21. State the methods of improving power factor?
Methods of improving power factor:
• By connecting static capacitors in parallel with the load operating at
lagging power factor.
• A synchronous motor takes a leading current when over excited and therefore
behaves like a capacitor.
• By using phase advancers to improve the power factor of induction motors. It
provides exciting ampere turns to the rotor circuit of the motor. By providing
more ampere-turns than required, the induction motor can be made to operate on
leading power factor like an overexcited synchronous motor.
22. State the factors, for the choice of electrical system for an aero turbine.
The choice of electrical system for an aero turbine is guided by three factors:
• Type of electrical output: dc, variable- frequency ac, and constant-
• Aero turbine rotational speed: constant speed with variable blade pitch,
nearly constant speed with simpler pitch- changing mechanism or variable speed
with fixed pitch blades.
• Utilization of electrical energy output: in conjunction with battery or other
form of storage, or interconnection with power grid.
23. What are the advantages of VSCF wind electrical system?
Advantages of VSCF wind electrical system are:
• No complex pitch changing mechanism is needed.
• Aero turbine always operates at maximum efficiency point.
• Extra energy in the high wind speed region of the speed – duration curve can
• Significant reduction in aerodynamic stresses, which are associated with
constant – speed operation.
24. Explain the terms real power, apparent power and reactive power for ac
circuits and also the units used.
• Real Power: It is the product of voltage, current and power factor i.e. P = V
I cos j and basic unit of real power is watt. i.e. Expressed as W or kW.
• Apparent power: It is the product of voltage and current. Apparent power = V
I and basic unit of apparent power is volt- ampere. Expressed as VA or KVA.
• Reactive Power: It is the product of voltage, current and sine of angle
between the voltage and current i.e. Reactive power = voltage X current X sinj
or Reactive power = V I sin j and has no other unit but expressed in VAR or
25. Define the following: Average demand, Maximum demand, Demand factor, Load
• Average Demand: the average power requirement during some specified period of
time of considerable duration is called the average demand of installation.
• Maximum Demand: The maximum demand of an installation is defined as the
greatest of all the demand, which have occurred during a given period. It is
measured accordingly to specifications, over a prescribed time interval during
a certain period.
• Demand Factor: It is defined as the ratio of actual maximum demand made by
the load to the rating of the connected load.
• Load Factor: It is defined as the ratio of the average power to the maximum