bt22o Ac theroy and meters

Question 1

where does an analog meter get its pointer movement

Answer 1

principals of electromagnetism

Question 2

what does a magometer do

Answer 2

tests insulation for its resistance

Question 3

what is the primary saftey function for all ohmmeters

Answer 3

not to be connected to an energized cct

Question 4

what adjustment is used to zero an analog meter movement

Answer 4

zero adjustment knob

Question 5

what happens if you ignore the zero adjustment knob on analog ohmmeters

Answer 5

readings wont be accurate

Question 6

digital ohhmeters dont read zero ohms with probes shorted. does it affect measurement accuracy?

Answer 6

no not high enough resistance

Question 7

on manual or analog digital meters the range is indicated by the position of the selector switch. How is the range indicated when the meter is autoranging type?

Answer 7

Display screen

Question 8

When an analog meter is set on Rx1 range and the meter rests on infinity what does it tell you about the resistance betweeen the probes

Answer 8

resistance is too high for range setting

Question 9

when a manually ranged meter is set for 20k ohms and the display screen is OL what does it tell you about the resistance between the probes?

Answer 9

too high for the setting. R>20k ohms, break in the cct, leads broken

Question 10

when an autoranged meter is set to its normal resistance function, and displays OL what does that indicate about the resistance between the probes?

Answer 10

resistance too high, leads broken, break in cct.

Question 11

is a high impedance meter or low impedance meter better for measuring faults

Answer 11

low

Question 12

what happens when an analog DC meter is connected with incorrect polarity

Answer 12

read downscale

Question 13

what is the purpose of having DC+ and DC- functions on the simpson meter

Answer 13

ensure the same polarity

Question 14

what happens when you connect a digital meter with the incorrect polarity

Answer 14

will read a negative number

Question 15

what is the largest number that can be shown on a 3 1/2 digital meter display

Answer 15

19.99. first digit can only be 1

Question 16

what happens when an analog DC voltmeter and digital is connected to a voltage that is higher then the range setting

Answer 16

analog- needle will go all the way right
digital- will read a negative number

Question 17

why is it bad to lean on metal while taking a voltage reading

Answer 17

easy path to ground. shock

Question 18

when taking voltage readings should you connect the common or line side of the cct first

Answer 18

common

Question 19

if your first meter is reading 0V should you assume the cct is de-energized and safe to touch

Answer 19

no. meter could be falty. test on known voltage first then test.

Question 20

does a high impedance meter draw more or less current from the cct then a low impedance meter

Answer 20

less current

Question 21

an analog meter has a 10k ohm/Vdc rating. what is the internal resistance of the meter if its set to a 30Vdc range?

Answer 21

300k ohms

Question 22

does the impedance of a typical digital meter change with each range change?

Answer 22

no its the same

Question 23

why do different meters give different readings when the connection to the supply contains a high resistance?

Answer 23

internal impedamnce has an affect on the readings. different answers depending on each meters impedance.

Question 24

list 3 or more reasons why its a bad idea to measure voltage in an electronic cct with a low impedance meter?

Answer 24

damage to meter
damage to self
inaccurate readings
malfunctions
misleading measurements

Question 25

list 5 habits or procedures that can reduce the risks when taking voltage measurements

Answer 25

double check the meter setting advoid being in contact with grounded surfaces plug in common side first take note of polarity test a known live terminal first

Question 26

A peice of electrical equipment has failed. a low impedance meter is used to measure the voltage across the supply side terminals of a disconnect located near the load. The normal voltage is 120-130Vdc. but meter only indicated 5V a) can you be sure the cct is safe to touch b what would be a cct problem if you got a shock.

Answer 26

a)no b) high resistance

Question 27

what do ammeters read and how do you hook it up.

Answer 27

currant.series

Question 28

list things about in-line ammeters

Answer 28

low internal resistance because a low resistance current shunt is connected directly across the meters input terminals. current reading across a shunt. connected in series with a load that will limit the current to a value that is within the meters max current reading

Question 29

list things about multimeters

Answer 29

low resistance current shunt connected across the current input jacks must pick right selection switch to get an accurate reading.

Question 30

why is it potentially dangerous to connect the leads of a typical multimeter, to the current input jacks?

Answer 30

damage the meter

Question 31

why must all in-line ammeters be connected in series with a load?

Answer 31

there is no resistance

Question 32

a 1 ohm resistor is used as a current shunt. with the cct operatinf normally the resistor drops 350mV. what is the current?

Answer 32

0.35A

Question 33

a manufactured current shunt is installed in the cct to a large DC motor. the shunt is marked 5000A, 100mV. If the shunt drops 75mV, what is the motor current?

Answer 33

25mV

Question 34

which meter has the higher internal resistance, a DC voltmeter or a DC ammeter?

Answer 34

DC voltmeter

Question 35

what is likely result if a digital meter is set to measure DC voltage, but the leads are plugged into the current jacks?

Answer 35

will create a short

Question 36

when is it possible to find a cct current from a voltage reading?

