electrical fundamentals book 2 Flashcards
1
Q
what occurs naturally in some minerals?
A
magnetism occurs naturally in some minerals
2
Q
give an example of a mineral that is magnetic
A
lodestone is a naturally occurring mineral that is magnetic
3
Q
what two ways can an artificial magnet be magnetised?
A
an artificial magnet can be magnetised through stroking it with a natural magnet or through passing an electric current through it
4
Q
give an example of a material that is easily magnetised
A
soft iron is easily magnetised
5
Q
will a material that is easily magnetised lose its magnetism easily?
A
yes, a material that is easily magnetised will also lose its magnetism easily. conversely, materials which are hard to magnetise do not lose their magnetism easily
6
Q
what are the properties of the minerals we use to make permanent magnets?
A
minerals that we use to make permanent magnets are hard to magnetise and do not lose their magnetism easily
7
Q
what do we call minerals that lose their magnetism easily?
A
temporary magnets
8
Q
what are the forces around a magnet called?
A
the forces around a magnet are called line of magnetic flux. these produce a magnetic field
9
Q
do the lines of magnetic flux travel from south to north or from north to south?
A
the lines of magnetic flux travel from a magnets north pole to its south pole
10
Q
what are the 4 properties of magnetic fields?
A
- lines of force tend to contract to the shortest possible length rather like an elastic band
- lines of force laterally repel one another
- the direction of lines of force is from north pole to south pole. they form closed loops which are completed by the lines passing through the magnet
- lines of force never cross over each other
11
Q
if 2 magnetic poles are opposite each other, what is the relationship between each other?
A
like poles will repel each other, opposite poles will attract
12
Q
what is the unit of magnetic flux?
A
the symbol for magnetic flux is the greek letter phi and its unit is the weber (Wb)
13
Q
what are the 2 laws of magnetism?
A
- the force between two magnetic poles at a fixed distance apart is proportional to their pole strengths
- the force between 2 magnetic poles of constant strength is inversely proportional to the square of the distance between them
14
Q
using the two laws of magnetism, give an equation for calculating the force of magnetising field between two magnets
A
force of magnetising field = (pole strength 1 in webers x pole strength 2 in webers)/distance in metres squared
15
Q
what is flux density?
A
the amount of flux per unit cross sectional area, anywhere in the magnetic field, at right angles to the lines of force
16
Q
what is the unit of flux density?
A
symbol B, measured in Tesla (T)
17
Q
how is flux density calculated?
A
flux density (tesla) = magnetic flux (weber)/area (metres squared)
18
Q
why would we use an electromagnet?
A
to obtain a large magnetic field
19
Q
what is the simple principle of electromagnetism?
A
a magnetic field is created by passing a DC current through a wire
20
Q
which rule can we use to determine the direction of current flow, the north pole and field direction through a conductor?
A
the right hand grasp rule can be used to determine the direction of north pole, direction of current flow and field direction
21
Q
what are the magnetic effects of 2 parallel conductors?
A
two parallel conductors with currents flowing in the same direction will attract if free to move, if currents are flowing in opposite directions they will repel
22
Q
what is a solenoid?
A
an electromagnetically controlled device in which the mechanical movement of a shaft or plunger is activated by a magnetising current
23
Q
what is magnetomotive force (MMF) and what is its electrical circuit equivalent?
A
the cause of a magnetic field, measured in ampere turns (At). its electrical circuit equivalent is electromotive force (EMF)
24
Q
give an equation for calculating MMF
A
MMF = current (amperes) x number of turns of wire through which current is flowing
25
what does the MMF value depend on?
amount of current and number of turns of wire through which it is flowing
26
what is a magnetic circuit?
A magnetic circuit is made up of one or more closed loop paths containing a magnetic flux. The flux is usually generated by permanent magnets or electromagnets and confined to the path by magnetic cores consisting of ferromagnetic materials like iron, although there may be air gaps or other materials in the path.
