Chapter 1 DC Generator (Generator Characteristics) 2022
DC generator Characteristics:
Following are the three most important characteristics of a dc generator:
1. Open Circuit Characteristics (OCC): this curve shows the relation between the
generated emf at no load (Eo) and the field current (If) at constant speed. It is also known
as magnetizing characteristics or no load saturation curve. Also called as no load
characteristics or magnetic characteristics.
2. Internal or total characteristics (E/Ia): this curve shows the relation between the
generated emf on load (E) and armature current (Ia). The emf is less than E0 due to the
demagnetizing effect of armature reaction. Therefore, this curve lies below OCC. This
characteristics is useful for designers.
3. External characteristics (V/IL): this curve shows the relation between the terminal
voltage (V) and load current (IL). The terminal voltage (V) will be less than E due to drop
in the armature circuit. Therefore, this curve lies below internal characteristics. This
characteristic is very important in determining the suitability of a generator for a given
purpose.
Open Circuit Characteristics (OCC)
The O.C.C. for a d.c. generator is determined as follows. The field winding of the d.c. generator
(series or shunt) is disconnected from the machine and is separately excited from an external d.c.
source as shown in Fig. (Below) (ii). the generator is run at fixed speed (i.e., normal speed). The
field current (If) is increased from zero in steps and the corresponding values of generated
e.m.f.(E0) read off on a voltmeter connected across the armature terminals. On plotting the
relation between E0 and If, we get the open circuit characteristic as shown in Fig. (Below) (i).
Figure: Circuit diagram and OCC
Electrical Machine I (SS 2022)
Chapter 1 DC Generator (Generator Characteristics) 2022
The following points may be noted from O.C.C.:
i.
When the field current is zero, there is some generated e.m.f. OA. This is due to the
residual magnetism in the field poles.
ii.
Over a fairly wide range of field current (up to point B in the curve), the curve is linear. It is
because in this range, reluctance of iron is negligible as compared with that of air gap. The
air gap reluctance is constant and hence linear relationship.
iii. After point B on the curve, the reluctance of iron also comes into picture. It is because at
higher flux densities, µr for iron decreases and reluctance of iron is no longer negligible.
Consequently, the curve deviates from linear relationship.
iv.
After point C the curve, the magnetic saturation of poles begins and E 0 tends to level off.
Characteristics of a separately excited DC generator
1. OCC
2. Load Characteristics
1. OCC: refers to no load conditions of generators
𝑷∅𝑵𝒁
Eg= 𝟔𝟎 𝑨 ; 𝑷, 𝒁, 𝑨 = 𝒄𝒐𝒏𝒔𝒕𝒂𝒏𝒕; 𝑬𝒈 ∝ ∅𝑵
a. Eg-N characteristics
If=constant, N=variable; Eg∝ 𝑁 (∅ 𝑖𝑠 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑎𝑠 𝐼𝑓 𝑖𝑠 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡. Therefore for constant field
current, Eg-N characteristics is a straight line passing through the origin as shown below.
b. Eg-If characteristics
N-constant; If=variable; 𝐸𝑔 ∝ ∅ (𝑠𝑖𝑛𝑐𝑒 𝑁 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 )
Figure: Eg-If characteristics
Electrical Machine I (SS 2022)
Chapter 1 DC Generator (Generator Characteristics) 2022
2. Load Characteristics/ Load saturation: The curve showing the relation between the
terminal voltage V and field current If when generator is loaded.
a. Internal characteristics: is the curve between emf actually induced in the
armature (after taking armature reaction into account) and the load current I L. This
curve lies below OCC.
b. External Characteristics: is the curve between terminal voltage V of the
generator and load current IL. it lies below Internal characteristics.
Figure: Circuit Diagram
Figure: Internal and External Characteristics
When IL=0; E0=Eg
As the load increases terminal voltage E0 starts drooping due to loading effect and the
following reasons.
