Voltage fluctuation is a common phenomenon in every part of
the country. The industrial units running round the clock,
usually face the problem of low and high voltage. 90% of industrial
load is of motors. Electric motors draw considerably high
current at low & high voltage. This higher current affects
the electrical motors (particularly smaller capacity motors
upto 7.5 H.P.) in three ways.
1. Higher current produces higher
losses in electrical motors which causes premature failure
of winding.
2. These higher losses of electric
motors also increase the losses of cables, switches, transformers
and other associated equipments.
3. For smooth continuous operation
of motors, overload relays are usually set at 20% higher setting.
ADVERSE
EFFECTS OF SINGLE PHASING
(a) The line
current increases by 1.5 times.
(b) If the relay
setting is at 15-20% higher than actual operating
current then the relay will take 4-6 minutes
to trip.
(c) The motor
(particularly smaller capacity motors upto
7.5 H.P.) cannot withstand this high current
for such a long time and in most of the cases
it burns out before the relay trips.
(d) Heavy short
circuit current flows through the relays,
contactors, cables etc.
(e) Under this
condition when the relay, contactor trips,
they produce sparks and some times bursts..
(f) This further
damages the switchgear and in some of the
cases may cause fire also.
With
the installation of the stabilizer and maintaining 390/400
volts, the motor will operate smoothly drawing 15-20% lesser
current and correspondingly the relay setting can be reduced
by 15-20%. In case single phasing occurs, the relay will trip
in 40-50 seconds. The motor can withstand the high current
for this period and will be safe. Also, the relays, contactors,
switchgears, etc. Incorporated with the motor will be safe.
