Electrical Machines Module 2'27

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Electrical MachinesModule 2.27

• Lecturer
• Dr Vesna Brujic-Okretic
• Ext 9676 and 9681
• Email mes1vb_at_surrey.ac.uk

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Induction motor

• The induction motor derives its name from the
  fact that ac voltages are induced in the rotor
  circuit by the rotating magnetic field of the
  stator
• An Induction motor operates on the principle of
  induction
• The rotor receives power from the stator due to
  Induction The rotor is not connected to an
  external source of voltage.
• It is important to understand the principle of
  rotating magnetic field in order to understand
  the operation of an Induction motor.
• The induction motor is the most commonly used
  type of AC motor. Its simple, rugged construction
  costs relatively little to manufacture.
• induction motors are often large motors and
  permanently mounted motors that drive loads at
  fairly constant speed.
• Examples washing machines, refrigerator
  compressors, bench grinders, table saws etc.

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Induction motor

• The stator in an AC motor is a wire coil, called
  a stator winding. It is built into the motor.
  When this coil is energized by AC power, a
  rotating magnetic field is produced.
• When a magnetic field comes close to a wire, it
  produces an electric current in that wire. This
  is called induction - as you remember (Faraday's
  law)
• In induction motors, the induced magnetic field
  of the stator winding induces a current in the
  rotor. This induced rotor current produces a
  second magnetic field necessary for the rotor to
  turn.

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Induction motor

• When a three-phase voltage is applied to the
  stator winding , a rotating magnetic field of
  constant magnitude is produced.
• This rotating field is produced by the
  contributions of space-displaced phase windings
  carrying appropriate time displaced currents
• These currents, which are time displaced by 120
  electrical degrees, are shown in the following
  slide

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Induction motor - rotating field

• iaImcos wt
• ibImcos (wt - 1200)
• icImcos (wt - 2400)

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Induction motor Rotor

• The induction rotor is made of a laminated
  cylinder with slots in its surface.
• The most common is the squirrel-cage winding.
  This entire winding is made up of heavy copper
  bars connected together at each end by a metal
  ring made of copper or brass. No insulation is
  required between the core and the bars. This is
  because of the very low voltages generated in the
  rotor bars.

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Induction motors Rotor

• squirrel cage rotors resemble the exercise wheels
  
• Configuration Several metal bars are placed
  within end rings in a cylindrical pattern.
  Because the bars are connected to one another by
  these end rings, a complete circuit is formed
  within the rotor.
• Another possible type is a wound rotor induction
  motor

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Induction motor Rotor and stator
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Induction motor Cut away view
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How does it work?

• Alternating current flowing in the stator causes
  the poles to change rapidly, from north to south
  and back again, thus generating a changing
  magnetic field.
• The rotating magnetic field generated in the
  stator induces a magnetic field in the rotor. The
  two fields interact and cause the rotor to turn.
  To obtain maximum interaction between the fields,
  the air gap between the rotor and stator should
  be very small.
• As you know from Lenz's law, any induced emf
  tries to oppose the changing field that induces
  it.
• In the case of an induction motor, the changing
  field is the motion of the resultant stator
  field. A force is exerted on the rotor by the
  induced emf and the resultant magnetic field.
  This force tends to cancel the relative motion
  between the rotor and the stator field. The
  rotor, as a result, moves in the same direction
  as the rotating stator field.

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How does it work?

• The lines of force of the rotor magnetic field
  are moving in the same direction as those of the
  stator, thus adding to the magnetic field, which
  keeps the rotor turning.
• It is, however, impossible for the rotor of an
  induction motor to turn at the same speed as the
  rotating magnetic field.
• If the speeds were the same, there would be no
  relative motion between the stator and rotor
  fields without relative motion there would be no
  induced voltage in the rotor.
• In order for relative motion to exist between the
  two, the rotor must rotate at a speed slower than
  that of the rotating magnetic field.
• The difference between the speed of the rotating
  stator field and the rotor speed is called slip.
  The smaller the slip, the closer the rotor speed
  approaches the stator field speed.

