UNIT V Design of Electrical Apparatus

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Outline
 Constructional details
 Winding design
 Output equation
 Choice of specific loadings
 Main dimensions
 Short circuit ratio
 Design of stator and rotor
 Design of Damper winding

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Advantages of Revolving Field
System
 Since the armature winding is stationary, load can
be directly connected.
 Easier to insulate stationary armature winding for
high voltage AC machines.
 More area is available for housing conductor in
stator than in rotor
 Due to the availability of more area in the stator,
cooling will be easier
 It requires only 2 slip rings if the output is to be
taken from rotor
 Less weight of field system so high speed can be
achieved

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Construction of synchronous
machines
 Synchronous machines are AC machines that have
a field circuit supplied by an external DC source
 In a synchronous generator, a DC current is applied
to the rotor winding producing a rotor magnetic field.
The rotor is then turned by external means
producing a rotating magnetic field, which induces a
3-phase voltage within the stator winding.

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machines
 In a synchronous motor, a 3-phase set of stator
currents produces a rotating magnetic field causing
the rotor magnetic field to align with it. The rotor
magnetic field is produced by a DC current applied
to the rotor winding.
 Field windings are the windings producing the main
magnetic field (rotor windings for synchronous
machines); armature windings are the windings
where the main voltage is induced (stator windings
for synchronous machines).

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Classification of Synchronous
Machines
 Salient pole/Projected Pole/Non- Cylindrical
Alternator
 Driven by water wheel or diesel Engine
 Operate at low speed
 Large number of poles are required to produce
frequency
 Hydro Power Station

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Machines

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Machines
 Non-Salient Pole/Non-Projected
Pole/Cylindrical Alternator
 Driven by steam & gas turbines
 Operate at very high speed
 Thermal Power Station

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Types of Synchronous Machines
(based on prime mover used)
 Hydro- generators
 Driven by water turbines
 Rating up to 750 MW
 Speed: 100 – 1000 rpm
 Turbo-alternators
 Driven by steam turbines
 High efficiency
 Ratings up to 1000 MW
 Speeds up to 3000 rpm

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Types of Synchronous
Machines
 Engine driven generators
 Driven by different forms of
internal combustion engines
 Ratings up to 20 MW
 Motors
 Plain synchronous machines or synchronous
induction machines

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Types of Synchronous
Machines
 Compensators
 Used for control of reactive power in power
supply
 Rated up to 100 MVAr

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machines
Two common approaches are used to supply a DC current to the field
circuits on the rotating rotor:
1. Supply the DC power from an
external DC source to the rotor by
means of slip rings and brushes;
2. Supply the DC power from a
special DC power source mounted
directly on the shaft of the
machine.
Slip rings are metal rings completely encircling the shaft of a machine but
insulated from it. One end of a DC rotor winding is connected to each of the
two slip rings on the machine’s shaft. Graphite-like carbon brushes
connected to DC terminals ride on each slip ring supplying DC voltage to
field windings regardless the position or speed of the rotor.

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machines

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Smooth Cylindrical Pole

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Run-Away Speed
 Speed which the prime mover would have, if it
is suddenly unloaded when working at it’s
rated load
 When suddenly unloaded racing occurs
 Speed governor is produced
 Pelton wheel - 1.8 times rated speed
 Francis turbine - 2 to 2.2 times rated speed
 Kalpan turbine - 2.5 to 2.8 times rated speedMaximum peripheral speed
for salient pole machine = 140 m/s
for Turbo alternators = 175
m/s

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Same as that of Induction machine
Ref PDF file
Output Equation

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Choice of specific magnetic loading
(Bav)
 Low Bav is preferred
 Iron loss- High Bav leads to High Iron loss;
 Voltage- High volt- Insulation takes larger space and teeth
is mechanically weak.
 Transient Short Circuit element- High Bav results in
decrease leakage reactance and increase Isc.
 High Bav is preferred
 Stability- P=EV/X; High Bav – Reduces X;
 Parallel operation
Range: 0.52 to 0.65
Wb/m2

