trns 220 Flashcards
1
Q
What do transformers change values of? and whats the one thing it cant change?
A
-voltage
-current
-impedance
-CANT CHANGE FREQUENCY
2
Q
All values of voltage,current and impedance in a transformer are ________________ to the turns ratio
A
proportional
3
Q
The primary winding of a transformer is connected to the __________________. The secondary is connected to the __________
A
power source
load
4
Q
A transformer that has a lower secindary voltage than primary voltage is called a _________
A
step down tranny
5
Q
A transformer that has a higher secondary voltage than primary voltage is called a ___________ transformer
A
step up
6
Q
An ______________ transformer has its primary and secondary windings electronically and mechanically seperated from eachother
A
isolation
7
Q
what is self induction
A
when a coil induces voltage into itself
8
Q
what is mutual induction
A
when a coil induces voltage into another coil
9
Q
isolation transformers help filter _______ and ____________ spikes between its primary and secondary side
A
voltage
current
10
Q
polarity dots are often added to schematic diagrams to indicate the _____________
A
tranny spolarity
11
Q
What 2 ways can transformers be connected as
A
-additibe polarity
-subtractive polarioty
12
Q
what causes magnetomotive force
A
(mmf) created by moving electrical charges
13
Q
the amt of mmf produces by eacg turn of a coil is ________________ to the current it carries
A
proportional
14
Q
The magnetizing force of a coil is equal to the _____________________
A
coil current in amps times the number of turns in a coil
15
Q
The path through which the field passes is called a _____________
A
magnetic cct
16
Q
______________ is the magnetic equivalent of mechanical friction or electrical resistance
A
reluctance
17
Q
equation for calculating current
A
emf divided by resistance
18
Q
equation for calculating flux
A
mmf divided by reluctance
19
Q
The term _____________ is used to decribe the material placed inside a coil to define the magnetic cct
A
core
20
Q
An open coil has an _________ core
A
air
21
Q
What materials for an iron core provide lower reluctance cct than air an dmakes it possible to direct/focus the field along a certain path
A
-ferrite
-iron
-magnetic steel
22
Q
A continuous loop of ________________ material provides the strongest field
A
low reluctance
23
Q
Openings in the core cct are called ___________
A
air gaps
24
Q
the amount of emf is directly proportional to the ___________at which the magnetic flux is changing and the number of turns in each coil.
A
25
what does the polarity of the emf depend on
polarity of:
-field relative to orientation of the coil
-increasing or decreasing
26
what is a flux line called
maxwell
27
in an electromagnetic coil what does a change in voltage do for current, flux and induced emf
-current changes
-flux changes
-induces an emf that opposes the change in current by oposing the change in supply voltage that caused it
28
what does self inductance do
-oposes an increase or decrease in current
29
what is the unit of inductance
Henery. based on the amount of emf induced into a coil for a specific rate of change in current
30
what happens when theres a high voltage spike
can cause damage to insulation and voltage sensitive electronic components
31
what force is responsible for all magnetic fields
mmf, magnomotive force
32
what changes in a magnetic material when it becomes magnetized
-domains are aligned along the samw axis (clusters)
33
how much mmf is generated by a coil with 100 turns when its carrying a 15 A current
1500mmf
(turns x amps)
34
what property is the magnetic equivalient of electrical resistance
reluctance
35
what effect does placing acurrent carying coil on a core made of magnetic material have on:
a) strengh of the field
b)shape of the field?
a) increases
b)more direct path.
36
whats faradays law of electromagnetic induction
-any change in magnetic flux passing through a coil will induce a emf into the coil
37
when current in a coil stops rising and starts to fall what happens to the polarity of the cemf
reverses
38
Is the induced cemf higher when a DC coil is turned on or off
when on cemf cant exceed applied voltage when off its limited by the rate at which the field collapses
39
differnece betweeen self inductance and mutal inductance
self- happens in a single cct or conductor
mutual- is about linking between 2 or more different ccts/conductors
40
when a conductor is looped around the core of an operating transformer, a voltage appears across the ends of the conductor. Where does this voltage come from?
- changing fiels passing through the coil
41
in a transformer cct what electrical quanity is a measure of field strength?
-induced voltage
42
what is a tap in a transformer
-connection made at somepoint in the winding
43
whats the primary in a transformer
-input side of the transfromer
44
whats the secondary in a transformer
-output side of the transformer
45
how can you calculate the volts/turns of a transformer winding?
V/T= total voltage across windings divided by the number of turns
46
how can you calculate the ampere-turns of a transformer winding?
