AC Electrical Machines: Three-Phase Induction Motor

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Questions and Answers

What is the purpose of the external resistances in the rotor circuit of a wound-rotor induction motor?

  • To increase the efficiency of the motor
  • To decrease the starting current
  • To increase the starting torque (correct)
  • To provide a braking force

What is the relationship between the number of poles (P) in a three-phase induction motor and the synchronous speed (Ns)?

  • Ns is directly proportional to the square of P
  • Ns is inversely proportional to the square of P
  • Ns is inversely proportional to P (correct)
  • Ns is directly proportional to P

What is the synchronous speed (Ns) of a four-pole induction motor operating at a frequency of 50 Hz?

  • 1500 rpm (correct)
  • 750 rpm
  • 3000 rpm
  • 1000 rpm

What is the function of the slip rings in a wound-rotor induction motor?

<p>To provide a path for the rotor current (A)</p> Signup and view all the answers

What is the significance of the laminated cylindrical core in the rotor of a wound-rotor induction motor?

<p>To reduce the magnetic losses (B)</p> Signup and view all the answers

What is the formula for the synchronous speed (Ns) of a three-phase induction motor?

<p>$Ns = (120 f/P)$ (B)</p> Signup and view all the answers

Which of the following is NOT a factor that determines the synchronous speed of a three-phase induction motor?

<p>Rotor resistance (A)</p> Signup and view all the answers

What is the primary function of the rotating magnetic field in a three-phase induction motor?

<p>To create a torque that drives the rotor (B)</p> Signup and view all the answers

What is the relationship between the rotating magnetic field's direction and the rotor's direction of motion?

<p>The rotor rotates in the same direction as the rotating magnetic field (B)</p> Signup and view all the answers

Why are currents induced in the rotor conductors of a three-phase induction motor?

<p>Due to the relative motion between the rotating magnetic field and the rotor conductors (C)</p> Signup and view all the answers

How does Lenz's law explain the movement of the rotor in a three-phase induction motor?

<p>Rotor currents oppose the change in magnetic flux caused by the rotating magnetic field (D)</p> Signup and view all the answers

What is the purpose of short-circuiting the rotor windings in a three-phase induction motor?

<p>To ensure that the rotor currents flow in the direction necessary to create torque (B)</p> Signup and view all the answers

What is the primary difference between a three-phase induction motor and a synchronous motor?

<p>A synchronous motor has a DC field winding on the rotor (D)</p> Signup and view all the answers

What is the relationship between the frequency of the rotor current (fr) and the slip (s)?

<p>fr = sf (C)</p> Signup and view all the answers

What is the frequency of the rotor current when the rotor is at standstill?

<p>f Hz (C)</p> Signup and view all the answers

What is the formula for calculating the slip?

<p>s = (Ns - N) / Ns (A)</p> Signup and view all the answers

A 4-pole induction motor is connected to a 50 Hz supply. What is the synchronous speed of the motor?

<p>1500 rpm (B)</p> Signup and view all the answers

A 6-pole induction motor operates at a slip of 5%. What is the rotor speed?

<p>850 rpm (C)</p> Signup and view all the answers

An induction motor is connected to a 60 Hz supply. The motor has 8 poles and rotates at 850 rpm. What is the slip of the motor?

<p>0.05 (D)</p> Signup and view all the answers

What is the main reason why three-phase induction motors are widely used in industries?

<p>They are very efficient and have a good power factor. (B)</p> Signup and view all the answers

Why does the rotor of an induction motor try to catch up with the stator field?

<p>To reduce the slip and the frequency of the rotor current (A)</p> Signup and view all the answers

What is the primary function of the rotor winding in a three-phase induction motor?

<p>To generate a magnetic field that interacts with the stator field. (B)</p> Signup and view all the answers

What happens to the frequency of the rotor current as the slip decreases?

<p>It decreases proportionally to the decrease in slip (B)</p> Signup and view all the answers

What is the purpose of using laminated silicon steel for the core of the stator in a three-phase induction motor?

<p>To improve the motor's efficiency by reducing iron losses. (D)</p> Signup and view all the answers

What is the key difference between a squirrel-cage rotor and a wound rotor in a three-phase induction motor?

<p>The squirrel-cage winding is permanently short-circuited while the wound rotor winding can be connected to external resistors. (D)</p> Signup and view all the answers

What is the primary advantage of using a three-phase induction motor over a direct current (DC) shunt motor?

<p>Three-phase induction motors are cheaper to build and maintain. (B)</p> Signup and view all the answers

Which of the following is NOT a disadvantage of a three-phase induction motor?

<p>High maintenance requirements. (A)</p> Signup and view all the answers

What is the purpose of the air gap between the stator and rotor in a three-phase induction motor?

<p>To allow for the rotor to rotate freely. (C)</p> Signup and view all the answers

What is the primary function of the end rings in a squirrel-cage rotor?

