IT 150 Midterm 2 ch 6-18
The amount of induced voltage/current depends on:
- amount of force in the magnetic field (number of flux lines cut ) - speed and angle of the conductor cutting magnetic lines of flux
the ability for a semiconductor to conduct a current can be controlled in two ways
- doping the material with an impurity or - applying heat to agitate a few electronsloose
barrier potential of gemanium
.4
barrier potential of silicon
.7
two useful ways to troubleshoot parallel circuit
1)using an ohmmeter to check actual vs measured resistance values 2)calculating total resistance then checking total current
Motor Failure in AC motor
- Unless equipped with brushes, AC electric motors are almost maintenance free
Reverse bias operation allows NO current to flow in either the P or N material (except a very small transient current [ie, micro Amps or nano Amps]) . . . . . . Why?
There are no "holes" to jump into at N side.
____________ motors are the most common stationary, big load "prime movers" used in industry .
Three-phase AC
the opposite of a generator
a motor
consists of a rotating magnetic field inside three sets of windings
a three phase generator
a device you can use to control the speed of an AC motor
a variable-frequency drive
in the circuit shown below, if LED need 10 mA to light up, what current the base B of transistor needs to activate the transistor? No calculation is needed to answer this question.
a very tiny current
A relay may have a ________ to control a _______.
a. small voltage, high voltage b. dc voltage, ac voltage c. small current, large current d. all answers are correct.
P type material is acceptor or donor
acceptor
amperages in parallel
add
voltages in series
add
what does the strength of an electromagnetic force depend on
add more turns in conductor, length in conductor, type of core material
The arrow always points _______ the direction of electron flow.
against
If the magnetic field moves, it is an . .
alternator
Checking voltage drops is ____________ method of troubleshooting a parallel circuit.
an impractical
in the direction against the black wall of the arrow is the
anode
The power-producing component of an alternator, generator, dynamo or motor. The armature can be on either the rotor or the stator
armature
Permanent Magnet DC Motors: - Require ___________ only
armature current
lines of force are most concentrated where?
at the poles
Adding trivalent elements produces: Select one: a. N-type crystal b. P-type crystal c. PN-type crystal d. None of the above
b
Highest induced voltages are produced when the coils cut the magnetic field at ________ angle a. parallel b. right c. between the parallel and right d. none of the above
b. right
Due to Lenz's Law: the magnetic field in the rotor opposes the magnetic field in stator. This is called __________ (equivalent to cemf).
back emf
many coils in a motor generate a large ________
back emf
increasing the load increases the slip causing a reduction in ______ and a corresponding increase in ______ and ________.
back emf. current (emf) and torque
A relatively small flow of electrons sent through the ____ of the transistor has the ability to exert control over a much larger flow of electrons through the ______
base, collector
add what elements to make a P type semiconductor
boron or gallium
what are individual currents called
branch currents
sliding contacts in the slip rings
brushes
how can a magnet be demagnetized
by A/C, Heat, Pounding. It scrambles the domains
A semiconductor is not a good ___________ Select one: a. conductor b. insulator c. a or b
c
Motor starting circuits are intended to protect motors until they build up ________ a. speed b. cemf c. all of the above d. none of the above
c
Slip in AC motors' speed occurs as a result of: a. jammed motor b. emf c. back emf d. overheated motor
c
The input voltages of a 3-phase AC motor: Select one: a. are identical and in phase b. non-identical and out of phase c. are identical but out of phase d. any of the above is possible
c
Transistors can function when they are operated with: Select one: a. Forward bias current b. Reverse bias current c. both a and b d. none of the above
c
in the direction going up the slant of the arrow is the
cathode
how to change the direction of the induced current
change the direction of the conductor
A single split slip ring called a _______ rectifies the current
commutator
Pure direct current (DC) voltage has a _______ amplitude vs time
constant
a synonym for back emf
counter emf
The motor converts electrical energy into mechanical energy. However, when armature begins to rotate, the motor also becomes a generator. This counter current is called
counterelectromotive force (cemf)
Magnetic field direction depends on __________ ________
current direction
Eddy current loss :
currents that are induced in the generator armature core
A circuit breaker a. is installed for safety b. opens overload circuits c. is often used in place of a fuse d. all of the above
d
In semiconductor terminology, doping Select one: a. is the process of adding impurities to pure semiconductor material b. results in extrinsic semiconductor c. changes the conduction traits of pure material d. all of the above
d
The stationary field poles and windings in an induction motor are called a ____. a. armature b. rotor c. split phase winding d. stator
d
What does the strength of induced voltage depend on? a. The number of magnetic lines of force cut by the coil b. The number of loops in the conductor coil c. The speed at which the conductors moves through the field d. any of the above.
