Electra

WHAT ARE THE FACTORS THAT AFFECT ALTERNATOR SIZE

                Alternators size determines the efficiency of the machine. As the power increases the efficiency of the alternator also increases.

Let us consider an example, if an alternator has got efficiency of 50% for 1kW, then the efficiency will be of 90% for 10MW alternator.

 So, for alternators in the range above 1000MW will have efficiency in the range of 99%, that’s highly efficient. Large machines have also got another advantage that as the power increases, power output per kilogram also increases.

When an alternator of 10MW rating weighs 20 Kilograms produces 50 W/kg, then an alternator of 10MW rating weighing 20000 kg will produce 500W/kg.

But the problem with the large machines is that they will have high power loss, this increases the cooling problem of the machine. Cooling alternators must be added to the machine to control the power loss and heat dissipation.

This certainly increases the cost of the total combination. Hence, the size of alternator is restricted to reduce the total cost.

Illumination terms in world

                                               Light is the best part in human’s innovation. We cannot imagine without the light. Light is nothing but an electromagnetic radiation. There are various terms in illumination.

1)      Luminous Intensity is the flux that is emitted by the source per unit solid angle.

2)      Luminous flux is the flux that is contained on unit solid angle of one candela. Its unit is lumen.

3)      Lumen hour is the amount of light delivered by a flux of one lumen in that one hour.

4)      Mean spherical candle power is the average of candle power of the source in all directions in all the planes.

5)      Mean hemispherical power candle power is defined to be the total flux emitted divided by the solid angle subtended at the point source.

6)      Reduction factor is just the ratio of the mean spherical candle power to the mean hemispherical candle power.

7)      Illumination is the ratio of the luminous flux per unit area which unit lumen per square metre.

PARALLEL OPERATION OF TRANSFORMERS- a overview

            The following conditions must be satisfied to perform parallel operation in single phase transformers

1.       The primary windings of the two transformers must be of same voltage rating and frequency.

2.       The polarity of the two transformers must be connected properly, else it may lead to some serious problems.

3.       The transformation ratio of both the transformers needs to be identical.

4.       Circulating current should be avoided, else the load sharing will differ from the KVA ratings of the transformers. This is achieved by having same X/R ratio.

5.       Circulating current can be avoided by having the equivalent impedance inversely proportional to the KVA rating. This is to be carried out when the two transformers have different KVA rating.

In case of three-phase transformers, the above said conditions along with the following conditions are to be satisfied

1. 1.       Same phase sequence must be maintained.
2. 2.       Individual transformer in the three-phase transformer needs to be of same construction which can be either core type or shell type.
3. 3.       Transformers must have same phase displacement between the primary and the secondary voltages.
4. 4.       Terminal voltage of the primary and the secondary must be used for the voltage ratio.

Introduction to ROTORS IN ALTERNATORS

                     In alternators, rotors are of two types namely, Salient-Pole type and Smooth-Cylindrical type.

Salient-Pole type is used for alternators of low and medium speed.

In this, the poles are bolted on heavy magnetic wheel, which is made of cast iron or steel. Eddy current develops heat, so as to minimize the heating effect, the poles and pole-shoes are laminated.

These generators can be easily recognized by their short axial length and large diameters. Smooth Cylindrical type is used in case of high speed alternators.

 Steam turbine-driven alternators use this rotor type as they are of high speed. Along the periphery of the rotor, number of slots is made at intervals for the field coils.

Rotor will be of smooth solid forged steel cylinder. Turbo generators can be recognized by very long axial lengths and small diameters. This construction gives a smooth operation with better balance.

Testing a TRANSFORMER

                 Two tests are carried out in transformers namely open-circuit test and short-circuit test.

 These tests are carried out to calculate certain parameters of the transformer – equivalent resistance and equivalent leakage reactance with respect to the primary winding, core-loss conductance and magnetic susceptance.

Without loading the transformer the above parameters can be calculated, hence the tests are very much economical. The performance of the transformer is calculated on these parameters.

Open-circuit test is done to determine core loss in the transformer. In this test, the high voltage winding is kept open as the other winding is connected to the supply.

In the low voltage winding, the wattmeter, ammeter and voltmeter are connected. Voltage is applied to the winding producing iron loss which gets recorded in the wattmeter. The no-load primary current is very small, so no copper loss. Hence the wattmeter reading represents only the core loss.

In short-circuit test, the low voltage winding is short circuited and applied with low voltage and then gradually increased to reach full load current. The core loss is considerably small as the mutual flux is very small compared with its normal value. Hence, the wattmeter reading represents the full-load copper loss.

What is meant by Parallel operation of Generators ???

               It is necessary to know how to make the use of generators effectively. In the substation the powers are taken to supply the consumers.

There is more number of generators connected in parallel to supply the desired load. The parallel operation of generators provides certain advantages.

It aids the generators to run at the full load capacity which results in the improvement in efficiency. There is the possibility of failure of generators. In that case other generators will supply that load which avoids the shutdown completely.

There are certain things to be kept in mind to run the generators in parallel. The polarities of the generators must be the same. The output voltage, frequency and the phase relation of the generators operating in parallel must be the same or else severe damage may occur to the generators. This are the main operating procedures for paralleling the generators.