3.12 DC Motors And Generators

Question 1

What type of devices are DC motors or generators?

Answer 1

Energy transfer devices

Question 2

How does a motor work?

Answer 2

Transferring electrical energy to kinetic energy of a turning armature (the motor effect)

Question 3

What is an armature in an electrical motor?

Answer 3

An armature that’s wrapped in turns of wire (a rotating coil or inductor)

Question 5

How does a an armature connect to a power source in a dc motor?

Answer 5

By using a split ring and a pair of brushes

Question 6

What happens when the brushes line up with the splits in a commutator of a dc motor?

Answer 6

It is momentarily disconnected from the supply but inertia carries it until it reconnects

Question 7

How does a generator work?

Answer 7

By spinning a conductor in an electromagnetic field it creates electricity

Question 8

What are the parts in a DC generator that stay still called?

Answer 8

Stator

Question 9

What are the parts in a C generator that rotate called?

Answer 9

Armature or rotor

Question 10

What are the stationary parts of a DC generator?

Answer 10

-the ring (yoke)
-two main magnetic poles
-excitation windings around each pole

Question 11

In a DC generator what is a steel ring also known as?

Answer 11

A yoke

Question 12

What is the purpose of the excitation windings in a DC generator?

Answer 12

To generate a stationary magnetic field in the stator

Question 13

DC generators with a high output will usually come with?

Answer 13

Inter poles and compensation windings

Question 14

What three parts does an armature consist of?

Answer 14

-the shaft
-the windings of the coil
-the commutator

Question 15

Why does a DC generator and motor have multiple separate coils wound around the armature?

Answer 15

So there is always at least one coil ideally situated in the magnetic field

Question 16

What happens when the brushes move across the commutator and go onto different sections?

Answer 16

It reverses the the polarity of the armature as the pass the neutral point

Question 17

What are the brushes on a DC generator or motor made from?

Answer 17

Carbon

Question 18

What happens when the brushes wear away over time?

Answer 18

They need to be replaced

Question 19

What is used to create the main field in DC machines?

Answer 19

Electromagnets

Question 20

Why is the armature core laminated in DC motors?

Answer 20

To minimise eddy currents

Question 21

What is meant by ‘armature cross field’?

Answer 21

As the field of the armature crosses the main field

Question 22

What happens to the cross field when the current in the armature increases?

Answer 22

It also increases

Question 23

What is the armature relation?

Answer 23

The impact of the armature cross field on the main field

Question 24

Where is electromagnetic induction at its highest in DC machines?

Answer 24

Under the poles

Question 25

Where is the electromagnetic induction at its lowest in DC machines?

Answer 25

Between the poles

Question 26

What is reactive sparking?

Answer 26

Where the brushes meet the coils in the neutral zone

Question 27

What does brush sparking cause?

Answer 27

High wear on the brushes and the commutator

Question 28

What are interpoles used for?

Answer 28

To cancel out the displacement of the neutral zone

Question 29

Where are the interpoles located?

Answer 29

Between the main poles

Question 30

What can be placed in the main poles to cover total armature cross field?

Answer 30

Compensating winding

Question 31

How are the interpoles and compensating windings connected?

Answer 31

In series (both supplied by the current from the armature)

Question 32

DC generators produce an output voltage which depends on what three main factors?

Answer 32

-number of turns on the armature -rotational speed -excitation current in the stator windings

Question 33

DC generators have a stated rotational sped with produces a particular output voltage, why can the current still fluctuate?

Answer 33

It depends on the external load that its powering

Question 34

What determines the current flow from a DC generator?

Answer 34

The direction of rotation

Question 35

Where can the rotation direction be found for a DC generator?

Answer 35

Marked on the casing or the main shaft

Question 36

What is the power output of a motor dependant on?

Answer 36

The motor rated torque and the rotational speed

Question 37

Output power is defined in terms of?

Answer 37

Horsepower

Question 38

To convert a calculated value of power from watts to horsepower, what do you have to do?

Answer 38

Divide by 746

Question 39

What generates torque in a motor?

Answer 39

Shaft rotation

Question 40

Torque is given in?

Answer 40

Force-distance units (lb-ft, n-m, etc.)

Question 41

What is stall torque?

Answer 41

When the shaft speed is zero or the motor stalls

Question 43

What is continuous torque?

Answer 43

The maximum torque at running conditions

Question 44

What is meant by race in a motor?

Answer 44

When the machine infinitely increases its speed

Question 45

What determines the the direction of rotation in a DC motor?

Answer 45

-direction of magnetic field -the flow of current in the armature

Question 46

What happens if you change the direction of the magnetic field and the direction of current flow in a DC motor?

Answer 46

Nothing it will spin the same way as you change both not just one

Question 48

Series wound motors have a very high?

Answer 48

Starting torque

Question 49

Why must series wound motors not be operated without load?

Answer 49

Because they can race causing the armature to break

Question 50

How are series wound machines limited during start-up?

Answer 50

By using resistors

Question 51

An operating series wound motor is also a?

Answer 51

Generator

Question 52

In a series wound motor what does the terminal voltage and the speed depend on?

Answer 52

The load

Question 53

Very high loads pose the risk of a?

Answer 53

Short circuit

Question 55

What is the most common DC motor?

Answer 55

The DC hunt wound motor

Question 56

When is a shunt wound motor used?

Answer 56

Whenever a constant speed at various loads is required

Question 57

What does a DC shunt wound motor consist of?

Answer 57

-stator with excitation windings -rotor with armature windings -set of carbon brushes

Question 58

What is meant by shunt characteristics?

Answer 58

-the speed is nearly independent from load -the motor reaches its maximum speed at no load operation

Question 59

What are the two types of electric starting systems for gas turbine aircraft?

Answer 59

-direct cranking electrical systems -starter generator systems

Question 60

Why are starter generator units economically desirable?

Answer 60

They’re a starter and a generator within a single unit