Answer 36

voltage drop over a known value or shunt

Question 37

draw a simple cct that has a supply a resistive load, an ammeter that will read the cct current and a voltmeter that will read the voltage across the load

Answer 37

pg 25 quesion 4

Question 38

explain why a cct will not function if a voltmeter is connected into the cct in place of an ammeter. b) will the voltmeter read if the supply is energized and the load is connected

Answer 38

wont read anything because voltmeters measure in parallel. b)the supply voltage

Question 39

what is power

Answer 39

rate of how work is done. transfer of energy

Question 40

what is apparent power

Answer 40

volt-amps (VA)-product of rms values of AC voltage and current without regard to phase angle. VA=ExI (Total voltsx total amps)

Question 41

what is active power

Answer 41

W or P- rate at which energy is transfered through the system and used to do work. measured in watts. P=ExI or cos

Question 42

what is reactive power

Answer 42

VARS- the rate at which energy is circulating through the system without doing any work. measured in volt-amps-reactance. VAR=ExIxsin

Question 43

What is phase angle

Answer 43

measure of th ephase shift between the voltage and the current wave form in electrical time degrees

Question 44

what is power factor.

Answer 44

pf- ratio of active power. pf=W divided by VA. or pf=cosphase angle

Question 45

A DC circiut supplies 15.8A at 230V. what is the power rate for the cct.

Answer 45

P=ExI P=3450W

Question 46

Two wattmeters are connected to read the power rate in a single-phase 120/240V three-wire cct supplying a resistive load. the L1 and L2 meter readinfs are 450 and 325W respectively. Find the cct current in L1 and L2 and the total active power

Answer 46

total power is added together= 775W total. L1= 3.75A L2= 2.71A

Question 47

what is most likely cause when a wattmeter reads downscale. how can the problem be corrected?

Answer 47

polarity is backwards. Deenergize and change them

Question 48

A wattmeter is rated at 1.5kW, has a 250v voltage element and a 5A current element. would it be acceptable to use this meter in a 2 wire 100Vdc cct supplying a 600W load?

Answer 48

no the power is too high

Question 49

an analog wattmeter is properly connected to a monitor a 240V load supplied from a 120/240Vac single phase 3 wire system. What is the voltage to ground at the voltage and meters current terminals of the meter?

Answer 49

120V to ground

Question 50

what is faradays law of electromagnetic induction

Answer 50

when the is a change in the number of lines of magnetic flux threadinf through a coil, an emf is inducted into the coil. the amt of emf that is induced is proportional to the rate at which this change occurs.

Question 51

what happens if there is a change in the total amount od magnetic flux passing through the coil

Answer 51

emf is induced into the coil

Question 52

what does the magnitude of th emf depends on

Answer 52

rate at which the magnetic flux changes

Question 53

what happens if a conductor cuts lines of flux

Answer 53

emf induced into conductor

Question 54

if a conductive loop encloses a magnetic field, under what 2 conditions will an emf be generated into the loop

Answer 54

increases when the field expands. decreases when the field collapses

Question 55

if a conductor is suspended in a magnetic field under what conditions will and emf be generated into the conductor

Answer 55

increase/decrease in magnetic field. move conductor.

Question 56

describe the relationship betweeen webbers or flux, and the emf induced by electromagnetic induction?

Answer 56

1 webber per second = 1 volt. more webbers= more voltage=more flux

Question 57

what angle results in the greatest and least emf relative to lines of flux

Answer 57

greatest emf-perpendicular 90 degrees least-parallel- zero degrees. pg 53 question 4c for visual.

Question 58

in flemmings right hand rule what do each fingers represent.

Answer 58

Thumb-thrust pointer-field middle-emf

Question 59

list 2 ways of reversing the polarity of the emf induced into a conductor that is cutting field

Answer 59

reversing polarity of field, or relitave direction of the conductor relative to the field

Question 60

list a method of reversing the polarity of the emf induced into the coil that is enclosinf a changing field

Answer 60

switch north and south poles

Question 61

in an elementry generator what part cuts the lines of flux

Answer 61

ends of conductive loop

Question 62

at a given instant, one side of the loop is passing under the north pole of the field. where will that side of the loop will be half of a revolution later

Answer 62

southpole, one revolution is 180 degrees

Question 63

why is it necessary to have slip rings and brushes

Answer 63

so wires dont get tangles and can rotate freely

Question 64

if you made an elementry genrator would it be a good source of electrical energy

Answer 64

no. too low of a voltage and wont turn fast enough

Question 65

why does the polarity of the output voltage change each time the loop passes through the zone that seperates the poles of the field magnent?

Answer 65

direction of the conductor is moving. changing the direction means changing polarity

Question 66

if the field induces 1 volt into each side of the loop what is the total voltage across the slip rings?

Answer 66

2v

Question 67

why does the output voltage vary as the loop rotates

Answer 67

cutting different amount of flux

Question 68

when is the output at a maxiumum and minimum in an elementary generator

Answer 68

max-90 degrees min- zero degrees

Question 69

what is a phasor diagram

Answer 69

mimics the movements of the roatating loop as it passes through the field.

