hardware consists of power module and control module. riving pulses are shown in figure The line to line voltage of the motor is shown in figure 2.14.The notches in the voltage waveform are due to the emf induced in the motor. 41

12 Fig 2.12 Hardware implementation Fig 2.13 PWM pulses 42

13 Fig 2.14 Line voltage with motor load 2.6 CONCLUION VI fed three-phase induction motor drive is simulated, fabricated and tested. The hardware is reduced since single phase rectifier is used. The reliability is increased by using microcontroller as the on chip intelligent controller. The speed is varied by varying the driving frequency of the inverter. The experimental results closely agree with the simulation results. 43