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MODULE CONTENTS
•
Introduction
•
Basic concepts of switch mode inverters
•
Pulse width modulating scheme
•
Square wave switching scheme
•
Single-phase inverters
•
Three-phase inverters
UNIT 5: DC-AC converters
Introduction
• To produce an ac output waveform from DC power supply
• Mainly applied in ac adjustable speed drives and ups
•
Frequency and magnitude of sinusoidal voltage can be controlled
•
Two categories (according to dc supply) – Voltage source inverters (VSI)
and Current source inverters (CSI)
•
VSIs are the most widely used as they behave as voltage sources
•
Only VSIs are considered
UNIT 5: DC-AC converters
Introduction
• Consider the basic electrical power conversion topology
• The voltage is desired to be sinusoidal with adjustable magnitude and
frequency
• That is achieved by means of switch mode inverter
UNIT 5: DC-AC converters
VSIs
•
VSIs are classified into three categories:• Pulse-width modulated inverters (PWM)
• Square-wave inverters (SW)
• Single-phase inverters with voltage cancellation (SP)
•
PWM – dc input voltage is constant in magnitude.
•
Used where adjustable ac output voltage magnitude and frequency are required.
•
SW – dc input voltage is controlled in order to control ac output voltage.
•
Used to control the frequency
•
SP – combines the characteristics of both PWM and SW.
•
Used where single phase output is required.
UNIT 5: DC-AC converters
Basic concepts
•
Pulse-width modulated switching scheme:
•
Discussed previously -> control signal generated from comparison, duty
ratio controlled to control average dc
•
In inverters, PWM is more complex. Sinusoid compared to triangular
waveform
•
Control signal used to modulate switch duty ratio
•
Has a frequency of f1 – desired fundamental frequency of inverter (also
called modulating frequency)
UNIT 5: DC-AC converters
Basic concepts
•
Pulse-width modulated switching scheme
UNIT 5: DC-AC converters
Basic concepts
•
Square-wave switching scheme
•
Each switch of the inverter leg is on for one half-cycle of desired output
frequency
•
The results of an output voltage waveform are presented below.
•
The peak values of the fundamental-frequency and harmonic components
can be obtained as:
UNIT 5: DC-AC converters
Basic concepts
•
Square-wave switching is a special case of sinusoidal PWM – when ma
becomes so large that the control voltage waveform intersects with the
triangular waveform
•
Each inverter switch changes its state only twice per cycle, which is
important at very high power levels.
•
However, it is not capable of regulating the output voltage magnitude.
•
Therefore, Vd must be adjusted in order to control the output voltage
magnitude of the inverter.
UNIT 5: DC-AC converters
Single-phase Inverter Output
•
Ripple in repetitive waveform refers to difference between instantaneous
values of waveform and its fundamental frequency component
•
Only fundamental-frequency component responsible for power transfer
•
Apply superposition to separate ripple components
•
Can calculate the current ripple
UNIT 5: DC-AC converters
Three-phase Inverters
•
Most frequently used three-phase inverter topology uses three legs – one
for each phase
UNIT 5: DC-AC converters
Three-phase Inverters - PWM
•
Three different regions of
operational control applicable to
PWM:
• Linear – output varies linearly
with ma
• Overmodulation – peak of
control voltages allowed to
exceed peak of the triangular
waveform
• Square wave
•
Modulation amplitude ratio is varied
as a function of the supply
UNIT 5: DC-AC converters
DC-AC Converters – Exercises
1. Explain – with the aid of suitable graph – the concept of PWM
overmodulation
2. Figure below gives a single-phase full bridge inverter, operating in a squarewave mode. The dc-voltage is 544V and the output voltage frequency
supplying the motor is 57 Hz. Inductance L = 150 mH. Solve for the peak
value of the ripple current in the output
UNIT 4: DC-DC converters