27
what is magnetising force?
the is the MMF per metre length of the magnetic circuit, given the symbol H, measured in ampere turns (At) per metre. i.e At/m
28
give an equation for calculating magnetising force
```
magnetising force (H) = MMF/length of the magnetic circuit (metres)
magnetising force (H) = (current (amperes) x number of turns of wire through which current is flowing)/length of magnetic circuit in metres
```
29
what is known as MMF per unit length?
magnetising force
30
what is reluctance?
the opposition to the establishment of a magnetic field in a material and is measured by ampere turns/weber (at/wb)
31
give an equation for calculating reluctance
reluctance (at/weber) = MMF(at)/magnetic flux (weber)
32
will an air gap have a bigger or smaller reluctance than an air gap?
an iron core focuses a magnetic field better than an air gap and therefore an air gap has a higher reluctance
33
if magnetic flux = MMF/reluctance, what does current = ?
current = voltage/resistance
34
in a magnetic circuit, if we increase the amount of turns on the input, what will increase?
the MMF (MMF = current (amperes) x number of turns of wire through which current is flowing)
35
what is meant when a magnet is saturated?
when a magnet is saturated, all of its domains are perfectly lined up with the direction of force and no further increase of flux density can be obtained
36
what is permeability?
the measure of ease with which a magnetic field can be established within a material, it is given the greek letter mu and has no units
37
give an equation for calculating permeability
permeability (mu) = flux density/magnetic field strength (H)
| can be put into a triangle to deduce either MMF or flux density
38
which will have the greater magnetic field strength, a coil with an iron core or air core?
a coil with an iron core will have a greater magnetic field strength because iron core has increased permeability and flux density
39
does a material with high reluctance magnetise easily?
no, it would be hard to magnetise (reluctance is the opposition to the establishment of a magnetic field in a material and is measured by ampere turns/weber (at/wb))
40
what type of material has high permeability?
for air and other nonmagnetic materials, permeability (mu) is approximately equal to 1, materials with relatively high permeability are irons (2000-3000) and very high permeability are nickel-iron alloys (approx 100,000)
41
list the 3 types of magnetic materials
1. ferromagnetic materials - pronounced magnetic effects, concentrated lines of flux - permeabilities much greater than 1 e.g. iron, nickel, cobalt etc
2. paramagnetic materials - no appreciable magnetic effects and generally considered nonmagnetic - permeability slightly greater than 1 e.g. platinum, manganese, chromium and aluminium
3. diamagnetic materials - minimum magnetic effects, tend to slightly antimagnetic - permeability of less than 1 e.g. bismuth and copper
42
which magnetic material has a permeability of just greater than 1?
paramagnetic materials, e.g. platinum, manganese, chromium and aluminium
43
what is the reason for magnetic shielding?
the sensitive mechanisms of electrical instruments and meters can be influenced by stray magnetic fields as there is no known insulator for magnetic flux. magnetic screens and shields (soft iron cases) provide the flux with a path of least reluctance and therefore the flux is diverted around the instrument
44
what material can we use when constructing a magnetic shield?
soft iron
45
if we drop a magnet, what could happen to its magnetism and why?
if a magnet is dropped, effective magnetism will be lost due to misalignment of its domains due to jarring
46
how should we store a horse shoe magnet?
a horse shoe magnet should be stored with a keeper, which is a soft iron bar used to connect the poles and give the lines of flux a path to circulate
47
do like forces attract or repel each other?
like forces will repel each other e.g. S-S or N-N
48
what metals are ferromagnetic?
ferromagnetic materials - pronounced magnetic effects, concentrated lines of flux - permeabilities much greater than 1 e.g. iron, nickel, cobalt etc
49
what is equivalent to resistance in the magnetic circuit?
in the magnetic circuit, reluctance (the opposition to the establishment of a magnetic field in a material and is measured by ampere turns/weber (at/wb)) is equivalent to resistance
50
what is equivalent to current in the magnetic circuit?
in the magnetic circuit, magnetic flux is equivalent to current
51
what is the unit of flux density?
