Download 03_AC Machines

SHAIFUL NIZAM MOHYAR
UNIVERSITI MALAYSIA PERLIS
SCHOOL OF MICROELECTRONIC ENGINEERING
2007/2008
3.0 Introduction of AC Machine
 Alternating current (ac) is the primary
source of electrical energy.
 It is less expensive to produce and transmit
than direct current. For this reason, and
because ac voltage is induced into the
armature of all generators, ac machines are
generally more practical.
 May function as a generator (mechanical to
electrical) or a motor (electrical to
mechanical)
DC Machine & AC machine
DC motor - ends of the coil connect to a
mechanical rectifier called commutator to
'rectify' the emf produced.
AC motor - No need rectification, so don’t need
commutator, just need split rings.
AC Motor
 As in the DC motor
case, a current is
passed through the
coil, generating a
torque on the coil.
 Since the current is
alternating, the motor
will run smoothly only
at the frequency of
the sine wave.
AC Generator
 This process can be described in terms of Faraday's law
when you see that the rotation of the coil continually
changes the magnetic flux through the coil and therefore
generates a voltage
Generator and Motor
How Does an Electric Generator
Work?
AC Machine
 Two major classes of machines;
(i) Synchronous machines.
(ii) Induction machines.
Synchronous Machine
Synchronous machines are ac machine that
have a field circuit supplied by an external dc
source.
 DC field winding on the rotor,
 AC armature winding on the stator
Origin of name: syn = equal, chronos = time
Synchronous machines are called
‘synchronous’ because their mechanical
shaft speed is directly related to the power
system’s line frequency.
Synchronous Machine
The frequency of the induced voltage is related to
the rotor speed by:
where P is the number of magnetic poles
fe is the power line frequency.
Typical machines have two-poles, fourpoles, and six-poles
Example 1
A hydraulic turbine turning at 200 r/min is
connected to a synchronous generator. If the
induced voltage has a frequency of 60 Hz, how
many poles does the rotor have?
Synchronous Machine
Construction
Energy is stored in the inductance
As the rotor moves, there is a change in
the energy stored
 Either energy is extracted from the
magnetic field (and becomes mechanical
energy – motor)
Or energy is stored in the magnetic field
and eventually flows into the electrical
circuit that powers the stator – generator
Synchronous Machine
DC field windings are mounted on the (rotating)
rotor - which is thus a rotating electromagnet
AC windings are mounted on the (stationary) stator
resulting in three-phase AC stator voltages and
currents
The main part in the synchronous machines are
 i) Rotor
 ii) Stator
Synchronous Machine
Rotor
There are two types of rotors used in synchronous
machines: cylindrical (or round) rotors and salient
pole rotors.
Salient pole rotors are less expensive than round
rotors.
Cylindrical ( round) rotor – low speed machines
(hydro-turbines)
 Salient-Pole rotor - high speed machines (steamturbines)
Synchronous Machine
Construction-Rotor
 i) Cylindrical (or round)
rotor
 i) Salient-pole
rotor
Synchronous Machine
 Synchronous machine rotors are simply rotating
electromagnets built to have as many poles as are
produced by the stator windings.
 DC currents flowing in the field coils surrounding each
pole magnetize the rotor poles.
 The magnetic field produced by the rotor poles locks in with
a rotating stator field, so that the shaft and the stator field
rotate in synchronism.
 Salient poles are too weak mechanically and develop too
much wind resistance and noise to be used in large, highspeed generators driven by steam or gas turbines. For
these big machines, the rotor must be a solid, cylindrical
steel forging to provide the necessary strength.
Synchronous Machine
 Axial slots are cut in the surface of the cylinder to
accommodate the field windings.
 Since the rotor poles have constant polarity they must be
supplied with direct current.
 This current may be provided by an external dc generator or
by a rectifier. In this case the leads from the field winding are
connected to insulated rings mounted concentrically on the
shaft. Stationary contacts called brushes ride on these slip
rings to carry current to the rotating field windings from the
dc supply. The brushes are made of a carbon compound to
provide a good contact with low mechanical friction. An
external dc generator used to provide current is called an
“exciter.
Synchronous Machine
Stator
The stator of a synchronous machine carries the
armature or load winding which is a threephase winding.
The armature winding is formed by
interconnecting various conductors in slots spread
over the periphery of the machine’s stator. Often,
more than one independent three phase winding is
on the stator. An arrangement of a three-phase
stator winding is shown in Figure below. Notice that
the windings of the three-phases are displaced from
each other in space.
Synchronous Machine –
Stator construction
Synchronous Machine
Magnetomotive Forces (MMF’s) and Fluxes Due
to Armature and Field Windings
Flux produced by a stator winding
Two Cycles of MMF around the Stator
Synchronous Generator
Equivalent circuit model – synchronous generator
 If the generator operates at a terminal voltage VT while
supplying a load corresponding to an armature current Ia,
then;
 In an actual synchronous machine, the reactance is much
greater than the armature resistance, in which case;
 Among the steady-state characteristics of a
synchronous
generator, its voltage regulation and power-angle
characteristics are the most important ones. As for
transformers, the voltage regulation of a synchronous
generator is defined at a given load as;
Synchronous Generator
Phasor diagram of a synchronous generator
The phasor diagram is to shows the relationship among
the voltages within a phase (Eφ,Vφ, jXSIA and RAIA) and
the current IA in the phase.
Unity P.F (1.0)
Synchronous Generator
Lagging P.F
Leading P.F.
