Here is an experiment on ‘Power Factor’ especially written for school and college students.
Aim of the Experiment:
Power factors improvement of single-phase load using capacitor bank.
To observe that by using capacitor bank with single phase load, power factor is improved.
Power factor is the ratio between the kW and the KVA drawn by the actual load power and the KVA is the apparent load power. It is a measure of how effectively current is being converted into useful work output and more particularly is a good indicator of effect of the load current on the efficiency of the supply system. For a given voltage and active power (kW) be current drawn is inversely proportional to the load.
All current causes losses in the supply and distribution system. A load with a power factor of 1.0 result in the most efficient loading of the supply and a load with a power factor of 0.5 will result in much higher losses in the supply system.
A poor power factor can be the result of either a significant phase difference between the voltage and current at the load terminals. Poor load current phase angle is generally the result of an inductive load such as an induction motor, power transformer, lighting ballasts and induction furnace. A distorted current wave form can be the result of a rectifier variable speed drive, discharged lighting or other electronic load.
A poor power factor due to inductive load can be improved by the addition of power factor correction.
Nowadays automatic power factor control panel are available in the market. The automatic power factor control panels are trouble free, ease of operation, long life and reliable. As you are aware, it has becomes mandatory to maintain high power factor. Electrical power charges constitute the major running cost of industrial units. The panel provides all the necessary and sufficient facilities to implement a reliable automatic power factor control system.
The panel is manufactured in sheet steel duly powder coated. Two doors are provided capacitive power factor correction is applied to circuits which include induction motor as a means of reducing the inductive component of the current and thereby reduce the losses in the supply. There should be no effect on the operation of the motor itself.
An induction motor draws current from the supply that is made up of resistive and inductive components. The resistive components are load current and loss current. The inductive components are leakage reactive and magnetizing current.
The current due to leakage reactance is dependent on the total current drawn by motor, but the magnetizing current is independent of the load on motor. The magnitude of current will typically be between 20% and 60% of the rated full load current of motor. The magnetizing current is the current that establishes the flux in the core.
The magnetizing current does not actually contribute to the actual work output of the motor. In order to reduce wasted energy in the distribution system, the consumer will be encouraged to apply power factor correction.
Power factor correction is achieved by the addition of capacitors in parallel with connected motor circuits and can be applied at the starter, or applied at the switch board. The resulting capacitive current is leading current and is used to cancel the lagging inductive current flowing from the supply.
As a large proportion of the inductive or lagging current on the supply is due to magnetizing current of induction motors, it is easy to correct each individual motor by connecting the correction capacitors to the motor starters. With static correction, it is important that the capacitive current is less than the inductive magnetizing current of the induction motor. In many installations employing static power factor correction, the correction capacitors are connected directly in parallel with motor windings.
When the motor is off line the capacitors are also off line. When the motor is connected to the supply, the capacitors are also connected providing correction at all times that the motor is connected to the supply.
This removes the requirement for any expensive power factor monitoring and control equipment. In this situation, the capacitor remains connected to the motor terminals as the motor slows down. An induction motor, while connected to the supply, is drawn by a rotating magnetic field in the stator which induces current into the rotor, when the motor is disconnected from the supply.
Static power factor correction should provide capacitive current equal to 80% of magnetizing current, which is essentially the open shaft current of the motor. Static correction is commonly applied by using on contactor to control both motors and capacitors. It is better practice of use two contactors, one for the capacitor and one for the motor.
Advantages of Power Factor Control:
For Electricity Board:
i. Reduction in line losses.
ii. Reduction in transformer rating.
iii. Reduction in KVA demand.
iv. Reduction in switch gear rating
v. More consumers can be served from the same equipment.
vi. Increased life of equipment.
For the Consumer:
i. Reduction in load current.
ii. Reduction in switch gear rating, rating.
iii. Penalty of low power factor is avoided.
iv. Penalty of excess maximum demand is avoided.
v. Increased life of the equipment.
vi. Overall reduction in electricity bills.
a. Consider the single phase inductive load (1– f) induction motor
b. Connect the voltmeter in parallel with supply. Ammeter (A) is connected in series with the inductive load
c. Now switch on the supply and note down the reading of voltmeter (V) and ammeter (I)
d. Switch off the supply and connect the capacitor, bank through ammeter (AC) as shown in figure
e. Switch on the supply and note down the reading of voltmeter and ammeters. Let it be I1 & IC the capacitor current IC leads the voltage by 90° the result current I1 is the vector sum of two currents I and IC and the angle of lag is now f1 which is less then f
f. The value of cos f1 is more than cos f. Hence power factor of load is improved from cos f to cos f1
By connecting the capacitor bank in parallel with load p.f. is raised
Power factor before (cos f) and after improvement (cos f1)