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.   ...

  Let       P = Number of poles of the generator                    Φ = Flux produced by each pole in webers (Wb)                    N = Speed of armature in r.p.m.                    Z = Total Number of Armature Conductors                    A = Number of parallel paths in which the 'Z' number of conductors are divided        So        A = P for lap type of winding                    A = 2 for wave type of winding        Now e.m.f. gets induced in the conductor according...

a) Conductor : It is the actual armature conductor which is under the influence of the magnetic field, placed in the armature slot. b) Turn : The two conductors placed in different slots when connected together,. forms a turn. While describing armature winding the number of turns may be specified from which, the number of conductors can be decided. Fig. 1 Single Turn        Z = 2 x Number of turns. c) Coil : For simplicity of connections, the turns are grouped together to form a coil. If coil contains only one one turn it is called single turn coil while coil more than one turn is called multiturn coil. Fig. 2 Armature coils        Hence if number of coils, along with of number of turns per coil are specified, it is possible to determine the total number of turns and hence total number of armature conductors 'Z' required to calculate  generated e.m.f. Example : A 4 pole, lap wound, d.c....

Basically there are two physical types of the windings. These are i) Single layer winding ii) Double layer winding. The sequential arrangement of coils around the armature is different for both these types of windings. 1.1 Single Layer Winding        In this type of winding, the complete slot is containing only one coil side of a coil. This type of winding is not normally used for machines having commutators. It is shown in the Fig. 1. Fig. 1 Single layer winding        In single layer windings permit the use of semienclosed and closed types of slots. Also the coils can be pushed through the slots from one end of the core and are connected during the process of windings at the other end. Here the insulation can be properly applied and consolidated which is advantageous in large output machines with high voltage.        The single layer windings used in high voltage machines...

Armature windings are classified into two different types namely i) Closed type winding ii) Open type winding. 1.1 Closed Type Winding        In this type of winding, a closed path is formed around the armature. The starting point of the winding is reached again after passing through all the turns. The current passing through closed type of winding is through brushes placed on commutator. The commutator segments are connected to various armature coils.        The armature current gets divided into different parallel paths. The current flowing through the coil changes continuously but from brush side the winding view remains same and polarity is maintained which is in effect due to use of commutator segments.        The closed type of winding is normally used in a.c. and d.c. commutator machines. This type of winding is usually double layer. 1.2 Open Type Winding   ...

Electrical Question 1: What are the different types of DC Generators ? Answer: DC Generators are characterized by the method in which the flux is produced in the magnetizing circuit ( Field Winding ) of the DC generator. The flux required for the DC Generator is produced by A Permanent magnet Field Coils excited by the external source Field Coils excited by the same source. Based on the method of excitation flux produced DC Generators are Characterized by Separately Excited DC Generator: In Separately excited DC Generator the field winding excitation current (which produces the flux) of the DC Generator is controlled by the External source such as battery or current from a small DC machine. Series Wound Generator: In Series Wound Generator the field winding of the DC Generator is connected in series of the armature winding of the DC machine.  The current which flows out from the DC Generator through the armature winding the same current flow through the ...

Electrical Question 1: Explain Armature reaction in DC machine ? Answer: Armature reaction is the effect of the magnetic field set up by the armature current on the distribution of the flux under the main poles of the DC machine. Emf will be induced in the armature of the dc machine by the Fleming's electromagnetic induction principle (Conductor and magnetic field required to produce emf). As the conductors of the armature are shorted current starts flowing and this current will produce a counter magnetic filed which opposes the main field. This effect of armature flux on the main flux so as to distort or weaken the main field flux is called armature reaction. Electrical Question 2: What is the effects of the armature reaction in a dc machine? Answer: Due the armature reaction the armature flux will have two impacts on the main field flux It partly weakens or distorts the main field flux It cross magnetises or distorts the main field flux Elect...

ElecticalQuestion 1: What is Electrical machine? Electrical machine is an electro mechanical device which converts input motive power ( prime mover like turbine in case of generator or electrical supply in case of dc motor) to output motive power (Electrical power in case of the generator or mechanical output in case of motor). This uses the principle of electro magentic induction principle. Electrical Question 2: What is the advantage of DC motors over AC motors? Answer: With the advert of the ac power generation and transmission over long distances with less losses using electrical transformers, dc machines were out ruled by the ac machines in industrial and home applications. However dc motors still exist in the industries because of some special properties of the dc machines where ac drives have failed to attain. DC motors have some special properties such as: High Starting Torque Excellent speed control capability For traction applications in electr...

An electrical machine is a device that convert the electrical energy into mechanical energy and vice versa. Transformer is also listed in electrical machine which change the voltage level of alternatering current without changing frequency. Take a look on this figure to understand the electrical machine. 1. Generator The Input is mechanical energy (from the prime mover), and the output is electrical energy.  An electrical generator is a device which converts the mechanical energy into electrical energy. The working principle of generator is that when a conductor rotate in magnetic field, an emf is induced in the conductor. A generator forces electrones to flow through an external circuit. There are two main parts of generator named as rotor and stator. In generator mechanical energy is applied to rotor. Rotor is connected to prime mover which is coupled to a turbine. The other source of mechanical energy are internal combustion engine, wind turbine, compressed air and hand c...

Q. Principle of operation of a generator? A. An electric generator is a machine that converts mechanical energy into electrical energy. An electric generator is based on the principle that whenever flux is cut by a conductor, an e.m.f. is induced which will cause a current to flow if the conductor circuit is closed. The direction of induced e.m.f. (and hence current) is given by Fleming’s right hand rule. Q. What is the role of a Commutator? A. The function of commutator is to facilitate the collection of current from armature conductors. It converts the alternating current induce in the armature conductors into unidirectional current in the external load circuit. Q. What is the function of brushes? A. The purpose of brushes is simply to lead current from the rotating loop or winding to the external stationary load. Q. What are the different types of generators? A. Generators are generally classified based on their methods of field excitation (i) Separately excited d.c....