Cycloconverter based induction motor drive; Braking and multi-quadrant operation of VSI drives; Variable frequency control from a current source;

1. BRAKING AND MULTI-QUADRANT OPERATION OF VSI
DRIVES; CYCLOCONVERTER BASED INDUCTION MOTOR DRIVE
VARIABLE FREQUENCY CONTROL FROM A CURRENT SOURCE
12/4/2017
1
Presented by:
Name Roll No. Enrollment No.
Nakum Dharemsh M. 46 150863109005
Nayakwade Ragini B. 47 150863109006
Parmar Ashish kumar 48 150863109007
(2160908 EPS II)
Laxmi Institute of Technology, Sarigam, Gujarat

2. BRAKING AND MULTI-QUADRANT OPERATION OF
VSI DRIVES
Multiquadrant Operation – Operation of Hoist in Four
Quadrant
* A motor operate in 2 modes – Motoring and braking
* Motoring - electrical energy to mechanical energy, support its
motion.
* (generator) braking – mech energy to electrical energy, oppose
the motion.
* Motor can provide motoring & braking for both forward & reverse
direction.
* Power developed by a motor is given by the product of speed &
torque.

4. Direction of motor & load torques and direction of speed are
marked by arrows.
* A hoist consists of a rope wound on a drum coupled to a motor
shaft. One end of a rope is tied to a cage which is used for
transporting material. Other end of the rope has a counter
weight.
* Weight of the counter weight chosen higher than the weight of
an empty case but lower than a fully loaded cage.
* Load torque TL2 in quadrants I & IV represent speed torque
charal of the loaded hoist. This torque is the diff. of torques
due to loaded hoist & counter weight.
* Load torque TL2 in quadrants II & III is the speed-torque
charal of an empty hoist. This torque is due to the diff in
torque of counter weight & empty hoist. This is –ve because
the counter weight is always higher than the empty cage.
*

5. The quadrant I operation – hoist requires the movement of the cage upward,
which corresponds to the +ve motor speed which is in CCW (counter
clockwise) direction. It will be obtained if motor produce +ve torque in CCW
direction equal to TL. Since developed power is +ve, this is forward motoring
operation.
* Quadrant IV operation is obtained when a loaded cage is lowered. Since the
weight of the loaded cage is > the counter weight. In order to limit the speed of
the cage within a safe value, motor must produce a +ve torque T = TL2 in anti
clockwise direction. Both power & speed are –ve, drive is in reverse braking.
* Quadrant II is obtained when an empty cage is moved up since a counter weight
is heavier than a empty cage, it is able to pull it up. In order to limit the speed
to safety value, motor must produce braking torque = TL2 in clockwise
direction. Since speed is +ve, developed power is, -ve. It is forward breaking
operation.
Quadrant III – empty cage is lowered since empty cage weight is < counter
weight motor produce a torque in clockwise direction. Since speed is –ve &
developed power is +ve, this is reverse motoring operation.

6. The circuit of a single phase input to single phase output
cycloconverter is shown inFigure 1.1. It is perhaps the simplest type of
cycloconverter and will be used in this thesis as the basis for the investigations
into the operation of the cycloconverter and fordeveloping techniques for
improving its performance.
It is classed as a 2-pulsecycloconverter because there are two phase
controlled pulses per mains cycle per output phase [10]. If more than one
output phase is needed, the single phase output circuit is just duplicated to
create the extra phases. If a neutral return is not required in a multiphase output
application, the transformer can be removed to simplify the circuit.
The Single Phase Input Cycloconverter

8.  The cycloconverter has four thyristers divided into a positive and
negative bank of two thrusters each. When positive current flows in
the load, the output voltage is controlled by phase control of the two
positive bank thyristors whilst the negative bank thrusters are kept
off and vice versa when negative current flows in the load.
Anidealised output waveform for a sinusoidal load current and a 45
degrees load phase angle is shown in Figure 1.2.
 It is important to keep the non conducting thyristor bank off at all
times, otherwise the mains could be shorted via the two thyristor
banks, resulting in waveform distortion and possible device failure
from the shorting current. A major control problem of the
cycloconverter is how to swap between banks in the shortest
possible time to avoid distortion whilst ensuring the two banks do
not conduct at the same time.

9. VARIABLE FREQUENCY DRIVE OR VFD
 It is interesting to know that the first A.C. drive (400 HP) based on thyratron
cycloconverter-fed WRIM was installed in 1932 by F.E. Alexanderson of
General Electric in the Logan Power Station of Pacific Gas and Electric
Company. From then industrial drives have evolved rapidly by dedicated
effort of many scientists and engineers all over the world resulting in
development of advanced drive technology such as Variable Frequency
Drive(VFD).VFD is a power electronics based device which converts a
basic fixed frequency, fixed voltage sine wave power (line power) to a
variable frequency, variable output voltage used to control speed of
induction motor(s). It regulates the speed of a three phase induction
motor by controlling the frequency and voltage of the power supplied to the
motor.

10. SINCE THE NUMBER OF POLE IS CONSTANT THE SPEED NS CAN BE VARIED BY
CONTINUOUSLY CHANGING FREQUENCY.

11. Working of Variable Frequency Drive
Any Variable Frequency Drive or VFD incorporates following three
stages for controlling a three phase induction motor.
Rectifier Stage
A full-wave power diode based solid-state rectifier converts three-
phase 50 Hz power from a standard 220, 440 or higher utility supply to either
fixed or adjustable DC voltage. The system may include transformers for high
voltage system.

12. Power electronic switches such as IGBT, GTO
or SCR switch the DC power from rectifier on and off to produce a
current or voltage waveform at the required new frequency. Presently
most of the voltage source inverters (VSI) use pulse width modulation
(PWM) because the current and voltage waveform at output in this
scheme is approximately a sine wave. Power Electronic switches such
as IGBT; GTO etc. switch DC voltage at high speed, producing a
series of short-width pulses of constant amplitude. Output voltage is
varied by varying the gain of the inverter. Output frequency is
adjusted by changing the number of pulses per half cycle or by
varying the period for each time cycle.
The resulting current in an induction motor simulates a sine wave of
the desired output frequency. The high speed switching action of a
PWM inverter results in less waveform distortion and hence decreases
harmonic losses.
Inverter Stage

13. CONTROL SYSTEM
 Its function is to control output voltage i.e. voltage vector of inverter being
fed to motor and maintain a constant ratio of voltage to frequency (V/Hz). It
consists of an electronic circuit which receives feedback information from
the driven motor and adjusts the output voltage or frequency to the desired
values. Control system may be based on SPWM (Sine Wave PWM),
SVPWM (Space Vector modulated PWM) or some soft computing based
algorithm.

14. APPLICATIONS OF VARIABLE FREQUENCY DRIVE
 They are mostly used in industries for large induction
motor (dealing with variable load) whose power rating
ranges from few kW to few MW.
 Variable Frequency Drive is used in traction system. In
India it is being used by Delhi Metro Rail Corporation.
 They are also used in modern lifts, escalators and
pumping systems.
 Nowadays they are being also used in energy efficient
refrigerators, AC’s and Outside-air Economizers.