Question 70

when a rotating phaor is used to genrate an AC sinwave what does the length of the phasor represent?

Answer 70

the max emf that is at that degree at that moment in time

Question 71

on a sine wave graph time is marked in degrees. why is it in degrees instead of seconds or minutes

Answer 71

each cycle has 360 degrees, time can change and degrees are constant

Question 72

time for one cycle is called the _______ of the wave

Answer 72

period

Question 73

the reciprocal of the time for one cycle is the _____________

Answer 73

frequency (Hz)

Question 74

the unit of Hertz represents _____________

Answer 74

1 cycle/second

Question 75

north american power systems operate at 60hz. what is the period of this AC sine wave

Answer 75

period= frequency divided by 1. 0.01667

Question 76

if the peak value of a sine wave is 100 what is the instantaneous value at -65 degrees?

Answer 76

einst=voltage peak x sin<0= -90.6307

Question 77

if the peak value of a sine wave is 170, what is the instantaneous value at 45 degrees?

Answer 77

einst= voltage peak x sin<0= 120.208

Question 78

if the instantaneous value of a sine wave is 65 at 45 degrees, what is the peak value?

Answer 78

einstant= emax x sin<0 =91.92

Question 79

if the instantaneous value of a sinewave is 350 at 60 degrees what is it at 135 degrees?

Answer 79

285.73

Question 80

pg 90 review questions

Answer 80

.

Question 81

how to find epk

Answer 81

epk=erms divided by 0.707

Question 82

how to find Ipk

Answer 82

Iaverage divided by 0.636

Question 83

how to find rms

Answer 83

avg divided by 0.9

Question 84

how to find irms

Answer 84

ipk x 0.707

Question 85

the opposition a resistor has to a AC current is (more,less,the same as) the oposition it has to a dc current

Answer 85

the same

Question 86

the opposition a coil has to a AC current is (more,less,the same as) the opposition it has to a DC current

Answer 86

more

Question 87

the opposition a coil has to a DC current is limited to the (inductive reactance of the coil, the resistance of the coil winding)

Answer 87

resistance of the coil winding

Question 88

adding an iron core to a coil will (increase,decrease,have no effect on) its inductive reactance

Answer 88

increase

Question 89

a capacitor (blocks,passes) DC current

Answer 89

blocks

Question 90

is it possible to calculate the watt demand of a coil or capacitor directly from voltage and current readings?

Answer 90

no, no active power or true power. only reactance.

Question 91

should a relay coil operate much the same as with either an AC or DC supply?

Answer 91

no, Dc oly has resistance to oppose DDC currant. AC has resistance and inductive AC current has magnetic field, DC cant build one

Question 92

suggest the difference betweeen ac and dc current that makes coil and caapaciroe loads behave so differently with these two supplies

Answer 92

ac current is alwasy changing. dc is constant.

Question 93

why should a capacitor be discharged before its handled

Answer 93

stores line voltage if its not grounded it can shock. can store up to peak voltage

Question 94

when current passes through a resistor what happens to the energy it carries

Answer 94

converts it into heat

Question 95

does a resistor have any different effect on an ac current compared to Dc

Answer 95

no

Question 96

when current passes through a coil, what happens to most of the energy it carries

Answer 96

concerts into a magnetic field

Question 97

capacitors are often called charge storage devices, what happens to energy carried by the current that entres a capacitor

Answer 97

static charge

Question 98

does a resistor conduct ac better then dc

Answer 98

the same

Question 99

under what conditions does inductance oppose current

Answer 99

when current changes

Question 100

what induced quality provides the current limiting effect of inductive reactance

Answer 100

induced counter emf

Question 101

what quantity is measured in henerys

Answer 101

inductance

Question 102

what is the unit of capacitance

Answer 102

microfarads

Question 103

what is the unit of capacitive reactance

Answer 103

ohm

Question 104

when is current blocked in a capacitive cct

Answer 104

capacitor coltage= to supply voltage and has opposite polarities

Question 105

can an inductor completley block current

Answer 105

no can only oppose and delay change

Question 106

an inductor opposes change in ______

Answer 106

current

Question 107

a capacitor opposes change in _____

Answer 107

voltage

Question 108

what current will a 2H inductor draw from a 208V 60Hz supply

Answer 108

XL=2xpiexfxl =753.9 I= E divided by Xl =0.275

Question 109

what current will a 250 micro farad capacitor draw from a 240v 50Hz supply

Answer 109

xc= 1 divided by 2x piexfxc=12.732 ohms e divided by xc=18.85A

Question 110

what total opposition in ohms if a current draws 10A from a 120V ac supply

Answer 110

12 ohms

Question 111

what is the correct name for total opposition to current in an ac cct.

Answer 111

impedance

Question 112

how to calculate total impedance

Answer 112

z= e divided by I

Question 113

capacitors_______ voltage

Answer 113

leads

Question 114

inductors________ voltage

Answer 114

lags

Question 115

7.1.3 review questions, 7.2.2 questions

Answer 115

pg 119, 124