Download DC Generator-Characteristics

DC Generator-Characteristics
Characteristics of DC Generators
• (i) Open Circuit Characteristic (O.C.C.),
• (ii) Internal or Total Characteristic
• (iii) External Characteristic.
• When the DC generator is driven by a prime
mover then an emf is induced in the
armature. The generated emf in the armature
is given by an expression
•
•
•
•
•
•
Here,
φ is the flux per pole,
P is the no. of poles,
N is the no. of revolution made by armature per minute,
Z is the no. of armature conductors,
A is no. of parallel paths.
• Generated emf is directly proportional to the
product of flux per pole and the speed of the
armature.
As the excitation current or field current (If) increases from its initial value, the
flux and hence generated emf is increased with the field current.
Magnetization curve of a DC generator represents the
saturation of the magnetic circuit. This curve is also
called saturation curve.
Magnetization curve of a DC generator does not start
from zero initially. It starts from a value of generated
voltage due to residual magnetism.
Residual Magnetism
In ferromagnetic materials, when current is reduced to
zero, there is still magnetic power left in those coils core.
This phenomena is called residual magnetism.
• Open Circuit Characteristic (O.C.C.) (E0/If)
• Open circuit characteristic is also known as magnetic
characteristic or no-load saturation characteristics
• Relation between generated emf at no load (E0) and
the field current (If) at the given fixed speed.
• The O.C.C. curve is just the magnetization curve and it
is practically similar for all type of generators.
• The data for O.C.C. curve is obtained by operating the
generator at no load and keeping speed constant. Field
current is varied and the corresponding terminal
voltage is recorded.
•
• Internal or Total Characteristic (E/Ia)
• Relation between the on-load generated emf (Eg) and the
armature current (Ia).
• The on-load generated emf Eg is always less than E0 due to
armature reaction.
• Eg can be determined by subtracting the drop due to
demagnetizing effect of armature reaction from no-load voltage
E0. Therefore, internal characteristic curve lies below O.C.C.
curve.
• 3. External Characteristic (V/IL)
• The external characteristic curve shows the relation between the
terminal voltage (V) and load current (IL).
• The terminal voltage V is less than generated emf Eg due to
voltage drop in the armature circuit.
• Therefore the external characteristic curve lies below the
internal characteristic curve.
• External characteristics are very important to determine the
suitability of a generator for a given purpose.
If there is no armature reaction and armature voltage drop, voltage will remain constant
for any load current.
AB - no-load voltage vs. load current IL.
Due to demagnetizing effect of armature reaction the on-load generated emf is less
than the no-load voltage.
AC -on-load generated emf Eg vs. load current IL i.e. internal characteristic.
AD -terminal voltage vs. load current i.e. external characteristic.
When load resistance is decreased in DC shunt generator, the load current
increases. But, load resistance can be decreased upto a certain limit,
beyond this limit any further decrease in load resistance results in
decreasing load current and terminal voltage. Consequently, the external
characteristic curve turns back as shown by dotted line in above figure.
The curve AB in above figure identical to open circuit characteristic (O.C.C.) curve. This is because, in DC series
generators field winding is connected in series with armature and load. Hence, here load current is similar to
field current. The curve OC and OD represents internal and external characteristic respectively.
• If series winding is adjusted so that, increase in load current causes increase in
terminal voltage then the generator is called to be over compounded.
• If series winding is adjusted so that, terminal voltage remains constant even the
load current is increased, then the generator is called to be flat compounded.
• If the series winding has lesser number of turns than that would be required to
be flat compounded, then the generator is called to be under compounded.
• Terminal voltage drop from no load to full load in a shunt
generator can be partially/fully/ over compensated by use of
an aiding series field (cumulatively compound). Aiding ampere
turns in series field automatically increase with the load,
compensating the armature voltage drop.
• Differential compounding is not used in practice as the
terminal voltage falls off steeply with load.