Simulation of Pwm Single to Three Phase Ac Converter

International Journal of Science, Engineering and Technology Research (IJSETR)

Volume 1, Issue 1, July 2012



SIMULATION OF PWM SINGLE TO THREE PHASE AC CONVERTER

Win Sandar

Abstract— Single to three phase converters are used to drive three phase machines (normally induction motors) from single phase supplies. In the proposed scheme, motor controller (MC3PHAC) is used to produce PWM signals. A 3-phase, 380V, 2HP Induction motor is used as load for testing the developed hardware. Pulse width modulation is widely used in power electronics to digitize the power so that a sequence of voltage pulses can be generated by the on and off of the power switches. The pulse width modulation inverter has been the main choice in power electronic for decades, because of its circuit simplicity and rugged control scheme PWM switching technique is commonly used in industrial applications PWM 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. Pulse width modulation or PWM is the mostly used method in motor control and inverter application., thus making the output filter smaller, cheaper and easier to implement. Conventionally, to generate this signal, triangle wave as a carrier signal is compared with the sinusoidal wave, whose frequency is the desired frequency. In this paper three-phase inverters and their operating principles are analyzed in detail. Finally the simulation results for a three-phase inverter using the PWM strategies described are presented .This project deals with implementing the basic theory of a Pulse Width Modulation Inverter (PWM) technique for Bipolar voltage switching , its Simulink modeling, estimating various designing parameters. The project will be commenced by a basic understanding of the circuitry of the PWM Inverter, the components used in its design and the reason for choosing such components in this circuitry. After this, it will be attempted to simulate a model circuit on any simulating software e.g. MATLAB and analyze the output waveforms and also varying the modulation index (M) for both Simulink Model and Implemented Design of Bipolar voltage switching

Index Terms—PWM,AC/DC/AC Converter , Simulation

Results .

I.INTRODUCTION

Power electronic converters use active semiconductors (e.g. MOSFET) and passive power semiconductors (e.g. diodes) and passive elements (e.g. inductors and capacitors) arranged in circuit structures to convert power from the form available from a source to that required by a load. The power source may be a DC source, a single–phase AC source, or a three-phase AC source with line frequency of 50, 60 or 400

Hz. It may also be an electric battery, a solar panel, an electric generator or a commercial power supply. The source feeds the input of the power converter, which converts the input power to the required form for a load. The load may be DC or AC, single–phase or three–phase, and may or may not need transformer isolation from the power source. The power converter, therefore, can be an AC/DC converter, a DC/DC converter, a DC/AC inverter or an AC/AC converter depending on the application. The main focus of the research

in this thesis has been on three-phase AC-DC power converters. These are converters that convert a three-phase input voltage into an isolated DC output voltage. The three phase AC voltage is typically obtained from the utility mains AC-DC power converters connected to the mainsvoltage can generate and inject current harmonics into theutility mains. This type of converter is required in those areas where the three phase electric power is not present. Most of these areas include domesticareas because most of the domestic use appliances arepowered by single phase electric power. One of thereasons of not giving three phase electric power to thedomestic customers is its high cost of connectioninstallation. But still, if any customer wants to run a threephase appliance on a single phase electric power, thesingle phase to three phase converter can be used. Thereason of using three phase power is that three phaseappliances are available in high ratings as compared toappliances available in single phase. AC drives, inverters and adjustable frequency drives all terms that are used to control the speed of AC motor. AC drives receive AC power and convert it to an adjustable frequency, adjustable voltage output for controlling motor operation. The three common inverter types are Current source inverter (CSI), Voltage source inverter(VSI), Pulse width modulation inverter(PWM).

The PWM single to three phase converter is shown in Figure .1.

Figure 1.Block diagram of single phase to three phase converter

II. Proposed Three Phase Converter

The converter proposed in this project convertsthe single phase electric power to three phase electricpower in two steps:

1)Power circuit: Conversion of single phase A.Cvoltage to D.C voltage i.e. from 220V, 50Hz single phase A.C supply to 309 V D.C supply.

2) Control circuit: Conversion of D.C voltage tothree phase A.C voltage i.e. from D.C 309 V to 380V,50Hz three phase A.C supply by using motorcontroller.

Both of these steps are described in detail below.

(a)Power circuit

A rectifier is an electrical device that converts current (AC), which periodically reverses direction, to current (DC), which is in only one direction, a process known as rectification. Rectifiers have many uses including as components of power supplies and as detector signals. Rectifiers may be made of solid state tube diodes, mercury arc valves, and other components. When only one diode is used to rectify AC (by blocking the negative or positive portion of the waveform), the difference between the term diode and the term rectifier usage, i.e, the term rectifier is

describes a diode used to convert AC to DC. The full bridge rectifier circuit is shown in Fig. 2.

Figure.2 Rectifier

The smoothing capacitor is used to remove theripples so that we can get a smooth D.C voltage. Thevalue of this capacitor can be found out by Eq. 1.

(1)

where C = Capacity of filtering capacitor

Po = Output power of the rectifier

Th = Hold-up time

V1 = Input D.C voltage i.e.

