Basic Principle Of Alternator

A.C. generators or alternators (as they are usually called) operate on the same fundamental principles of electromagnetic induction as d.c. generators. They also consist of an armature winding and a magnetic field. But there is one important difference between the two. Whereas in d.c. generators, the armature rotates and the field system is stationary, the arrangement in alternators is just the reverse of it. In their case, standard construction consists of armature winding mounted on a stationary element called stator and field windings on a rotating element called rotor.

The details of construction are shown in Fig.

alternator

The stator consists of a cast-iron frame, which supports the armature core, having slots on its inner periphery for housing the armature conductors. The rotor is like a flywheel having alternate N and S poles fixed to its outer rim. The magnetic poles are excited (or magnetised) from direct current supplied by a d.c. source at 125 to 600 volts. In most cases, necessary exciting (or magnetising) current is obtained from a small d.c. shunt generator which is belted or mounted on the shaft of the alternator itself. Because the field magnets are rotating, this current is supplied through two sliprings. As the exciting voltage is relatively small, the slip-rings and brush gear are of light construction. Recently, brushless excitation systems have been developed in which a 3-phase a.c. exciter and a group of rectifiers supply d.c. to the alternator. Hence, brushes, slip-rings and commutator are eliminated.

When the rotor rotates, the stator conductors (being stationary) are cut by the magnetic flux, hence they have induced e.m.f. produced in them. Because the magnetic poles are alternately N and S, they induce an e.m.f. and hence current in armature conductors, which first flows in one direction and then in the other.

Hence, an alternating e.m.f. is produced in the stator conductors
(i)whose frequency depends on the number of N and S poles moving past a conductor in one second and
(ii)whose direction is given by Fleming's Right-hand rule.

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Working Principle of 1-phase Induction Motor

1-PHASE INDUCTION MOTOR

For the motoring action, there must exist two fluxes which interact with each other to produce the torque. In d.c. motors, field winding produces the main flux while d.c. supply given to armature is responsible to produce armature flux. The main flux and armature flux interact to produce the torque.

       In the single phase induction motor, single phase a.c. supply is given to the stator winding. The stator winding carries an alternating current which produces the flux which is also alternating in nature. This flux is called main flux. This flux links with the rotor conductors and due to transformer action e.m.f. gets induced in the rotor. The induced e.m.f. drives current through the rotor as rotor circuit is closed circuit. This rotor current produces another flux called rotor flux required for the motoring action. Thus second flux is produced according to induction principle due to induced e.m.f. hence the motor is called induction motor. As against this in d.c. motor a separate supply is required to armature to produce armature flux. This is an important difference between d.c. motor and an induction motor.

      Another important difference between the two is that the d.c. motors are self starting while single phase induction motors are not self starting.

       Let us see why single phase induction motors are not self starting with the help of  a theory called double revolving field theory.

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Double Revolving Field Theroy

single phase induction motor

According to this theory, any alternating quantity can be resolved into two rotating components which rotate in opposite directions and each having magnitude as half of the maximum magnitude of the alternating quantity.

       In case of single phase induction motors, the stator winding produces an alternating magnetic field having maximum magnitude of Φ_1m.

       According to double revolving field theory, consider the two components of the stator flux, each having magnitude half of maximum magnitude of stator flux i.e. (Φ_1m/2). Both these components are rotating in opposite directions at the synchronous speed N_s which is dependent on frequency and stator poles.

       Let Φ_f  is forward component rotating in anticlockwise direction while Φ_b  is the backward component rotating in clockwise direction. The resultant of these two components at any instant gives the instantaneous value of the stator flux at the instant. So resultant of these two is the original stator flux.

FIGURE.1- Stator flux and its two components

       The Fig. 1 shows the stator flux and its two components Φ_f  and Φ_b. At start both the components are shown opposite to each other in the Fig.1(a). Thus the resultant Φ_R = 0. This is nothing but the instantaneous value of the stator flux at start. After 90^o , as shown in the Fig. 1(b), the two components are rotated in such a way that both are pointing in the same direction. Hence the resultant Φ_R is the algebraic sum of the magnitudes of the two components. So Φ_R = (Φ_1m/2) + (Φ_1m/2) =Φ_1m. This is nothing but the instantaneous value of the stator flux at θ = 90^o as shown in the Fig 1(c). Thus continuous rotation of the two components gives the original alternating stator flux.

