Characteristics of D.C. Generators

The d.c. generators have following characteristics in general,

1) Magnetization characteristics

2) Load characteristics

1.Magnetization Characteristics

       This characteristics is the graph of generated no load voltage E against the field current I_f, when speed of, generator is maintained constant. As it is plotted without load with open output terminals it is also called No load characteristics or Open circuit characteristics.

       E_o V_s I_f  is magnetization characteristics

       Where  E_o = No load induced e.m.f.

       But for generator,

       E = (ΦPNZ)/(60A)

.^..     E α   Φ                 with (PNZ) / (60A)  constant

.^..     E  α   I_f                    as   Φ  α   I_f

       Thus induced e.m.f. increases directly as I_f  increases. But after certain I_f  core gets saturated and flux also remain constant though I_f  increases. Hence after saturation, voltage also remains constatnt.

*GOOD TO KNOW:
 Thus characteristics is linear till saturation and after that bends such that voltage remains constant though I_f  increases.

        The characteristics is shown in the BELOW figure.

magnetisation characteristics

Now the induced e.m.f. also varies with speed.

       Actually,             E   α   N Φ

       So if magnetization characteristics for various speeds are plotted we will get family of parallel characteristics as shown in the BELOW FIG.1. For lower speeds, generated voltages are less so characteristics for lower speeds are below the characteristics for higher speeds.

 

fig.1

2 .Load Characteristics

These are further divided into two categories,

1) External characteristics

2) Internal characteristics

       The external characteristics is the graph of the terminal voltage V_t against load current I_L.

       The internal characteristics is the graph of the generated induced e.m.f. against the armature current I_a.

*GOOD TO KNOW:  While plotting both the characteristics, the speed N of the generator is maintained constant.

*GOOD TO KNOW: In most of the cases, the shunt field current is very small as compared with load current I_L. Hence in practice, the internal characteristics shows the graph of induced e.m.f. E against load current I_L, instead of I_a, neglecting I_sh.

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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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