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Flashcards in Review Deck (59)
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1

What is a generator?

A device that converts mechanical energy to electrical energy

2

What does a commutator in an AC generator do?

-Changes AC voltage to DC voltage
-Provides connection to the outside circuit via brushes
-Provides connection for the windings in the armature

3

Describe series field windings.

-Large wire with few turns
-Low resistance

4

Describe shunt field windings.

-Small wire with many turns
-High resistance

5

What is the output voltage of a generator proportional to?

-Strength of magnetic field of the pole pieces
-Speed of rotation of the armature
-# of turns of wire in armature

6

What happens when load is added to a series generator?

Output voltage increases

7

What happens when load is added to a shunt generator?

Output voltage decreases

8

What are the characteristics of a self-excited shunt generator?

It has residual magnetism in the pole pieces which produces initial output voltage

9

How is the output voltage of a DC generator that does not contain a separate voltage regulator generally controlled?

A large value variable resistor is connected in series with the shunt field

10

A DC generator is over-compounded when...

The output voltage is higher at full load than it is at no load

11

What determines the voltage regulation of a DC generator?

The resistance of the armature

12

What does it mean when a DC machine is cumulative-compounded?

The series and shunt field aid each other in the production of magnetism

13

What determines countertorque in a generator?

-strength of the magnetic field of the armature
-strength of the magnetic field of the pole pieces

14

______ is the twisting or bending of the magnetic lines of flux of the pole pieces.

Armature reaction

15

What are interpoles?

Small pole pieces located between the main field poles and connected in series with the armature.

16

What procedure can be used to set the brushes to the neutral plane position in a DC machine?

-Apply AC to the armature
-Connect an AC voltmeter across the shunt field leads
-Adjust brushes until the meter indicates 0 V

17

The DC motor known as a “constant speed motor” is the ______ motor.

Shunt

18

What type of motor should never be operated under a no-load condition?

Series

19

What determines the speed regulation of a DC motor?

Armature resistance

20

What is the method for reversing the direction of rotation for a DC motor?

Reverse the connection of the armature leads (or the field leads)

21

When a motor is connected differential compound, the ______ determines the direction of rotation of the motor at no load or light load.

Shunt field

22

In a compound DC motor control circuit, the field loss relay will disconnect the ______ if the power is lost in the ______.

armature, shunt field

23

The rotating part of an alternator is called the ______.

Rotor

24

The non-rotating part of an alternator is called the ______.

Stator

25

In an alternator, what does the rotor do and what does it produce?

-Provides the magnetism needed to induce voltage into the stator windings
-Connects to some source of external DC to excite the alternator

26

Connections to the armature of an alternator is normally done by using ______.

Sliprings

27

An alternator containing eight poles per phase is turning at 900 RPM. What is the output frequency?

f = PS/120

f = (8)(900)/120

f = 60 Hz

28

In an alternator, using a ______ is one method of preventing induced voltage from becoming excessive.

Field discharge resistor

29

What are the three factors that determine output voltage of an alternator?

-Strength of magnetic field of the rotor
-Speed of rotation of the rotor
-# of turns of wire in armature

30

What must be done to a synchronous motor to make it have a leading power factor?

Overexcitation of the rotor

31

Why should a synchronous motor never be started with DC?

It won’t start

32

What are 5 characteristics of a synchronous motor?

-Not an induction motor
-Must have DC excitation
-Operates at constant speed from full load to no load
-Operates at synchronous speed
-Has the ability to correct its own power factor

33

What are amortissuer windings in a three phase synchronous motor?

Squirrel cage bars used to start the motor

34

A three-phase motor has three slip rings on its rotor shaft. What type of motor is this?

Wound rotor motor

35

What is the formula to calculate percent slippage?

Percent slip = (SynchSpeed-RotorSpeed) / SynchSpeed x 100

36

What three things determine the amount of torque produced by an induction motor?

-Phase angle difference between rotor and stator fields
-Strength of magnetic field of the rotor
-Strength of magnetic field of the stator

37

What is synchronous speed?

The speed of the rotating magnetic field

38

What two factors determine synchronous speed of a three-phase motor?

-Number of stator poles per phase
-Frequency of the applied voltage

39

What are the three major types of three-phase motors?

-Squirrel cage induction
-Wound rotor motor
-Synchronous motor

40

What is the operating principle of all three-phase motor?

Rotating magnetic field

41

What is a motor?

A device that converts electrical energy into mechanical energy

42

How do you reverse the direction of rotation of a three-phase motor?

Switch any two leads

43

The wound rotor motor is popular in industry because it has ______ starting torque and ______ starting current.

High, low

44

In a DC motor, what determines torque?

-Strength of magnetic field of the pole pieces
-Strength of magnetic field of the armature

45

What factors determine CEMF in a DC motor?

-Strength of magnetic field of the pole pieces
-Speed of rotation of the armature
-# of turns of wire in armature

46

What are the three major types of single-phase split-phase motors?

-Resistance-start induction-run
-Capacitor-start induction-run
-Capacitor-start capacitor-run

47

What are the two separate windings in split-phase motors?

Run windings and start windings

48

In a split-phase motor, the ______ winding is made of many turns of small wire and is located at the top of the stator core.

Start

49

In a split-phase motor, the ______ winding is made of few turns of large wire and is located at the bottom of the stator core.

Run

50

Motors that are hermetically sealed use what to disconnect from the start winding?

Centrifugal switch

51

The capacitor-start induction-run motor has a(n) ______ connected in series with the centrifugal switch and start windings.

AC electrolytic capacitor

52

What is special about the capacitor-start capacitor-run motor?

The start windings remain energized

53

In multi-speed fan motors, what permits the motor to be operated without damage?

High impedance of the stator windings

54

What type of single-phase motor will operate on AC or DC voltage?

Universal motor

55

Similar to DC series motor, the universal motor has the same construction but also has...

Compensating windings (conductive compensation and inductive compensation)

56

Speed regulation of a universal motor is ______.

very poor

57

What are the advantages of a wound rotor motor over a squirrel cage motor?

-High starting torque and low current
-Speed control

58

What is the principle of operation of a wound rotor motor?

Rotating magnetic field

59

What is the principle of operation of a squirrel cage motor?

Induction