Year 2 D1 Flashcards
Faraday’s law of electromagnetic induction
the magnitude of voltage induced in a turn of wire is proportional to the rate of change of flux passing through (or linked with) that turn.
Faraday’s formula
e =Blv
e = voltage generated
B = flux density of the field in tesla
l = active length of the conductor in the field in m
v = velocity of the conductor at right angles to the magnetic field in meters per second
Flux density
of the magnetic field is normally controlled by the magnitude of the current flowing through the field coils. Normally controlled by a series field rheostat
Lenz’s Law
the direction of the induced emf must be such that any current resulting from it will develop a flux that will oppose any change in the original flux
Induced voltage
anytime the conductor cuts flux lines at the rate of 1 weber per second, an emf of 1 volt will be induced in the conductor
The frequency of the generated emf
is directly proportional to the number of field poles in the alternator
Formula for frequency is
f = P X N / 120 f = frequency of the electrical waveform in cycles per second or Hz P = number of poles in the alternator N = speed of the alternator in rpm
transformers have efficiencies in
95% to 99% range
AC
periodically changes in both magnitude and direction
Cycle
the distance from one point on a waveform to where it starts to repeat itself is a cycle
Alternation
one cycle of a sine wave contains two alternations
Period
is the time it takes to complete one cyle
the period of a waveform is equal to the number of seconds per cycle
Frequency
of a waveform is the number of complete cycles that occur in one second.
it is the reciprocal of the period
Amplitude
of the waveform is the max height of the waveform above or below the zero axis
peak value
max positive or negative value of the waveform
Peak to Peak
is twice the peak value
Instantaneous value
of the waveform is constantly changing
e = Em sin(theta) i = Im sin(theta)
Average value
of a waveform is .637 time peak
average value for one complete sine wave is zero
Effective value
of an AC waveform is equal to the DC source that would give the same heating effect in a resistive load.
.707 times peak value
in an AC cct effective resistance is made up of
pure ohmic resistance skin effect losses hysteresis losses eddy current losses dielectric losses radiation losses
opposition to AC current flow is called
impedance
The peak power in an AC lighting circuit is 1200 W. what is the effective power in the cct.
Peff = .5 x Pm
.5 x 1200 W = 600 W
electromagnetic induction
the process of generating a voltage by cutting a magnetic field with a conductor or by cutting a conductor with a magnetic field
Inductance formula
E = - L times (delta I/delta t)
Inductors are built by
winding a conductor into a coil to increase the inductive effect. chokes, coils and reactors
Air-core inductors
high-frequency applications and some high voltage protection systems
what is the symbol
Iron-core inductors
have a ferrite core often used as smoothing chokes in power supplies
variable inductors
Used in welding machines
tuners in radios
factors affecting inductance and formula
L = N^2uA/l L inductance in henries N number of turns in the coil u permeability of the core in webers/amp turn metre l length of the core in metres
Inductance is the property of an electric cct
that opposes any change in current in that cct
time constant in a RL cct
during each time constant the current increases by 63.2% of the difference between its starting value and its steady state
After 5 time constants reaches 99.33% or steady state
t = L/R
t length of one time constant in seconds
L inductance of the cct in henries
R resistance of the cct in ohms
mutual induction
is the process of inducing a voltage in a coil by changing current in another coil
DC current will only induce a voltage in an
Most come use is the automobile ignition coil
other coil in the instant it is turned on or off.