```
symbol B, measured in Tesla (T)
flux density (tesla) = magnetic flux (weber)/area (metres squared)
```
52
which magnetic quantity uses the greek symbol phi?
magnetic flux is given the symbol phi, and its unit is the weber
53
do the lines of magnetic flux cross over each other?
no, in line with the 4th property of magnetic fields
| 4. lines of force never cross over each other
54
what are the 2 laws of electromagnetic induction?
1. faradays law - when a magnetic flux through a coil is made to vary, an emf is induced in the coil and the magnitude of this emf is proportional to the rate of change of flux
2. lenz law - the direction of an induced emf is always such that it tends to set up a current opposing the motion or the change of flux responsible for inducing that emf
55
what is another name for back EMF?
self induced voltage or self induction
56
what happens to back EMF when current reaches its maximum?
no back EMF is apparent when current is at maximum
57
what happens to the induced voltage in an inductor when the current starts to rise?
decreases exponentially once current starts to rise
58
state lenz law
the direction of an induced emf is always such that it tends to set up a current opposing the motion or the change of flux responsible for inducing that emf
59
in a LR (resistor, inductor) circuit, if the back EMF = max, what is the voltage across the resistor?
voltage accrocs resistor is at zero when EMF = max
60
in an LR circuit, when the switch is closed, total voltage will always be equal to:
V = Vr (voltage across resistor) + Vl (voltage across inductor)
61
in an LR circuit, when the switch is open, voltage will be equal to:
Vr + Vl = 0 or Vl = Vr
62
state the formula for inductance
inductance (L) = Vl (induced voltage)/(di (change in current)/dt (change in time))
given in amps per second
63
what is the unit for inductance?
the unit for inductance is the Henry (H)
64
what is inductance?
the ability of a conductor to induce voltage in itself when the current changes, is its inductance
65
how are inductors classified?
inductors are classified according to their core type. normally one of 2 basic types
1. iron-core inductor (formed around iron core)
2. core-inductor (formed around nonmagnetic material)
66
name the 2 main types of inductor
1. iron-core inductor (formed around iron core)
| 2. core-inductor (formed around nonmagnetic material)
67
what are the 5 main factors which affect the inductance of an inductor?
1. the number of turns in the coil
2. the diameter of the coil
3. the coil length
4. the type of material used in the core
5. the number of layers of winding in the coils
68
if we increase the number of turns in an inductor - will the inductance increase or decrease?
the inductance will increase.
| inductance varies as the square of the number of turns
69
which will have the greatest inductance, an inductor with widely spaced coils or narrowly spaced coils?
narrowly spaced coils produce greater inductance. more flux linkages
doubling the length of a coil while keeping the same number of turns halves the inductance value
70
will an inductor with an air core produce more or less magnetic lines of force than one with an iron core?
air core will produce less magnetic lines of force than one with an iron core. iron core is more permeable than air
inductance of a coil increases directly as the permeability of the core material increases
71
why would an inductor with 3 layers of windings have more inductance than that with only 1 layer?
a coil can be made more inductive by winding it in 3 layers. the increased number of layers improves flux linkage even more.
72
explain the hysteresis loop
study hysteresis loop
73
what is meant by remanence?
when current is zero but there is still some flex density (magnetism) left in the iron core
74
what is meant by coercive force?
the point where the flux density begins to rise positively negatively past 0. this process is repeated for every positive and negative pulse of current
75
what is meant by retentivity?
the ability of a material to retain magnetism
| retentivity = coercive force/remanence
76
how do you calculate inductance total (Lt) of inductors in series?
the sum of all inductors in series e.g. Lt = L1 + L2 + L3 etc
inductors are added similarly to resistors
77
how do you calculate inductance total (Lt) of inductors in parallel?
