INVERTERS REFERENCE 1. Power Electronics-(CH-8) M.S. Jamil Asghar 2. Power Electronics- Circuits, Devices & Applications (CH-10) (2nd edition) M.H. Rashid

General concept APPLICATIONS Un-interruptible power supply (UPS) Converts DC to AC power by switching the DC input voltage (or current)in a pre-determined sequence so as to generate AC voltage (or current) output. APPLICATIONS Un-interruptible power supply (UPS) Industrial (induction motor) drives Traction HVDC

Types of inverter BLOCK DIAGRAM Voltage Source Inverter (VSI)-a) Square wave b) PWM Current Source Inverter (CSI)

What are the various types of Inverters What are the various types of Inverters? Inverters can be broadly classified into two types. They are Voltage Source Inverter (VSI) Current Source Inverter (CSI) When the DC voltage remains constant, then it is called voltage inverter(VSI) or voltage fed inverter (VFI). When input current is maintained constant, then it is called current source inverter (CSI) or current fed inverter (CFI). Some times, the DC input voltage to the inverter is controlled to adjust the output. Such inverters are called variable DC link inverters. The inverters can have single phase or three-phase output. A voltage source inverter is fed by a stiff dc voltage, whereas a current source inverter is fed by a stiff current source. A voltage source can be converted to a current source by connecting a series inductance and then varying the voltage to obtain the desired current. A VSI can also be operated in current-controlled mode, and similarly a CSI can also be operated in the voltage control mode. The inverters are used in variable frequency ac motor drives, uninterrupted power supplies, induction heating, static VAR compensators, etc.

Voltage source inverter (VSI) with variable DC link DC link voltage is varied by a DC-to DC converter or controlled rectifier. Generate “square wave” output voltage. Output voltage amplitude is varied as DC link is varied. Frequency of output voltage is varied by changing the frequency of the square wave pulses The DC-link quantity is then impressed by an energy storage element that is common to both stages, which is a capacitor C for the voltage DC-link or an inductor L for the current DC-link.

Variable DC link inverter (2) Advantages 1.Simple waveform generation 2.Reliable Disadvantages 1.Extra conversion stage 2.Poor harmonics

VSI with fixed DC link DC voltage is held constant. Output voltage amplitude and frequency are varied simultaneously using PWM technique. Good harmonic control, but at the expense of complex waveform generation

Operation of simple square-wave inverter To illustrate the concept of AC waveform generation

Operation of simple square-wave inverter(2)

Single Phase Half-bridge inverter Also known as the “inverter leg”. Basic building block for full bridge, three phase and higher order inverters. G is the “centre point”. Both capacitors have the same value. Thus the DC link is equally “spilt”into two.

Single Phase Half-bridge inverter (2) The top and bottom switch has to be “complementary”, i.e. If the top switch is closed (on), the bottom must be off, and vice-versa. •In practical, a dead time as shown below is required to avoid “shoot-through” faults.

Single Phase Half-bridge inverter with R Load

Single Phase Half-bridge inverter with RL Load Mode 1: Mode 2:

Single Phase Half-bridge inverter with RL Load Mode-3: Mode-4:

Single Phase Half-bridge inverter with RL Load

Single Phase Half-bridge inverter with RL Load

Single Phase Half-bridge inverter with RC Load

Single-phase, full-bridge Inverter (1) Full bridge (single phase) is built from two half-bridge leg. The switching in the second leg is “delayed by 180 degrees” from the first leg.

Single-phase, full-bridge Inverter (2)

Single-phase, full-bridge Inverter (2)

Performance Parameter

Performance Parameter

Performance of square-wave inverter Study of harmonics requires understanding of wave shapes. Fourier Series is a tool to analysis wave shapes.

Performance of square-wave inverter

Spectra of Square Wave

Spectra of Square Wave

Drawbacks of Square Wave For fixed voltage the output of the inverter cannot be controlled. The output voltage contains appreciable harmonics, THD is also very high. Pulse Width Modulation Technique A better square wave notching is known as PWM technique. Both amplitude and frequency can be controlled independently. Very flexible Modulation Techniques are: 1.SPWM 2.UPWM 3.SinPWM

Single Pulse Width Modulation (SPWM)

Uniform /Multiple In a uniform PWM inverter, the output voltage has several pulses of equal width in each half-cycle. No of output pulses in each half-cycle, N =fc/2fref =mf/2

Sinusoidal Pulse Width Modulation Bipolar Switching Scheme : 1. VrefVc, S1 and S2 are on and Vo=V 2. VrefVc, S3 and S4 are one and Vo=-V

Sinusoidal Pulse Width Modulation Unipolar Switching Scheme : 1. VrefVc, S1 is on and vref  Vc, S4 is on 2. -VrefVc, S3 is on and –Vref  Vc, S2 is on

Unipolar Switching Scheme :