d
Which of the followings is considered as a loss resulting of resistance in the windings? a. copper loss b. I2R loss c. none of the above d. a or b
d
a. produces heat b. flows in the core of an armature windings c. causes a loss of energy d. all of the above
d
n comparing magnetic circuits with electrical circuits, which of the following comparisons is correct? a. magnetic force is comparable to voltage b. magnetic reluctance is comparable to resistance c. magnetic flux is comparable to current d. all of the above
d
which of the followings contributes to rotation of rotor in dc motors? a. current following in the armature b. magnetic field generated by armature c. magnetic field generated by field magnet d. all of the above
d
two ways to double the frequency
double field poles or speed
The number of cycles per second can be increased by: a. increasing the size of the generator b. increasing the rotational speed of generator c. increasing the number of field poles d. any of the above e. b or c
e
What is the purpose of field windings in the operation of a dc motor? a. producing current in the field windings b. producing magnetic field c. inducing current in rotor d. all of the above e. b and c
e
three applications of magnetism
electric motors, electric generator, MRI
electro magnetic induction is
electricity made from magnetism
applications of soft magnets
electromagnet materials in electric motors, magnetic shields, solenoids
source of magnetism comes from
electrons spinning
When resistors of equal value are connected in series, they will develop voltage drops of_____ value.
equal
N type has an excess or lack of electrons
excess. it is a donor
Direction of flow around a field magnet:
exterior: south to north interior: north to south
Adding one of the elements makes pure silicon lattices into one (of two) semiconductor types : N-type, P-type This type of semiconductors is called _______
extrinsic
A dc motor also generates an EMF that is in opposite direction to the applied current. Select one: True False
false
LEDs are different from silicon diodes in the sense they can not control the direction of current like a silicon diode. Select one: True False
false
Materials such as wood, plastics or glass can provide magnetic shield. true or false
false
The squirrel cage rotor receives current directly from input power line. Select one: True False
false
While high voltage can damage a motor, low voltage can not. Select one: True False
false
"The amount of voltage induced in a coil is directly proportional to the rate of change of the magnetic field with respect to the coil"
faradays law
Emitter-base: always _____ bias_____ resistance
forward, Low
doubling the number of field poles will double the
frequency
doubling the speed will double the ________
frequency
If the wire coils (armature) move, it is a . . .
generator
in a simple AC generator, load is exactly equal to the
generator output
eddy currents have a loss due to
heat
Synchronous speed formula
hertz x 60sec / poles = synch speed
when an ohmmeter is connected to troubleshoot parallel what does it mean if it reads infinity and zero
if it reads infinity it would indicate an open switch, if it reads zero resistance it would indicate a short circuit
Troubleshooting Series Circuits w/Voltmeter Switch
if switch is open it will read voltage across the switch but none across resistor, if switch is closed it will read no voltage across switch but read voltage across resistor
which direction is the current blocked on a diode schematic
in the direction not going up the slant of the arrow
in a circuit, resistors of equal value are connected in series. One of the resistors malfunctions and shorts the circuit. The voltage drops across the remaining resistors will:
increase
The most useful tool for troubleshooting motors is the
inductive ammeter
refers to that part of the sine wave that is represented at the output terminals of each generator at that instant of time
instantaneous polarity
Silicon atoms combine to make perfect crystalline lattice networks . . . (covalent bond between atoms) . . . Undoped, these lattices are pure called "______" pure insulators
intrinsic
this tool turns DC into AC
inverter
when load on AC increases what happens to slip
it increases, the speed between the two deceases
How much is the voltage drop across a shorted resistor?
it is equal to zero
in a parallel circuit if a resister goes out what happens to the coltage in the resistor on the other side
it stays the same
Magnetic field induced around the conductor is opposed by the existing field.
lenz's law
The total resistance in a parallel circuit is always ____________ the smallest resistance in the circuit.
less than
first two laws of magnetism
like poles repel unlike poles attract
The greater the ______, the greater the "slip" between the speed of the rotating magnetic field and the actual speed of the rotor
load
to have current in a simple AC generator you must have
load
In ferromagnetic materials like iron, there are regions, called ________ ________, where the magnetic fields of the electrons all point in the same direction
magnetic domains
The larger the current and/or the permeability, (or the shorter the distance between current and the location where the magnetic field is needed,) the larger the ________________
magnetic field
higher density flux stronger _________________
magnetic field
reed relay uses a _________ to close its contacts
magnetic field
Magnetic shields change the path of ____________ thus protecting a shielded object.
magnetic flux
invisible magnetic field lines
magnetic flux
this provides a concentrated path for magnetic lines to flow and is a redirecting source of magnetic lines
magnetic shielding
what is the total current called
mainline current
an impractical method for troubleshooting a parallel circuit
measuring voltage drops
a generator turns ________ energy into __________ energy
mechanical into electrical
Back emf reduces the speed of rotor. This speed reduction is called
motor slip
electromagnetic induction is based on what
moving a magnet toward or away from a conductor without touching it
The spin of an electron creates a strong magnetic field, which can have one of two possible orientations:
opposite spin (non ferromagnetic materials, aluminium and copper) same direction spin (ferromagnetic materials, iron, cobalt, nickel)
In P-type semiconductors, voltage applied forces "holes" (positive charges) to flow in direction
opposite to flow of electrons.
