MAYURBHANJ SCHOOL OF ENGINEERING
AY-2021-2022
ELECTRICAL ENGINEERING DEPARTMENT
5th Semester
SUB-Energy Conversion - II
TOPIC-ALTERNATOR
by
Er. Sailesh Barik
CONSTRUCTION OF ALTERNATOR
Stationary Armature - Rotating Field
An alternator has 3 phase winding on the stator and
DC field winding on the rotor.
STATOR
Stationary part of the machine.
It is built up of Sheet-Steel Lamination Core (Stampings) with slots
to hold the armature Conductor
Armature winding is connected in STAR
ROTOR:
There are two types of rotor
i) Salient Pole type {Projected Poles}
ii) Non - Salient Pole type {Non – Projected Poles} Smooth Cylindrical Type
Salient Pole type {Projected Poles}
It is also called Projected Poles.
Poles are mounted on the larger circular frame.
Made up of Thick Steel Laminations.
Field Winding are connected in series.
Ends of the field winding are connected to the DC Supply through Slip Rings
Features
Large Diameter and short Axial Length.
Poles are Laminated to reduced
Eddy Current Losses
DAMPER WINDING
Pole faces are provided with damper winding
Damper winding is useful in preventing Hunting
EMF generated will be sinusoidal
Copper Bar
II) NON SALIENT POLE TYPE
Smooth cylindrical rotor or TURBO ALTERNATOR
field winding used in high speed alternators driven by steam turbines .
Features
Smaller diameter and larger axial length compared to salient pole type machines, of the same rating.
Less Windage loss.
Speed 1200 RPM to 3000 RPM.. Better Balancing..
Noiseless Operation
Flux distribution nearly sine wave
Frequency 50 Hz
Ns = 120 F / P
Poles | 2 | 4 | 6 |
Speed | 3000 | 1500 | 1000 |
EMF Equation
Where,K
c
= cos
(α/
2),
K
d
= {sin
(mβ/
2)} / {m sin
(β/
2)}
f = PNs/120, Hz;
Φ
= flux per pole, Wb
T
ph
= Turns in series per phase
= (
No. of slots * No. of cond. per slot) / (2 x 3)
phd c ph
T f K K E
44.4
EMF Equation of an Alternator
Let
Φ = Flux per pole, Wb
P = Number of Poles
Ns = Synchronous Speed in RMP
Z = Total Number of Conductors or coil sides in series / Phase
Z = 2T
T = Number of coils or Turns per phase
Tph = Turns in series per phase
= ( No. of slots * No. of cond. per slot) / (2 x 3)
Zph = Conductor per phase
Zph = Z / 3. No. of phase 3
Kc or Kp = Pitch factor or coil span factor
Kd = Distribution factor
Kp = Cos (α / 2 )
Kd = Sin (mβ / 2)
m Sin(β / 2)
ARMATURE WINDING
3 Phase alternator carry 3 sets of winding arranged in slots
Open circuited
6 terminals
Can be connected in Star or Delta
Armature Winding Classification
Single Layer and Double Layer Winding
Single- layer winding
• One coil-side occupies the total slot area
• Used only in small ac machines
Double- layer winding
• Coil-sides in two layers
• Double-layer winding is more common used above about 5kW machines
The advantages of double-layer winding over single layer winding:
a. Easier to manufacture and lower cost of the coils
b. Fractional-slot winding can be used
c. Chorded-winding is possible
d. Lower-leakage reactance and therefore , better performance of the machine
e. Better emf waveform in case of generators
POLE – PITCH
It is the distance between the centres of pole
faces of two adjacent poles is called pole pitch.
Pole pitch = 180 Phase angle
COIL :
A coil consists of two coil sides.
