Big review practice questions Flashcards
Magnetic lines exit one point and enter another. In which direction does this happen?
North to South
What is flux density?
concentration of flux lines
Name 3 ferromagnetic materials?
Iron
Nickel
Cobalt
There are two groups of magnets, what are they?
permanent
temporary
What does the term saturation refer to?
Max flux lines
The material that the coil is wrapped around determines what?
magnetizing ability
What does a high coercive force indicate?
difficulty to demagnetize
How is a hysteresis loop formed?
by A/C current
The term hysteresis in electrical terminology means?
“to lag” between a change in value or direction
Normally how many times per second do the domains in the core undergo a periodic reversal?
60
The core will become warm partly due to the constant field of reversal. Some of the energy is being lost due to heat. This is known as a ?
hysteresis loop
What size of hysteresis loop would be preferred for motors, transformers, alternators and other equipment used in A/C circuits?
small area
Does wood have a high or low reluctance?
high
Does iron have a high or low reluctance?
low
If AC equipment is an assembly of thin sheets of iron called?
laminations
whenever mines if current pass through any kind of metal they generate a current in the metal. What is this current called?
eddy currents
Unit for magnetic flux?
weber
The amount of residual magnetism depends on the _____________ is the steel used?
retentivity
The strength of a magnetic field is represented by the __________ of the lines of force?
density
The magnetic strength of a coil can be measured in ____________?
ampere turns
Equal magnetizing forces can be produced by ____________ current and few turns or ___________ current and many turns.
large
small
When iron oxide is mixed with oxides of manganese, cobalt, nickel, copper or zinc and then is pressed and fired resulting temporary magnetic materials called?
ferrites
Unit for flux density?
Tesla
Magnetic cores are usually constructed from materials with a high ________ and low ___________?
Permeability
Reluctance
The MAGNETIC flux in a circuit is directly proportional to the _____________ and inversely proportional to the ____________ of the magnetic circuit?
mmf
reluctance
What is one way to tell the difference between the high voltage leads and low voltage leads of magnetic cores?
size of wire
hysteresis losses are caused from what two things?
heat
vibration
What is the curie temperature for iron?
770 degrees celsius
1420 Fahrenheit
What happens to the flux density when the air gap is widened?
it becomes lower
Some of the lines of force leave and enter the coil before reaching the ends of the coil. That is referred to as _________________?
leakage flux
Symbol for reluctance?
Rm
What is the symbol for magnetizing force?
H
Motors convert _________ energy to ___________ energy.
electrical
mechanical
Generators convert ____________ energy to _____________ energy.
mechanical
electrical
What controls the magnitude of the current through the field coils?
field rheostat
motors and generators need magnetic poles. what are these poles called?
field poles
The moveable part of a motor or generator is called?
rotor
