Electricity One Seven CH7

Question 1

Faraday used Qersted’s discovery that electricity could be used to produce motion: thus:

Answer 1

The first electric motor was built in 1821

Question 2

An electric motor transforms electricity into rotary motion to:

Answer 2

Perform useful work

Question 3

Torque is:

Answer 3

a twisting or turning force exerted on the loop that causes it to rotate around its axis

Question 4

The four principal parts of an elementary DC motor are:

Answer 4

the magnetic field, movable conductor, commutator and brushes

Question 5

DC motor can be made to start by itself by:

Answer 5

giving it an armature that contains two or more loops

Question 6

The armature:

Answer 6

is the rotating element of the DC motor

Question 7

The leads from the armature coils are connected to the raised portions of commutator segments known as:

Answer 7

Risers

Question 8

The brushes used on the commutator are made up of soft carbon material containing large amounts of:

Answer 8

Graphite

Question 9

Although brushes are designed to be long lasting, they are always made to wear out faster than the commutator because:

Answer 9

It is cheaper and easier to replace brushes

Question 10

The field assembly consists of:

Answer 10

Pole pieces and field coil

Question 11

Most motor housing consists of three parts. What are they?

Answer 11

Field frame and 2 end frames

Question 12

The rotor or armature core is made of laminated soft steel to:

Answer 12

Minimize Eddy currents and hysteresis losses

Question 13

Lap windings are used:

Answer 13

For low voltage; high current motors

Question 14

Wave windings are used:

Answer 14

For high voltage, low current motors

Question 15

The role of (CEMF) counter-electromotive force plays a very important role in the operations of a motor. What is it?

Answer 15

The CEMF opposes the line voltage, therefore, it acts to cut down the short-circuit current which the line voltage could potentially cause through the low-resistance armature

Question 16

(CEMF) Counter-electromotive force tends to:

Answer 16

Regulate motor speed

Question 17

When a load is applied to the motor, the motor tends to slow down. However, this reduces CEMF (counter-electromotive force) which:

Answer 17

increases the EMF supplied to the motor to bring it back to the right speed

Question 18

Power can be described electrically in watts or:

Answer 18

Mechanically in horsepower

Question 19

When a motor has no load, it needs little torque, and so, speeds up to build up EMF in order to reduce the armature current, and thus:

Answer 19

Reducing the torque

Question 20

Series motors are used to do heavy work on:

Answer 20

ALL OF THE ABOVE

1. Cranes
2. Hoists
3. Subway trains

Question 21

The direction of rotation in a DC motor can be reversed if the current of

Answer 21

(“Both are CORRECT” is the answer)
1.the field is reversed
2. the armature is reversed

Question 22

When motors are first started, a high current flows due to the low DC resistance of the armature coils. In order to protect against this:

Answer 22

Starters and controllers are used

Question 23

Drum controllers are used with motors that require:

Answer 23

ALL OF THE ABOBE

1. Frequent starting
2. Frequent stopping and reversing
3. Frequent speed changing

Question 24

Synchronous speed in an AC motor is:

Answer 24

The natural rate at which the magnetic field rotates about the stator

Question 25

The basic synchronous motor:

Answer 25

“BOTH A AND B ARE CORRECT” is the answer

1. Turns at a synchronous speed with the stators rotating the magnetic field
2. Is locked into position by the attractive force of the stator field

Question 26

The starting torque of a synchronous motor is:

Answer 26

Zero

Question 27

A two-pole motor that works on 60 cycles power has a synchronous speed of:

Answer 27

3600 RPM

Question 28

Synchronous motors can also be made self-starting by:

Answer 28

Providing them with a damper winding over their regular winding

Question 29

Induction motors use:

Answer 29

Electromagnetic induction to magnetize their rotors

Question 30

The squirrel-cage in an induction motor is located in the:

Answer 30

Rotor

Question 31

For the induction motor to work:

Answer 31

the rotor must have slip and turn at a rate slightly less than that of the rotating field

Question 32

The double-squirrel cage motor provides:

Answer 32

higher starting torque than the conventional squirrel-cage rotor

Question 33

The wound-rotor motor differs from other induction motors in that

Answer 33

it uses resistance in series with its rotor windings through slip ring and brush arrangement

Question 34

The stator and rotor of an induction motor acts like

Answer 34

Windings of a transformer

Question 35

Since the induction motor relies on electromagnetic induction, its rotor cannot run:

Answer 35

At synchronous speed

Question 36

Losses are created when the molecules of an iron bar remain aligned in an external magnetic field after the position of the bar in the field has been changed. This is called:

Answer 36

Hystersis losses

Question 37

The AC repulsion motor:

Answer 37

resembles a DC motor and an inductive motor

Question 38

The universal motor operates equally well on:

Answer 38

AC or DC

Question 39

The hysteresis synchronous motor uses:

Answer 39

a cobalt-steel rotor that, through hystersis effects, develops a greater torque start

Question 40

The repulsion start induction motor begins as a repulsion motor, but:

Answer 40

Runs as a wound rotor motor