Placed in two separate slots
SLOT PITCH:
It is the phase angle between two adjustment slots
COIL SPAN OR COIL PITCH
It is the distance between two coil sides of a coil
Full Pitch and Short Pitch Winding
Full Pitch Winding
If the coil span is equal to pole pitch then the winding is called Full Pitch Winding
Coil Span = Pole Pitch
Short Pitch Winding
If the coil span is less than Pole
Pitch is called Short pitch
winding
e1 V
e2 V
e1 V
e2 V
e2 V
CONCENTRATED AND DISTRIBUTED WINDING
Advantages of Short Chorded winding or Chorded Pitch Winding
Slot Angle : The angular displacement between any two
adjacent poles in electrical degree
Slot angle (β) = 180
(Number of slots / Pole)
PITCH FACTOR OR COIL SPAN FACTOR OR SHORT CHORDED FACTOR
Kp OR Kc
Pitch factor is defined as the ratio EMF induced in the Short pitch winding to the EMF induced in the full pitch winding
E V
E V
E V
α
α/2
A
B
D
C
2E
Vector Sum EMF = AB
= AC + CB
AD = BD
Kp = Cos (α / 2)
α/2
Kp = AC + CB
AD + DB
DISTRIBUTION FACTOR OR BREATH FACTOR (Kd)
E in coil 2
E in coil 1
β
β
E in coil 3
Arithmetic Sum of EMF = AB + BC + CD
From Vector diagram AB = Ax + xB
= r Sin (β/2) + r Sin (β/2)
AB = 2 r Sin (β/2)
AB = BC = CD = 2 r Sin (β/2)
Arithmetic Sum of EMF = 3 x (2 r Sin (β/2) )
If there are ‘m’ slots for distribution, then
Arithmetic Sum /phase of the EMF = m x (2 r Sin (β/2) )
Vector Sum of EMF AD = AE + ED
Vector Sum of EMF AE = ED = r Sin (mβ/2)
Vector Sum of EMF = 2r x (Sin (mβ/2))
Armature Leakage Reactance(XL)
Three major components -Slot leakage reactance, end winding leakage reactance and tooth tip leakage reactance.
Synchronous reactance / phase
Xs = XL + Xa
, where
Xa is the fictitious armature reaction reactance.
Synchronous impedance/phase
Zs = (Ra + jXs).
�
Causes of Voltage drop in Alternator
Armature Reaction
Effect of the armature flux on the main field flux.
Armature Reaction effect depends upon the PF of the Load
UPF - cross magnetizing.
Lag PF - demagnetizing.
Lead PF - magnetizing
UPF (Pure Resistive Load)
cross magnetizing
N
S
Main Flux Φf
Armature Flux Φa
Main Flux
Φf
Eph
Induced EMF due to Main Flux Φf
Iaph
Φa
Lagging PF (Purely Inductive Load)
Demagnetizing
N
S
Main Flux Φf
Armature Flux Φa
Main Flux
Φf
Eph
Induced EMF due to Main Flux Φf
Ia
Armature Flux
Φa
Load current
Lag the Voltage by
90
Main Flux
Decreases
DC excitation
Lead PF (Purely Capacitive Load)
Magnetizing
N
S
Main Flux
Φf
Eph
Induced EMF due to Main Flux Φf
Ia
Armature Flux
Φa
Main Flux Φf
Armature Flux Φa
Load current
Lead the Voltage by
90
Main Flux
Increases
DC excitation
1.Direct loading method
2. Synchronous impedance method or E.M.F. method
3. Ampere-turns method or M.M.F. method
4. Zero power factor method or Potier triangle method
5. ASA modified from of M.M.F. method
6. Two reaction theory
VOLTAGE REGULATION
Voltage Regulation of an alternator is defined as the change in terminal voltage from NO load to full load divided by full-load voltage.
% Voltage Regulation = E0 – V x 100
V
There are different methods available to determine the voltage regulation of an alternator,
Direct loading method
The prime mover drives the alternator at its synchronous speed.
The star connected armature is to be connected to a three phase load
The field winding is excited by separate d.c. supply.
To control the flux i.e. the current through field winding, a rheostat is inserted in series with the field winding.
Eph α Φ
..... (From e.m.f. equation)
For high capacity alternators, that much full load can not be simulated or directly connected to the alternator. Hence method is restricted only for small capacity alternators.
Synchronous Impedance Method or E.M.F. Method
The method is also called E.M.F. method
The method requires following data to calculate the regulation.
2. Open circuit characteristics which is the graph of open circuit voltage against the field current. This is possible by conducting open circuit test on the alternator.
3. Short circuit characteristics which is the graph of short circuit current against field current. This is possible by conducting short circuit test on the alternator.
