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