Chapter 15 - Electromagnetic Machines Flashcards
Magnetic Flux Density =
F/IL
Magnetic Field Lines must
Not cross, Go from North to South and be close near magnet
Anti-clock wise coil has what polarity
North
Clockwise coil has what polarity
South
Permeance is
Ability to transmit magnetic flux
Faraday’s Law =
e.m.f. is proportional to rate of change of flux linkage
Flux Linkage(equation) =
NBAcos(x)
Lenz’s Law =
the induced current always opposes the change causing it
Forces between two poles tend to make …
Flux paths shorter and straighter.
Flux behaves as…
An elastic string.
Flux loops round ..
Current.
Flux is measured in ..
Wb - Weber
E.M.F. =
- the rate of change of flux linkage over time.
Ratio of alternating voltages across two coils =
Ratio of their numbers of turns
Permeance =
Flux / Driving current turns
Unit of Permeance
WbA^-1
Shorter and thicker circuits are better or worse?
Better
How do you make a generator from a transformer
Cut out the section of the core with the primary coils and spin it.
Electromagnetic machines have …
At least one magnetic circuit and one electric circuit linked round one another.
In an electromagnetic machine the electric circuit needs …
High conductance, increased by larger cross-sectional area and smaller length.
In an electromagnetic machine the magnetic circuit needs …
High permeance, increased by higher cross-sectional area and a lower length of magnetic circuit.
How does a transformer work?
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.
How is an induced e.m.f in an alternator made?
Moving flux linking a conductor
How do induction motors work?
Rotating flux-inducing currents in a conducting rotor - producing a force on the rotor.
Is a motor also a generator?
Yes, a back e.m.f. opposing the supply is generated as the motor turns.
