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M3 > 3.18 AC Motors > Flashcards

3.18 AC Motors Flashcards

1
Q
A
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2
Q

What are the two main categories of AC motors?

A

Synchronous motors and asynchronous motors (induction motors)

These categories differ primarily in their rotor speeds relative to the stator magnetic field.

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3
Q

Define synchronous motors.

A

Machines whose rotor speeds are equal to the speeds of the stator magnetic field

These motors do not experience slip.

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4
Q

Define asynchronous motors.

A

Machines where rotors rotate at speeds less than the synchronous speed

These are commonly referred to as induction motors.

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5
Q

What is the primary advantage of using AC motors?

A

They cost less than DC motors and require less maintenance

They also eliminate issues with brushes and commutators.

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6
Q

What is the purpose of damper windings in synchronous motors?

A

To provide starting torque and minimize hunting

They are typically made of heavy copper bars inserted in the rotor pole faces.

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7
Q

What is the Load Angle in synchronous motors?

A

The difference between the speed of the stator magnetic field and the rotor speed

It equates to the ‘Slip’.

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8
Q

What distinguishes salient pole machines from non-salient pole machines?

A

Salient pole machines have radially arranged pole shanks; non-salient pole machines have cylindrical rotors

Salient pole machines are suitable for low speeds, while non-salient ones are for high speeds.

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9
Q

True or False: Synchronous motors have starting torque.

A

False

Synchronous motors require external starting equipment to reach rated speed.

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10
Q

What is the primary construction of induction motors?

A

They consist of a stator and a rotor

The stator has three-phase windings and the rotor typically has a squirrel cage design.

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11
Q

How is torque produced in induction motors?

A

By the induced EMF in the rotor due to the stator’s rotating magnetic field

The rotor current opposes the rotating magnetic field, creating torque.

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12
Q

What is the formula for slip in induction motors?

A

Slip = Synchronous speed - Actual speed

Slip is essential for the operation of induction motors.

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13
Q

What is the typical phase arrangement in a three-phase induction motor?

A

Three coils placed 120° apart

This arrangement ensures smooth rotation of the magnetic field.

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14
Q

What is the primary characteristic of induction motors?

A

They are self-starting and robust in construction

They are also economical and easy to maintain.

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15
Q

Fill in the blank: Synchronous motors operate at a _______ speed.

A

[constant]

Their speed is determined by the number of poles and supply frequency.

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16
Q

Which type of induction motor has its stator made up of two windings placed at right angles?

A

Two-phase induction motor

These motors are not common due to impracticality in modern applications.

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17
Q

What effect does increasing load have on low-impedance induction motors?

A

It does not greatly reduce speed but increases torque

This characteristic helps maintain consistent speed despite load changes.

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18
Q

What is the synchronous speed for a two-pole machine at 60 Hz?

A

3600 RPM

This speed is determined by the frequency of the applied voltage.

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19
Q

What happens if the rotor speed equals synchronous speed in an induction motor?

A

No EMF is induced in the rotor, leading to no torque generated

This prevents the rotor from reaching synchronous speed.

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20
Q

What are the characteristics of synchronous motors compared to induction motors?

A

Synchronous motors are sensitive to load changes and have no starting torque

Induction motors, on the other hand, are less sensitive to load variations.

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21
Q

What is the relationship between rotor speed and the magnetic field in AC motors?

A

If the rotor reaches the same speed as the magnetic field, there is no induction and the motor torque drops.

22
Q

What happens when the rotor lags behind the rotating field in an AC motor?

A

The rotor cuts the flux lines, producing torque.

23
Q

What term is used to describe the difference in speed between the motor and the field?

A

‘Asynchronous’

24
Q

What are the two viable methods for controlling the speed of an AC induction motor without rebuilding it?

A
  • Change the frequency
  • Use a wound rotor
25
Q

Why is changing frequency not viable for aircraft AC induction motors?

A

AC induction motors are designed for a specific speed in terms of construction.

26
Q

What is a wound rotor induction motor?

A

A special type of 3-phase AC induction motor designed to provide high starting torque and variation in speed.

27
Q

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

A

To connect external resistance in the rotor circuit.

28
Q

How does varying the resistance in the rotor circuit of a wound rotor motor affect its speed?

A

Increasing resistance decreases speed; decreasing resistance increases speed.

29
Q

What is the effect of increased rotor resistance on torque in a wound rotor motor?

A

More voltage is induced in the rotor circuit, producing necessary torque and increasing it.

30
Q

How is the direction of rotation changed in a 3-phase induction motor?

A

By reversing any two of the three wires.

31
Q

What controls the direction of rotation in a single-phase induction motor?

A

A starting capacitor.

32
Q

Fill in the blank: The rotor follows the _______ phase in AC motors.

A

[leading]

33
Q

True or False: The speed of a wound rotor motor is equal to the synchronous speed of the rotor.

A

False

34
Q

What is a single-phase induction motor?

A

A motor that runs on single-phase AC and is used for small low-output applications

Examples include communication equipment, fans, refrigerators, and power tools.

35
Q

What are the two kinds of single-phase motors?

A
  • Induction motors
  • Series motors
36
Q

Describe the characteristics of a series AC motor.

A
  • Varying-speed machine
  • Low speeds for large loads
  • High speeds for light loads
  • Very high starting torque
37
Q

What is the primary difference between a single-phase induction motor and a series AC motor?

A

The single-phase induction motor has only one stator winding that generates a non-rotating field.

38
Q

What is the purpose of the capacitor in a capacitor start motor?

A

To create a phase difference between the primary and auxiliary windings for self-starting.

39
Q

What is synchronous speed?

A

The speed at which the magnetic field rotates in a rotary machine, determined by frequency and number of poles.

40
Q

What are the two types of induction motors?

A
  • Single-phase induction motor
  • Three-phase induction motor
41
Q

How does the rotor in an induction motor generate torque?

A

The rotor flux lags behind the stator flux, creating a torque that causes rotation.

42
Q

What is the role of the starting capacitor in a capacitor start induction motor?

A

Provides additional starting torque by being connected in parallel during startup.

43
Q

Fill in the blank: A shaded pole motor uses a _______ wound in a small notch in the stator pole piece.

A

shading coil

44
Q

What happens when the shading coil in a shaded pole motor is energized?

A

It produces a field opposite to that of the main field, inducing rotation in the rotor.

45
Q

True or False: A single-phase induction motor is self-starting.

A

False

46
Q

What is the function of the centrifugal switch in a capacitor motor?

A

Turns off the starting capacitor after the motor reaches 70-80% of its rated speed.

47
Q

List the categories of induction motors.

A
  • Single-phase induction motor
  • Split-phase induction motor
  • Capacitor start induction motor
  • Capacitor start and capacitor run induction motor
  • Shaded pole induction motor
  • Three-phase induction motor
  • Squirrel-cage induction motor
  • Slip ring induction motor
48
Q

What is the effect of connecting a capacitor to the auxiliary winding?

A

It creates a 90-degree phase difference necessary for starting the motor.

49
Q

What is the primary difference between squirrel-cage and wound induction motors?

A

Squirrel-cage motors have a simple rotor design, while wound motors have windings on the rotor.

50
Q

How can the direction of rotation in a three-phase motor be changed?

A

By altering the phase sequence of the three-phase supply.

51
Q

What is the importance of slip in an induction motor?

A

It is necessary to induce rotor current and generate torque.

M3 (19 decks)

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