Page 348 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

This method of switching provides a constant bit delay and fixed data rate channel between the sender and
receiver. The full channel capacity is dedicated for the duration of a connection. When data is to be transferred
from sender to receiver, firstly the sender sends a request to the switching station for the establishment of a
connection. The receiver replies with an acknowledgment. After receiving the acknowledgment signal the sender
starts the data transmission. This switching is commonly used for voice circuits. The public switched telephone
network, Datakit, B channel of ISDN, Optical mesh network, etc are some of the examples of circuit-switched
networks.
Circuit Switching Diagram
In this type of switching, there is a set of switches connected with physical links. Here once the dedicated path is
established between the sender and receiver, it stays the same until one of the users terminates the connection.
Fixed data is transmitted and this type of switching is highly used to transfer voice data. The network consists of
the switching offices with permanent links between them. Whenever is connection requested the communication
links are dedicated to the terminals forming the transmission route. This dedicated link is maintained until the
connection is terminated. Other users will be able to use this link only when it is terminated by the sender or
receiver.
There are three phases in the establishment of a circuit switching network. They are – circuit establishment, Data
transfer and circuit disconnect.

Pulse Width Modulation (PWM) Techniques

Introduction

A common control method in power electronics for managing the output voltage of converters, particularly DC/
AC inverters, is pulse width modulation (PWM). The basic concept behind PWM is to adjust the output pulse
width in order to regulate the average output voltage. With PWM, a fixed DC input voltage source can produce a
sinusoidal output waveform with variable frequency and amplitude.
PWM methodologies in inverters provide fine control over the output voltage waveform in VSIs, enabling accurate
voltage regulation as well as current regulation. This is vital for numerous applications where precise voltage
control is necessary for top performance, including motor drives, renewable energy systems, and uninterruptible
power supplies (UPS).
With the usage of PWM, it is also possible to control the output waveform’s harmonic distortions which ultimately
leads to improved power quality and lowering system losses. In contrast to the fundamental square-wave
modulation techniques, PWM in inverters offers advantages in terms of improved control over output voltage,
frequency, and harmonics.

The common PWM methods, as well as their impacts on inverter performance, harmonic content, and distortion,
are covered in single-phase inverters and three-phase inverters in the section below.
Variable Voltage Variable Frequency (VVVF) Lift Drive

Variable Voltage Variable Frequency (VVVF) Lift Drive employs frequency inverter technology which regulates input
voltage and frequency throughout the journey, drawing much less current during acceleration and deceleration.
Operating characteristics of different motor drives

When compared with other modes of drives, one can see that the VVVF drive offers low starting current
(approximately 1.8x of the rated current), high power factor (i.e. better efficiency in power supply) with good
ride quality and floor leveling. It can reduce motor starting currents by as much as 50 to 80% compared with
conventional motor drives. Further, wear and tear of the equipment can also be less during start/stop of the motor
by using VVVF motor drive.

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CITS : E & H - Electronics Mechanic - Lesson 223 - 236