13-11-2012, 03:55 PM
Effects of unbalance voltage on induction motor current and its operation performance
Effects of unbalance.pdf (Size: 168.88 KB / Downloads: 58)
Power system unbalance is a common phenomenon in electrical grids especially in case
of 11KV systems. Reasons for unbalance are
• Incomplete transposition of transmission lines
• Open delta transformer connections
• Single phase loads
• Blown fuses on capacitor banks
• Railway traction loads
Induction motor being the most popular one in the industry, it is very important to carry
out studies about the effects of power quality in the efficiency and reliability of induction
motors. The paper tries to study the effect of voltage unbalance on the motor 3-phase
currents. It further studies heating in the motor windings and necessary derating that is
required. It also studies the effect of voltage unbalance on the motor torque, efficiency.
Methodology of analysis
• To study the voltage unbalance we resolve the motor unbalance 3-phase voltages
in to positive sequence voltages and negative sequence voltages by the method of
symmetrical components.
• We apply this positive sequence and negative sequence voltages to respective
induction motor equivalent circuits.
• We arrive at positive sequence current and negative sequence current.
• Then we arrive at 3 currents and current unbalance factor.
• We study the effect of voltage unbalance on reduction in motor torque and its
additional heating, and reduced efficiency.
Positive and Negative sequence equivalent circuits
If the voltages applied to the terminals of a three phase induction motor are unbalanced,
the motor performance will be modified. The most direct approach to the problem is by
the analysis of the set of unbalanced voltages into two symmetrical sets of balanced
voltages of positive and negative phase-sequence respectively. Each set produces
corresponding balanced currents, and the synthesis of the two sets of current vectors
represents the actual currents produced in the three stator phases by the original
unbalanced voltages. The behaviour of the machine to the positive -sequence voltage is
essentially the same as for normal balanced operation
Derating of the induction machine:
In order to prevent the motor overheating due to unbalanced voltages the motor has to be
operated below its rated output power which is called derating of the induction motor. As
per NEMA guidelines, operating a motor for any length of time at voltage unbalance
above 5% is not recommended. Any amount of unbalance makes a motor run hotter. The
NEMA standard says that once unbalance reaches 5%, the temperature begins to rise so
fast that protection from damage becomes impractical. The simplest protection as
proposed by the NEMA standard, is to derate the motor-to reduce its output horsepower
load so it can tolerate the extra heating imposed by the unbalanced supply. one of the way
to develop a derating curve is as below.
Conclusion:
• Even a small voltage unbalance will result in large current unbalance during
the running of motor by a factor of 6 times.
• Negative phase sequence components will lead to heating of motor
• Negative phase sequence currents leads to reduction in motor output torque.
• Motor is forced to run at higher slip leading to increased rotor loss and
reduced efficiency.
• Electricity boards should look in to this phenomenon seriously where
irrigation pump sets suffer the voltage imbalance, even 1% loss of efficiency
for the country like India would mean a great loss. 5 HP irrigation pump sets
with a quantity of 6 Lakh approximate, the loss of power would be 22MW and
annual wastage of Rs.47.5 millions (At 3 Rs/unit, 4 Hrs of Pump working and
6 month’s season)