A/T= winding current x # of turns
47
an ampere-turn is a unit of _____________--
magnomotive force (mmf)
48
what are the most common construction arrangements in a transformer
-shell
-core
49
whats a shell-type transformer
-primary and secondary windings are wound into a form in alternating layers, results in a single loop which is then enclosed by the iron core
50
What is a core-type transformer
- core iron is a single loop and the windings encircle the core legs.
51
what material can transformer coils be made from
-copper
-aluminum
52
in large shell type transformers how are the windings aranged
-in concentric layers or pancake coils
53
in small transformers how are the windings arranged
-duct sticks are inserted berween the layers to create gaps through the coils for more effective cooling
54
what is vaccum pressure impregnation used for
improve varnish protection
55
what are taps used for
so turns ratio can be adjusted to compensate for a supply voltage that is slightly avove or below the rated voltage
56
Taps with a higher voltage rating then the nominal are called ____________ taps
above normal
57
Taps rated for less than the nominal are called ____________- taps
below normal
58
What happens to other taps when one is energized
-when one is all are
59
where are the taps on small dry type transformers
individual leads for the main and tap connections
60
where are the taps on larger dry type transformers
- seperate tap connections accessible right on the winding assembly.
61
when are shielded transformers used
when neccesary to provide total isolation
62
what precautions must be taken with lead-type transformer taps?
- make the right connection
-dont connect a supply from one tap to another
-make sure they are isolated because they are live
63
what precautions must be used when the taps are short tabs mounted directly on the winding assembly?
- when any tap is energized they all are
-winding insulation is brittle
-easily damaged
-careful when tightening lugs
-support stud with second wrench
64
What special hazard exists with transformers tapped for multiple standard voltages?
- when one is energized they all are to hookup the right voltafe to the right tab
65
what precautions must be taken with tap switches?
- denergize the transformer before switching off bc it can create an arc blast capsulating magnetic form
66
what is the purpose of the typical on-line tap changer?
- regulate voltage without denergizing
67
how are dual windings usually installed
- in series with a centre tap
68
transformers that are not liquid filled are _______
dry types
69
an ____________ type transformer has no enclosure
open
70
largre dry type transformers are usually _________ type
core
71
most highvoltage distribution transformers are filled with a ________________
dielectric (insulating) liquid
light mineral oil
acts like a coolant
72
what improves heat transfer
fins
tubes
73
what is used to allow the connecting conductors to pass through the container wall without causing an electrical short or an oil leak
bushings on top and smaller low voltage bushings on the side
74
when oil filled transformers are mounted indoors what requirement do they have to meet
they are a fire hazard abd must be inside a fireproof vault
75
do liquid or dry types have a longer service life
-liquid able to handle temperary overloads better
76
where are open type transformers normally found
-equipment enclosures
-smaller appliances
77
most small dry type transformers have a _________-- coil and core construction
shell
78
Extra low voltage control transformers can be mounted directly into a knockout on an electrical box. What are the high and low voltage connections made to transformers of this type?
Exposed leads or terminals
extra-low voltage can be on outside of the box
79
Why is it unsafe to mount a transformer on or near conbustable material?
- generate heat
-during fauls or faliures can get hot enough to start a fire
80
how do you acess the connections for a typical mid-size dry type transformer
removable panels
81
why are oil filled transformers rarly used indoors
-fire hazard
-need a fire vault
82
what benifets does transformer oil provide?
-longer service life
-better at handling temperaty overloads
83
what is the purpose of a berm around a large high-voltage transformer
-contains the oil if the tank leaks
84
in transformer windings the lower the resistance the __________ turns
less
85
What does the letter H and X indicate on transformer windings
h- high voltage winding. more resistance smaller wire, less current
x- low voltage winding, less resistance, bigger wire more current
86
Whats the standard location for H1 in transformers with fixed terminals or bushings Drawings use a ___________ view
-on the right when facing high voltage side
-top view
87
What does H2 indicate
tap with the lowest voltage rating, H3 is the next
88
X1 has __________ instantaneous polarity as H1
the same
89
if H1 and X1 are at the same end, the transformer has a _____________ connection. if they are at opposite ends the transformer is said to have a _________ connection
-subtractive
-additive
90
the connection arangement ___________ affect the actual terminal polarity
does not
91
AC voltages and currents have a __________ polarity relative to one and another
instantaneous
92
what is instantaneous polarity
- polarity of voltages or the firection of the currents has to be compared at the same instant in time
93
X1 terminal must have the same instantanous polarity as ___________
H1
94
transformers are listed by _______ with the primary voltage first
voltage
95
At the instant current entres H1 supply from the supply, it exits _______ of the load
x1
96
On a transformer with dual secondary windings, which terminal has the same instantaneous polarity as X1?