- Armature reaction weakens the main flux so that actual emf generated E on the load
is less than that of E0 (no load emf)
Electrical Machine I (SS 2022)
Chapter 1 DC Generator (Generator Characteristics) 2022
-
Drop due to armature resistance (IaRa=ILRa)
Curve 2=internal characteristics; curve 3=external characteristics and curve 1=curve in absence
of armature reaction and armature drop.
Characteristics of series wound DC Generator:
Figure: Circuit diagram and characteristics
1. OCC: curve between E0 and field current and is shown by curve 1
2. Internal characteristics: relation between generated emf E and armature current Ia(=Ise=IL)
Due to armature reaction (demagnetizing effect) flux in the machine will be less than flux at no
load. Hence E<E0 (E∝ ∅). Therefore internal characteristics lie below OCC as shown by curve 2.
3. External characteristics: gives relation between terminal voltage V and load current I L.
V=Eg-Ia(Ra+Rse). External curve will lie below internal curve by amount Ia(Ra+Rse) drop as
shown by curve 3
From external characteristics it is observed that the terminal voltage first increases with the
increase in load current, reaches the maximum value and finally decreases.
If the load circuit resistance is reduced sufficiently, the terminal voltage may fall to zero. So if
the series generator is operated in initial straight line portion of the characteristics it gives
voltage approximately proportional to the current and if it is operated in drooping portion of the
curve it gives approximately constant current irrespective of external load resistances.
Electrical Machine I (SS 2022)
Chapter 1 DC Generator (Generator Characteristics) 2022
Characteristics of shunt Generator:
Figure: circuit diagram and characteristics
Characteristics of compound wound DC Generator
Figure: Circuit Diagram and Characteristics curve
Over compounded: if series excitation becomes more prominent than shunt, terminal voltage rise
with increase in load.
Flat compound: terminal voltage remains constant.
Under compound: shunt field excitation is more prominent, full load terminal voltage is less than
no load terminal voltage.
Electrical Machine I (SS 2022)
Chapter 1 DC Generator (Generator Characteristics) 2022
Voltage Regulation: change in terminal voltage of a generator between full load and no load at
constant speed.
Percentage voltage regulation=
𝑵𝒐 𝒍𝒐𝒂𝒅 𝒕𝒆𝒓𝒎𝒊𝒏𝒂𝒍 𝑽𝒐𝒍𝒕𝒂𝒈𝒆−𝑭𝒖𝒍𝒍 𝒍𝒐𝒂𝒅 𝑻𝒆𝒓𝒎𝒊𝒏𝒂𝒍 𝑽𝒐𝒍𝒕𝒂𝒈𝒆
𝑭𝒖𝒍𝒍 𝑳𝒐𝒂𝒅 𝒕𝒆𝒓𝒎𝒊𝒏𝒂𝒍 𝑽𝒐𝒍𝒕𝒂𝒈𝒆
𝒙𝟏𝟎𝟎
Q1. A shunt generator is rated at 240 V at full load; at no load the voltage is 252 V. calculate %
voltage regulation.
Solution:
Given data: VFL=240 V; VNL=252 V
% VR=
252−240
240
𝑥100 = 5%
If VNL-VFL is small = good regulation
If VNL-VFL is large= poor regulation.
Critical Load Resistance:
If a straight line OB passing through origin is drawn, then its point of intersection with external
characteristics gives terminal voltage along Y-axis and current along X-axis when the resistance
of external load circuit is equal to the resistance represented by the slope of this line. i.e) Load
resistance=gradient of line OB.
If series of such lines are drawn, the point of intersection with the external characteristics and
passing through origin “O” neglecting the initial ordinates due to residual magnetism such as
OC, is drawn, the resistance represented by the slope of this line is called CRITICAL
RESISTANCE
Electrical Machine I (SS 2022)
Chapter 1 DC Generator (Generator Characteristics) 2022
Critical resistance: is the maximum resistance for given speed with which the generator will be
able to excite.
If resistance in the external load circuit is greater than this, as shown by slope of line OD,
generator will not excite because OD has no point of intersection with external characteristics
and so it will deliver no current.
