CHAPTER 1 INTRODUCTION

1 CHAPTER 1 INTRODUCTION 1.1 GENERAL Induction motor drives with squirrel cage type machines have been the workhorse in industry for variable-speed applications in wide power range that covers from fractional power to multi-megawatts. These applications include pumps and fans, paper and textile mills, subway and locomotive propulsions, electric and hybrid vehicles, home appliances, heat pumps and air conditioners, rolling mills, wind generation, etc. Based on the construction of the rotor, induction motors are broadly classified in two categories: squirrel cage motors and slip ring motors. The stator construction is same in both motors. Induction motor speed control methods are pole changing, stator voltage control, supply frequency control, eddy-current coupling, Rotor resistance control and Slip power recovery (Dubey 2009). Almost 90% of induction motors are squirrel cage motors. This is because the squirrel cage motor has a simple and rugged construction. The rotor consists of a cylindrical laminated core with axially placed parallel slots for carrying the conductors. Each slot carries a copper, aluminum, or alloy bar. If the slots are semi-closed then these bars are inserted from the ends. These rotor bars are permanently short-circuited at both ends by means of the end rings. Variable voltage / Variable frequency (V/f) speed control of squirrel cage motor, the torque developed by the motor is directly proportional to the magnetic field produced by the stator. So, the voltage

2 applied to the stator is directly proportional to the product of stator flux and angular velocity. This makes the flux produced by the stator proportional to the ratio of applied voltage and frequency of supply. By varying the frequency, the speed of the motor can be varied. Therefore, by varying the voltage and frequency by the same ratio, flux and hence, the torque can be kept constant throughout the speed range. Stator Voltage (V) [Stator Flux ( )] X [Angular Velocity ( )] (1.1) V x 2 f (1.2) Therefore, V f V =k f (1.3) Where, k is the constant This makes the constant V/f as most common speed control method of an induction motor. An Insulated-Gate Bipolar Transistor (IGBT) combines the low conduction loss of a Bipolar Junction Transistor (BJT) with the switching speed of a power Metal Oxide semiconductor Field Effect Transistor technology (MOSFET), an optimal solid state switch. The IGBT technology offers a combination of these attributes (Rashid Muhammad 2003). The IGBT is, in fact, a spin off from power MOSFET and the structure of an IGBT closely resembles that of a power MOSFET. The IGBT has high input impedance and fast turn on speed like a MOSFET. IGBTs exhibits an on voltage and current density comparable to a bipolar transistor while switching much faster. IGBTs are replacing MOSFETs in high voltage applications where conduction losses must be kept low. In the switched mode Pulse Width Modulation (PWM) inverter, the IGBT can be operated in the hundreds of kilohertz range. Although turn on speeds are very fast, turn off of the IGBT is slower than a MOSFET. The IGBT exhibits a current fall time or tailing.

3 The tailing restricts the devices to operating at moderate frequencies (less than 50 khz) in traditional square waveform Pulse-Width Modulation, switching applications. At operating frequencies between 1 and 50 khz, IGBTs offer an attractive solution over the traditional bipolar transistors, MOSFETs and thyristors. Compared to thyristors, the IGBT is faster, has better dv/dt immunity and, above all, has better gate turn off capability. While some thyristors such as Gate Turn Off thyristors are capable of being turned off at the gate, substantial reverse gate current is required, whereas turning off an IGBT only requires that the gate capacitance be discharged. A thyristor has a slightly lower forward on voltage and high surge capability than an IGBT. MOSFETs are often used because of their simple gate drive requirements. Since the structure of both devices is so similar, the change to IGBTs can be made without having to redesign the gate drive circuit. IGBTs, like MOSFETs, are transconductance devices and can remain fully on by keeping the gate voltage above a certain threshold. Forward Voltage Drop (Volt) Figure 1.1 Reduced Forward Voltage Drop of IGBT Realized When Compared to a MOSFET with Similar Ratings

4 As shown in figure 1.1 using an IGBT in place of a power MOSFET dramatically reduces the forward voltage drop at current levels above 12 amps. By reducing the forward drop, the conduction loss of the device is decreased. The gradual rising slope of the MOSFET in figure 1.1 can be attributed to the relationship of V DS to R DS(on). The IGBT curve has an offset due to an internal forward biased p n junction and a fast rising slope typical of a minority carrier device. It is possible to replace the MOSFET with an IGBT and improve the efficiency and/or reduce the cost. The feature that limited the IGBT from serving a wide variety of applications was its relatively slow turn off speed when compared to a power MOSFET. While turn on is fairly rapid, initial IGBTs had current fall times of around three microseconds. The turn off time of an IGBT is slow because many minority carriers are stored in the N region. When the gate is initially brought below the threshold voltage, the n contains a very large concentration of electrons and there will be a significant injection into the P+ substrate and a corresponding hole injection into the N region. As the electron concentration in the N region decreases, the electron injection decreases, leaving the rest of the electrons to recombine. Therefore, the turn off of an IGBT has two phases: an injection phase where the collector current falls very quickly, and a recombination phase in which the collector current decrease more slowly. PWM technique is widely used in variable-speed motor drives, especially after fast switching the high power rating Insulated Gate Bipolar Transistor come up, which enable a higher switching frequency and thus, better performance in dynamic response and reduction in size, weight and acoustic noise of the system are achievable. However, as the Electromagnetic Compatibility (EMC) regulation becomes more stringent, EMI becomes a major concern for inverter driven motor drives, particularly when this kind of motor drives is used in electric drives. This is because the conducted and

