04-07-2012, 03:38 PM
INVERTERS
INVERTERS.ppt (Size: 301 KB / Downloads: 53)
INTRODUCTION
An inverter is an electrical device that converts direct current (DC) to alternating current (AC); the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits.
Solid-state inverters have no moving parts and are used in a wide range of applications, from small switching power supplies in computers, to large electric utility high-voltage direct current applications that transport bulk power
Inverters are commonly used to supply AC power from DC sources such as solar panels or batteries.
The inverter performs the opposite function of a rectifier.
The conversion can be achieved either by controlled turn-on and turn-off devices (e.g: BJT’s, MOSFET’s, etc..
For low and medium power outputs, the above mentioned power devices are suitable but for high power outputs, thyristors should be used.
CLASSIFICATION OF INVERTERS
Classification According to the Nature of Input Source:
i) Voltage Source Inverters (VSI).
ii) Current Source Inverters (CSI).
Incase of VSI, the input to the inverter is provided by a ripple free DC voltage source, where as CSI, the voltage source is first converted into a current source and then used to supply the power to the Inverter.
(b) Classification According to the Wave shape of the output voltage:
i) Square-wave inverter
ii) Quasi-Square wave Inverter
iii) Pulse-width modulated (PWM) Inverters
A square wave Inverter produces a square wave AC voltage of a constant magnitude. The output voltage of this type of Inverter can only be varied by controlling the input DC voltage. Square wave AC output voltage of an Inverter is adequate for low and medium power applications.
The second method, pulse-width modulation (PWM) uses a switching scheme within the inverter to modify the shape of the output voltage waveform.
THYRISTOR INVERTER CLASSIFICATION:
The Thyristors Inverters can be classified in the following categories:
i) According to the method of commutation.
ii) According to the connections.
(a) Classification According to the Method of Commutation:
According to the method of commutation, the SCR inverters can mainly be categorized in two types, viz.
i) Line commutated inverters
ii) Forced commutated inverters.
Line Communication Inverters : In case of AC circuit, AC line voltage is available across the devices. When the current in the SCR goes through a natural zero, the device is turn-off.
Forced Commutated Inverters : In case of DC circuits, since the supply voltage does not go through the zero point, some external source is required to commutate the device.
These Inverters are further classified as :
a) Auxiliary Commutated Inverters
b) Complimentary Commutated Inverters
b) Classifications According to the Connections:
According to the connections of the Thyristors and commutating components, the Inverters can be classified mainly three groups.
1) Series Inverters
2) Parallel Inverters
3) Bridge Inverters: Bridge Inverters are further classified as
i) Half Bridge
ii) Full Bridge
ADVANTAGES
Pulse Width Modulated (PWM):
i) A wider speed range is possible.
ii) Can be used with more than one motor.
iii) Input power factor is good at all frequencies.
iv) Diode input stage allow ride through on input power interruption.
2. Current Source Inverter:
i) Capable of regeneration back to the AC line because DC link polarity can be reversed.
ii) Large DC link filter Inductor and regulated power supply act as a current limiter, making it easier to apply protective fuses.
iii) Ability to ride through power link interruption.
iv) The converter grade thyristors are sufficient.
v) Current Source Inverter does not generate radio frequency interface.
APPLICATIONS
Variable speed AC motor drives.
Induction heating.
Aircraft power supplies.
Uninterruptible power supplies (PUS).
High voltage DC transmission lines.
Battery-vehicle drives.
Regulated voltage and frequency power supplies, etc.