Voltage build up for shunt generator
Figure: Building up of voltage of a shunt generator at no load
The generator excites itself due to residual magnetism and develops the voltages as discussed
below:
OP=shunt filed resistance
✓ When generator is started a small emf is induced due to residual magnetism (OA). This
emf causes a flow of current 𝑜𝑎′ in the field circuit
✓ When generated emf =𝑎′ 𝑎" ; produces field current ob', which in turn produces a high
voltage b'b". hence effect is cumulative and value of induced emf and field current
increases until these reach point D (Intersection point of field resistance line and OCC)
OB=maximum value of Eg and generator cannot generate more than this voltage.
Conditions for self-excitation:
Conditions required to be fulfilled before a series or shunt generator excites itself are:
1. There must be some residual magnetism in the field system
Electrical Machine I (SS 2022)
Chapter 1 DC Generator (Generator Characteristics) 2022
2. The residual magnetism must be in proper direction. The field coils should be connected
with the armature in such a way that current flowing through them should increase the
induced emf by the residual magnetism.
3. For series wound generator, the resistance of the external circuit should be less than the
critical resistance
4. For shunt wound generator, resistance in field must be less than critical resistance for
field circuit and resistance in the load circuit must be greater than critical resistance for
load circuit.
Causes of failure to build up voltage:
1. No residual magnetism: the start of the buildup process needs some residual magnetism
in the magnetic circuit of the generator. If there is little or no residual magnetism,
because of inactivity or jarring (vibrating) in shipment (transportation), no voltage will be
induced that can produced field current.
2. Reversal of field connections: the voltage induced owing to residual magnetism acts
across the field and results in flow of current in the field coils in such a direction as to
produce magnetic flux in the same direction as the residual flux. Reversal of connection
of the field windings destroys residual magnetism which causes the generator failure to
build up voltage.
3. In case of dc series wound generators, the resistance in the load circuit may be more
than its critical resistance, which may be due to
a. Open circuit
b. High resistance of load circuit
c. Faulty contact between brushes and commutator
d. Dirty/ Greasy commutator surface.
4. In case of shunt wound generator
a. The resistance of the shunt field circuit may be greater than critical resistance
b. Resistance in the load circuit may be lower than critical resistance
c. Speed of rotation may not be equal to rated one.
Solutions:
In case of generator which is starting for the first time-it may be that no voltage will build up
because
-poles have no residual magnetism
-Poles have retained some residual magnetism but field winding connections are reversed so that
the magnetism developed by the field winding on start has destroyed the residual magnetism and
the machine cannot build up.
Electrical Machine I (SS 2022)
Chapter 1 DC Generator (Generator Characteristics) 2022
In both the cases field coils must be connected to a dc source (a storage battery) for a short while
to magnetize the poles. The application of external source of direct current to the field is called
FLASHING of the field.
Determination of OCC at different speed (N2) from OCC drawn for some other speed (N1)
𝑃∅𝑁𝑍
Eg=
60 𝐴
; P,Z, A=constant; if ∅ = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
Eg∝ N
Let E1=induced emf from OCC at a speed N1 for excitation I Amperes
Induced emf at speed N2 and for same excitation of I Amperes is given by
𝐸2 𝑁2
=
𝐸1 𝑁1
𝑵
𝑵
E2=𝑵𝟐 𝒙E1 =HD=HC x 𝑵𝟐
𝟏
𝟏
Critical Speed: is that speed for which the given shunt field resistance will represent critical
field resistance.
Or
Is the minimum speed below which the shunt generator fails to excite.
Electrical Machine I (SS 2022)
Chapter 1 DC Generator (Generator Characteristics) 2022
Curve 1 represents OCC at given speed
OA=tangent line to OCC
𝐵𝐶 𝑁𝑐
=
𝐴𝐶
𝑁
𝑩𝑪
NC=𝑨𝑪 𝒙𝑵
Electrical Machine I (SS 2022)