Electrical World

Consider two d.c. series generators in parallel as shown in the Fig. 1. Fig. 1        If no load voltages  E 1 and  E 2 of the two generators are initially same, generators supply equal currents and have equal resistance for its field winding. If E 1 > E 2 then I 1 > I 2 which causes the field of generator 1 strengthened while that of 2 is weakened which will decrease  E 2 further. A stage is reached where generator 1 will take entire load and will also supply power to generator 2 which will start running as a motor. The two machines will form a short circuited loop which will increase the current indefinitely. This can be avoided by use of equalizer bar through which equal currents are passed in the two similar generators to the load. The slight difference in the currents therefore being confined to the loop made by the armatures and the equalizer bar.

Consider two compound generators 1 and 2 running in parallel as shown in the Fig. 1. Fig. 1        As the voltage characteristics for the usual compound generators are rising, their parallel operation is unstable in the absence of corrective devices.        Let us consider that each generator is taking proper share of load. If for some reason, the generator 1 takes increased load slightly then the current passing through its series field winding increases strengthening its field to increase the generated e.m.f. This causes generator 1 to take still more load. If system load is assumed to be constant then the load on generator 2 will decrease weakening its series field due to less current passing through its field winding which results in further decrease in its load. This effect is cumulative which leads generator 1 to take the entire load and generator 2 will be driven as motor. The circuit breakers of at...

For stable parallel operation, the most suitable type of d.c. generator is shunt generator as it has slightly drooping characteristics. If there is any tendency for a generator to supply more or less than its proper share of load it changes system voltage which certainly opposes this tendency. This restores the original division of load. Thus the shunt generators automatically remains in parallel, once they are paralleled.        If any of the generators is taken out of service then firstly its field is weakened while the field of other generators is strengthened till the ammeter associated with that generator reads zero. After this, the breaker and the concerned switch are opened to take the generator out of service. This procedure of connecting and disconnecting a generator from service prevents any shock and sudden disturbance to the prime mover or the system itself.        If field of any generator is weake...

The generating station either of d.c. type or of a.c. type, normally consists of number of smaller generators operating in parallel instead of large single unit capable of supplying the maximum load demand. These smaller units can run in single or in various parallel combinations to supply actual load for following different reasons. 1. Continuity and Reliability of Service        The most important requirement for any generator is its continuity. This can not be achieved if there is a single unit as in case of breakdown of prime mover or generator. there will be total unavailiability of the supply. This also affects the reliability of the supply. The requirement of uninterrupted supply is important in case of industrial loads as otherwise the economy may get affected. Thus instead of having a single large unit, Number of small generators are operated in parallel. 2. Maintenance and Repair It is required to have a periodic checking an...

In case of d.c. generating stations, there are heavy thick copper bars which acts as positive and negative terminals for the entire stations. These bars are called bus bars. While connecting the d.c. generators in parallel, the positive and negative terminals of the generators should be respectively connected to the positive and negative terminals of bus bars. If the generator is connected with reverse polarity to the bars, it results in short circuit which will cause damage to the commutators and brushes, ultimately shutting down the station. Before making the parallel operation, it should be checked for reversal polarity for the generators otherwise breakers are tripped off as a result of heavy fault current.        Consider the bus bar with positive and negative terminals as shown in the Fig. 1. The generators are connected to these bus bars for parallel operation. Fig. 1         The generator 1...

Separately Excited Generators :       As a separate supply is required to excite field, the use is restricted to some special applications like electro-palting, electro-refining of materials etc. Shunt Generators :        Commonly used in battery charging and ordinary lighting purposes. Series Generators :        Commonly used as boosters on d.c. feeders, as a constant current generators for welding generator and arc lamps. Cumulatively Compound Generators :        These are used for domestic lighting purposes and to transmit energy over long distance. Differential Compound Generators :       The of this type of generators is very rare and it is used for special application like electric arc welding.

Consider the field magnetization characteristics of a d.c. shunt generator shown in the Fig. 1. Fig. 1 Concept of critical resistance        The Fig. 1 shows that generator voltage builds in step till point A. This point is intersection of field resistance line with the open circuit characteristics (O.C.C.). The voltage corresponding to point A is the maximum voltage it can generate. If the slope of field resistance line is reduced by decreasing the field resistance, the maximum voltage generator can build will be higher than that corresponding to point A. Similarly if the slope of field resistance line is increased by increasing the field resistance, the maximum voltage generator can build will be less that that corresponding to point A i.e. corresponding to point B.        If now the slope of the field resistance line is increased in such a way that it becomes tangential to the lower part of the open c...

