[Sharma, 3(8): August, 2014] ISSN: 2277-9655 Scientific Journal Impact Factor: 3.449 (ISRA), Impact Factor: 1.852 http: // www.ijesrt.com(C)International Journal of Engineering Sciences & Research Technology [106] IJESRT INTERNATIONAL JOURNAL OF ENGINEERING SCIENCES & RESEARCH TECHNOLOGY Unipolar and Bipolar SPWM Voltage Modulation Type inverter for Improved Switching Frequencies Amit Kumar Sharma * , Ashok Kumar Sharma & Nidhi Vijay Department of Electrical Engineering, University College of Engineering, Kota, India amy.sharma21@gmail.com Abstract This paper attempts an in-depth analysis of switching loss, waveform quality and voltage linearity characteristics of the modern PWM methods. SPWM or sinusoidal pulse width modulation is widely used in power electronics to initialize the power so that a sequence of voltage pulses can be generated by the on and off of the power switches. SPWM techniques are characterized by constant amplitude pulses with different duty cycle for each period. The width of this pulses are modulated to obtain inverter output voltage control and to reduce its harmonic content. In SPWM a unipolar and bipolar SPWM voltage modulation type is selected because these types of methods offers the advantage of effectively doubling the switching frequency of the inverter voltage, thus making the output filter smaller, cheaper and easier to implement. Keywords: Modelling, Simulation, SPWM, Voltage profile Introduction Of all the modern power electronics converters, the Voltage Source Inverter (VSI) is perhaps the most widely utilized device with 3 power ratings ranging from fractions of a kilowatt to megawatt level. A voltage–fed inverter (VFI) or more generally a voltage–source inverter (VSI) is one in which the dc source has small or negligible impedance. The voltage at the input terminals is constant. A current–source inverter (CSI) is fed with adjustable current from the dc source of high impedance that is from a constant dc source [1], [2], [3]. The VSI consists of six power semiconductor switches with anti-parallel feedback diodes. It converts a fixed DC voltage to three phase AC voltages with controllable frequency and magnitude. Since the VSI has discrete circuit modes for each set of switch states, generating an output voltage with correct frequency and magnitude requires an averaging approach. In the widely utilized Pulse Width Modulation (PWM) methods, the inverter output voltage approximates the reference value through high frequency switching. In AC motor drive applications, typically a rectifier device converts the AC three phase line voltages to DC voltage. Following the rectifier voltage passive filtering stage, the PWM-VSI interfaces the DC source with the AC motor to control the shaft speed/position/torque. Some industrial applications of inverters are for adjustable-speed ac drives, induction heating, standby aircraft power supplies, UPS (uninterruptible power supplies) for computers, HVDC transmission lines, etc [1],[4]. When utilized in such applications, the device is often termed as converter (opposite of inverter), hence PWM-VSC. In all cases, power flow is controlled by the inverter switching device gate signals in a manner to obtain high performance, improved efficiency, and reliable operation. Although its main circuit topology is quite simple, a modern PWM-VSI drive involves an overwhelming level of technology and intelligence. From the semiconductor power switching devices such as Insulated Gate Bipolar Transistors (IGBTs) operating at frequencies as high as many tens of kilohertz to the microcontrollers and Digital Signal Processors (DSPs) that process the control signals at speeds beyond many tens of megahertz, most components of a state of the art PWM-VSI drive involve advanced technologies. The costumer's increasing demand for multifunctionality, precision performance, efficiency, and reliability and user friendliness has motivated engineers to build a significant amount of intelligence into the microcontrollers and DSPs of the PWM-VSI drives. Load parameter estimation, fault diagnostics, high performance vector control, observer based shaft encoder less speed control, energy efficiency optimization etc. algorithms have been developed and built into the modern PWM-VSI drives. With their