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Choice of specific electric loading
(ac)
 Low ac is preferred
 Copper loss- High ac- loss increased
 Temperature Rise- High ac- Temp rise increased
 Voltage- High ac is suitable for low volt. Machine.
 Synchronous Reactance- ac affects XL and
armature reaction
 Stray Load Loss- High ac – loss steeply increased
 High value of ac machine has poor volt. regulation,
low s/ckt current and instability.
 Salient pole m/c- 20K to 40 KA/m
 Turbo alternators- 50K to 75 KA/m

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Design of Salient Pole
Machines
 Main Dimensions – Stator Diameter (D) and
Stator Length (L)
 Selection of Diameter depends upon
 Types of poles used and allowable speed
 Types of poles- 2 types
 Round pole and Rectangular pole
 The ratio of pole arc to pole pitch is in between
0.6 to 0.7 for round pole and 1 to 3 for
rectangular pole.
 Diameter is decided based on peripheral speed.
 Rotor should be designed to withstand
centrifugal force produced under runaway

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 Main Dimensions: D & L
D: Depends on type of pole & Va
 Two types of salient poles:
 Round pole , Rectangular Pole
 Round Poles:
 Ratio: b/τ=0.6 to 0.7
 For round poles,square pole shoes can be
used. Hence L = b
 Thus L/ τ=0.6 to 0.7

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Rectangular Poles:
Ratio: L/τ=1 to 5,
Maintained as 3 for economic field
system
Value of Peripheral Speed at runaway
speed:
Depends on type of pole attachment
For Bolted pole structure: Va=50m/s
Dovetail construction: Va= 80 m/s

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Stator Winding (Armature)
 Star connected with neutral earthed.
 It eliminates all triple frequency harmonics
from the line voltage.
 Present practice is to use double layer lap or
wave winding.
 Fractional slot windings are used to reduce the
higher order harmonics.

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Armature Design (stator)
 Armature windings are 2 types
 Single layer
 Double layer

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Single Layer Winding

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Double Layer Winding

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Advantages of Single and Double
layer windings
 Double layer windings have more advantages over
single layer windings
 Easy manufacture of coils
 Lower cost
 Fractional slot windings can be used
 Single layer winding Advantages
 Higher efficiency
 Quieter operation
 Modern practice all over the world, double layer
winding is preferred.

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Number of Armature Slots (S)
 Factors
 Balance windings- unbalance may lead to overheat
 Cost- If S is small leads to fewer coils to wind,
insulate.
 Temperature- If S is Small and it gives rise to high
internal temperature.
 Leakage Reactance- If S is small, it is increased.
 For High voltage and large capacity machines, it is
desirable to use a larger slot pitch. (25 mm- 60 mm)
 In a Salient pole m/c, number of slots/pole/phase- 2 to
4.

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Turns per phase and Conductor
section
 Turns per phase is always calculated from
induced EMF equation.
 Tph = Eph/ (4.44*f*flux*Kw)
 Area of the conductor is calculated from
current and permissible current density.

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Length of Air gap (lg)
 It influences the performance of the machine.
Large Lg offers large reluctance to magnetic
flux path.
 Adv of larger air gap (smaller X and higher
SCR)
 Better cooling
 Better voltage regulation
 High stability limit
 Smaller UMP
 Quiet operation

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Estimation of Air gap length (lg)
 Length of air gap is calculated from the MMF
required for air gap equation.
 MMF required across the air gap is approx. 80
% of the no load field mmf.
 ATfo= ATa * SCR
 ATfo = 2.7 * (Iph *Tph* Kw/ P ) * SCR
 MMF required for Air gap ATg= 0.8 * ATfo
 800000 * Bg* Kg* lg = 0.8 * ATfo
 Lg= 0.8 * ATfo/ (800000 * Bg* Kg* lg)

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Factors Affecting Size
 Efficiency: Power increases efficiency also increases.
 1 KW – 50%, 10 MW– 90%, 100 MW- 98%
 Power output per kg increases as the alternator power
increases.
 Cooling: large machines produce high power loss per
unit surface area W/m2
 Upto 50 MW circulating cold air system is adequate.
1000 MW machines need hydrogen or water cooled
system.
 Cost: fixes upper limit of machine size.
 Speed: low speed machines always bigger than large
machine.

