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MIT International Journal of Electronics and Communication Engineering, Vol. 3, No. 2, August 2013, pp. 83–86 ISSN No. 2230-7672 ©MIT Publications 83 Advanced Techniques for Controlling Output Voltage of Inverter Amit Kumar Sharma M.Tech SITM, Lucknow, UP, INDIA E-mail: [email protected] Vivek Mishra Asst. Prof ECE Deptt. TMU, Moradabad, UP, INDIA E-mail: [email protected] Amit Sharma Neeraj Kaushik M.Tech IIMT, Meerut, UP, INDIA E-mail: [email protected] Manas Singhal M.Tech IIMT, Meerut, UP, INDIA E-mail: [email protected] ME Scholar NITTTR, Chandigarh, INDIA E-mail: [email protected] ABSTRACT The full-bridge pulse-width-modulation (PWM) single-phase inverter is widely used in uninterruptable power supplies (UPS), wind and solar power dc-ac interfacing, stand-alone voltage regulators in distributed power systems, and many other applications. In many applications it is required that the output of the inverter must be varied as demanded by the load. The main goal of this report is to study the various methods for controlling the output voltage of the inverter. Though the output voltage can be controlled externally to the inverter either from DC side or from AC side but these methods needs external component for controlling the inverter output. 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. This method is called the internal voltage control of the inverter. This paper mainly gives the brief of –Single Pulse Modulation, Multiple Pulse Modulation, Sinusoidal Pulse Width Modulation (SPWM) or Carrier based Pulse Width Modulation Technique., Modified sinusoidal pulse width modulation. Keywords: PWM, Modulation index, Undermodulation, SPWM, pulsating. I. INTRODUCTION to control the output voltage of inverter in various applications. This paper gives the brief of techniques used to control the DC-AC inverters are electronic devices used to produce AC output voltage of inverter. power from low voltage DC energy (from a battery or solar panel). This makes them very suitable for when required to use II. OUTPUT VOLTAGE CONTROL AC power tools or appliances but the usual AC mains power is TECHNIQUES not available. It is the first and foremost requirement that the output of three phase inverter should be purely sinusoidal[1]. The various methods for the control of output voltage of However the waveforms of the practical inverter are non- inverters can be classified as: sinusoidal and contain certain harmonics. These harmonics are (a) External control of ac output voltage generated by the semiconductor devices used in implementing (b) External control of dc input voltage three phase inverter is in the generation of harmonics. The (c) Internal control of the inverter. power electronics devices when used in practical circuits like converter/controller circuits generate harmonics in the output The first two methods require the use of peripheral voltage supply. The harmonics generate undesirable effects in components whereas the third method requires no external power supply circuits like generation of humming noises[2], components. Mostly the internal control of the inverters is de-rating of the machines and torque pulsations which make dealt, and so the third method of control is discussed in great the operation of the machine pulsating. This is also important detail in the following section. MIT International Journal of Electronics and Communication Engineering, Vol. 3, No. 2, August 2013, pp. 83–86 ISSN No. 2230-7672 ©MIT Publications 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. This method is called the internal voltage control of the inverter. signal of amplitude Vc with triangular carrier signal Vcar. The frequency of the control signal determines the fundamental frequency of ac output voltage. The amplitude modulation index is defined as: (1) 2.1 External Control of ac Output Voltage In this type of control as shown in Figure 2.1 an ac voltage controller is used to control the output of inverter . Through the firing angle control of ac voltage controller the voltage input to the ac load is regulated. 84 The rms value of ac output voltage (2) where (3) Figure 2.1: External control of ac output voltage By varying the control signal amplitude Vc from 0 to Vcar the pulse width ton can be modified from 0 sec to T/2 sec and the rms value of output voltage Vo from 0 to Vs. 2.2 External Control of dc Input Voltage When the available voltage souecr is ac then the dc voltage input to the inverter can be controlled through fully controlled rectifier, uncontrolled rectifier and chopper,ac voltage controller and uncontrolled rectifier as showh in Figure 2.2(a), (b) and (c). Figure 2.3: Gating signals of Single pulse-width modulated inverter 2.5 Sinusoidal Pulse Width Modulation (SPWM) Figure 2.2: External control of DC input voltage 2.3 Internal Control of Inverter Inverter output voltage can also be adjusted by exercising a control with in the inverter itself.Pulse width modulation is the most commonly used technique to control the output voltage of inverter, the various techniques are: 2.4 Single-Pulse-Width-Modulation In single pulse width modulation control, there is only one pulse per half cycle and the output rms voltage is changed by varying the width of the pulse. The gating signals of single pulse-width modulation are shown in Figure 2.3. The gating signals are generated by comparing the rectangular control In sinusoidal pulse width modulation there are multiple pulses per half-cycle and the width of the each pulse is varied with respect to the sine wave magnitude. Figure 2.4 shows the gating signals and output voltage of SPWM with unipolar switching. In this scheme, the switches in the two legs of the full-bridge inverter are not switched simultaneously, as in the bi-polar scheme. In this unipolar scheme the legs A and B of the full-bridge inverter are controlled separately by comparing carrier triangular wave vcar with control sinusoidal signal vc and -vc respectively. This SPWM is generally used in industrial applications. The number of pulses per half-cycle depends upon the ratio