AC Voltage and/or Current Waveform Measuring Device :
oscilloscope
in 3 phase AC motors input voltages are identical but
out of phase
how to create a magnetic field around the conductor
pass an electric current through a conductor
another name for N type semiconductors
pentavalent
The ability of a material to conduct magnetic lines of force is called______
permeability
if something can obtain and sustain a magnetic field it is said to have
permiability
The number of separate electric currents energizing the coils independently, out of step, is known as the
phase of the motor
adding these two elements produces an N type
phosphorous or aresenic
In N-type semiconductors, voltage applied forces electrons to flow towards the ________ pole, similar to conduction of current in a wire
positive
total power consumed is equal to
power of individual components added. this is for series and parallel
three methods to measuring resistance in a parallel circuit
product over sum method • reciprocal of sum of reciprocals method • equal resistances method
The magnetic flux remaining in the magnetized element is called
residual magnetism
Base-collector: always _____ bias ___ resistance, ____ current when transistor is OFF
reverse, High, low
the basis of torque in many devices
rotation
most commonly used elements for semiconductors
silicon, then germanium. they both have four valence electrons
This difference between the synchronous and actual speed is called ______ . . . (approximately 5%).
slip
parallel resistors greatest loss is with what size resistor
smallest resistor
a magnet that can be reversable is called a _______ magnet. a magnet that is permanent is called a _______ magnet
soft, hard.
_______ is an electrical device that uses a small voltage/current to connect a larger voltage/current
A relay
________ is a special type of electromagnet composed of an electromagnetic coil, a ferromagnetic plunger, and a case. When energized, the magnetic field attempts to center the plunger between its ends.
A solenoid
The stationary part of an alternator, generator, dynamo or motor
stator
voltages in parallel
stay the same
amperage in series
stays the same
three uses of transistors
switches, relays, amplifiers
this is the rotational speed of a magnetic field in AC motors
synchronous speed
what are the three sets of windings referred to as
the armature of the generator
what is this a function of: Adding armature coils and corresponding commutator segments increases torque
the inverse square law
what does the armature provide power for
the load circuit
how is iron made into a magnet
the magnetic domains that may be canceling each other out are exposed to external magnetic field which cause the domains to point the same direction
where does magnetism come from
the motion and properties of electrons
electromagnetic force that attempts to center a sliding ferromagnetic core within an energized coil of wire
the solenoid effect
total power consumed in parallel circuits is equal to...
the sum of individual values of power consumed by each element
kirschoffs law:
the sum of voltage drops equals the voltage applied (in series)
when there are two sine waves on A/C power supply that are out of phase what is happening that is key
their peaks occur at different times
left hand rule:
thumb points direction of current and fingers in the rotation of magnetic field
P type is also called
tivalent, electron acceptor
when electric current passes through a coil in the magnetic field the magnetic force produces a ______ which turns the DC motor
torque
kirchoffs current law
total current into a node equals the total current out of the node AKA current in equals current out
A _________ is a semiconductor device used to amplify and switch electronic signals and electrical power.
transistor
two things semiconductors are used in
transistors and diodes
AC motors can not be directly driven from batteries. Select one: True False
true
Adding armature coils and corresponding commutator segments increases torque true or false
true
Current can flow only in one direction through the LED true or false
true
Field magnet could be a wound stator or a permanent magnet. true or false
true
Generators convert electrical energy into mechanical energy. T or F
true
In reverse biased mode. very little current can flow in a diode. Select one: True False
true
is this an AC or DC Induction motor: first, induction of current by rotor, then magnetic Repulsion/Attraction
AC
speed control AC vs DC
AC uses variable frequency drive, DC uses voltage variation
Lenz's Law states: magnetic field induced around the conductor is opposed by the existing field. Select one: True False
true
a diode is made of N type and P type and flows in one direction and blocks flow in other direction, true or false
true
armature is stationary and magnetic field is rotational true or false
true
in dc motors, as armature speed increases, cemf increases. true or false
true
magnetic field can go through both conductors and nonconductors true or false
true
transformers are used to step up for cities and step down for residential true or false
true
two magnetic fields cannot occupy the same space at the same time true or false
true
the back emf reduces the current flow true or false
true, when a motor is running the back emf will reduce the current flow. this explains why lights "undim"
Adding a second loop of wire (and commutator poles) superimposes a second sine wave over the first (but 90°out of phase). This is called
two-phase DC generator
a typical electromagnetic relay uses a ______ circuit to turn on a _____
use a small voltage to switch a large voltage
Pulsating direct current (DC) voltage has a ______ amplitude vs time
varying
How much voltage will be found across a blown fuse?
