Induction Machine
Week 5 Class
Electrical Rotating Machines
ELEC 10131
Electrical Machines Vijay Khatri
Induction Machine
FIGURE 13-1 Super-E, premium efficiency induction motor rated 10 hp, 1760 r/min, 460
V, 3-phase, 60 Hz. This totally-enclosed fan-cooled motor has a full-load current of 12.7 A,
efficiency of 91.7%, and power factor of 81%. Other characteristics: no-load current: 5 A;
locked rotor current: 85 A; locked rotor torque: 2.2 pu; breakdown torque: 3.3 pu; service
factor 1.15; total weight: 90 kg; overall length including shaft: 491 mm; overall height: 279
mm. (Courtesy of Baldor Electric Company)
Electrical Machines Vijay Khatri
Induction Machine
Electrical Machines Vijay Khatri
Induction Machine Rotor
• Rotor (Squirrel cage)
Electrical Machines Vijay Khatri
Induction Machine Rotor
• Wound Rotor
Electrical Machines Vijay Khatri
Induction Machine Operating Principle
• Principle of operation
Electrical Machines Vijay Khatri
Induction Machine Operating Principle
• Magnet is moving at speed of v
• A voltage E = Blv is induced in each conductor
while it is being cut by the flux
• The induced voltage immediately produces a
current I which flows down a conductor
underneath the pole face, through the end bars
and back through the other conductors
• The current carrying conductor experiences a
mechanical force.
• The force acts in a direction to drag the
conductor along with the magnetic field.
Electrical Machines Vijay Khatri
Induction Machine Operating Principle
• Modified Ladder
Electrical Machines Vijay Khatri
IM Stator
•6 salient poles
•Three identical sets of windings mechanically
spaced at 120o
•The windings are connected in Y
Electrical Machines Vijay Khatri
Induction Machine Stator Flux pattern
Electrical Machines Vijay Khatri
IM Stator Flux Instant 1
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IM Stator Flux Instant 2
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IM Stator Flux Instant 3
Electrical Machines Vijay Khatri
IM Stator Flux Instant 4
Electrical Machines Vijay Khatri
IM Stator Flux Instant 5
Electrical Machines Vijay Khatri
IM Stator Flux Instant 6
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IM Actual Stator
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IM Actual Stator
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IM actual Stator
Electrical Machines Vijay Khatri
IM Speed of Rotating Field
120 * f
ns =
p
ns synchronous speed rpm
f frequency of the source
p number of poles
(Problem)
Electrical Machines Vijay Khatri
IM Starting Characteristics
• When a 3φ voltage is applied at the stator, a
rotating magnetic field is produced
• The rotating field induces a voltage in the rotor
circuit
• The induced voltage creates a large circulating
current
• The current carrying conductors in a magnetic
field are subjected to strong mechanical force
• The sum of all forces produce a torque which
drags the rotor in the direction of the rotating
field
Electrical Machines Vijay Khatri
IM Rotor Acceleration
• As rotor accelerates from standstill in the
direction of rotating field, it picks up speed
• The relative velocity between the rotating
field and the rotor decreases
• The frequency, induced voltage and rotor
current decreases (compared to standstill)
• The speed never catches/equal to the
rotating field.
Electrical Machines Vijay Khatri
IM Under Load
• If acceleration was under no load and load is
now applied.
• The motor slows down, the rotor frequency
and induced voltage increase leading to more
rotor current.
• Increased current causes increased torque
until equilibrium is reached.
• In equilibrium, the speed does not change
Electrical Machines Vijay Khatri
IM Slip
ns − n
slip =
ns
Electrical Machines Vijay Khatri
