Unit-1
Basic concepts of electrical machines
Introduction
Advantages of Synchronous Machines
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Synchronous generators can be interconnected or operated in parallel to
share the load.
Load can be shared easily, Load factor is improved, cost/unit decreases.
Machines can be connected in parallel only when the polarities of them
same and frequency should be same.
Asynchronous generators require converters to change ac to dc and then
dc to ac at required frequency.
In SM armature conductors rotate in dc magnetic field and emf is
produced.
Because of DC field, it will readily supply reactive power.
Induction generators can not supply reactive power because it has no dc
field.
Basics of electrical machines
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Consider conductor carries current towards us and is represented by a dot.
Apply right hand thumb rule to find the direction of magnetic field . i.e the
direction of M.F is counter clock wise direction.
Similarly consider other conductor carries current away from us is
represented as cross.
Apply right hand thumb rule to find the direction of magnetic field
produced. In this case the direction will be clock wise direction.
What happens when two conductors carrying current in one direction.
▪ The magnetic flux out side the conductors will increase and M.F between
the conductors will decrease.
▪ when two conductors carrying current in opposite direction, the magnetic
flux out side the conductors will decrease and M.F between
the conductors will increase.
Basics of electrical machines
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Consider solinoid. When current flowing through coil M.F is produced .
The direction can be found by right hand thumb rule.
Keeping curl of fingers of right hand in the direction of current thumb
shows the direction of flux.
The flux leaving the coil is north pole and flux entering the coil is south
pole .
What happens if conductor placed in the magnetic field.
When current carrying conductor placed in the M.F, force is produced. The
direction is found by flemings left hand rule.
F  IXB
or
F = BIle sin( )
le = effectivelength of conductor
 = angle b / w I and B
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Let us see the mmf distribution of current carrying distributed winding.
To analyze, cut the stator and open the armature winding.
Apply right hand thumb rule the direction of flux or mmf b/w the
conductors can be found. Adding all mmfs of all coils to get air gap mmf.
Keeping more conductors in more slots more sinusoidal mmf is produced.
Basic Working Principles of Rotating Machines
• Faradays law of electromagnetic induction:
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Emf induced α rate of change of flux linkages.
Basic requirements:
i) magnetic field, set of conductors, relative space variation or time
variation b/w set of conductors and magnetic field.
There are two possible methods.
i) M.F is time varying and conductors are stationary. Called statistically
induced emf. Ex- T/F
Ii)M.F is stationary and conductors are being moved. Called dynamically
induced emf. Ex-rotating m/c.
The magnitude of dynamically induced emf is given by flux cutting law.
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when conductor moving in the M.F emf is produced
 
emf  V  B
e = BleV sin( )
 is angle b / w B and V
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where le is effective length of conductor falling in the magnetic field.
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Assume conductors rotating in clockwise direction in the M.F. emf is
produced in the conductors. The direction of emf can be found from
flemings right hand rule.
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Let us see the behavior of the EMF in the conductor, when it is moving in
the magnetic field.
let us assume conductor moving in clockwise direction.
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When conductor at position 1, angle b/w B and V is 180 deg sin(θ)=0.Emf
induced is zero.
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At position 2 angle b/w B and V is 90 deg, sin(θ)=1, maximum emf is
produced.
At position 3 angle b/w B and V is 0 deg, sin(θ)=0, emf is 0.
At position 4 angle b/w B and V is -90 deg, sin(θ)=-1, negative maximum
emf is produced.
The emf induced follows sinusoidal wave form.
Advantages of keeping field on rotor
Relation b/w frequency and speed
➢ There are two possibilities in construction. One is
➢ the field winding of alternator is on the rotor and armature winding is on
stator .
➢ 2nd one is field winding on stator and armature winding is on rotor.
Advantages of stationary armature and rotating field:
➢ The field winding is placed on rotor. Dc is applied to the field winding
through 2 slip rings.
➢ Power required for field is less, so ,size↓, inertia↓.
➢ Only 2 slip rings are required, so ,loses↓.
➢ The voltage rating of field ↓ ,so insulation ↓ so size ↓.
➢ As size ↓,inertia ↓, so rotor can run at higher speeds→ can be used for
Higher power output.
➢ Easier to insulate stationary armature winding for high voltages.
➢ No centrifugal forces are developed on stationary armature.
➢ Cooling can be provided easily for stationary armature for high voltages.
➢ Stationary 3 phase armature can be directly connected to load
Without slip rings and brushes, saving the losses.
Relation b/w frequency and speed
N=rotor speed in rpm
P=no of poles
f= frequency of emf.
No of cycles/revolution= P/2
No of revolutions/sec=N/60.
No of cycles /sec = cycles/rev * rev/sec
f =
f =
P N
NP
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=
2 60 120
NP
120 f
 Ns =
120
P
Concepts of flux linkages
EMF equation of alternator
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Consider a solenoid with N1 turns and current I1 flowing in the coil. Consider
another solenoid have N2 turns and current I2 is flowing in the coil.
the mmf produced = NI
flux is given by  = mmf = NI
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
the coil axis of the solenoid is called
field axis.
let θ is angle b/w two coil axis.
when θ =0, flux linkages will be max.
when θ=90 deg flux linkage=0.
therefore flux linkages ψ follows
cosine wave.
ψ= φ cos(θ).
v
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Consider the case of m/c. the pole axis called field axis and 90 deg to that is
called quadrature axis. When θ=0, flux linkage is max. when θ=90, flux linkage=0
when rotor rotates at speed ω rad/sec, Flux linkages can be written as
d
d
( cost )
= −T
 =  cos =  cos(t ) and emf = −T
dt
dt
T is number of turns;
e = T sin (t )
Where  = 2f so
e = 2fT  sin(t )
e = E m ax sin(t )
e = E m ax sin(t )
ctor
E rms =
E m ax
sin(t )
2
2f T
sin(t )
2
= 4.44 f T sin(t )
=
= 4.44 f T c os(t −
E rms = 4.44f T

2
)
Erms is rms value of induced emf.
Some more factors will be added to this equation to account for
distribution factor and pitch factor. Which are
K d = distribution factor
K p = pitch factor
E rms = 4.44 k p k d  f T
E rms = 4.44 k w  f T
where k w = k p k d called winding factor