bt22o Ac theroy and meters Flashcards
where does an analog meter get its pointer movement
principals of electromagnetism
what does a magometer do
tests insulation for its resistance
what is the primary saftey function for all ohmmeters
not to be connected to an energized cct
what adjustment is used to zero an analog meter movement
zero adjustment knob
what happens if you ignore the zero adjustment knob on analog ohmmeters
readings wont be accurate
digital ohhmeters dont read zero ohms with probes shorted. does it affect measurement accuracy?
no not high enough resistance
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?
Display screen
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
resistance is too high for range setting
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?
too high for the setting. R>20k ohms, break in the cct, leads broken
when an autoranged meter is set to its normal resistance function, and displays OL what does that indicate about the resistance between the probes?
resistance too high, leads broken, break in cct.
is a high impedance meter or low impedance meter better for measuring faults
low
what happens when an analog DC meter is connected with incorrect polarity
read downscale
what is the purpose of having DC+ and DC- functions on the simpson meter
ensure the same polarity
what happens when you connect a digital meter with the incorrect polarity
will read a negative number
what is the largest number that can be shown on a 3 1/2 digital meter display
19.99. first digit can only be 1
what happens when an analog DC voltmeter and digital is connected to a voltage that is higher then the range setting
analog- needle will go all the way right
digital- will read a negative number
why is it bad to lean on metal while taking a voltage reading
easy path to ground. shock
when taking voltage readings should you connect the common or line side of the cct first
common
if your first meter is reading 0V should you assume the cct is de-energized and safe to touch
no. meter could be falty. test on known voltage first then test.
does a high impedance meter draw more or less current from the cct then a low impedance meter
less current
an analog meter has a 10k ohm/Vdc rating. what is the internal resistance of the meter if its set to a 30Vdc range?
300k ohms
does the impedance of a typical digital meter change with each range change?
no its the same
why do different meters give different readings when the connection to the supply contains a high resistance?
internal impedamnce has an affect on the readings. different answers depending on each meters impedance.
list 3 or more reasons why its a bad idea to measure voltage in an electronic cct with a low impedance meter?
damage to meter
damage to self
inaccurate readings
malfunctions
misleading measurements
list 5 habits or procedures that can reduce the risks when taking voltage measurements
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
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)no
b) high resistance
what do ammeters read and how do you hook it up.
currant.series
list things about in-line ammeters
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
list things about multimeters
low resistance current shunt connected across the current input jacks
must pick right selection switch to get an accurate reading.
why is it potentially dangerous to connect the leads of a typical multimeter, to the current input jacks?
damage the meter
why must all in-line ammeters be connected in series with a load?
there is no resistance
a 1 ohm resistor is used as a current shunt. with the cct operatinf normally the resistor drops 350mV. what is the current?
0.35A
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?
25mV
which meter has the higher internal resistance, a DC voltmeter or a DC ammeter?
DC voltmeter
what is likely result if a digital meter is set to measure DC voltage, but the leads are plugged into the current jacks?
will create a short
when is it possible to find a cct current from a voltage reading?
voltage drop over a known value or shunt
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
pg 25 quesion 4
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
wont read anything because voltmeters measure in parallel.
b)the supply voltage
what is power
rate of how work is done. transfer of energy
what is apparent power
volt-amps (VA)-product of rms values of AC voltage and current without regard to phase angle. VA=ExI (Total voltsx total amps)
what is active power
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
what is reactive power
VARS- the rate at which energy is circulating through the system without doing any work. measured in volt-amps-reactance. VAR=ExIxsin<theta
What is phase angle
measure of th ephase shift between the voltage and the current wave form in electrical time degrees
what is power factor.
pf- ratio of active power. pf=W divided by VA. or pf=cosphase angle
A DC circiut supplies 15.8A at 230V. what is the power rate for the cct.
P=ExI P=3450W
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
total power is added together= 775W total.
L1= 3.75A
L2= 2.71A
what is most likely cause when a wattmeter reads downscale. how can the problem be corrected?
polarity is backwards. Deenergize and change them
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?
no the power is too high
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?
120V to ground
what is faradays law of electromagnetic induction
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.
what happens if there is a change in the total amount od magnetic flux passing through the coil
emf is induced into the coil
what does the magnitude of th emf depends on
rate at which the magnetic flux changes
what happens if a conductor cuts lines of flux
emf induced into conductor
if a conductive loop encloses a magnetic field, under what 2 conditions will an emf be generated into the loop
increases when the field expands. decreases when the field collapses
if a conductor is suspended in a magnetic field under what conditions will and emf be generated into the conductor
increase/decrease in magnetic field. move conductor.
describe the relationship betweeen webbers or flux, and the emf induced by electromagnetic induction?
1 webber per second = 1 volt.
more webbers= more voltage=more flux
what angle results in the greatest and least emf relative to lines of flux
greatest emf-perpendicular 90 degrees
least-parallel- zero degrees.
pg 53 question 4c for visual.
in flemmings right hand rule what do each fingers represent.