Principle of Operation of a D.C. Generator         All the generators works on a principle of dynamically induced e.m.f. This principle nothing but the Faraday's law of electromagnetism induction. It states that, 'whenever the number of magnetic lines of force i.e. flux linking with a conductor or a coil changes, an electromotive force is set up in that conductor or coil.' The change in flux associated with the conductor can exist only when there exists a relative motion between a conductor and the flux. The relative motion can be achieved by rotating conductor with respect to flux or by rotating flux with respect to a conductor. So a voltage gets generated in a conductor, as long as there exists a relative motion between conductor and the flux.        Such an induced e.m.f. which is due to the physical movement of coil or conductor with respect to flux or movement of flux with respect to coil or conduc...

  It consists of a permanent magnet with two poles. A single turn rectangular coil is kept in the vicinity of the permanent magnet. The coil is made up of conducting material like copper or aluminium.        The coil is made up of the two conductors namely a-b and c-d. Such two conductors are connected at one end to from a coil as shown in the Fig.1 Fig. 1 Single turn coil        The remaining two ends are to be connected to the rings mounted on the shaft, called slip rings C 1 and C 2 . Slip rings also rotate along with armature of a machine. The two brushes P and Q are resting on the slip rings, just making a contact with slip rings. The brushes P and Q are stationary. The slip ring and brush assembly is required to collect the e.m.f. induced in the rotating coil and make it available to the stationary external resistance. The overall construction is shown in the Fig. 2. Fig. 2   Single ...

We have seen that there are number of armature conductors, which are connected in specific manner as per the requirements, which is called armature winding. According to the way of connecting the conductors, armature winding has basically two types namely, a) Lap winding          b) Wave winding 1.1 Lap winding        In this case, if connection is started from conductor in slot 1 then connections overlap each other as winding proceeds, till starting point is reached again.        Developed view of part of the armature winding in lap fashion shown in the Fig. 1. Fig. 1 Lap Winding        As seen from the Fig. 1, there is overlapping of coils while proceeding. Note : Due to such connection, the total number of conductors get divided into 'P' number of parallel paths, where P = number of pole sin the machine.   ...

For general lighting purpose in shops, offices, houses, schools etc. Single phase a.c. supply is commonly used. Hence instead of d.c. motors, the motors which work on single phase a.c. supply are very popularly in use. These a.c. motors are called single phase induction motors. The numerous domestic applications use single phase motors. The power rating of such motors is very small. Some of them are even fractional horse power motors, which are used in applications like small toys, small fans, hair dryers etc. This chapter explains the construction, working principle and applications of various types of single phase induction motors.  Construction of Single Phase Induction Motors        Similar to a d.c. motor, single phase induction motor has basically two main parts.one rotating and other stationary. The stationary part in single phase induction motors is called stator while the rotating part is called rotor.    ...

Speed Control of D.C. Series Motor (Part2)        2. Rheostatic Control        In this method, a variable resistance (R x ) is inserted in series with the motor circuit. As this resistance is inserted, the voltage drop across this resistance (I a R x ) occurs. This reduces the voltage across the armature. As speed is directly proportional to the voltage across the armature, the speed reduces. The arrangement is shown in the Fig 1(a). As entire current passes through R x , there is large power loss. The speed-armature current characteristics with changes in R x are shown in the Fig 1(b). Fig. 1 Related Articles :

Speed Control of D.C. Series Motor (Part1)               The flux produced by the winding depends on the m.m.f. i.e. magnetomotive force which is the product of current and the number of turns of the winding through which current is passing. So flux can be changed either by changing the current by adding a resistance or by changing the number of turns of the winding. Let us study the various methods based on this principle. 1. Flux Control        The various methods of flux control in a d.c. series motor are explained below : 1.1 Field Divertor Mehtod        In this method the series field winding is shunted by a variable resistance () known as field divertor. The arrangement is shown in the Fig. 1(a).        Due to the parallel path of R x , by adjusting the value of R x , any amount of current can be diver...