5 radiated EMI noise may cause malfunction of other electronic equipment of the electric drives. Many research works show that the switching (dv/dt) voltage transient and (di/dt) current transient are the sources of EMI noise. Higher the switching (dv/dt) and (di/dt), higher is the EMI emission. Since, switched mode PWM inverter techniques can significantly reduce the switching (dv/dt) and (di/dt), it is conceivable that the EMI noise generated by a inverter could be reduced by switching sequence of the switched mode PWM inverter (Maxime moreau et al 2009). Much effort of earlier work on the EMC performance of power electronic systems has tended to concentrate on switched mode power supply and may not be directly applicable to PWM motor drives, which is more complicated in terms of its power stage construction, external connections to the motor and supply, control circuit, and operation modes. Some recent researchers works focusing on the mechanism of EMI noise generation and propagation have been published. However, these efforts concentrate only on the hard-switching inverter. Figure 1.2 Common-mode and Differential-mode Current Paths in a Typical PWM Drive

6 Electromagnetic Interference (EMI) is any undesirable electromagnetic emission or any electrical or electronic disturbance, manmade or natural, which causes an undesirable response, malfunctioning or degradation in the performance of electrical equipment. The conducted EMI noise in a PWM inverter can be viewed as consisting of two parts, viz. Differential Mode (DM) noise and Common Mode (CM) noise, which are illustrated in figure 1.2. The (dv/dt) at the midpoints of the three legs of the inverter is normally identified as CM noise source. The (dv/dt) caused by the switch turn on/turn off, coupled through the parasitic capacitance between the Insulated gate Bipolar Transistor collector and the module base-plate that is normally grounded through the heat-sink, generate CM noise current. The CM noise current flows into the ground and through the stray capacitance inside the motor to the motor frame and back to the source via the power mains. The CM noise current also flows into the ground and through the stray capacitance inside the power supply and back to the noise source. The (di/dt) in the dc bus is normally identified as DM noise source. This change of current is also caused by the switching operation of the PWM inverter. The DM noise current flows into power supply and back to the inverter. The DM current flows also through the motor phase windings, and through the stray capacitance inside the motor, and then back to the power mains via the dc bus and the rectifier. The stray capacitance between stator windings has been relatively small and negligible. EMI noise measured by a standard network, known as Line Impedance Stabilization Network (LISN), is used to provide standard load impedance to the noise source. The voltage across this load is measured as conducted noise emission of the device. Seen from the LISN, the whole system could be simplified as the equivalent noise source, noise path and load (LISN serves as the load).

7 Figure 1.3 Block Diagram of Simplified Noise Propagation Noise source is time-variant. Noise path is also time-variant and non-linear due to the switching operation. The noise measured at LISN is determined by the excitation of the noise source and the response of the noise transmission network (Muttaqi et al 2008) as shown in figure 1.3. In frequency domain, if the noise source is expressed by its transfer function as N(s), and the noise transmission network is expressed by its transfer function as Z(s), then the noise F(s) measured at LISN can be expressed in frequency domain as the product of N(s) and Z(s). F(s) = N(s) * Z (s) (1.4) If the magnitude of each item in the above formula is represented in db, then the following equation is evolved 20log F(s) 20log N(s) 20log Z(s) (1.5) Switched-mode DC-AC Inverters are used in ac motor drives and uninterruptible ac power supply, where the objective is to produce ac output whose magnitude and frequency can both be controlled. Since the input to switched-mode inverter is fixed dc voltage source, such inverters are referred to as voltage source inverters. So, Voltage Source Inverter (VSI) is used in very high power ac motor drives. It is classified into three categories Viz. 1) Switched mode PWM Inverter 2) Square-wave Inverter and 3) Single phase

