Question

3- Write the types of DC generator?

Answer 1

D.C generators may be classified as

1. Separately excited generator

2. Shunt generator

3. Series generator

4. Compound generator

Shunt field Series field Shunt machine 其 Series machine F. F, 5」 Shunt field Series field Compound machine Se, Se, F. F: Field coils for different d.c machines.

Separately excited generator - In a separately excited generator field winding is energised from a separate voltage source in order to produce flux in the machine. As long as the machine operates in unsaturated condition the flux produced will be proportional to the field current.

IL A2 Field coil 0000 R. A1 Supply for field 6 Connection of separately excited generator.

Shunt generator - In these type of DC generators, the field windings are connected in parallel with armature conductors, as shown in the figure below. In shunt wound generators the voltage in the field winding is same as the voltage across the terminal

I. IL IL A A1 L 00007 FI Field coil R w AZ Shunt generator.

Series generator -In series d.c generator, there is one field winding wound over the main poles with fewer turns and large cross sectional area. Series winding is meant to be connected in series with the armature and naturally to be designed for rated armature current. Obviously there will be practically no voltage or very small voltage due to residual field under no load condition (Ia = 0). However, field gets strengthened as load will develop rated voltage across the armature with reverse polarity, is connected and terminal voltage increases. Variation in load resistance causes the terminal voltage to vary. Terminal voltage will start falling, when saturation sets in and armature reaction effect becomes pronounced at large load current. Hence, series generators are not used for delivering power at constant voltage. Series generator found application in boosting up voltage in d.c transmission system.

Isc IL elle Series Field L la + V D Eg ARMATURE 09 Series Wound Generator

Compound generator - A compound generator has two separate field coils wound over the field poles. The coil having large number of turns and thinner cross sectional area is called the shunt field coil and the other coil having few number of turns and large cross sectional area is called the series field coil. Series coil is generally connected in series with the armature while the shunt field coil is connected in parallel with the armature.

As introduced earlier, compound machines have both series and shunt field coils. On each pole these two coils are placed as shown in topmost figure(field coils for DC machines). Series field coil has low resistance, fewer numbers of turns with large cross sectional area and connected either in series with the armature or in series with the line. On the other hand shunt field coil has large number of turns, higher resistance, small cross sectional area and either connected in parallel across the armature or connected in parallel across the series combination of the armature and the series field. Depending on how the field coils are connected, compound motors are classified as short shunt and long shunt types as shown in figures below.

Series field O Shunt field 00000 Shunt field Series field Short shunt connection. Long shunt connection.