Input A.C voltage (rms) x √2

V2 = Input D.C voltage which can hold output voltage

Ƞ= Efficiency

These 309V D.C produced will be given to the

Switching devices. The switching devices the heart of three phase inverter. It takes a D.C voltage and uses six switches

arranged in three legs for switching purpose. The switching devices used is show n in fig. 3

Figure 3.Switching devices

In fig. 3, S1-S6 are six switching devices and A, B, C are the three phase output terminals that will be connected to the motor terminals. There are two types of switching devices that could be used IGBT or MOSFET. In this project IGBTs are used instead of MOSFETs . This selection is made on the conditions for which these two devices are to be used. Mostly the decision is made keeping two factors in mind 1) Switching frequency 2) Voltage bearing capability of devices .

(b) Control circuit

The control circuit is that part of the converter in which electrical components are used to control the switching of the six MOSFETs to generate three phase 380V, 50Hz voltage with exact phase shift of 120o. To see the control circuit at a glance, block diagram of control circuit is shown in fig. 1.The method used to generate control signals throughmicrocontroller is pulse width modulation (PWM) as shown in fig. 4.

Figure.4 Pulse width modulation with switching devices

III.PULSE WIDTH MODULATION IN INVERTER

Output voltage from an inverter can also be adjusted by exercising a control within the inverter itself. The most efficient method of doing this is by pulse-width modulation control used within an inverter. In this method, a fixed dc input voltage is given to the inverter and a controlled ac output voltage is obtained by adjusting the on and off periods of the inverter components. This is the most popular method of controlling the output voltage and this method is termed as Pulse-Width Modulation (PWM) Control. The advantages possessed by PWM techniques are as under:

(i) The output voltage control with this method can be obtained without any additional components.

(ii) With the method, lower order harmonics can be eliminated or minimized along with its output voltage control. As higher order harmonics can be filtered easily, the filtering requirements are minimized.

The main disadvantage of this method is that SCRs are expensive as they must possess low turn-on and turn-off times. PWM inverters are quite popular in industrial applications. PWM techniques are characterized by constant amplitude pulses. The width of these pulses is however modulated to obtain inverter output voltage control and to reduce its harmonic content. The different PWM techniques are as under:

(a) Single-pulse modulation

(b) Multiple pulse modulation

Here we studied about Carrier based Pulse Width Modulation for open loop control of three-phase induction motor drive.

A.The Carrier-Based Pulse Width Modulation (PWM) Technique

As mentioned earlier, it is desired that the ac output voltage vo = vaN follow a given waveform (e.g., sinusoidal) on a continuous basis by properly switching the power valves. The carrier-based PWM technique fulfils such a requirement as it defines the on and off states of the switches of one leg of a VSI by comparing a modulating signal vc (desired ac output voltage) and a triangular waveform vΔ (carrier signal). In practice, when vc vΔ the switch S+ is on and the switch S- is off; similarly, when vc vΔ the switch S+ is off and the switch S- is on.

A special case is when the modulating signal vc is a sinusoidal at frequency fc and amplitude v^c, and the triangular signal vΔ is at frequency fΔ and amplitude v^Δ. This is the sinusoidal PWM (SPWM) scheme. In this case, the modulation index ma (also known as the amplitude-modulation ratio) is defined as

(2)

and the normalized carrier frequency mf (also known as the frequency-modulation ratio) is

(3)

Figure.5 (a) shows the amplitude of the fundamental component of the ac output voltage v^01 satisfying the following expression:

(4)

(b) for odd values of the normalized carrier frequency mf the harmonics in the ac output voltage appear at normalized frequencies fh centered around mf and its multiples, specifically,

l=1, 2,3,… (5)

Where k = 2, 4, 6….for l =1, 3, 5….. ; and k =1, 3, 5 ….for

l =2, 4, 6…. ;

(c) The amplitude of the ac output voltage harmonics is a function of the modulation index ma and is independent of the normalized carrier frequency mf for mf > 9;

(d) The harmonics in the dc link current (due to the modulation) appear at normalized frequencies fp centered around the normalized carrier frequency mf and its multiples, specifically,

l=1,2… (6)

Where k = 2, 4, 6….for l =1, 3, 5….. ; and k =1, 3, 5 ….for

l =2, 4, 6….;

Additional important issues are:

(a) For small values of mf (mf < 21), the carrier signal vΔ and the modulating signal vc should be synchronized to each other (mf integer), which is required to hold the previous features; if this is not the case, sub harmonics will be present in the ac output voltage;

(b) For large values of mf (mf > 21), the sub harmonics are negligible if an asynchronous PWMtechnique is used, however, due to potential very low-order sub harmonics, its use should be avoided; finally

(c) In the over modulation region (ma > 1) some intersections between the carrier and the modulating signal are missed, which leads to the generation of low-order harmonics but a higher fundamental ac output voltage is obtained; unfortunately, the linearity between ma and v^01 achieved in the linear region Eq. 4. does not hold in the over modulation region, moreover, a saturation effect can be observed Fig. 6