must have an alternating magnetic field so that the coil is continually cutting the lines of flux
Inductance of inductors in series
cct is the sum of the individual inductances
Inductance of inductors in parallel
cct is the sum of the reciprocals of the individual inductances
Coulomb’s law of electrostatic force
the electric force between two point electric charges is directly proportional to the product of the two charges and inversely proportional to the square of the spacing between them
Coulomb’s law of electrostatic force formula
F = k((Q1 x Q2)/d^2)
Electric field intensity
E = V/d E = the intensity of the field expressed in volts per metre
dielectric strength
the maximum field intensity the dielectric is able to withstand without breakdown
Capacitance of the capacitor
Unit for capacitance is farad F
is C = Q/V
C the capacitance of the capacitor in farads
Q the charge on either plate in coulombs
V the voltage between the plates
Define farad
an electric circuit has a capacitance of 1 farad when a charge of 1 coulomb on the plates is required to raise the voltage between the plates by 1 volt
A capacitors working voltage is
the max usable DC voltage, beyond which damage to the dielectric results
Three thing affect the capacitance of a capacitor
capacitance is directly proportional to the plate area
inversely proportional to the spacing between the plates
on the type of dielectric between the plates
dielectric constant
the ratio of the capacitance of a capacitor with a certain dielectric between its plates to one with the same physical dimensions but with only air between its plates
permittivity
is a measure of the capacitance between opposite ends of a unit length and unit cross-section of a dielectric material
formula for calculating the capacitance of a capacitor
C = (8.85 x 10^-12 x K x A)/d C capacitance of the capacitor in farads K dielectric constant A area of each plate in square metres d distance between the plates in metres 8.85 x 10^-12 permittivity of free space
Types of capacitors
fixed
variable
trimmer
please note the curved end of a capacitor symbols is usually connected to common
Three types of fixed capacitors
Oil-filled
Dry, self-healing
Electrolytic capacitors
Page 90 of D-1
special capacitor
Electrolytic capacitor
polarity-sensitive, use only in DC circuits to avoid puncturing
electrolytic capacitor puncturing
when it is hooked up back to front the electrolytic action reverses and aluminum oxide on one plate will break down and begin to form on the other plate as the dielectric diminishes the capacitor will conduct
Two ways to vary the capacitance of a variable capacitor
vary the effective area of the plates
vary the distance between the plates
capacitor problems
shorting
leaky dielectric or weak spots in the dielectric
opens
strays when rapid switching is required stray capacitance will limit how fast voltage can change
Dielectric absorption
if a capacitor is discharged and then let sit for a few min, a charge may build up on the plates again. When a charge is removed from the plates the orbitals of the dielectric try to return to their original shape this takes time, after the initial discharge a charge can be re-established on the plates as the orbitals return to their original shape
Safe discharge of capacitors
always discharge it and short-cct the leads
CEC requires the capacitors be provided with a means by which can be automatically discharged.
time constant for an RC cct
t = RC
t the length of one time constant in seconds
C the capacitance of the capacitor in farads
R the resistance of the resistor in ohms
Capacitance is the property of an electric cct
that opposes any change in the voltage across that cct
the total capacitance of capacitors connected in series is always less than that of the
smallest individual capacitor. this is because connecting capacitors in series has the same effect as increasing the distance between the plates of one capacitor
for capacitors in series
the total capacitance is the reciprocal of the sum of each capacitor
for capacitors in parallel
the total capacitance is the sum of each individual capacitor
Inductive reactance
Xl = 2(pi)fL
reactive (wattless) power
is the rate at which energy is transferred back and forth between the source and the inductor
Unit is VARs
Active power
is transferred to the load and dissipated as energy in some other form
Unit is Watt
Q factor is
the ratio or inductive reactance to its resistance
Saturable reactors
use a small DC current to control the flow of a larger AC current
capacitive reactance formula
Xc = 1/(2pifC)
VARs leading
in a capacitive cct, the currents leads the voltage, and capacitive VARs is stated as VARs leading
VARs lagging
In an inductive cct, current lags the voltage, and inductive VARs is stated as being VARs lagging.
Reactive power
= volts x amps x sin(theta)
when the phase angle is 0 then the reactive power is zero the cct is purely resistive
when the phase angle is 90 then there is no true power the cct is purely reactive
Inductive reactance makes VARs
lagging
Capacitive reactance makes VARs
leading
Impedance is the term
for total opposition to current flow in an AC cct
Power factor is
Watt/VA