8 Inverter with voltage reduction. The switched mode PWM Inverter is inherent by a noise source that makes abrupt voltage transitions (high dv/dt) accompanied by switching actions. Coupled with the stray capacitance of the load machine the high frequency current is generated, which can affect the operation of nearby equipment due to the conducted and radiated EMI. An increase in (dv/dt) increases the EMI level. The EMI is transmitted in two forms Viz.1) Radiated noise and 2) Conducted noise (Tihanyi 1995) and (Williams 1996). This EMI noise is filtered by new Active Common mode Electromagnetic Interference Filter (ACEF). In conventional passive filter, both inductance and capacitance are varied, so the system will not be in stable condition. Proposed filter is a new ACEF which makes the system stable, and filter out the EMI noise. The simulated results have been compared with the prototype hardware results for validation. 1.2 PROBLEM IDENTIFICATION Squirrel-cage induction motor has a number of advantages when compared with dc motor. Some of the advantages: ruggedness, lower maintenance requirement, better reliability, lower cost, weight, volume, inertia, higher efficiency and the ability to operate in dirty and explosive environments. Because of their advantages, induction motors are more widely used in applications requiring constant speed. In industries, wide range of speed control is achieved by switched mode PWM inverter fed squirrel-cage motor drive. In conventional methods, speed control is not achieved precisely. Also EMI noise and harmonics problems are occurred. In conventional methods, speed control does not involve solid state devices like IGBT, MOSFET and Silicon Control Rectifier (SCR) etc. IGBT is voltage control device and also low switching losses. In conventional method harmonics and

9 EMI noise is reduced by filter connected across the load. But due to sudden variation in load will cause damage to the load side filter. In conventional EMI noise reduction technique, passive or active filter is connected with load. In these types of filters, both the inductor and capacitor are variable parameters. So the system will not remain in stable condition. Conducted EMI noise measurement, EMI filter connected in between phase to phase is called as a common mode noise measurement and also EMI filter connected in between phase to ground is called as a differential mode noise measurement. The radiated EMI noise is mitigated by metal cabinet. In switched mode PWM inverter, inherent EMI noise is generated during (switch turn ON-OFF period) high (dv/dt) transient period. This conducted EMI noise is mitigated using Active Common mode EMI filter connected in front of the source. It is possible to construct an active EMI filter independent of the source voltage of the equipment. Thus, ACEF can be used in any application regardless of working voltage. 1.3 OBJECTIVE OF THE THESIS The main objectives of this research are to develop and design ACEF to reduce the conducted EMI noise as well as harmonics in switched mode PWM inverter fed induction motor drive. The main proposed ACEF for quasi-square wave mode PWM inverter fed drive can be outlined as follows: i. Development of a new switching sequence control by single Pulse width modulation Technique which is used to reduce the lower order harmonics. ii. Development of a new quasi-square wave mode PWM inverter for wide range of speed control applications.

10 iii. iv. Development of a new ACEF connected on the source side for mitigation of inherent noise present in the quasi-square wave mode PWM inverter. It is used in any application regardless of working voltage. In ACEF, the inductance value is varied and the capacitance value is kept constant to reduce the Conducted EMI noise. v. The simulation and experiment results have been compared for the proposed ACEF. It is found that there is a close agreement between them. vi. Development of ACEF with various output filters configurations to study the effects on the proposed system. 1.4 OUTLINE OF THE THESIS Introduction to the thesis in Chapter 1, has addressed the commonmode and differential-mode current paths in a typical PWM drive and the of path EMI noise propagation. A brief introduction about the EMI noise generated in switched mode PWM inverter and two transmission form of EMI noise. Further, the squirrel cage induction motor along with speed control methods and its applications is outlined. A valid comparison is made among the power devices such as IGBT, MOSFET and Thyristors. Due to low conduction loss and high switching speed, IGBT is predominantly used in inverters. This thesis is composed of seven chapters. The overall organization of the rest of the chapters is as follows: Chapter 2 outlines the Literature review about the recent research trends of the Active Common mode Electromagnetic interference Filter for switched mode PWM inverter fed AC drive. Electromagnetic interference noise generation, suppression techniques and EMI filter design are presented.

11 The conduction modes of switched mode PWM inverter fed drive topology with its equivalent circuits are reviewed in Chapter 3. The concept of PWM technique and the various PWM techniques to obtain output voltage control and to reduce its harmonic content are discussed. The effects of PWM switching frequency and effects related to switch Turn-On and Turn-Off transient period of switched mode PWM inverter is presented. Chapter 4, describes the EMI caused by both Capacitive and inductive coupling currents, and Conducted EMI measurement setup deals with methods of suppression of conducted EMI noise using active EMI filters and Noise measurements. Chapter 5 deals with proposed active common mode EMI filter analysis for switched mode PWM inverter fed three phase induction motor. The various configurations adopted for the proposed active common mode filter has been presented. It also discusses the main points of the work completed with the results to justify the direction taken in the research. The conclusion of the main body of the work, including the applicability, use and effectiveness of the active common mode EMI noise mitigation for switched mode PWM inverter-fed AC drive is outlined. Chapter 6 deals with proposed ACEF with various configurations of output filters. The effects of these configurations have been verified by simulation and validated by hardware implementation and in Chapter 7 the conclusion of this research is presented and the scope for future work and directions are discussed. 1.5 SUMMARY The need for the mitigation of electromagnetic interference in the PWM inverter driven motor drives has been discussed. The significant roles

12 of switched mode PWM Inverter techniques over the conventional switching techniques were emphasized for the PWM drives. The conducted EMI noise in a PWM inverter with common mode and differential mode current paths were illustrated. The concept of simplified noise propagation relating the EMI noise was summarized. The problem identification and its objective of the research work are described.