Thumb-thrust
pointer-field
middle-emf
list 2 ways of reversing the polarity of the emf induced into a conductor that is cutting field
reversing polarity of field, or relitave direction of the conductor relative to the field
list a method of reversing the polarity of the emf induced into the coil that is enclosinf a changing field
switch north and south poles
in an elementry generator what part cuts the lines of flux
ends of conductive loop
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
southpole, one revolution is 180 degrees
why is it necessary to have slip rings and brushes
so wires dont get tangles and can rotate freely
if you made an elementry genrator would it be a good source of electrical energy
no. too low of a voltage and wont turn fast enough
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?
direction of the conductor is moving. changing the direction means changing polarity
if the field induces 1 volt into each side of the loop what is the total voltage across the slip rings?
2v
why does the output voltage vary as the loop rotates
cutting different amount of flux
when is the output at a maxiumum and minimum in an elementary generator
max-90 degrees
min- zero degrees
what is a phasor diagram
mimics the movements of the roatating loop as it passes through the field.
when a rotating phaor is used to genrate an AC sinwave what does the length of the phasor represent?
the max emf that is at that degree at that moment in time
on a sine wave graph time is marked in degrees. why is it in degrees instead of seconds or minutes
each cycle has 360 degrees, time can change and degrees are constant
time for one cycle is called the _______ of the wave
period
the reciprocal of the time for one cycle is the _____________
frequency (Hz)
the unit of Hertz represents _____________
1 cycle/second
north american power systems operate at 60hz. what is the period of this AC sine wave
period= frequency divided by 1. 0.01667
if the peak value of a sine wave is 100 what is the instantaneous value at -65 degrees?
einst=voltage peak x sin<0= -90.6307
if the peak value of a sine wave is 170, what is the instantaneous value at 45 degrees?
einst= voltage peak x sin<0= 120.208
if the instantaneous value of a sine wave is 65 at 45 degrees, what is the peak value?
einstant= emax x sin<0
=91.92
if the instantaneous value of a sinewave is 350 at 60 degrees what is it at 135 degrees?
285.73
pg 90 review questions
.
how to find epk
epk=erms divided by 0.707
how to find Ipk
Iaverage divided by 0.636
how to find rms
avg divided by 0.9
how to find irms
ipk x 0.707
the opposition a resistor has to a AC current is (more,less,the same as) the oposition it has to a dc current
the same
the opposition a coil has to a AC current is (more,less,the same as) the opposition it has to a DC current
more
the opposition a coil has to a DC current is limited to the (inductive reactance of the coil, the resistance of the coil winding)
resistance of the coil winding
adding an iron core to a coil will (increase,decrease,have no effect on) its inductive reactance
increase
a capacitor (blocks,passes) DC current
blocks
is it possible to calculate the watt demand of a coil or capacitor directly from voltage and current readings?
no, no active power or true power. only reactance.
should a relay coil operate much the same as with either an AC or DC supply?
no, Dc oly has resistance to oppose DDC currant. AC has resistance and inductive AC current has magnetic field, DC cant build one
suggest the difference betweeen ac and dc current that makes coil and caapaciroe loads behave so differently with these two supplies
ac current is alwasy changing. dc is constant.
why should a capacitor be discharged before its handled
stores line voltage if its not grounded it can shock. can store up to peak voltage
when current passes through a resistor what happens to the energy it carries
converts it into heat
does a resistor have any different effect on an ac current compared to Dc
no
when current passes through a coil, what happens to most of the energy it carries
concerts into a magnetic field
capacitors are often called charge storage devices, what happens to energy carried by the current that entres a capacitor
static charge
does a resistor conduct ac better then dc
the same
under what conditions does inductance oppose current
when current changes
what induced quality provides the current limiting effect of inductive reactance
induced counter emf
what quantity is measured in henerys
inductance
what is the unit of capacitance
microfarads
what is the unit of capacitive reactance
ohm
when is current blocked in a capacitive cct
capacitor coltage= to supply voltage and has opposite polarities
can an inductor completley block current
no can only oppose and delay change
an inductor opposes change in ______
current
a capacitor opposes change in _____
voltage
what current will a 2H inductor draw from a 208V 60Hz supply
XL=2xpiexfxl =753.9
I= E divided by Xl =0.275
what current will a 250 micro farad capacitor draw from a 240v 50Hz supply
xc= 1 divided by 2x piexfxc=12.732 ohms
e divided by xc=18.85A
what total opposition in ohms if a current draws 10A from a 120V ac supply
12 ohms
what is the correct name for total opposition to current in an ac cct.
impedance
how to calculate total impedance
z= e divided by I
capacitors_______ voltage
leads
inductors________ voltage
lags
7.1.3 review questions, 7.2.2 questions
pg 119, 124
