CHARACTERISTICS OF D.C GENERATORS

By -

The three important characteristics of DC generator are

1. Open circuit characteristic or Magnetization curve or No – load saturation Curve

Open circuit characteristic is the relation between the No-load generated emf in the armature, and the field exciting current at a fixed speed. It is the magnetization curve for the material of electro- magnets. It is same for separately excited or self excited machine.

2. Internal or total characteristic

This characteristic curve gives the relation between the emf generated in the armature and the armature current.

3. External characteristic

This gives the relation between the terminal voltage and the load current. This characteristic takes into account the voltage drop due to armature circuit resistance and the effect of armature reaction. This characteristic is of importance in judging the suitability of generator for a particular purpose. This characteristic is also referred to as performance characteristics or voltage regulating curve.

image

The circuit diagram for obtaining the OCC is shown in figure 1.9. Irrespective of the type of the DC machine, namely, shunt, series, compound, the shunt field winding is disconnected and excited from an external source.

image

As iron is unsaturated under low excitation current condition, emf increases as flux increases with increase in excitation current. As the field current increases further the iron starts saturating, the emf will not increase proportionately as the flux is not varying proportionately with the current. This is shown by the knee ‘pq’ of the characteristic curve shown in figure 1.10. A further increase in field current leads to saturation of iron and the flux remains almost constant and hence the induced emf will also remain constant. This is shown by the region ‘qr’ in the figure1.10. Figure 1.10

image

CRITICAL RESISTANCE FOR SHUNT GENERATOR

When the armature is rotating with armature open circuited, an emf is induced in the armature because of the residual flux. When the field winding is connected with the armature, a current flows through the field winding ( in case of shunt field winding, field current flows even on No-load and in case of series field winding only with load) and produces additional flux. This additional flux along with the residual flux generates higher voltage. This higher voltage circulates more current to generate further higher voltage. This is a cumulative process till the saturation is attained. The voltage to which it builds is decided by the resistance of the field winding as shown in the figure 1.11. If field circuit resistance is increased such that the resistance line does not cut OCC like ‘om’ in the figure 1.11, then the machine will fail to build up voltage to the rated value.

The slope of the air gap line drawn as a tangent to the initial linear portion of the curve represents the maximum resistance that the field circuit can have beyond which the machine fails to build up voltage. This value of field circuit resistance is called critical field resistance. The field circuit is generally designed to have a resistance value less than this so that the machine builds up the voltage to the rated value.

image

CONDITIONS FOR VOLTAGE BUILD UP OF A GENERATOR

Following are the conditions necessary for the voltage build up of a self excited generator.

(i) Residual magnetism must be present.

(ii) For the given direction of rotation, the field coils must be properly connected to the armature so that the flux produced by the field current reinforces the residual flux.

(iii) Its field resistance must be less than the critical field resistance.

CRITICAL SPEED

Critical speed of a generator is that speed for which the field circuit resistance becomes the critical field resistance.

image

Relation between induced emf and terminal voltage will provide an insight into the performance of the machine. The terminal voltage of the machine under loaded condition reduces from the no-load induced emf value because of the armature circuit voltage drop and armature reaction. Further the contact drop of the brushes will have to be taken into account. Usually a brush contact drop of 1volt is considered for a brush. This will be constant throughout the operating range of the machine.

Comments

Post a Comment

Popular posts from this blog

Total Loss in a D.C. Generator

By -
Image
Total Loss in a D.C. Generator The various losses occurring in a generator can be sub-divided as follows : where R a = resistance of armature and interpoles and series field winding etc. This loss is about 30 to 40% of full-load losses. Field copper loss. In the case of shunt generators, it is practically constant and I 2 R (or V I sh ). In the case of series generator, it is = I s e 2 R s e where R s e is resistance of the series field winding. This loss is about 20 to 30% of F.L. losses. (iii) The loss due to brush contact resistance. It is usually included in the armature copper loss. (b) Magnetic Losses (also known as iron or core losses), (i) hysteresis loss, W h µ B max f and ( ii ) eddy current loss, W e µ B max f These losses are practically constant for shunt and compound-wound generators, because in their case, field current is approximately constant. Both these losses total up to about 20 to 30% of F.L. losses. (c) Mechanica l Losses. These consist of :...
Read more

Transformer,three phase:Wye/Delta or Y/∆ Connection and Delta/Wye or ∆/Y Connection.

By -
Image
Wye/Delta or Y/∆  Connection The main use of this connection is at the substation end of the transmission line where the voltage is to be stepped down. The primary winding is Y -connected with grounded neutral as shown in Fig. 33.6. The ratio between the secondary and primary line voltage is 1/ 3 times the transformation ratio of each trans- former. There is a 30° shift between the primary and secondary line voltages which means that a Y - D transformer bank cannot be paralleled with either a Y - Y or a D - D bank. Also, third harmonic currents flows in the D to provide a sinusoidal flux. Delta/Wye or ∆/Y Connection This connection is generally employed where it is necessary to step up the voltage as for example, at the beginning of high tension transmission system. The connection is shown in Fig. 33.7. The neutral of the secondary is grounded for providing 3-phase 4-wire service. In recent years, this connection has gained considerable popularity because it can be used to s...
Read more

Testing of d.c.Machines:Brake Test

By -
Image
Brake Test It is a direct method and consists of applying a brake to a water-cooled pulley mounted on the motor shaft as shown in Fig. 31.1. The brake band is fixed with the help of wooden blocks gripping the pulley. One end of the band is fixed to earth via a spring balance S and the other is connected to a suspended weight W 1. The motor is running and the load on the motor is adjusted till it carries its full load current. The simple brake test described above can be used for small motors only, because in the case of large motors, it is difficult to dissipate the large amount of heat generated at the brake. Another simple method of measuring motor output is by the use of poney brake one form of which is shown in Fig. 31.2 ( a ). A rope is wound round the pulley and its two ends are attached to two spring balances S 1 and S 2. The tension of the rope can be adjusted with the help of swivels. Obviously, the force acting tangentially on the pulley is equal to the differ ence ...
Read more