of the frequency of carrier signal (fc) to the modulating sinusoidal signal. The frequency of control signal or the modulating signal sets the inverter output frequency (fo) and the peak magnitude of control signal controls the modulation index ma which in turn controls the rms value of output voltage. The area of each pulse corresponds approximately to the area under the sine wave between the adjacent midpoints of off MIT International Journal of Electronics and Communication Engineering, Vol. 3, No. 2, August 2013, pp. 83–86 ISSN No. 2230-7672 ©MIT Publications 85 Fig 2.5(g) shows the output waveform of inverter. Observe that there are four pulses in each half cycle. Figure 2.4: Sinusoidal Pulse Width Modulation periods on the gating signals. If ton is the width of nth pulse, the rms value of output voltage can be determined by: (4) 2.6 Multiple Pulse Width Modulation Principle: Multiple pulses are used to reduced the harmonic contents and to control the output voltage. the width of all the pulses is same. In this modulation there are multiple number of output pulses per half cycle and all pulses are of equal width. The Figure 2.5: Waveforms of Multiple Pulse Width Modulation gating signals are generated by comparing a rectangular reference with a triangular reference. The frequency of the Thus by varying the width, output rms voltage can be reference signal sets the output frequency (fo) and carrier changed. frequency (f c). The number of pulses per half cycle is determined by p = fc/2f0 Advantages The rms ac output voltage 1. Distortion factor is reduced compared to single pulse V0 = vs(pα/π)1/2 (5) modulation. Where α = duty ratio = ton/T 2. As value of p increases amplitude of lower harmonics reduces. The variation of modulation index (MI) from 0 to 1 varies the pulse from 0 to π/p and the output voltage from 0 to Vs. Disadvantages 2.7 Waveforms and Explanation Figure shows the waveform of multiple pulse width modulation. Figure 2.5(a) shows the carrier signal and the reference signal. The signal is a triangular waveform and reference signal is the DC voltage. They are compared and the pulsed waveform of Figure 2.5(b) is obtained. The width of the pulses can be varied by changing the amplitude of the DC reference signal. Figure 2.5(c) and (d) shows two 50 Hz out of phase masking square wave. The pulsed waveform of Figure 2.5(b) is ANDed with these masking signals to obtain the base drivers. Figure 2.5(e) shows the base drive of T1 and T2. Similarly Figure 2.5(f) shows the base drives of T3 and T4 . 1. With increase in number of pulse switching losses are increased. 2. Control scheme is complex. 2.8 Modified Sinusoidal PWM (MSPWM) The widths of the pulses near peak of the sine wave do not change much when modulation index is changed. Hence carrier is suppressed at +30° and –30° in the neighborhood of peak of sine wave. Such scheme is shown in Figure 2.6. Observe that the triangular wave is present for the period of first 60° and last 60° of the half cycle of sine wave. The middle 60° of the sine wave do not have the triangular wave. Hence the generated PWM has less number of pulses. The rms value is more for the same modulation index. MIT International Journal of Electronics and Communication Engineering, Vol. 3, No. 2, August 2013, pp. 83–86 ISSN No. 2230-7672 ©MIT Publications 86 Output voltage from an inverter can be control 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. This method is called the internal voltage control of the inverter. Pulse width modulation is the most adaptable method which control the output voltage of the inverter by exercising the internal control of the inverter . the various PWM techniques include: 1. Single pulse modulation. 2. Multiple pulse width modulation. 3. Sinusoidal pulse width modulation. 4. Modified sinusoidal pulse width modulation. We also note that though there is various advantage Figure 2.6: Waveforms of Modified Sinusoidal PWM of PWM techniques, it also poses some of the limitations The harmonic contents are also reduced. This control the most important limitation is that by the implementation scheme also reduced switching losses. The implementation of of these techniques the switching loss in the inverter increases. this scheme is relatively complex then sine PWM. 2.9 Phase Displacement Control The two inverters are connected in parallel. Output of one inverter is phase shifted with respect to another inverter. The [1] net output depends upon phase shift between the two inverter output. III. CONCLUSION [2] The report presents output voltage control of inverter. There are various techniques to control the output of inverter whether they are three phase or single phase The various methods for [3] the control of output voltage of inverters can be classified as: (a) External control of ac output voltage [4] (b) External control of dc input voltage (c) Internal control of the inverter. The first two methods require the use of peripheral [5] components whereas the third method requires no external components. Mostly the internal control of the inverters is dealt, and so the third method of control is discussed in great detail in this report. REFERENCES M. Depenbrock, “Pulse width control of a three-phase inverter with non-sinusoidal phase voltage of a three-phase PWM inverter”, Proc. IEEE Int. semiconductor Power Conversion Conf., Orlando, Florida, USA, pp. 399-403, 1977. G. Dong, “Sensorless and efficiency optimized induction motor control with associated converter PWM schemes”, Ph.D. Thesis, Faculty of Graduate School, Tennessee Technological University, Dec. 2005. A.M. Hava, “Carrier based PWM-VSI drives in the overmodulation region”, Ph.D. Thesis, University of Wisconsin-Medison, 1998. A.M. Hava, “Kerkman R.J. and Lipo T.A”, A High Performance Generalized Discontinuous PWM Algorithm, IEEE Trans. on Industry Appl., Vol. 34, No. 5, Sept./Oct. 1998. S. Haykin, “Neural Networks”, ND Prentice Hall, New York, USA, Holmes D.G., 1996. The significance of zero space vector placement for carrier-based PWM schemes, IEEE Trans. Industry Appl.,Vol. 32, No. 5, pp. 1122-1129 Sept./ Oct. 2004.