Synchronous Generator
Power and Torque
In generators, not all the mechanical power going into a
synchronous generator becomes electric power out of
the machine
The power losses in generator are represented by
difference between output power and input power shown
in power flow diagram below
Synchronous Generator
Losses
Rotor
- resistance; iron parts moving in a magnetic field causing
currents to be generated in the rotor body
- resistance of connections to the rotor (slip rings)
Stator
- resistance; magnetic losses (e.g., hysteresis)
Mechanical
- friction at bearings, friction at slip rings
Stray load losses
- due to non-uniform current distribution
Synchronous Generator
The input mechanical power is the shaft power in the generator given by
equation:
The power converted from mechanical to electrical form internally is given
by
The real electric output power of the synchronous generator can be
expressed in line and phase quantities as
and reactive output power
Synchronous Generator
In real synchronous machines of any size, the armature
resistance RA is more than 10 times smaller than the
synchronous reactance XS (Xs >> RA). Therefore, RA can
be ignored
Synchronous Generator
Synchronous Generator
Power flow
Synchronous Generator
Example 2
 A three-phase, wye-connected 2500 kVA and 6.6 kV
generator operates at full-load. The per-phase
armature resistance Ra and the synchronous
reactance, Xd, are (0.07+j10.4). Calculate the
percent voltage regulation at
(a)0.8 power-factor lagging, and
(b)0.8 power-factor leading.
Induction Machine
 The machines are called induction machines because of the
rotor voltage which produces the rotor current and
the rotor magnetic field is induced in the rotor windings.
 Induction generator has
many disadvantages and
low efficiency. Therefore
induction machines are
usually referred to as
induction motors.
Induction Machine
 Induction motors use shorted wire loops on a rotating armature
and obtain their torque from currents induced in these loops by
the changing magnetic field produced in the stator (stationary)
coils.
 The current in the stator coil is
in the direction shown and
increasing. The induced voltage
in the coil shown drives current
and results in a clockwise torque.
Induction Machine
Induction in Armature Coils
Induction Machine
 A large percentage of small AC motors are classed as
induction motors. This implies that there is no current
supplied to the rotating coils. These coils are closed loops
which have large currents induced in them because of their
low resistance.
 An induction motor must achieve
a rotating magnetic field to
continue to exert a torque on the
armature coils. In this example,
the rotating field is achieved by
the extra coils on the pole pieces.
Induction Machine
Induction Machine
There are two different types of induction motor
rotors that can be placed inside the stator.
1. Squirrel cage – the conductors would look like
one of the exercise wheels that squirrel or
hamsters run on.
2. Wound rotor – have a brushes and slip ring at
the end of rotor
 The magnetic field's rotation of induction motors is
given by
Induction Machine
1. Squirrel cage – the conductors would look like
one of the exercise wheels that squirrel or hamsters
run on.
Induction Machine
2. Wound rotor – have a brushes and slip ring at
the end of rotor
Induction Machine - Operation
 The stator’s rotating field cuts the rotors conductors
thereby inducing voltages in the rotor circuit.
 The induced (Faraday) voltages cause currents to
flow in the rotor.
 The rotor’s currents produce a rotating (rotor) field
which is always aligned (travels with) the stator’s
rotating field.
 The whole process is essentially that of a
transformer.
 The induction motor is sometimes referred as a
rotating transformer .
Induction Machine - Operation
Speed of rotation (synchronous speed)
P is the number of magnetic poles designed into the
machine,
fe is the power line frequency.
The Concept of Rotor Slip
The voltage induced in a rotor bar of an
induction motor depends on the speed of the
rotor relative to the magnetic fields
1. Slip speed – defined as the difference between
synchronous speed (magnetic field's speed) and
rotor speed.
nslip = nsync - nm
nslip = slip speed of the machine
nsync = speed of the magnetic fields
nm = mechanical shaft speed of motor
The Concept of Rotor Slip
2. Slip – defined as the relative speed expressed on a perunit (or sometimes as percentage) basis
If the rotor turns at synchronous speed, s = 0,
while if the rotor is stationary (standstill), s = 1.
Mechanical speed (rotor's speed) can be expressed in term
of synchronous speed and slip as below:
The Electrical Frequency on
the Rotor
The rotor frequency can be expressed as
fr = sfe
where
fr = rotor frequency
s = slip
fe = electrical frequency
Alternative to find fr is defined as below
Induction Motor
– Equivalent Circuit
Same as a transformer
Stator is connected to the ac source, and the rotor’s
voltage and current are produced by induction.
The primary of the transformer corresponds to the
stator of the machine, whereas the secondary
corresponds to the rotor
Stator and Rotor as Coupled Circuits
Induction Motor
– Equivalent Circuit
Induction Motor
– Power and Torque
The power flow diagram
Induction Motor
– Power and Torque
Example 3
A 480-V, 50Hz, 50hp, three phase induction motor is drawing 60A at
0.80 PF lagging. The stator copper losses are 2 kW, and the rotor
copper losses are 700W. The friction and windage losses are
600W, the core losses are 1800 W, and the stray losses are
negligible. Fine the following quantities:
a. The air gap power PAG
b. The power converted Pconv
c. The output power Pout
d. The efficiency of the motor
Induction Motor
– Equivalent Circuit
Induction Motor
– Equivalent Circuit
Induction Motor
– Power and Torque
The output power can be found as
Pout = Pconv – PF&W – Pmisc
The induced torque or developed torque:
Induction Motor
– Power and Torque
Example 4
A 460 V, 25-hp, 60Hz, four pole, Y-connected induction
motor has the following impedances in ohms per phase
referred to the stator circuit:
R1 =0.641Ω
R2 =0.332Ω
X1 =1.106Ω X2 =0.464Ω Xm =26.3Ω
The total rotational losses = 110 W, Rotor slip = 2.2% at
rated voltage and frequency.
Find the motor's
i) Speed, ii) Stator Current, iii) Power factor, iv) Pconv,
v) Pout vi)  ind, vii) load and viii) Efficiency