Classification of DC Generators:
DC generators are classified as:
- Separately Excited Generator
- Shunt Generator
- Series Generators and
- Compound Generators
Separately Excited DC Generator:
In a separately excited generator field winding is energized from a separate voltage source to produce flux in the machine. So long the machine operates in unsaturated conditions the flux produced will be proportional to the field current. To implement a shunt connection, the field winding is connected in parallel with the armature. It will be shown that subject to the fulfillment of certain conditions, the machine may have sufficient field current developed by its shunt connection.Series Excited DC Generator:
In the Series d.c machine, there is one field winding wound over the main poles with fewer turns
and a 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 minimal voltage due to the residual field under no load condition (Ia = 0).
However, the field gets strengthened as the 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. The terminal voltage will start falling when saturation sets in and the armature reaction effect become pronounced at a large load current. Hence, series generators are
not used for delivering power at a constant voltage. Series generator found application in boosting
up the voltage in d.c transmission system.
Compound DC Generator:
A compound generator has two separate field coils wound over the field poles. The coil
having a large number of turns and a thinner cross-sectional area is called the shunt field coil and the
other coil having a few numbers of turns and a 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. If the series coil is left alone without any connection, then it
becomes a shunt machine with the other coil connected in parallel.
 |
| Field coils for Different DC Machines |
Characteristics of a Separately Excited Generator:
- No load or open-circuited Characteristics.
- Load Characteristics.
No load or open-circuited Characteristics:
In this type of generator field winding is excited from a separate source, hence field current is
independent of armature terminal voltage. The generator is driven by a
prime mover at rated speed, say n rps. With switch S in opened condition, the field is excited via a
potential divider connection from a separate d.c source, and the field current is gradually increased.
The field current will establish the flux per pole φ. The voltmeter V connected across the
armature terminals of the machine will record the generated emf [Eg=(pz/60a)φn=kφn]. As field current is increased, Eg will increase.' Eg' versus 'If' plot at constant speed n is shown in the figure.
 |
| Connection of Separately Excited Generator |
It may be noted that even when there is no field current, a small voltage (OD) is generated
due to residual flux. If field current is increased, φ increases linearly initially and O.C.C follows
a straight line. However, when saturation sets in, φ practically becomes constant, and hence Eg
too becomes constant. In other words, O.C.C. follows the B-H characteristic, hence this
characteristic is sometimes also called the magnetization characteristic of the machine.
 |
| No load and load Characteristics of Separately Excited Generator |
It is important to note that if O.C.C is known at a certain speed nl, O.C.C at another speed n2 can
easily be predicted. It is because, for a constant field current, the ratio of the generated voltages
becomes the ratio of the speeds as shown below.
Eg2/Eg1=generated voltage at n2/generated voltage at n1
ie. Eg2/Eg1=n2/n1;if field current is constant.
 |
| OCC at Different Speeds |
Load Characteristics of Separately Excited Generator:
Load characteristic essentially describes how the terminal voltage of the armature of a generator
changes for varying armature current Ia. First, at rated speed, rated voltage is generated across the
armature terminals with no load resistance connected across it (i.e., with S opened) by adjusting
the field current. So for Ia = 0, V = Eo should be the first point on the load characteristic. Now
with S closed and by decreasing RL from an infinitely large value, we can increase Ia gradually
and note the voltmeter reading. The voltmeter reads the terminal voltage and is expected to decrease
due to various drops such as armature resistance drop and brush voltage drop. In an
uncompensated generator, the armature reaction effect causes an additional voltage drop. While noting
down the readings of the ammeter A2 and the voltmeter V, one must see that the speed remains
constant at the rated value.
Characteristics of Shunt Generator:
Shunt generators are self-excited type, their field winding is energized by their own terminal voltage. The field coil (F1, F2) along with a series-external resistance is connected in parallel with the machine's armature terminals (A1, A2). Let us first qualitatively
explain how such a connection can produce sufficient voltage. Suppose there exists some residual
field. Therefore, if the generator is driven at rated speed, we should expect a small voltage
( kφn ) to be induced across the armature. But this small voltage will be directly applied across
the field circuit since it is connected in parallel with the armature. Hence a small field current
flows producing additional flux. If it so happens that this additional flux aids the already existing
residual flux, total flux now becomes more generating more voltage. This more voltage will
drive more field current generating more voltage. Both field current and armature-generated
voltage grow cumulatively.
This voltage growth and the final value to which it will settle down can be understood by referring to the figure below where two plots have been shown. One corresponds to the O.C.C. at rated
speed and is obtained by connecting the generator in a separately excited fashion as detailed in the
preceding section. The other one is the V-I characteristic of the field circuit which is a straight
line passing through the origin and its slope represents the total field circuit resistance.
 |
| Shunt Generator |
 |
| Voltage buildup in Shunt Generators |
Initially, the voltage induced due to residual flux is obtained from O.C.C and given by Od. The
field current thus produced can be obtained from the field circuit resistance line and given by Op. In
this way, the voltage build-up process continues along the staircase. The final stable operating point
(M) will be the point of intersection between the O.C.C. and the field resistance line. If the field
circuit resistance is increased, the final voltage decreases as a point of intersection shifts toward the left.
