INVERTERS Dc to ac converters Change a dc input voltage to a symetric ac output voltage of desired magnitude and frequency (fixed or variable

Variable output voltage can be obtained by varying input dc voltage and maintaining the gain of the inverter const.If the input dc voltage is fixed gain of the inverter is varied output voltage is varied Inverter gain is defined as ratio of the ac output voltage to dc input voltage Output voltages of ideal inverters are sinusoidal Output voltages of practical inverters are non sinusoidal and contain certain harmonics For low and medium power applications,square wave or quasi square wave voltages may be acceptable For high power applications low distorted sinusoidal waveforms

Applications Variable speed ac motor drives Induction heating Standby power supplies Ups Typical power outputs of inverters 1.120 v at 60hz 2.220v at 50hz 3.115v at 400hz for three phase 1.220v to 380v at 50hz 2.120v to 208v at 60hz ,115v to 200v at 400hz

Inverters can be broadly classified into two types 1. single phase inverters 2.three phase inverters These inverters generally use PWM control signals For producing an ac output voltage VFI input voltage remains constant CFI - input current remains constant Resonent pulse inverter if the output or current of the inverter is forced to pass through zero by creating an LC resonent circuit

Single-phase half-bridge inverter

Operational DetailsConsists of 2 choppers, 3-wire DC sourceTransistors switched on and off alternatelyNeed to isolate the gate signal for Q1 (upper device)Each provides opposite polarity of Vs/2 across the load

3-wire DC source

Q1 on, Q2 off, vo = Vs/2Peak Reverse Voltage of Q2 = Vs

Q1 off, Q2 on, vo = -Vs/2

Waveforms with resistive load

Look at the output voltagerms value of the output voltage, Vo

Fourier Series of the instantaneous output voltage

rms value of the fundamental component

When the load is highly inductive

Turn off Q1 at t = To/2Current falls to 0 via D2, L, Vs/2 lower+Vs/2-+Vs/2-

Turn off Q2 at t = ToCurrent falls to 0 via D1, L, Vs/2 upper+Vs/2-+Vs/2-

Load Current for a highly inductive loadTransistors are only switched on for a quarter-cycle, or 90

Fourier Series of the output current for an RL load

Fundamental Output PowerIn most cases, the useful power

DC Supply CurrentIf the inverter is lossless, average power absorbed by the load equals the average power supplied by the dc source.

For an inductive load, the current is approximately sinusoidal and the fundamental component of the output voltage supplies the power to the load. Also, the dc supply voltage remains essentially at Vs.

DC Supply Current (continued)

Performance ParametersHarmonic factor of the nth harmonic (HFn)for n>1Von = rms value of the nth harmonic componentV01 = rms value of the fundamental component

Performance Parameters (continued)Total Harmonic Distortion (THD)Measures the closeness in shape between a waveform and its fundamental component

Performance Parameters (continued)Distortion Factor (DF)Indicates the amount of HD that remains in a particular waveform after the harmonics have been subjected to second-order attenuation.

for n>1

Performance Parameters (continued)Lowest order harmonic (LOH)The harmonic component whose frequency is closest to the fundamental, and its amplitude is greater than or equal to 3% of the amplitude of the fundamental component.

Single-phase full-bridge inverter

Operational DetailsConsists of 4 choppers and a 3-wire DC sourceQ1-Q2 and Q3-Q4 switched on and off alternatelyNeed to isolate the gate signal for Q1 and Q3 (upper)Each pair provide opposite polarity of Vs across the load

Q1-Q2 on, Q3-Q4 off, vo = Vs + Vs -

Q3-Q4 on, Q1-Q2 off, vo = -Vs - Vs +

When the load is highly inductiveTurn Q1-Q2 off Q3-Q4 off

Turn Q3-Q4 off Q1-Q2 off

Load current for a highly inductive load

Example 6.3 MultiSim7

Example 6.3 using the scope

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