Transcript Lecture VI - AC Machines II

LECTURE
VI
mohd hafiz ismail [email protected] 04-9798330 level II jejawi
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INDUCTION
MACHINE
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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.
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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
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Induction Machine
1. Squirrel cage – the conductors would look like one of
the exercise wheels that squirrel or hamsters run on.
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Induction Machine
2. Wound rotor – have a brushes and slip ring at the end
of rotor
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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 .
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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.
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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
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The Concept of Rotor
Slip
2. Slip – defined as the relative speed expressed on a per-unit
(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 follow:
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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
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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
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Induction Motor – Equivalent
Circuit
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Induction Motor – Power and
Torque
The power flow diagram
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Induction Motor – Power and
Torque
Example
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
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Induction Motor – Equivalent
Circuit
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Induction Motor – Equivalent
Circuit
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Induction Motor – Power and
Torque
The output power can be found as
Pout = Pconv – PF&W – Pmisc
The induced torque or developed torque:
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Induction Motor – Power and
Torque
Exercise
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
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EMT462 Electrical System Technology
hafizism february 2007
EMT462 Electrical System Technology
hafizism february 2007