1/Lt = 1/L1 + 1/L2 + 1/L3 etc
| inductors are added similarly to resistors
78
explain mutual inductance of coils in series
Mutually connected series inductors are classed as either “aiding” or “opposing” the total inductance depending whether the coils are cumulatively coupled (in the same direction) or differentially coupled (in opposite direction). mutual inductance is positive, increasing the total inductance when the coils are series aiding. mutual inductance is negative when series opposing, reducing the total inductance. note the dotted ends of the coils
79
what is copper loss?
copper loss is wasted power due to inherent resistance of an inductor
80
give an equation for calculating copper loss
(current squared) x resistance of windings
81
in addition to copper loss, iron core inductors suffer two iron losses, what are they called?
hysteresis loss and eddy current loss
82
what is hysteresis loss?
the power consumed in reversing the magnetic field of the inductor core each time the direction of current in the inductor changes
83
what is eddy current loss?
circulating currents in the core caused by magnetic field in coil dissipate power in from of heat
84
what is the inductive time constant in an LR DC circuit?
the time taken for the circuit current to reach 63.2% of its maximum value
time constant = T
85
how do we calculate the inductive time constant in an LR DC circuit?
time constant (T) = inductance (L, Henrys)/resistance (R, ohms)
86
how many time constants, in practice, does it take for current to build to a maximum in an LR DC circuit?
5T seconds (5 time constants), conversely, it would take 5 T seconds for current to drop to zero after the switch was opened
87
what use does an air core inductor have?
to concentrate a magnetic field or control radio frequencies
88
what use does an iron core inductor have?
used for 60 Hz and audio frequency
89
what use does a variable powdered iron core inductor have?
variable inductor used for tuning process
90
what use does a iron core transformer inductor have?
step up and down ac voltage in a transformer
91
how many degrees in a circle?
360 degrees or 2 rad
92
how many degrees are there in half a cycle?
90 degrees or 1 rad
93
what is a cycle?
in a sin wave, where the value passes from zero to maximum, bad to zero, then negatively to maximum and back to zero. one cycle
94
how many degrees is 1/4 of a cycle?
90 degrees or 1 rad/2
95
what is the formula and unit for frequency?
frequency f = 1/time period
| given in Hz
96
what is the formula and unit for time period?
period T = 1/frequency
| given in seconds
97
how many hertz is 1kHz?
1000 Hz
98
what is another word for the highest peak of a wave form?
maximum or amplitude
99
the peak to peak value is what?
sum of maximum positive peak value and maximum negative peak value
100
what is the formula for calculating the RMS value of a sin wave?
RMS value of a sin wave = 0.707 x peak value
101
what is the formula for calculating the average value of a sin wave?
average value of a sin wave = 0.637 x peak value
102
if 2 waves start and pass through the same points on the x axis, they are said to be what?
in phase
103
if 2 waves start and pass through the same points on the x axis at different instants, they are said to be what?
out of phase
104
what is phase difference/displacement?
angular measure of difference between phases
105
how do you calculate power in an AC circuit?
1. P= IV
2 P=I(SQUARED)R
3. P=V(SQUARED)/R
106
what data does an appliance have written on its electrical rating plate?
voltage or current in RMS values and average power written in VA (volts amps)
107
what are effective values
RMS values of AC voltage and current have the same heating effect as equal values of DC voltage and current and are sometimes referred to as effective values
108
what are the main characteristics of a star system?
- 4 conductors (3 phase, 1 neutral)
- high voltage, low current
- line current = phase current
- 2 voltages available, line and phase
- phase voltage 115v, line voltage (route 3 (1.73)) x 115 = 199.18V
- neutral is reference
109
what are the main characteristics of a delta system?