whatever the source voltage is
can an ammeter be used to check the full current across a switch in the process of troubleshooting a parallel circuit
yes
types of overload protection
• Locked rotor damage • Fuses • Circuit Breakers • Thermal-overload Devices • Bimetallic devices
disadvantages to AC motors
• Speed control is simpler and accurate in DC motors: - The speed of DC motors is much easier to control simply by turning the supply voltage up or down. DC: Excellent for variable speed operations. In AC motors the speed depends on the frequency of the alternating current that drives it. It needs a variable-frequency drive to control the speed. • Though relatively simple, induction motors can be fairly heavy and bulky because of their coil windings. • Unlike DC motors, AC motors can't be driven from batteries or any other source of DC power (solar panels, for example) without using an inverter (a device that turns DC into AC). That's because AC motors need a changing magnetic field to turn the rotor.
inverter turns _____ into _____
D/C into A/C
is this an AC or DC Induction motor: A simple magnetic Repulsion/Attraction machine
DC
two things that use hard magnets
DC motors, loud speakers
this predicts how a magnetic field will interact with an electric circuit to produce an electromagnetic force (EMF) : a phenomenon called electromagnetic induction
Faradays law
The magnetic field component of an alternator, generator, dynamo or motor. The field can be on either the rotor or the stator and can be either an electromagnet coil or a permanent magnet.
Field
__________ (electromagnets) allow the generator to produce varying filed strength and substantially more power compared to permanent magnets
Field coils
third law of magnetism
Magnetic attraction force varies inversely with the square of distance between poles. (Inverse Square Law)
The motor effect was discovered by who when
Michael Faraday circa 1821.
for a _____ transistor, base current must consist of electrons flowing from emitter to base (against the emitter arrow symbol, just like the lamp current)
NPN
Copper losses
P = I^2R power loss Conductors have resistance, power loss in form of heat)
Forward bias operation allows current to flow in the ___ type material by holes and in the ____-type material by electrons
P, N
power in a series circuit
P=IR, P=IE, P=sum of Ps
Mechanical losses
Rotational or mechanical losses can be caused by bearing friction (main shaft), brush friction on the commutator
The rotating part of an alternator, generator, dynamo or motor
Rotor
__________________ motors are BY FAR the most common "prime movers" in the world. They are used in a wide variety of industrial, household, and shop tools and/or appliances . . .
Single-phase AC
motor installation and protection
- Dedicated circuit - Adjustable, slowtripping branch current protection - Stop/Start Pushbutton Control - Run protection - Set to motor "service factor" (125% or 1.25 for example)
universal motors
- Can operate on either AC or DC: • Typically series wound • Used for light loads due to large amount of sparking at the brushes • Small fans • Powered Drill
High accuracy in position control through DC Servo (Stepper) Motors:
- Must be paired with controllers which provide closed loop control about: • Speed • Degrees of rotation (armature position in tenths of a degree)
Speed Regulation:
- Some motors develop full power under load - Other motors must be brought up to speed before a load is applied.
Application of permanent magnet DC motors
Starter motor in automobiles, windshield wipers, washer, for blowers used in heaters and air conditioners, to raise and lower windows, it also extensively used in toys. Because of its permanent magnet, these motors are used where there is no need of speed control of motor by means of controlling its magnetic field.
_______ means the speed of the stator's rotating magnetic field.
Synchronous speed
advantages of AC motors
• The biggest advantage of AC induction motors is their sheer simplicity. They have only one moving part, the rotor, which makes them low-cost, quiet, long-lasting, and relatively trouble free. (more reliable) DC motors, by contrast, have a commutator and carbon brushes that wear out and need replacing from time to time. The friction between the brushes and the commutator also makes DC motors relatively noisy (and sometimes even quite smelly). • DC motors are more vulnerable to harsh industrial settings (dust, explosives). AC can be operated in explosive atmospheres. • AC power lines can be found everywhere. Most motors are AC. • Higher power factor: input power/output power less current used to do the same work
The magnitude of the voltage produced in all generators is dependent on a number of factors:
• The strength of the magnetic field • The speed at which the conductor cuts the magnetic field (armature rotational speed) • # of conductor loops (or conductor length) within the magnetic field