13. DC to AC Converters

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1 13. DC to AC Converters Inverters Inverter is a device which converts DC voltages (or current) to AC voltages (or current).inverter converting voltage is called VOLTAGE SOURCE INVERTER (VSI), while inverter converting current is called CURRENT SOURCE INVERTER (CSI). The output of the inverter could be at any desired frequency, voltage or current. Power transistor such as BJT, MOSFET and IJBT, are widely used in low and medium power inverters. Thyristors or SCRs inverters are used for high power inverter. So there are many types of inverters available in industry today, and each of them is designed to suit particular application or to meet designed performance requirements. Generally, inverters are categorized into two mains types of voltage source inverters (VSI) and current source inverter (CSI). For VSI inverter, there are the following common types used in industry: 1- Parallel inverter. 2- Single phase inverter. -Single-phase half-bridge inverter. -Single-phase full-bridge inverter. 3-Three-phase inverters. 1

2 13.1 Parallel Inverter The Parallel Inverter and Parallel capacitor inverter: Transistor parallel inverter The simplest type of voltage source inverter used in low power applications and UPS system for computers is the parallel inverter shown in Figure Fig.13.1 Transistor parallel inverter circuit. The basic single- phase parallel inverter circuit consists of two transistors Tr 1 and Tr 2 that are alternatively turned ON and OFF. Assuming Tr 1 conducts and Tr 2 OFF, Tr 1 current flows through the primary winding (a-b) of the output transformer Tx and produces an induced voltage on the secondary winding. Hence, supplies a load with positive voltage + Vdc. When Tr 1 switched off (ib1 o) and Tr 2 is switched on by applying current ib 2 in its base, current flows through the primary winding (c-b) of the output transformer Tx and produce V dc at the secondary of Tx. Hence a square wave of amplitude V dc is generated. This square or rectangular wave can be approximated to sine wave by using additional filter. Waveforms of this inverter are shown in Fig.13.2 for pure resistive load. 2

3 Base Current Tr 1 i b1 t Base Current Tr 2 i b2 t Conducting Tr 1 Tr 2 Device Sequence Upper Lower Fig.13.2 Output voltage and current waveforms for the parallel transistor inverter Thyristor parallel inverter The basic single- phase parallel inverter circuit consists of two SCRs : SCR 1 and SCR 2, an inductor L, an output transformer, and commutating capacitor C as depicted in Fig The output voltage and current are V o and I o respectively. The function of L is to make the source current constant. During the working of this inverter, capacitor C comes in parallel with the load via the 3

4 transformer. So it is called a parallel capacitor inverter. The operation of this inverter can be explained in the following modes. Fig.13.3 Thyristor parallel capacitor inverter circuit. Mode I In this mode, SCR1 is conducting while SCR 2 is OFF. In this case current flows in the upper half of the primary winding (section a-b) of the output transformer. As a result an induced voltage on the secondary is produced and supplied to the load. At this time the voltage between a and c terminals, V ac, is equal two times the dc supply voltage.in other words total voltage across primary winding is 2 V dc. Now the capacitor C charges to a voltage of 2V dc with upper plate as positive. Mode II When SCR 2 is turned ON by applying a trigger pulse to its gate m at this time (t0), capacitor voltage 2Vdc appears as a reverse bias across SCR 1, it is therefore turned OFF. A current I o begins to flow through SCR 2 and lower half of primary winding. Now the capacitor has charged (upper plate as negative) from +2V dc to -2V dc at time tt 1. Load voltage also changes from V dc at t0 to V dc at tt 1. 4

5 Mode III When capacitor has charged to V dc, SCR 1 may be tuned ON at any time. When SCR 1 is triggered, capacitor voltage 2V dc applies a reverse bias across SCR 2, it is therefore turned OFF. After SCR 2 is OFF, capacitor starts discharging, and charged to the opposite direction, the upper plate as positive. These procedures (Modes) proceed and repeat. The inductor L in Fig.13.1 is used also to limit the commutating current during commutation process. In the absence of L, The capacitor will charged and discharged very rapidly during each of SCR s conduction. The result is that the SCR may have no time to turn off. The waveforms of the SCR and capacitor voltages and currents are shown in Fig.13.4 (a) and (b) respectively. (b) Fig.13.4 Waveforms of the SCR and capacitor (a) voltage, and (b) current for the parallel inverter. (b) 5

6 13.2 Harmonic Analysis of the Output Voltage Waveform: The output voltage waveform of the inverter is a square wave (fig.13.5) which is non-sinusoidal. Since this wave is periodic, its harmonic content can be found using Fourier series as follows: The Fourier coefficients are, The square wave is defined as, Fig.13.5 The dc component is: 0 6

7 When n is even (2, 4, 6 ) cosn 1, b n 0. When n is odd (3, 5, 7 ) cosn -1 b n The Fourier series is given by: + Hence the output (load voltage) can be represented by Fourier series as, or 0 + The fundamental component (n1) is, The R. M. S value of the fundamental component, (peak value of the first harmonic) Similarly, the peak amplitude of 3 rd harmonic n1 can be evaluated as, And the R.M.S value of the 3 rd harmonic is, 7

8 And so on, the R.M.S. values for the 5 th, 7 th,. odd harmonics can be evaluate. Hence, the harmonic spectrum of the square wave is as shown in Fig.13.6.It is clear from this figure that: Harmonic amplitudes decrease as n increases. No even harmonics. Nearest harmonic is the third, if the fundamental is 50 Hz, then the third harmonic is 150Hz and the fifth harmonic 250 Hz as shown in Fig Fig.13.6 Harmonic spectrum of the square wave. 8

9 Fig.13.7 The square wave and its odd harmonics. 9

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Lecture Note. DC-AC PWM Inverters. Prepared by Dr. Oday A Ahmed Website: https://odayahmeduot.wordpress.com

Lecture Note. DC-AC PWM Inverters. Prepared by Dr. Oday A Ahmed Website: https://odayahmeduot.wordpress.com Lecture Note 10 DC-AC PWM Inverters Prepared by Dr. Oday A Ahmed Website: https://odayahmeduot.wordpress.com Email: 30205@uotechnology.edu.iq Scan QR DC-AC PWM Inverters Inverters are AC converters used