- 3 conductors
- low voltage high current
- line voltage = phase voltage
- only line voltage available
- line current = (route 3(1.73)) x phase current
- B (yellow) is reference
110
what is the neutral line?
star system only, conductor which connects the star points of generator and load. under balanced conditions, carries 0 current. in reality, small amount of unbalance is to be expected so small current expected
111
what must the neutral line be capable of?
carrying full load current in a simple distribution network if severe unbalance occurs. i.e. if one or more phases is disconnected
112
what is symmetry in AC?
voltage waveforms are of equal magnitude, of the same frequency and equal phase displacement
113
what is balance in AC?
when currents in the conductors are equal in magnitude and their phasor sum is zero. balance is achieved when the phases of a voltage symmetrical 3 phase AC system are fully loaded. to achieve balance, it is necessary to equally load the individual phases and for the phase voltages to be sinusoidal
114
aircraft three phase AC generators are used because:
robust construction and good power to weight ratio
115
how are aircraft AC generators connected and what is their output?
- three phase star connected
- 115V AC (phase voltage)
- 200V AC (line voltage)
- 400 Hz frequency (typical)
116
give examples of non sinusoidal wave forms
- square wave
- triangle wave
- sawtooth wave
117
give two more names for sawtooth waveform
- ramp voltage
| - timebase
118
what is typical 3 phase AC rotation and what is the phase displacement?
- ABC (RYB), BCA (YBR), CAB (BRY)
| - 120 degree displacement
119
voltage dropped across an inductor is a reaction against...
the change in current through it
120
what is the relationship between the applied voltage and the current through an inductive circuit
voltage leads the current by 90 degrees or pi/2 radians (sinusoidal waveforms). voltage is at a peak when current is zero and vice versa
121
define inductive reactance
an inductors electrical resistance when used in an AC circuit. a ratio of voltage to current, given the symbol XL and expressed in ohms
122
how would you calculate inductive reactance?
1. XL = V/I
| 2. XL = 2 x pi x frequency (Hz) x inductance, L (Henrys)
123
what does the mnemonic CIVIL mean?
current leads voltage in a capacitative circuit
| current lags voltage in an inductive circuit
124
what is the abbreviation for inductive reactance?
XL
125
in a capacitative circuit, does current lead voltage?
yes (CIVIL)
126
what is the unit for frequency?
Hz
127
what is reactive power sometimes known as?
watless power
128
what is the symbol for capacitative reactance?
XC
129
how would you calculate capacitive reactance?
1. XC = V/I
| 2. XC = 1/(2 x pi x frequency (Hz) x capacitance, C (farads)
130
what is capacitative reactance?
is a measure of a capacitor's opposition to AC. measured by the ration of RMS voltage to RMS current, denoted as XC
131
is power through a capacitive circuit useful?
no, power can be seen to equally alternate between cycles of positive and negative. thus, no useful work has been done.
132
in a series circuit, is the current the same through all components?
yes, because there is only one path for current to flow
133
what is used as the reference phasor for all diagrams of series circuits?
current phasor because is a constant in series circuits
134
how is applied voltage calculated in a LR circuit?
V applied = square root of (VLsquared + VRsquared)
| calculated by pythagoras' theorem
135
what s the angle called that is between the applied voltage and current, in the phasor diagram for an LR circuit?
phase angle
calculated by tan theta = VL/VR
or tan theta = XL/R
136
in a capacitive circuit, what is the relationship between current and voltage?
current leads voltage by 90 degrees or pi/2 radians
137
as frequency changes, what happens to capacitive reactance?
as frequency increases, capacitive reactance decreases
138
what is impedance?
impedance is the total opposition to current flow in the circuit
139
how is impedance calculated?
impedance Z = V(RMS VOLTS)/I (RMS CURRENT)measured in ohms
140
what would power be in a circuit with both resistance and inductance?
phase angle would be somewhere between 0 and pi/2 rads. there would be more positive power than negative power, which will represent a dissipation on energy in the resistor
141
how do you calculate alive power?
I(squared) x R (total resistance of circuit)