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Rotating field velocity

• The velocity of the rotating magnetic field of
  the stator can be calculated with the formula
  below (we shall not go into details of how it is
  derived, but it is simple, and follows from the
  equations for poly-phase machines)
• Ns120fs / p
• where p is the number of poles and fs is the
  applied frequency of the stator magnetic field
• When the stator is supplied by a balanced
  three-phase source, it will produce a magnetic
  field that rotates at synchronous speed
  determined by the above eq.

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Slip

• We said the rotor reacts to the magnetic field,
  but does not travel at the same speed.
• We also said the rotor speed actually lags behind
  the speed of the magnetic field. It runs at the
  speed Nr which is close to the speed of the
  stator field, Ns when the motor is running light,
  but decreases as the load is increased
• The term slip quantifies the slower speed of the
  rotor in comparison with the rotating speed of
  the stator magnetic field and is expressed
  mathematically as
• S(Ns-Nr)/Ns

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Slip

• The rotor is not locked into any position and
  therefore will continue to slip throughout the
  motion.
• The speed of the rotor depends upon the torque
  requirements of the load. The bigger the load,
  the stronger the turning force needed to rotate
  the rotor.
• The turning force can increase only if the
  rotor-induced e.m.f. increases
• This e.m.f. can increase only if the magnetic
  field cuts through the rotor at a faster rate.
• To increase the relative speed between the field
  and the rotor, the rotor must slow down.
• Therefore, for heavier loads the induction motor
  turns slower than for lighter loads. The amount
  of slip increases proportionally with increase in
  load

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Slip

• Actually only a slight change in speed is
  necessary to produce the current changes required
  to accommodate the changes in load. This is
  because the rotor windings have a low resistance.
  
• As a result, induction motors are called
  constant-speed motors
• A typical T-w characteristic for the cage-type
  induction motor is shown on the following slide

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Speed-Torque Characteristics
torque
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Analysis of operation

• On start-up the slip is s1 and the starting
  torque (also known as a breakaway torque) is
  sufficiently large to accelerate the rotor (the
  rotor has previously been 'locked' - stationary)
• As the rotor runs up to its full-load speed the
  torque increases in essentially inverse
  proportion to the slip
• the start-up and running curves merge to give the
  characteristic as shown on the previous slide
• After the torque reached its maximum, it rapidly
  falls to zero, at the synchronous speed, Ns
• Looking backwards as rotor speed falls below Ns
  the torque increases almost linearly to a maximum
  dictated by the full load (plus rotor losses)
• the speed only falls a little when the load is
  raised from 0 to its full value - this is a
  normal operating region

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Analysis of operation

• the induction motor may be regarded as a
  constant speed machine (similarly to a shunt DC
  motor)
• Other key features
• The maximum speed is a synchronous speed, Ns,
  independent of the applied voltage
• Torque is proportional to the V2 at an arbitrary
  speed
• When operating at 90-95 Ns heat losses are at
  minimum

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Types Applications

• There are a variety of different types of
  induction motors, differing mainly by the number
  of phases and the winding type.
• Some of the more common names are
• shaded pole, split phase, capacitor start, two
  value capacitor, permanent split capacitor, two
  phase, three phase star, three phase delta, and
  three phase single voltage
• APPLICATIONS
• Most widely used AC motor squirrel-cage
• Big squirrel-cage motors - in factories
• small squirrel-cage motors - in home appliances

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Important considerations

• Factors to be considered when selecting an
  induction motor
• speed range and speed variation
• acceleration/deceleration characteristics
  (starting torque/full load torque ratio this can
  vary a lot from a few percent to several times)
  to have enough starting torque to overcome
  static friction, accelerate the load up to the
  full working speed and handle the maximum
  overload
• duty cycle the percentage of the time the motor
  is loaded
• the power required to drive the load
• The operating speed of the motor is determined by
  the point at which the power (or torque) that the
  motor can supply intersects with the power (or
  torque) that the load can absorb, mechanically
• the motor must satisfy mechanical requirements
• heating properties