Characteristic Of Shunt Wound Dc Generator 2u3m6e

Characteristic of Shunt Wound DC Generator Magnetic Characteristic • Critical Load Resistance • Internal Characteristic • External Characteristic

In shunt wound DC generators the field windings are connected in parallel with armature conductors as shown in figure below. In these type of generators the armature electric current Ia divides in two parts. One part is the shunt field electric current Ish flows through shunt field winding and the other part is the load current IL goes through the external load.

Three most important characteristic of shunt wound dc generators are discussed below:

Magnetic or Open Circuit Characteristic of Shunt Wound DC Generator This curve is drawn between shunt field current(Ish) and the no load voltage (E0). For a given excitation electric current or field current, the emf generated at no load E0 varies in proportionally with the rotational speed of the armature. Here in the diagram the magnetic characteristiccurve for various speeds are drawn. Due to residual magnetism the curves start from a point A slightly up from the origin O. The upper portions of the curves are bend due to saturation. The external load resistance of the machine needs to be maintained greater than its critical value otherwise the machine will not excite or will stop running if it is already in motion. AB, AC and AD are the slops which give critical resistances at speeds N1, N2 and N3. Here, N1 > N2 > N3.

Critical Load Resistance of Shunt Wound DC Generator This is the minimum external load resistance which is required to excite the shunt wound generator.

Internal Characteristic of Shunt Wound DC Generator The internal characteristic curve represents the relation between the generated voltage Eg and the load current IL. When the generator is loaded then the generated voltage is decreased due to armature reaction. So, generated voltage will be lower than the emf generated at no load. Here in the figure below AD curve is showing the no load voltage curve and AB is the internal characteristic curve.

External Characteristic of Shunt Wound DC Generator AC curve is showing the external characteristic of the shunt wound DC generator. It is showing the variation of terminal voltage with the load current. Ohmic drop due to armature resistance gives lesser terminal voltage the generated voltage. That is why the curve lies below the internal characteristic curve.

The terminal voltage can always be maintained constant by adjusting the of the load terminal.

When the load resistance of a shunt wound DC generator is decreased, then load current of the generator increased as shown in above figure. But the load current can be increased to a certain limit with (upto point C) the decrease of load resistance. Beyond this point, it shows a reversal in the characteristic. Any decrease of load resistance, results in electric current reduction and consequently, the external characteristic curve turns back as shown in the dotted line and ultimately the terminal voltage becomes zero. Though there is some voltage due to residual magnetism. We know, Terminal voltage Now, when IL increased, then terminal voltage decreased. After a certain limit, due to heavy load current and increased ohmic drop, the terminalvoltage is reduced drastically. This drastic reduction of terminal voltage across the load, results the drop in the load current although at that time load is high or load resistance is low. That is why the load resistance of the machine must be maintained properly. The point in which the machine gives maximum electric current output is called breakdown point (point C in the picture).

Types of D.C Machines [DC Generators & DC Motors] Types of D.C Machines [DC Generators & DC Motors] A DC machine is an electromechanical energy conversion device. It requires magnetic flux conductors and the relative motion for the energy conversion. Based on the production of magnetic flux (i.e., exciting the field winding) the DC machines are classified as follows.

1.Permanent Magnet Type D.C Machines

This type of machines are of low rating and consists of the magnetic poles fixed in the inner periphery and the armature coils feed or being fed by the supply in case of generator and motor respectively.

2.Electromagnet Type D.C Machines

A) Separately Excited DC Generator

In the separately excited machine the field windings are being fed by a separate D.C source (battery) and the. e.m.f generated in case of generator would be the sum of the supply voltage and the armature resistance drop.

B) Self Excited DC Generator

In case of self excited machines, the field winding is connected to the armature, so that the armature could feed the field coils. Let us consider, these conditions of connecting field winding in case of generators.

a) Shunt Wound DC Generators

If the field winding is connected across the armature winding then the machine is called the shunt machine. In the shunt generator, as the shunt winding has to overcome the generated voltage it has to be made with higher turns of lower cross-sectional conductors. They have higher resistance as compared to the series coils,but the current is less. Let, Rsh = Shunt winding resistance Ish = Current flowing through the shunt field Ra = Armature resistance Ia = Armature current IL = Load current V = Terminal voltage Eg = Generated emf

Ia=Ish + IL

Shunt field current, Ish = V/Rsh Voltage across the load, V = Eg-Ia Ra Power generated, Pg= Eg×Ia Power delivered to the load, PL = V×IL

b) Series Wound DC Generators

In this type of generator the field coils are connected in series with the armature terminals and the conductors would be of higher cross- section and with lesser number of turns.

The current flowing through the coil would be the same as that of the armature current. So, it is made with higher cross-sections.

From above connections, Ia = Isc = IL=I (say) Voltage across the load, V = Eg -I(Ia×Ra) Power generated, Pg = Eg×I Power delivered to the load, PL = V×I

c) Compound Generator

The combination of two windings i.e., series winding and shunt field winding is considered as a compound generator. In the compound generator, normally the field of the shunt will be more'than the series field and will be less than the individual shunt machine. The same is the case with the series field also.

Based on the type of connection of the shunt field to the armature and series field it is classified as,

(i) Long shunt, (ii) Short shunt.

(i) Long Shunt Compound Wound DC Generator

In this type of compound machine, the series current and the armature current is made , same and the shunt connection is made after the series connection is done.

Series field current = Armature current

Shunt field current, Ish=V/Rsh Armature current, Ia= series field current, Isc= IL+Ish Voltage across the load, V=Eg-Ia Ra-Isc Rsc=Eg-Ia (Ra+Rsc) [∴Ia=Ics] Power generated, Pg= Eg×Ia Power delivered to the load, PL=V×IL

(ii) Short Shunt Compound Wound DC Generator

In this type of machine, the series field current is made same to that of the line current. The connection of shunt field is done first and then to the series field.

Series field current, Isc = IL Shunt field current, Ish = (V+Isc Rsc)/Rsh Armature current, Ia = Ish + IL Voltage across the load, V = Eg - Ia Ra - Isc Rsc Power generated, Pg = Eg×Ia Power delivered to the load, PL=V×IL

Whatever the connection of machine long or short shunt machines, if the flux produced by the series field aids with the shunt field then the machine is called cumulative compound machine or if the series field opposes the shunt field then the machine is called the differential compound machine.