DC-AC Converter (Inverter)

Demonstrative Video


Aim:

To study the operation of DC - AC converter (Inverter) using R-L load. To observe the output waveforms (Simulation) using MATLAB Simulink.

Simulation tools required:

MATLAB

Theory :

A power inverter, or inverter, is an electronic device or circuitry that changes direct current (DC) to alternating current (AC). The input voltage, output voltage and frequency, and overall power handling depend on the design of the specific device or circuitry. The inverter does not produce any power; the power is provided by the DC source. The input to the inverter is a direct dc source or dc source derived from an AC source. For example, the primary source of input power may be utility AC voltage supply that is converted to DC by a rectifier with a filter capacitor and then ‘inverted’ back to AC using an inverter. Here, the final AC output may be of a different frequency and magnitude than the input AC of the utility supply. A voltage source is called stiff, if the source voltage magnitude does not depend on the load connected to it. All voltage source inverters assume a stiff voltage supply at the input. If the input is a voltage source, then the inverter is called a Voltage Source Inverter (VSI). Similarly, in a Current Source Inverter (CSI), the input to the circuit is a current source. The VSI circuit has direct control over ‘the output (AC) voltage’ whereas a CSI directly controls ‘the output (AC) current’. Figure.1 shows the power circuit diagram for single-phase bridge voltage source inverter. In this four switches (in 2 legs) are used to generate the AC waveform at the output. Semiconductor switches like IGBT, MOSFET or BJT are used. Four switches are sufficient for a resistive load as the load current io is in phase with the output voltage vo. However, in the case of RL loads where io is not in phase with vo, diodes are connected in anti-parallel with the switches which allow the conduction of the current in the reverse direction even when the switch is turned OFF. These diodes are called Feedback Diodes since the energy is fed back to the (DC) source.


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A typical power inverter requires a relatively stable DC power source capable of supplying enough current for the intended power demands of the system. The input voltage depends on the design and purpose of the inverter.

I. Square Wave Modulation: for R = 12.5 ohms, L = 6 mH, DC voltage source of 100 V


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Pulse generators showing square wave pulses


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Observation table:

S. no Input Voltage Output voltage Fundamental (RMS) Output voltage THD Output Voltage (RMS) At 150Hz Output Current Fundamental (RMS) Output Current (RMS) At 150 Hz Output Current THD
1 (Square Wave Mode) 100 V
2 (Unipolar PWM) MI=1.0 100 V
3 (Unipolar PWM) MI=0.6 100 V
4 (Unipolar PWM) MI=0.2 100 V

II. Unipolar PWM: for R = 12.5 ohms, L = 6mH, DC voltage source of 100V


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Block parameters for generating sine PWM


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Results:

I. Square Wave Modulation:

  1. Attach the Matlab circuit Diagram of Square wave modulation
  2. Attach the waveforms of a) Output Voltage, b) output current
  3. Attach the FFT plot of output Voltage and output current

II. Bi-polar PWM

  1. Attach the Matlab circuit Diagram of Unipolar wave modulation
  2. Attach the waveforms of a) Output Voltage ,b) output current ( M.I = 0.2)
  3. Attach the FFT plot of output Voltage and output current ( M.I = 0.2)