16-01-2013, 04:28 PM
ON LINE POWER FACTOR IMPROVEMENT OF INDUCTION MOTORS
ON LINE POWER.pdf (Size: 376.65 KB / Downloads: 33)
Abstract
This paper describes the
design and implementation of a simple scheme
to achieve on line and dynamic power factor
improvement of single and three phase induction
motors subjected to variable loads. The
pro osed scheme employ-s an FC-TCR type of
static compensator along with associated
control circuits. The results show that,
under all loading conditions, a power factor
of almost unity is achieved automatically.
Furthermore, the waveform of supply current
can be kept almost sinusoidal. The response
time of the proposed scheme to motor load
changes is quite reasonable. The paper
provides details regarding design, implementation,
analysis and operation of such dynamic
power factor compensators.
Introduction
Induction motors are commonly used in
domestic, industrial and commercial applications.
The power factor of such motors is
usually low and depends upon the load and the
motor parameters. If the motor is coupled to
a fixed load, its power factor can be improved
by a single fixed capacitor connected
across the motor terminals. For variable
load drives, the power factor may be improved
by using switched capacitors. Such a scheme,
however, only provides capacitance variation
in discrete steps and therefore optimum power
factor improvement is not always obtained.
Compensator Design
For an induction motor coupled to a
variable load (or any other variable load
which is to be compensated), the first step
in the design rocedure is to determine the
load VAR requirements. Since such requirements
can vary for variable loads, one has to
determine the variation of the motor reactive
voltamperes (Q,) over the entire range of
operation considering the possible overload
conditions also. Using such data, let us
define Qmax and Qmin as the maximum and the
minimum per phase reactive power of the load.
Obviously for an inductive load, one has to
provide capacitive compensation. For dynamic
loads. the amount of such capacitive compensation
will vary but will be in the range of
Qmin < Qc < Qmax. This necessitates the provision
of a variable capacitance for optimum
power factor compensation. Such a smooth
variation of capacitance required for dynamic
power factor correction can be obtained 'by
using a static compensator employing a fixed
capacitor in parallel with a thyristor
con+rolled reactor [I].