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Constructional feature of synchronous generator ;
A synchronous generator or alternator consists of an armature winding and a magnetic
field. Armature winding mounted on a stationary element called stator and field
winding on a rotating element called rotor.
The stator consists of a cast iron frame which supports the armature core having slots
on its inner periphery for housing the armature conductor. The rotor is like a flywheel having alternate N and S poles fixed to its outer rim. The magnetic poles are excited
from direct current supplied by a d.c source which is belted or mounted on the shaft of
the alternator itself. Because the field magnets are rotating, this current is supplied
through two slip rings. As the exciting voltage is relatively small, the slip ring and
brushes gear are of light construction.
Types of synchronous generator :
There are two types of rotor used in synchronous generator. Accordingly, A
synchronous generator may be classified as:
(i) Salient pole type
(ii) smooth cylindrical type.
Salient pole type has a large number of projecting poles having their cores bolted or
dovetailed on to a heavy magnetic wheel of cast iron or steel of good magnetic
quality. Such generators are characterized by their large diameter and short axial
length.
Smooth cylindrical type runs at very high speed. The rotor consists of a smooth solid
forged steel cylinder having a number of slots milled out at intervals along the outer
periphery for accommodating field coils. Such rotors are designed mostly for 2 pole
turbo generator running at 3600 rpm. These are characterized by small diameter and
long axial rotor length. The cylindrical construction of the rotor gives better balance
and quieter operation and less windage losses.
E.M.F equation of synchronous machine :
Let Z = no. of conductor or coil sides in series / phase.
= 2T – where T is the no of coils or turns/phase.
P = No. of poles.
F = frequency of induced e.m.f in Hz.
F = flux /pole in webers
Kd = distribution factor = sin m /2 / msin /2
KC & KP = Pitch or coil span factor = cos /2
Kf = form factor = 1.11 – if e.m.f is assumed sinusoidal
N = rotative speed of the rotor in r.p.m
In one revolution of the rotor (i.e in 60/N seconds) each stator conductor is cut by a flux of
F p webers
dF = F p and dt = 60/N seconds
\ average e.m.f induced per conductor
= dF/ dt = F p / (60/N) = F Np / (60) volts
Now, we know that f = pN/120
Or N = 120f /p
Substituting this value of N above, we get average e.m.f per conductor
=F p / (60) x 120f/p = 2 f F volts
if there are Z conductors in series / phase then
Average e.m.f /phase = 2fF Z volts
= 4fFT volts
R.M.S value of e.m.f / phase = 1.11 x f FT
= 4.44 fF T volts
This would have been the actual value of the voltage induced. All the coils in a phase are :
(i) full pitched and
(ii) concentrated or bunched in one slot.
But the actually available voltage is reduced in the ratio of the these two factors.
\ actually available voltage / phase.
= 4.44 k C kd fF T volts
=4.44 kf k C kd fF T volts
gregorymasila answered the question on February 22, 2018 at 17:00
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