Electra

Shunt Motor Versus Compound Motor

                            The field winding of the motor connected in parallel to the armature is known to be Shunt Motor.

Here the field consists of many turns of fine wire since it does not carry the entire armature current. The current path is divided to go to armature and field.

The Shunt motor is a constant speed motor because speed of the shunt motor remains practically constant with varying loads.

It is used where the constant speed operation is necessary. It is widely used in Lathe machines where one speed is maintained constant for a longer period.

The compound motor consists of both the shunt and series winding which is required for certain applications. Both the fields can be varied to operate the motor nearer to series or shunt motor.

When it operated nearer to series motor it is known as cumulative compound and when it is nearer to shunt motor it is known as differential compound.

Series motor has the disadvantage that we cannot operate it in decreasing light loads which can be overcome by this compound motor. It is also used in applications where there is a fluctuating load.

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.

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.    

Concept of Commutation

                      Commutator plays a major role in the case of DC generators. It is necessary to have some idea about commutation that will fetch you in having depth knowledge in DC Generator.

Alternating current alternates in either direction. It is necessary to reverse that one. The reversal of current which can be achieved by means of short circuiting the brushes is known as commutation.

There must be certain period of time for this operation to take place. It is known as time of commutation. This reversal of current produces some effects. The self induced emf is developed when the brushes are short circuited which is the Reactance voltage.

It opposes that reversal which ultimately results in sparking of the brushes. The commutation must be improved to eliminate this effect. The Resistance commutation can be implemented by providing high resistance brushes.

Still we can have better commutation by means of interpoles. It can also be called as emf commutation. It produces external emf which cancels the reactance voltage. The interpole is connected in series with the armature.

It main advantage is that in increase the sparking limit of the machine which result in the creation of the smaller interpole machine with that output.

Concept of Armature Reaction

               The main concept in the DC Generator is the armature reaction. It is the simple thing that we can capture easily. The Main Field system produces the field.

 The armature current also develops its own flux. Since armature is revolving within the main field system it shows some effect on the main field. This effect of flux developed by armature on the main flux is described as an armature reaction.

There are two possibilities based on the effect. It weakens the main flux or it distorts the main flux. The both effects can be described as demagnetizing and cross magnetizing respectively. It is necessary to nullify the effects of these effects.

Demagnetizing effect can be nullified by providing additional turns on each pole. Cross magnetizing effect can be nullified by providing compensating windings in the pole shoes and it is connected in series with the armature. The armature reaction in general is produced by the load current.