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Effect of Power factor of the
load
 Unity power factor: EMF is distortional.
 Zero pf lagging: Weakening the main flux, less
emf is generated. To compensate this increase
field excitation.
 Zero pf leading: Armature reaction is totally
magnetising, which results in greater induced
emf.

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Voltage Regulation
 Definition: Rise in voltage when full load is
removed.
 Field excitation and speed remaining same.
 Methods of Finding voltage regulation:
 Synchronous impedance method or EMF
method
 Ampere Turn or MMF method
 Zero power factor or Potier method
 All these methods require
 Ra, OCC, SCC characteristics curve.

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Short Circuit Ratio (SCR)
 SCR is defined as the ratio of filed current
required to produce rated open circuit voltage
and field current required to produce rated
short circuit current.
 It is reciprocal of synchronous reactance (Xd)
 Modern turbo alternator, SCR is normally 0.5 -
0.7.
 Hydroelectric Generator, 1-1.5.

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Effect of SCR on Machine
Performance
 Voltage Regulation : Low SCR- Higher Xd, Poor
voltage regulation due to higher changes in
voltage under load.
 Stability: Max. power output is inv. proportional
to Xd. Low SCR- lower stability limit.
 Parallel operation: low SCR machines is difficult
to operate in parallel.
 Self Excitation: Machine feeding long
transmission line should not be designed with a
small SCR. It leads to large voltage on open
circuit.

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Advantages of Higher SCR value
Machine
 Higher stability limit.
 Better voltage regulation
 Better cooling effect (lg is higher)
 DISADV
 Costly
 Present Trend
 Low SCR with recent advancement in fast acting
control
and excitation systems.

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Steady state stability limit

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Elimination of Harmonics
 Harmonics means non sinusoidal wave form
 Multiples of Fundament frequency
 Elimination methods
 Distribution- windings are distributed not
concentrated
 Use Short pitched coils- Adv- save cost, pure
sine.
 Skewing the pole face
 Fractional slot windings
 Large air gap length

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Synchronous Motor
 150 KW to 15 MW and 150 -1800 rpm
 Characteristic features
 Runs only at synchronous speed or not at all
 Not self starting
 Capable of operating under a wide range of
power factors, both lagging and leading.
 Working Principle: Magentic locking.

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Comparison b/w Synchronous and
Induction motor
Factor Synchronous motor Induction motor
Speed Constant average speed whatever
the load
Falls with increase in
load
Power factor Wide range of power factor Runs with a lagging pf
Starting Not Self staring Self starting
Changes in
applied
voltage
Don’t affect synchronous motor
torque
Affect the induction
motor torque
D.C field
Excitation
Required Not required
Cost Costly Cheap
Suitability Low speed dives and Power factor
correction and voltage regulation.
High speed drives

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Applications of Synchronous
Motor
 Power factor correction
 Constant speed application: pumps,
compressors, blowers, line shafts, rubber and
paper mills.
 Voltage Regulation: by varying its field
excitation voltage rise and drop can be
controlled.

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Main Dimension Problem

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Height of Pole
 By estimating of full load Field MMF
 ATfo= ATa * SCR
 Find Height of field winding required
 Total field winding area= ATfl/ Current density
 hf= total winding area/ depth of winding
 Space factor= copper area/ total space reqd
 Space factor- 0.8 to 0.9
 Height of pole
 hpl= hf+ clearance for winding in mm

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Design of Damper Winding
 Damper windings are provided in pole face of rotor. It
is copper or Al bar housed in slots of pole faces. Ends
are short circuited by end rings.
 Purpose:
 In Syn. Gr. It is provided to suppress the negative
sequence field and damp out the oscillations when the
machines starts hunting.
 In Syn. Motor. To provide starting torque and to
develop damping power when the machine starts
hunting.
 Design:
 1. Area of Damper winding 2. Number of damper
winding 3. Dia and Length of damper winding.

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Problem from Damper winding
Design

UNIT V Design of Electrical Apparatus

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