03-05-2014, 03:13 PM
Effects of Voltage Unbalance on Induction Motors
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ABSTRACT
This paper investigates the effects of different voltage magnitude
unbalances with the same voltage unbalance factor 0 on the
stator loss and rotor copper loss and total copper loss of three
classes of three-phase induction motor. Under six different
voltage magnitude unbalance conditions, the copper losses of
three different three-phase induction motor under full load
conditions has been analyzed through simulation. Following the
investigations, the importance of the positivesequence voltage
and the negative-sequence voltage have been pointed out. In the
present work, the relative percentage increase in the stator
copper loss, rotor copper loss and total copper losses have been
computed for the motor working under unbalanced and balanced
voltages. For this analysis MATLAB environment using Slmnlink
toolbox has been used. In this paper the three definitions (Line
voltage unbalance in percent-LWR, Phase voltage unbalance in
percent-PWR and Voltage unbalance factor-VUF) of voltage
magnitude unbalance have been considered.
INTRODUCTION
Due to the incomplete transposition of transmission lines,
unbalanced loads, open delta transformer connections, blown
kses on three-phase capacitor hanks and so on, lead to power
system unbalance putting worries for the power companies.
Even unproportional unbalance in the line currents are caused
by a small unbalance in the line voltages. .
Because of various techno-economic benefits the induction
motors are widely used in industrial, commercial and
residential systems. Once the voltage is unbalanced, the ill
effects on induction motors will cause enormous impacts.
CALCULATION OF VOLTAGE UNBALANCE
FACTOR
Table I gives the tabulation of three phase voltages and their
positive and negative sequence voltage components in six
different cases of voltage magnitude unbalances with VUF
equal to 4%, 8% and 12%. The table also gives the values of
LVUR and PVUR for each of the unbalance cases. PVUR and
LVUR are not able to suggest the further action to he taken in
case of voltage unbalance situation whereas the VUF is
calculated using positive and negative sequence components.
Hence by comparing the balanced voltages with the positive
sequence component, the situation of voltage unbalance can be
estimated.
CONCLUSIONS
Following conclusions can he drawn based on present work.
Not only the negative sequence hut positive sequence
voltage also has to he considered for analyzing the effects
of voltage unbalance on induction motor operating
performance.
In case of equal VUF, the status of the voltage unbalance
can be estimated by the positive-sequence component.
With the same VUF, a higher positive-sequence voltage
leads to a lower copper losses.
With the same VUF, each voltage unbalance case would
lead to a different degree of copper losses of an
induction motor. Under-voltage unbalance cases cause the
more copper losses.