<p>To provide a path for current flow between the rotor bars. (D)</p> Signup and view all the answers

Flashcards

Three-Phase Induction Motor

An electric motor widely used in industries, operating at constant speed under load.

Stator

The stationary part of a three-phase induction motor, housing the stator winding.

Rotor

The rotating part of the induction motor that carries the rotor winding.

Air Gap

The small distance between the stator and rotor, typically from 0.4 mm to 4 mm.

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Squirrel Cage Rotor

A type of rotor with parallel slots filled with conductive bars connected at both ends.

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Wound Rotor

A rotor type that uses windings instead of bars and requires external connections.

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Advantages of Induction Motors

They include simple construction, low cost, minimal maintenance, high efficiency, and self-starting torque.

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Disadvantages of Induction Motors

Notable cons include constant speed operation and lower starting torque compared to DC motors.

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Wound type induction motor

A type of induction motor with a cylindrical core and 3-phase rotor winding.

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Synchronous speed (Ns)

The speed at which the rotating magnetic field in a motor revolves.

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Star-connected rotor winding

Configuration where rotor windings are connected in a star formation.

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Starting torque

The initial torque provided to start an induction motor.

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Slip rings

Electrical contacts that allow current transfer to the rotor's windings.

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Rotor Direction

The rotor runs in the same direction as the stator field to reduce speed difference.

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Slip

The difference between synchronous speed and rotor speed, expressed as a percentage.

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Frequency of Rotor Current

At standstill, the rotor current frequency matches the supply frequency; changes with slip when rotor moves.

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Standstill Condition

When the rotor is stationary, slip equals 1, and rotor frequency is equal to the supply frequency.

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Increased Rotor Speed

When the rotor runs faster, the slip decreases and the rotor current frequency changes accordingly.

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Slip Calculation

Slip can be calculated using the formula: s = (Ns - N) / Ns, where N is rotor speed.

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Rotor Frequency (fr)

Frequency of rotor current when rotor speed is at a specific value; it’s calculated using fr = sf.

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Resultant Flux

The total magnetic flux resulting from summing individual flux components.

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Rotating Magnetic Field

A magnetic field that rotates around the stator at synchronous speed.

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Induced e.m.f.

Electromotive force generated in the rotor due to the rotating field.

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Torque

The rotational force produced by current-carrying rotor conductors.

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Lenz's Law

A law stating that induced currents oppose the change that created them.

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Stationary Rotor

The rotor that remains still while the magnetic field rotates.

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Mechanical Force

The force acting on the rotor due to the interaction with the stator's magnetic field.

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Study Notes

AC Electrical Machines: Three-Phase Induction Motor

  • Three-phase induction motors are the most common type of electric motor used in industry.
  • They maintain a relatively constant speed from no load to full load.
  • The motor consists of a stator and a rotor.
  • The stator houses a three-phase winding (stator winding).
  • The rotor has a short-circuited winding (rotor winding).
  • The machine converts electrical energy into mechanical energy.

Advantages

  • Simple and rugged construction
  • Relatively inexpensive
  • Low maintenance requirements
  • High efficiency
  • Good power factor
  • Self-starting torque

Disadvantages

  • Constant speed characteristic - Changing speed is difficult.
  • Lower starting torque compared to DC shunt motors.

Construction: Stator

  • A steel frame encloses a hollow cylindrical core.
  • The core is made of thin, laminated silicon steel to minimize hysteresis and eddy current losses.
  •  Slots in the core house the stator windings.

Construction: Rotor

  • Squirrel Cage Type:
    • Consists of a cylindrical laminated core with parallel slots.
    • Copper or aluminum bars are placed within each slot.
    • The bars are shorted together at each end by end rings.
    • This forms a permanently short-circuited winding.
  • Wound Type:
    • Has a laminated cylindrical core.
    • The core carries a three-phase winding similar to the stator winding.
    • The rotor winding terminals are connected to slip rings mounted on the rotor shaft.
    • External resistance is incorporated in the rotor circuit during starting to increase torque.
    • Gradually reduced as the motor accelerates to reach rated speed.

Speed of Rotating Magnetic Field

  • The speed at which the magnetic field rotates is called synchronous speed (Ns).
  • For a P-pole motor, with a frequency (f) of the supply current, the synchronous speed is calculated by this formula: Ns = (120 × f) / P

Slip

  • The difference between the synchronous speed (Ns) and the actual rotor speed (N) is called slip (s).
  • Slip is usually expressed as a percentage of synchronous speed. s = [(Ns - N) / Ns] × 100
  • Slip is typically small (0.1% to 3%) in induction motors.

Frequency of Rotor Current

  • At standstill (s = 1), the rotor frequency (fr) is the same as the supply frequency (f).
  • When the rotor rotates, the rotor frequency is related to the slip and supply frequency by the following equation: fr = s × f

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Electrical Machines (AC) - Three Phase Induction Motor (PDF)

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