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Level 2 > bt22o Ac theroy and meters > Flashcards

bt22o Ac theroy and meters Flashcards

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1
Q

where does an analog meter get its pointer movement

A

principals of electromagnetism

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2
Q

what does a magometer do

A

tests insulation for its resistance

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3
Q

what is the primary saftey function for all ohmmeters

A

not to be connected to an energized cct

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4
Q

what adjustment is used to zero an analog meter movement

A

zero adjustment knob

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5
Q

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

A

readings wont be accurate

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6
Q

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

A

no not high enough resistance

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7
Q

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?

A

Display screen

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8
Q

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

A

resistance is too high for range setting

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9
Q

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?

A

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

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10
Q

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

A

resistance too high, leads broken, break in cct.

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11
Q

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

A

low

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12
Q

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

A

read downscale

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13
Q

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

A

ensure the same polarity

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14
Q

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

A

will read a negative number

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15
Q

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

A

19.99. first digit can only be 1

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16
Q

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

A

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

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17
Q

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

A

easy path to ground. shock

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18
Q

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

A

common

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19
Q

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

A

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

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20
Q

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

A

less current

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21
Q

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

A

300k ohms

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22
Q

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

A

no its the same

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23
Q

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

A

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

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24
Q

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

A

damage to meter
damage to self
inaccurate readings
malfunctions
misleading measurements

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25
Q

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

A

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

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26
Q

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.

A

a)no
b) high resistance

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27
Q

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

A

currant.series

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28
Q

list things about in-line ammeters

A

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

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29
Q

list things about multimeters

A

low resistance current shunt connected across the current input jacks

must pick right selection switch to get an accurate reading.

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30
Q

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

A

damage the meter

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31
Q

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

A

there is no resistance

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32
Q

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

A

0.35A

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33
Q

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?

A

25mV

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34
Q

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

A

DC voltmeter

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35
Q

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

A

will create a short

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36
Q

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

A

voltage drop over a known value or shunt

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37
Q

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

A

pg 25 quesion 4

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38
Q

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

A

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

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39
Q

what is power

A

rate of how work is done. transfer of energy

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40
Q

what is apparent power

A

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

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41
Q

what is active power

A

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

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42
Q

what is reactive power

A

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

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43
Q

What is phase angle

A

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

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44
Q

what is power factor.

A

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

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45
Q

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

A

P=ExI P=3450W

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46
Q

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

A

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

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47
Q

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

A

polarity is backwards. Deenergize and change them

48
Q

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?

A

no the power is too high

49
Q

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?

A

120V to ground

50
Q

what is faradays law of electromagnetic induction

A

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.

51
Q

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

A

emf is induced into the coil

52
Q

what does the magnitude of th emf depends on

A

rate at which the magnetic flux changes

53
Q

what happens if a conductor cuts lines of flux

A

emf induced into conductor

54
Q

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

A

increases when the field expands. decreases when the field collapses

55
Q

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

A

increase/decrease in magnetic field. move conductor.

56
Q

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

A

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

57
Q

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

A

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

58
Q

in flemmings right hand rule what do each fingers represent.

A

Thumb-thrust
pointer-field
middle-emf

59
Q

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

A

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

60
Q

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

A

switch north and south poles

61
Q

in an elementry generator what part cuts the lines of flux

A

ends of conductive loop

62
Q

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

A

southpole, one revolution is 180 degrees

63
Q

why is it necessary to have slip rings and brushes

A

so wires dont get tangles and can rotate freely

64
Q

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

A

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

65
Q

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?

A

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

66
Q

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

A

2v

67
Q

why does the output voltage vary as the loop rotates

A

cutting different amount of flux

68
Q

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

A

max-90 degrees
min- zero degrees

69
Q

what is a phasor diagram

A

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

70
Q

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

A

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

71
Q

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

A

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

72
Q

time for one cycle is called the _______ of the wave

A

period

73
Q

the reciprocal of the time for one cycle is the _____________

A

frequency (Hz)

74
Q

the unit of Hertz represents _____________

A

1 cycle/second

75
Q

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

A

period= frequency divided by 1. 0.01667

76
Q

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

A

einst=voltage peak x sin<0= -90.6307

77
Q

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

A

einst= voltage peak x sin<0= 120.208

78
Q

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

A

einstant= emax x sin<0
=91.92

79
Q

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

A

285.73

80
Q

pg 90 review questions

A

.

81
Q

how to find epk

A

epk=erms divided by 0.707

82
Q

how to find Ipk

A

Iaverage divided by 0.636

83
Q

how to find rms

A

avg divided by 0.9

84
Q

how to find irms

A

ipk x 0.707

85
Q

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

A

the same

86
Q

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

A

more

87
Q

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

A

resistance of the coil winding

88
Q

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

A

increase

89
Q

a capacitor (blocks,passes) DC current

A

blocks

90
Q

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

A

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

91
Q

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

A

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

92
Q

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

A

ac current is alwasy changing. dc is constant.

93
Q

why should a capacitor be discharged before its handled

A

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

94
Q

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

A

converts it into heat

95
Q

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

A

no

96
Q

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

A

concerts into a magnetic field

97
Q

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

A

static charge

98
Q

does a resistor conduct ac better then dc

A

the same

99
Q

under what conditions does inductance oppose current

A

when current changes

100
Q

what induced quality provides the current limiting effect of inductive reactance

A

induced counter emf

101
Q

what quantity is measured in henerys

A

inductance

102
Q

what is the unit of capacitance

A

microfarads

103
Q

what is the unit of capacitive reactance

A

ohm

104
Q

when is current blocked in a capacitive cct

A

capacitor coltage= to supply voltage and has opposite polarities

105
Q

can an inductor completley block current

A

no can only oppose and delay change

106
Q

an inductor opposes change in ______

A

current

107
Q

a capacitor opposes change in _____

A

voltage

108
Q

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

A

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

109
Q

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

A

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

110
Q

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

A

12 ohms

111
Q

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

A

impedance

112
Q

how to calculate total impedance

A

z= e divided by I

113
Q

capacitors_______ voltage

A

leads

114
Q

inductors________ voltage

A

lags

115
Q

7.1.3 review questions, 7.2.2 questions

A

pg 119, 124

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