Speed Control of D.C. Shunt Motor (Part3)  3. Applied Voltage Control         Multiple voltage control : In this technique the shunt field of the motor is permanently connected to a fixed voltage supply, while the armature is supplied with various voltages by means of suitable switch gear arrangements.        The Fig. 1 shows a control of motor by tow different working voltages which can be applied to it with the help of switch gear.  Fig. 1  Multiple voltage control        In large factories, various values of armature voltages and corresponding arrangement can be used to obtain the speed control. 1.1 Advantages of Applied Voltage Control 1. Gives wide range of speed control. 2. Speed control in both directions can be achieved very easily. 3. Uniform acceleration can be obtained. 1.2 Disadvantages of Applied Voltage Control 1. Arrangement is ex...

Speed Control of D.C. Shunt Motor (Part2)  2. Armature Voltage Control Method or Rheostatic Control        The speed is directly proportional to the voltage applied across the armature. As the supply voltage is normally constant, the voltage across the armature can be controlled by adding a variable resistance in series with the armature as shown in the Fig. 1. Fig. 1 Rheostat control of shunt motor        The field winding is excited by the normal voltage hence I sh is rated and constant in this method. Initially the reheostat position is minimum and rated voltage gets applied across the armature. So speed is also rated. For a given load, armature current is fixed. So when extra resistance is added in the armature circuit, I a remains same and there is voltage drop across the resistance added ( I a R). Hence voltage across the armature decreases, decreasing the speed below normal value. By varyi...

Speed Control of D.C. Shunt Motor (Part1) 1. Flux Control        As indicated by the speed equation, the speed is inversely proportional to the flux. The flux is dependent on the current through the shunt field winding. Thus flux can be controlled by adding a rheostat (variable resistance) in series with the shunt field winding, as shown in the Fig. 1. Fig. 1 Flux control of shunt motor        At the beginning the rheostat R is kept at minimum indicated as start in the Fig. 1. The supply voltage is at its rated value. So current through shunt field winding is also at its rated value. Hence the speed is also rated speed also called normal speed. Then the resistance R is increased due to which shunt field current I sh decreases, decreasing the flux produced. As N α   (1/ Φ ), the speed of the motor increases beyond its rated value.         Thus by this method, the sp...

To change the speed as per the requirements, it is not possible to increase the voltage or currents beyond certain limit. These limits are called ratings of the motor.        The maximum voltage that can be applied to the motor, safely is called rated voltage or normal voltage of the motor. While changing the applied voltage, one should not apply the voltage more than the rated voltage of the motor.        Similarly maximum current that field winding can carry, safely is called rated field current of the notor. Hence while changing the flux, one should not increase field current beyond its rated value. This is important rating as far as shunt motor is concerned. In a series motor, the entire armature current flows through the series field winding. The armature current is decided by the load and it can not be changed by changing the resistance of the armature circuit. So the maximum current that armature winding...

According to the speed equation of a d.c. motor we can write,        The factors Z, P, A are constants for a d.c. motor.        But as the value of armature resistance R a and series field resistance R se is very small, the drop I a R a and (R a + R se ) is very small compared to applied voltage V. Hence neglecting these voltage drops the speed equation can be modified as,        Thus the factors affecting the speed of a d.c. motor are, 1. The flux Φ 2. The voltage across the armature 3. The applied voltage V        depending upon these factors the various methods of speed control are, 1. Changing the flux Φ by controlling the current through the field winding called flux control methods. 2. Changing the armature path resistance which in turn changes the voltage applied across the armature called rheostatic control. 3. Changi...

The armature reaction is nothing but the effect of flux produced by armature conductors on the distribution of main flux due to the field poles.        In case of all the d.c. machines it can be seen that armature m.m.f. is approximated by a symmetrical triangular waveforme with axis as interpolar axis. Thus armature m.m.f. and field m.m.f. are displaced in space by 90 o . This will cause distortion in main field flux which is called cross magnetising effect of armature reaction. The armature m.m.f. lags behind field m.m.f. with respect to the direction of rotation in case of motors and vice versa in case of generatos.        The Fig. 1 shows the developed form indicating distribution of main field flux, armature flux and the respective m.m.f.s The armature m.m.f. is zero at the pole centers and maximum at interpolar axis. Assuming air gap to be uniform then the distribution of flux due to armature current only...

Instead of just stating the applications, the behaviour of the various characteristics like speed, starting torque etc., which makes the motor more suitable for the applications, is also stated in the Table .1 Table 1