INVERTERS a Inverters are also known as DC

INVERTERS a) Inverters are also known as DC to AC converters. b) The purpose of an inverter is to convert DC supply into AC supply, so that an AC load can work properly. d) An inverter is also necessary to step-up a low DC voltage to much higher AC voltage, through a step-up transformer. e) Output voltage waveforms of ideal inverters should be sinusoidal. But practically they are non-sinusoidal, squarewave, quasi-square-wave, modified sinewave and low distorted sinewave. f) Inverters are commonly used in battery operated power systems, solar electricity installation, UPS and AC drives. g) Inverters are also need to change frequency for motor drives.

Single-Phase Half-Bridge Inverter Can be in 2 types : a) Single voltage source with 2 series capacitors in parallel. Disadvantage of this circuit is the needs of 2 capacitors.

b) Two voltage sources in series. Disadvantage of this circuit is the needs of 2 voltage sources.

Let’s consider the (a) circuit; with 2 series capacitors. The rms output voltage can be found from, Rms output with resistive load

Let’s consider the (a) circuit; with 2 series capacitors. Mode of operations with R load : Mode 1 : S 1 conducts, S 2 Off Waveforms with resistive load

Let’s consider the (a) circuit; with 2 series capacitors. Mode of operations with R load : Mode 2 : S 2 conducts, S 1 Off Waveforms with resistive load

Let’s consider the (a) circuit; with 2 series capacitors. Mode of operations with R load : Mode 1 : S 1 conducts, S 2 Off Waveforms with resistive load

Let’s consider the (a) circuit; with 2 series capacitors. Mode of operations with R load : Mode 2 : S 2 conducts, S 1 Off Waveforms with resistive load

Let’s consider the (a) circuit; with 2 series capacitors. Mode of operations with R load : Mode 1 : S 1 conducts, S 2 Off Waveforms with resistive load

Let’s consider the same circuit with inductive load. RL Mode 1 : S 1 conducts, S 2 Off Io will increase gradually (builds up) to maximum. At t=T/2 Mode 1

Let’s consider the same circuit with inductive load. RL Mode 2 : S 1 Off, S 2 On, D 2 conducts The voltage across the D 2 keeps the Q 2 reverse biased thus keep it OFF even been triggered. Io will decrease gradually to zero, then only S 2 starts to conducts. Mode 2

Let’s consider the same circuit with inductive load. RL Mode 3 : S 1 Off, S 2 conducts Io will increase gradually (builds up on different side) to maximum. at t=T Mode 3

Let’s consider the same circuit with inductive load. RL Mode 4 : S 1 On, S 2 Off, D 1 conducts The voltage across the D 1 keeps the Q 1 reverse biased thus keep it OFF even been triggered. Io will decrease gradually to zero, then only S 1 starts to conducts. Mode 4

For an inductive load, the current cannot change immediately with the output voltage. When Q 1 is turned off, the load current would continue flow through D 2, load and the lower capacitor. Similarly, when Q 2 is turned off, the load current flows through D 1, load and the upper capacitor.

The instantaneous output voltage can be expressed in Fourier series as,

For an RL load, the instantaneous load current can be found as,

Performance Parameters of Inverters The output of practical inverters contain harmonics and the quality of an inverter is normally evaluated in terms of the following performance parameters.

Example 1: • Single phase half-bridge inverter has a resistive load of R=4. 5 and the DC input voltage is Vs = 48 V. Determine : a) b) c) d) e) f) RMS output voltage, Vo. RMS output voltage at the fundamental frequency, V 1. Output power Po. Peak current of each transistor, Ip. Average current of each transistor, Iavg. Total harmonic distortion, THD.

Single-Phase Bridge Inverter Half bridge, Vout=Vs/2 Full bridge, Vout=Vs Load current with highly inductive load

Single-Phase Bridge Inverter Half bridge, Vout=Vs/2 Full bridge, Vout=Vs Mode 1 Load current with highly inductive load Mode 1

Single-Phase Bridge Inverter Half bridge, Vout=Vs/2 Full bridge, Vout=Vs Mode 2 Load current with highly inductive load Mode 2

Single-Phase Bridge Inverter Half bridge, Vout=Vs/2 Full bridge, Vout=Vs Mode 3 Load current with highly inductive load Mode 3

Single-Phase Bridge Inverter Half bridge, Vout=Vs/2 Full bridge, Vout=Vs Mode 4 Load current with highly inductive load Mode 4

Operation Quadrants on the Vo – Io Plane for Single-Phase Inverter If Vo is filtered out to be sinusoidal and Io is the current to an inductive load, then during switching period the polarities of current and voltage vary over all 4 quadrants. Powerflows from converter into the load in 1 st & 3 rd quadrants. Power flows out from the load into the converter in 2 nd & 4 th quadrants. Inverse connected diodes (free-wheeling diodes) Are connected to allow bidirectional current flow.

Example 2: a) Draw a single phase bridge inverter with Vs = 48 V, R = 4. 8 b) Find its: a) RMS output voltage, Vo. b) RMS output voltage at the fundamental frequency, V 1. c) Output power Po. d) Peak current of each transistor, Ip. e) Average current of each transistor, Iavg. f) Total harmonic distortion, THD.

Three-Phase Inverter • Normally used for high-power applications. • The gating signals of a single-phase inverters should have advance or delay by with respect to each other to obtain balanced 3 -ph. A 3 -ph output can be obtained from a configuration of 6 transistors and 6 diodes.

1. Circuit for 3 -phase Inverter