       Both the components are rotating and hence get cut by the motor conductors. Due to cutting of flux, e.m.f. gets induced in rotor which circulates rotor current. The rotor current produces rotor flux. This flux interacts with forward component Φ_f  to produce a torque in one particular direction say anticlockwise direction. While rotor flux interacts with backward component Φ_b to produce a torque in the clockwise direction. So if anticlockwise torque is positive then clockwise torque is negative.

       At start these two torque are equal in magnitude but opposite in direction. Each torque tries to rotate the rotor in its own direction. Thus net torque experienced by the rotor is zero at start. And hence the single phase induction motors are not self starting.

Torque speed characteristics

       The two oppositely directed torques and the resultant torque can be shown effectively with the help of  torque-speed characteristics. It is shown in the Fig.2.

FIGURE.2-Torque-speed characteristic

 

It can be seen that at start N = 0 and at that point resultant torque is zero. So single phase motors are not self starting.

       However if the rotor is given an initial rotation in any direction, the resultant average torque increase in the direction in which rotor initially rotated. And motor starts rotating in that direction. But in practice it is not possible to give initial torque to rotor externally hence some modifications are done in the construction of single phase induction motors to make them self starting.

       Another theory which can also be used to explain why single phase induction motors is not self starting is cross-field theory.

 

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Introduction to D.C motors,Principle of operation of D.C motors

dc motor

 

 

Introduction:

A motor is a device which converts an electrical energy into the mechanical energy . The energy conversion process is exactly opposite to that involved in a d.c. generator. In a generator the input mechanical energy is supplied by a prim mover while in a d.c. motor, input electrical energy is supplied by a d.c. supply. The construction of a d.c. machine is same whether it is a motor or a generator.

Principle of Operation of a D.C. Motor :

The principle of operation of a d.c. motor can be stated in a single statement as 'when a current carrying conductor is placed in a magnetic field' it experiences a mechanical force'. In a practical d.c. motor, field winding produces a required magnetic field while armature conductors play a role of a current carrying conductors and hence armature conductors experience a force. As a conductors are placed in the slots which are in the periphery, the individual force experienced by the conductors acts as a twisting or turning force on the armature which is called a torque. The torque is the product of force and the radius at which this force acts. So overall armature experiences a torque and starts rotating. Let us study this motoring action in detail.

 Consider a single conductor placed in a magnetic field as shown in the Fig .1. The magnetic field is produced by a permanent magnet but in a practical d.c. motor it is produced by the field winding when it carries a current.

figure.1

  Now this conductor is excited by a separate supply so that it carries a current in a particular direction. Consider that it carries a current away from an observe as shown in the Fig. 1. Any current carrying conductor produces its own magnetic field around it. hence this conductor also produces its own flux, around. The direction of this flux can be determined by right hand thumb rule. For direction of current considered, the direction of flux around a conductor is clockwise. For simplicity of understanding, the main flux produced by the permanent magnet is not shown in the Fig. 2.

Now there are two fluxes present,

1. The flux produced by the permanent magnet called flux.

2. The flux produced by the current carrying conductor.

      There are shown in the Fig.2. Form this, it is clear that on one side of the conductor, both the fluxes are in same direction. In this case, on the left of the conductor there is gathering of the flux lines as two fluxes help each other. As against this, on the right of the conductor, the two fluxes are in opposite direction and hence try to cancel each other. Due to this, the density of the flux lines in this area gets weakened. So on the left, there exists high flux density area while on the right of the conductor there exists low flux density area as shown in the Fig. 2.

FIGURE-2

 

 

 

  This flux distribution around the conductors acts like a stretched rubber band under tension. This exerts a mechanical force on the conductor which acts from high flux density area towards low flux density area. i.e. from left to right for the case considered as shown in the Fig. 2.

 

FIGURE-3

 

*GOOD TO KNOW: 

In the practical d.c. motor, the permanent magnet is replaced by a field winding which produces the required flux called main flux and all the armature conductors, mounted on the periphery of the armature drum, get subjected to the mechanical force. Due to this, overall armature experiences a twisting force called torque and armature of the motor starts rotating.

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