Zs is calculated.
Ra measured and Xs obtained.
For a given armature current and power factor, Eph determined - regulation is calculated.
Synchronous Impedance
Regulation Calculation
Zs = √(Ra)2 + (Xs)2
Xs = √(Zs)2 - (Ra)2
Xs
Eph = √ (Vph Cos Φ + Ia Ra)2 + (Vph Sin Φ ± Ia Xs)2
Phasor Diagram of a loaded Alternator
Lagging PF Load
Vph
IaRa
IaXs
Eph
Ia
IaZS
O
A
B
C
Φ
Eph = √ (Vph Cos Φ + Ia Ra)2 + (Vph Sin Φ + Ia Xs)2
Vph Cos Φ
Vph Sin Φ
IaRa
Synchronizing and Parallel operation
Necessary Condition for Synchronization
The process of switching of an alternator to another alternator or with a common Bus bar without any interruption is called Synchronization
CONDITIONS FOR PARALLEL OPERATION
1. The terminal voltage of the incoming machine must be same as that of bus bar Voltage.
2. The frequency of the generated voltage of the incoming machine must be same as that of bus bar frequency.
3. The phase Sequence voltage of the incoming machine must be same as that of bus bar.(R Y B).
Advantages of Parallel operation
Continuity of supply is possible when Breakdown or Shut down for maintenance of alternator in generating station
Repair and Maintenance of individual machine can be carried out one after the other without effecting the normal routine work
Depending upon the load requirement any number of alternator can be operated and the remaining can be put off
It is economical and improves the efficiency of the generating station
New alternator can be connected in parallel, when the demand increases. This reduces the capital cost of the system.
Methods of Synchronization of alternator
Three Methods
Conditions Should Satisfy
1. Voltage �2. Frequency
3. Phase Sequence
Existing Alternator
Incoming Alternator
L1
L2
L3
Alternator 1
Alternator 2
R
R
R’
Y
Y’
Y
B
B’
B
Main Switch
Synchronizing
Switch
Bus Bar
V
V
Bus Bar Voltage
Incoming Voltage
Dark lamp
method
Alternator 1 is already (Exciting) connected with the Bus Bar and Supplying power to load
Alternator 2 is Incoming Alternator
Voltage of Incoming Alternator SHOULD be same to that of Exciting Alternator
V1 = V2 Voltage SAME
Phase Sequence
3 Lamps Glowing Uniformly together and becoming dark together Phase Sequence
is correct
LAMP Flickering together in uniform
Frequency
Difference in frequency Lamp will be glow DARK and BRIGHT alternatively
Speed of alternator 2 should be adjusted
Demerits
It is not possible to judge whether the incoming alternator is fast or slow.
The lamp can be dark even through a small value of voltage may present across the
Terminals.
Existing Alternator
Incoming Alternator
L1
L2
L3
Alternator 1
Alternator 2
R
R
R’
Y
Y’
Y
B
B’
B
Main Switch
Synchronizing
Switch
Bus Bar
V
V
Bus Bar Voltage
Incoming Voltage
Bright Lamp
Method
Lamps are cross connected
Lamps will GLOW the BRIGHTEST when two voltage are in PHASE (V2)
V1 = V2 Voltage SAME
Phase sequence same LAMPS will start Flickering in uniform
Switch is closed at the middle of the Brightest period of the lamp
Existing Alternator
Incoming Alternator
Alternator 1
Alternator 2
R
R
R’
Y
Y’
Y
B
B’
B
Main Switch
Synchronizing
Switch
Bus Bar
V
V
Bus Bar Voltage
Incoming Voltage
Synchroscope
Method
Slow
Fast
Synchroscope
LAMP Flickering together in uniform
Synchroscope consists of STATOR and ROTOR
The ROTOR is connected to the INCOMING alternator
The STATOR is connected to the EXISTING alternator
The pointer is attached to the rotor. The pointer will indicate the correct time of
closing the switch. (12’O Position)
Frequency Different the pointer will rotate
Anti clock wise ---- Frequency of INCOMING alternator is LOW
Clock wise ---- Frequency of INCOMING alternator is Higher
THANK YOU