Question

Could some give a deep descripition about the power factor.
1. How the power factor will be less?
2.What should we do inorder to increase the power factor?
3.How do we maintain a good power factor in a 3 phase motor without using the capacitor?

Answer 1

Solution 1- First I describe what is the power factor- the power factor is defined as the ratio of the real power flowing to the load to the apparent power in the circuit and it is a dimensionless number in the closed interval of ?1 to 1.

Real power is the capacity of the circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit.

HOW POWER FACTOR WILL BE LESS

1. Approx 90% of the industrial load consists of Induction Machines (1-? and 3-?). Such machines draw magnetizing current to produce the magnetic field and hence work at low power factor. For Induction motors, the pf is usually extremely low (0.2 - 0.3) at light loading conditions and it is 0.8 to 0.9 at full load.

2.Varying Load in Power System We know that load on the power system is varying. During low load period, the supply voltage is increased which increase the magnetizing current which causes the decreased power factor

3. Electrical discharge lamps (High-intensity discharge lighting) Arc lamps (operate very low power factor)

4. Industrial heating furnaces it also works on the low power factor and draws more current from the supply.

5. Harmonic Currents The presence of harmonic currents in the system also reduces the power factor. In some cases, due to improper wiring or electrical accidents, a condition known as 3-? power imbalance occurs. This results in low power factor too.

Solution 2.- To increase power factor- Methods are given below-

1. Capacitor Bank It is the simplest method. This is applied in areas where large inductive loads (lagging currents) are present. Static capacitors are used which produce capacitive reactance that cancels out the inductive reactance of the lagging current.

2. Synchronous Condenser When a synchronous motor is overexcited, it draws leading current. In a way, it behaves like a capacitor. When such a motor is over excited and runs at no load, it is called a Synchronous Condenser

3. Phase Advancer This can be used only for Induction Motors We know that stator winding draws lagging current in a motor. This current is drawn from the main supply. Hence, to improve pf, we supply this lagging current from an alternative source. This alternative source is the phase advancer.

Solution 3. To maintain good power factor in a 3 phase motor- it can be done without capacitor bank

1. Pase Advancer it used only for Induction Motors. A phase advancer is basically an AC exciter. It is mounted on the same shaft as the main motor and connected in the rotor circuit. It supplies exciting ampere turns to the rotor circuit at slip frequency. This improves the power factor. if we supply more amp-turns than needed, the motor will operate in an overexcited state (at leading pf)

2. Synchronous Condenser- The most attractive feature is that it allows stepless pf correction. In a static capacitor, the leading kVAR supplied is constant. But in a synchronous condenser, we can vary the field excitation and hence control the amount of capacitive reactance produced. Synchronous Condensers are used in large factories, industries, and major supply substations.