Chapter 15 - Electromagnetic Machines

Question 1

Magnetic Flux Density =

Answer 1

F/IL

Question 2

Magnetic Field Lines must

Answer 2

Not cross, Go from North to South and be close near magnet

Question 3

Anti-clock wise coil has what polarity

Answer 3

North

Question 4

Clockwise coil has what polarity

Answer 4

South

Question 5

Permeance is

Answer 5

Ability to transmit magnetic flux

Question 6

Faraday’s Law =

Answer 6

e.m.f. is proportional to rate of change of flux linkage

Question 7

Flux Linkage(equation) =

Answer 7

NBAcos(x)

Question 8

Lenz’s Law =

Answer 8

the induced current always opposes the change causing it

Question 9

Forces between two poles tend to make …

Answer 9

Flux paths shorter and straighter.

Question 10

Flux behaves as…

Answer 10

An elastic string.

Question 11

Flux loops round ..

Answer 11

Current.

Question 12

Flux is measured in ..

Answer 12

Wb - Weber

Question 13

E.M.F. =

Answer 13

• the rate of change of flux linkage over time.

Question 14

Ratio of alternating voltages across two coils =

Answer 14

Ratio of their numbers of turns

Question 15

Permeance =

Answer 15

Flux / Driving current turns

Question 16

Unit of Permeance

Answer 16

WbA^-1

Question 17

Shorter and thicker circuits are better or worse?

Answer 17

Better

Question 18

How do you make a generator from a transformer

Answer 18

Cut out the section of the core with the primary coils and spin it.

Question 19

Electromagnetic machines have …

Answer 19

At least one magnetic circuit and one electric circuit linked round one another.

Question 20

In an electromagnetic machine the electric circuit needs …

Answer 20

High conductance, increased by larger cross-sectional area and smaller length.

Question 21

In an electromagnetic machine the magnetic circuit needs …

Answer 21

High permeance, increased by higher cross-sectional area and a lower length of magnetic circuit.

Question 22

How does a transformer work?

Answer 22

In a transformer, an alternating current in the primary coil produces alternating flux in an iron core, which induces an alternating e.m.f. in the secondary coil.

Question 23

How is an induced e.m.f in an alternator made?

Answer 23

Moving flux linking a conductor

Question 24

How do induction motors work?

Answer 24

Rotating flux-inducing currents in a conducting rotor - producing a force on the rotor.

Question 25

Is a motor also a generator?

Answer 25

Yes, a back e.m.f. opposing the supply is generated as the motor turns.