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Power Point Representation on

Electrical Machines II

​

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Content

  • Introduction
  • Why induction motor (IM)?
  • Classification of Motor
  • Rotating Magnetic Field
  • Principle of operation
  • Induction motor speed
  • Induction Motors and Transformers
  • Disadvantages
  • References

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Introduction

  • Three-phase induction motors are the most common and frequently encountered machines in industry
    • simple design, rugged, low-price, easy maintenance
    • wide range of power ratings: fractional horsepower to 10 MW
    • run essentially as con•sintadnucttisopnemedotforrom no-load to full load
    • Its speed depends on the frequency of the power source
      • not easy to have variable speed control
      • requires a variable-frequency power-electronic drive for optimal speed control

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••Why induction motor (IM)?

  • –Robust; No brushes. No contacts on rotor shaft
  • –High Power/Weight ratio compared to Dc motor
  • –Lower Cost/Power
  • –Easy to manufacture
  • –Almost maintenance-free, except for bearing and other mechanical parts

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Classification of Motor

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Rotating Magnetic Field

  • Balanced three phase windings, i.e. mechanically displaced 120 degrees form each other, fed by balanced three phase source
  • A rotating magnetic field with constant magnitude is produced, rotating with a speed

sync

synchronous speed in rpm (revolutions per

minute)

120 fe

P is the no. of poles and n Pis called the

Where fe is thensusypnpcly=frequency and rpm

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Synchronous speed

P

50 Hz

60 Hz

2

3000

3600

4

1500

1800

6

1000

1200

8

750

900

10

600

720

12

500

600

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Rotating Magnetic Field

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Rotating Magnetic Field

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Rotating Magnetic Field

2

2

M

M

M

M

3 B

3 B

Bnet (t) = Ba (t) + Bb (t) + Bc (t)

= BM sin(ωt)∠0° + BM sin(ωt −120°)∠120° + BM sin(ωt − 240)∠240°

= BM sin(ωt)xˆ

−[0.5B sin(ωt −120°)]xˆ −[

sin(ωt −120°)]yˆ

−[0.5B sin(ωt − 240°)]xˆ +[

sin(ωt − 240°)]yˆ

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Rotating Magnetic Field

4 4

4 4

4

4

4

4

net M

M

M

M

M

M

M

M

M

3 B

3 B

3 B

3 B

B (t) = [B sin(ωt) + 1 B sin(ωt) +

cos(ωt) + 1 B sin(ωt) −

+[−

sin(ωt) − 3 B cos(ωt) +

cos(ωt)]xˆ

sin(ωt) − 3 B cos(ωt)]yˆ

= [1.5BM sin(ωt)]xˆ −[1.5BM cos(ωt)]yˆ

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Rotating Magnetic Field

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Principle of operation

  • This rotating magnetic field cuts the rotor windings and produces an induced voltage in the rotor windings
  • Due to the fact that the rotor windings are short circuited, for both squirrel cage and wound-rotor, and induced current flows in the rotor windings
  • The rotor current produces another magnetic field
  • A torque is produced as a result of the interaction of those two magnetic fields

Where τind is the induced torque and BR and BS are the magnetic flux densities of the rotor and the stator respectively

τind

= kBR × Bs

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Induction motor speed

  • At what speed will the IM run?
    • Can the IM run at the synchronous speed, why?
    • If rotor runs at the synchronous speed, which is the same speed of the rotating magnetic field, then the rotor will appear stationary to the rotating magnetic field and the rotating magnetic field will not cut the rotor. So, no induced current will flow in the rotor and no rotor magnetic flux will be produced so no torque is generated and the rotor speed will fall below the synchronous speed
    • When the speed falls, the rotating magnetic field will cut the rotor windings and a torque is produced

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Induction motor speed

  • So, the IM will always run at a speed lower than the synchronous speed
  • The difference between the motor speed and the synchronous speed is called the Slip

Where nslip= slip speed

nsync= speed of the magnetic field

nm = mechanical shaft speed of the motor

nslip = nsync − nm

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The Slip

Notice that : if the rotor runs at synchronous speed

s = 0

if the rotor is stationary

s = 1

Slip may be expressed as a percentage by multiplying the above

eq. by 100, notice that the slip is a ratio and doesn’t have units

s = nsync − nm

nsync

Where s is the slip

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Induction Motors and Transformers

  • Both IM and transformer works on the principle of induced voltage
    • Transformer: voltage applied to the primary windings produce an induced voltage in the secondary windings
    • Induction motor: voltage applied to the stator windings produce an induced voltage in the rotor windings
    • The difference is that, in the case of the induction motor, the secondary windings can move
    • Due to the rotation of the rotor (the secondary winding of the IM), the induced voltage in it does not have the same frequency of the stator (the primary) voltage

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Disadvantages

  • –Essentially a “fixed-speed” machine
  • –Speed is determined by the supply frequency
  • –To vary its speed need a variable frequency supply

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References

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Thanks