The field circuit resistance line which is tangential to the O.C.C. is called the critical field
resistance. If the field circuit resistance exceeds the critical value, the machine will fail to
excite and no voltage will be induced.
 |
| Critical Field Resistance |
 |
| Critical Speed |
Essential requirements for Voltage Buildup in Shunt Generators:
A shunt generator driven by a prime mover, can not build up voltage if it fails to comply with any of
the conditions listed below.
- The machine must have some residual field. To ensure this one can at the beginning excite the field separately with some constant current. Now removal of this current will leave some amount of residual field.
- The field winding connection should be such that the residual flux is strengthened by the field current in the coil. If due to this, no voltage is being built up, reverse the field terminal connection.
- The total field circuit resistance must be less than the critical field resistance.
Load Characteristics of Shunt Generators:
With switch S in open condition, the generator is practically under no load condition as the field current is pretty tiny. The voltmeter reading will be Eo. In other words, Eo and Ia = 0 is the first point in the load characteristic. To load the machine S is closed and the load resistances decreased so that it delivers load current IL. The drop in the terminal voltage will be caused by the usual IaRa drop, brush voltage drop, and armature reaction effect. Apart from these, in the shunt generator, as terminal voltage decreases, field current hence φ also decreases causing an additional drop in terminal voltage. Remember in a shunt generator, the field current is decided by the terminal voltage by virtue of its parallel connection with the armature. The plot of terminal voltage versus armature current is called the load characteristic. |
| Load Characteristics of Shunt Generator |
Characteristics of Series Generators:
- Magnetic Characteristics
- Internal Characteristics
- External Characteristics
Magnetic Characteristics:
In series generators, the field winding is connected in series with the armature. It is made of a thick copper conductor as the whole load current passes through it. The curve which shows the relation between no load voltage and the field excitation current is called Magnetizing Characteristics. As during no load, the load terminals are open-circuited, there will be no field current in the field since, the armature, field, and load is series connected, and these three make a closed loop of circuit.
Internal Characteristics:
The internal characteristics curve gives the relation between the voltage generated in the armature and the load current. This curve is obtained by subtracting the drop due to the demagnetizing effect of the armature reaction from the no-load voltage. So, the actual generated voltage ( Eg) will be less than the no-load voltage (E0). That is why the curve slightly drops from the open circuit characteristic curve.
External Characteristics:
The external characteristic curve shows the variation of terminal voltage (V) with the load current ( IL). The terminal voltage of this type of generator is obtained by subtracting the ohmic drop due to armature resistance (Ra) and series field resistance ( Rsc) from the actually generated voltage ( Eg).
Terminal voltage V = Eg - I(Ra + Rsc)The external characteristic curve lies below the internal characteristic curve because the value of the terminal voltage is less than the generated voltage. It can be observed from the characteristics of the series wound DC generator, that with the increase in load (load is increased when load-current increases) the terminal voltage of the machine increases. But after reaching its maximum value it starts to decrease due to the excessive demagnetizing effect of the armature reaction. This phenomenon is shown in the figure by the dotted line. The dotted portion of the characteristics gives approximately constant current irrespective of the external load resistance. This is because if the load is increased, the field current is increased as the field is series connected with the load.
 |
| Characteristics of Series Generator |
Characteristics of Compound Generators:
Compound machines have both series and shunt field coils. On each pole, these two coils are placed. Series field coil has low resistance, fewer
numbers of turns with a large cross-sectional area, and is connected either in series with the armature
or in series with the line. On the other hand, the shunt field coil has a large number of turns, higher
resistance, small cross-sectional area and is 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.
 |
| Short Shunt Connection |
 |
| Long Shunt Connection |
Series field coil may be connected in such a way that the mmf produced by it aids the shunt
field mmf then the machine is said to be a cumulative compound machine, otherwise, if the series
field mmf acts in opposition with the shunt field mmf – then the machine is said to be a differential
compound machine. In a compound generator, the series field coil current is load-dependent. Therefore, for a
cumulatively compound generator, with the increase of load, flux per pole increases. This in turn
increases the generated emf and terminal voltage. Unlike a shunt motor, depending on the
strength of the series field mmf, the terminal voltage at full load current may be the same or more than
the no-load voltage. When the terminal voltage at rated current is the same that at no load condition,
then it is called a level compound generator. If the terminal voltage at rated current is more
than the voltage at no load, it is called an over-compound generator. The load characteristic of a
cumulative compound generator will naturally be above the load characteristic of a shunt
generator At a load current higher than the rated current, the terminal
voltage starts decreasing due to saturation, armature reaction effect, and more drop in armature
and series field resistances. For the differentially compounded generator where the series field mmf opposes the shunt field
mmf, the terminal voltage decreases fast with the increase in the load current.
 |
| Load Characteristics of DC Generators |