Figure . 5: The half-bridge VSI. Ideal waveforms for the SPWM

(a) carrier and modulation signals:(b) ) switch S+ state;(c) switch S- state;(d) ac output voltage

Figure .6Voltage control by varying ma

B.SPWM for Full Bridge VSI

Fig. 7: The full-bridge VSI. Ideal waveforms for SPWM

(a)carrier and modulating signals; (b) switch S1+ state; (c) switch S2+ state; (d) ac output voltage

SPWM for Three Phase VSI

This is an extension of the one introduced for single-phase VSIs. In this case and in order to produce 120⁰ out-of-phase load voltages, three modulating signals that are 120⁰ out of phase [17] are used. Fig. 8 shows the ideal waveforms of three-phase VSI SPWM. In order to use a single carrier signal and preserve the features of the PWM technique, the normalized carrier frequency mf should be an odd multiple of 3. Thus, all phase voltages (vaN , vbN , and vcN ) are identical but 120⁰ out of phase without even harmonics; moreover, harmonics at frequencies a multiple of 3 are identical in amplitude and phase in all phases.For odd multiple of 3 values of the normalized carrier frequency mf , the harmonics in the ac output voltage appear at normalized frequencies fh centered around mf and its multiples, specified .

l=1,2… (7)

Where l =1, 3, 5…..….for k = 2, 4, 6 ; and l =2, 4, 6….for k =1, 5, 7 ….; such that h is not a multiple of 3. Therefore, the harmonics will be at mf ± 2, mf ± 4 . . . 2mf ± 1, 2mf ±5 . . ., 3mf ± 2, 3mf ± 4. . ., 4mf ±1, 4mf ± 5 . . .

For nearly sinusoidal ac load current, the harmonics in the dc link current are at frequencies given by

l=1,2,… (8)

Where l = 0, 2, 4…. for k=1, 5, 7….and l =1, 3, 5…. for k = 2, 4, 6 …. such that h = l * mf ± k is positive and not a multiple of 3. For instance, Fig. 7h shows the sixth harmonic (h = 6), which is due to h = (1 * 9) - 2 – 1 = 6. The identical conclusions can be drawn for the operation at small and large values of mf as for the single-phase configurations. However, because the maximum amplitude of the fundamental phase voltage in the linear region (ma <=1) is vi/2 , the maximum amplitude of the fundamental ac output line voltage is v^ab1 = (√3vi)/2.

Therefore, one can write, 0<ma≤1(9)

Fig. 8: The three-phase VSI. Ideal waveforms for the SPWM

(a) carrier and modulating signals; (b) switch S1 state; (c)switch S3 state; (d) ac output voltage

IV. MALAB SIMULATION MODEL

We design PWM single to three phase converter using MALAB and Simulink model for 2hp.We can see simulation result and the switching pulse. Gate pulses to drive for switching devices are shown in figure .8.

Simulink Model and PWM Waveforms

Figure (9) simulink model for PWM single to three phase converter with

Load

Figure (10) Simulimnk model for PWM single to three phase converter

without load

Figure.9 Gating pulses for switching devices

V. SIMULATION RESULTS

Figure.11 wavesforms of single to three phase converter with load

Figure .12Wavesforms of single to three phase converter without load

The output voltage depends on the modulation index . It can calculate with equation (9).

Table.1 Inverter output voltage for different modulation index

Vdc(V) / ma / Vab-inv(V)
309 / 0.2 / 53.52
309 / 0.4 / 107.04
309 / 0.6 / 160.56
309 / 0.8 / 214.08
309 / 1 / 267.6

VI.CONCLUSION

The single phase to three phase voltage converter presented in this research by which three phase motor is driven is made up of six MOSFETs and PWM technique is used for its switching purpose. If the required output voltage isexactlydesired ,the step-up transformer or DC converter are used to increase the dc supply to the switching devices. In power electronic systems, simulations are mainly performed to analyze and design the circuit configuration and applied control strategy.

References

[1] L.R. Sinclair and J. Dunton: Practical Electronics Handbook, 6th ed. New York: Newnes,

[2] Wilson, T.G. “The evolution of power electronics”- Power Electronics, IEEE Transactions on, Volume 15, Issue 3, May 2000, pp. 439-446

[3] Bose, B.K. “Power electronics-a technology review” - Proceedings of the IEEE, Volume 80, Issue 8, Aug. 1992, pp. 1303

[4] Rashid, H.M. “Power electronics- Circuits, Devices, And Applications” –Prentice-Hall, New Jersey, 1993.

[5]Mohan, N., Undeland, T., Robbins, W., “Power electronics- Converters, Applications, and Design” – John Wiley & Sons, Inc. 1995.

[6] Hart, D.W., “Introduction to Power Electronics” – Prentice Hall, New Jersey

[7] Jim Doucet,Dan Eggleston,Jeremy Shaw, “DC/AC PureSine Wave Inverter” MQP Terms ABC 20062007.

Manuscript received Oct 15, 2011.

Win Sandar,Department of Electrical Power Engineering , Mandalay Technological University, (e- mail :).Man-dalay,Myanmar,Phone/Mobile No 09402648912