X3
97
where should you look for information before connecting any equipment?
nameplate
98
would the instantaneous polraity of the X1 terminal vhange if a transformer were converted from additive to subtractive connection arrangement?
no
99
larger single-phase transformers are usually core types with 2 seperate windings assemblies, each containing _______ of the primary and _____ of the secondary winding
half
half
100
The start and finish of high voltage windings are identified as:
H1 H2
101
CEC requires the secondary winding to be ___________
grounded, for 3 wire systems its always applied to the centre connection
102
Why does changing taps change the output voltage?
different amount of turns
103
If the high voltage winding has 4 taps, marked H2,H3,H4 and H5, the connection with the lowest voltage ratings will be H1 to __________
H2
104
A transformer is rated 600:120/240V. How should the secondary windings be connected to supply a 120V load?
in parallel
105
What is the purpose of turns ratio compenstation?
-without, output voltage would be low
-with, the full load is close to nameplate rating
106
What effect will turn ratio compensation have if a step-down transformer is connected for a step-up operation?
- work backwards
107
A transformer has dual 120V secondary windings, and the X1 and X2 markers have accidentally been interchanged. What effect does this have when the windings are connected:
a)in series
b)in parallel
a) would get 0 volts, volts would cancel eachother out. Would read 120 for 120 but read 0 for 240V
b)it would create a short in the cct, would trip out from the high current
108
A 480:120/240V transformer is used to supply a 2 wire 120V cct. What would be wrong with using only one of the windings, instead of both winfings in parallel?
coukd overload secondary transformer
109
What is one advantage and one disadvantage to using a 3 wire 120/240V cct instead of 120V cct to supply a group of 120V loads
A- less current
D- more wires
110
A 600:120/240V transformer is used to supply a 2 wire 240V cct. What differences would it make if the seondary ground were applied to X4 instead of X2-X3?
-shock hazard
-would be 240 instead of 120
111
why are instrument transformers a better solution
-safer
-easier to apply bc it can they isolate the instruments from the power cct voltages and currents
112
when are potential transformers used
-when voltages are too high
-reasons for isolation
113
where is the voltmeter conneted
-directly across the output terminals of the PT
-the actual power cct voltage is the meter reading times the PT voltage ratio
114
when are current transformers used
--when the current is too high, the voltage is too high, or there is some other reason to avoid connecting an ammeter or other current monitoring instrument directly into the power cct.
-connected across the output of a ct is directly propotional to thecurrent across the output of the a ct is directly proportional to the current through the power cct conductior
115
why are instrument transformers not used in DC systems?
-dont work on dc
-not changing magnetic fields
116
Typical PTs have secondary windings rated at what voltages?
120V
117
typical CTs have secondary windings rated at what cuurent?
5A
118
A PT transformer is normally connected to the power cct through a set of _____________________
-protective fuses
119
the primary of the CT is always connected in _______________- with the load
series
120
The secondary windings of a CT must be connected to a _____________- load such as an armature or shorted with a jumper
low impedance
121
Voltage induced into a CT winding is ______________________ to be high enough to be dangerous and may cause insulation failure that destroys the CT.
less likely
122
Bushing CTs dont have have primary terminals but what do they have instead
-polarity mark on one side that is normally turned to the face the supply
123
The primary of a strap type CT is protected by the _________________________
power cct fuses
124
The secondary windings of a CT is grounded where
at one end
125
How much voltage does a ungrounded terminal have
almost no voltage to ground
126
Why is one terminal of a PT or CT connected to ground?
-prevent voltage- ground from rising due to capacitance couplings, refrence it to ground
127
What is likely to happen if the ground is removed from the secondary of an instrument transformer?
-high voltages
128
What are the letter and number markings at the polarity marked terminals of a typical PT?
H1, X1 and sometimes a dot
129
Sould the H1 terminal of a PT be connected to the power cct or the instruments?
-power cct
130
When the current flows into the polarity marked terminal of a PT or CT, will it flow into, or out, the polarity secondary terminal?
out of X1
131
when its safe to open the short-circuting device provided in the secondary cct of a CT?
when its enrgized
132
What meter would be used to measure tge output of a PT, and how should it be connected?
- voltmeter in paralell
133
What meter would be used to measure the output of a CT, and how should it be connected
amp meter in series
134
what is a capacitive coupling?
-if not grounded voltages can add together
135
volts/turn= _________________
volts/turn
136
The ____________ of a transformer is connected of supply voltage
primary
137
The ___________ of a transformer is connected to the load
secondary
138
The primary voltage of a step up transformer will be ________ than the secondary voltage
lower
139
The primary voltage terminals of a step down transformer will be _________ than the secondary voltage
higher
140
The higher voltage terminals of a transformers always marked with the letter ___________
H
141
The lower voltage terminals are marked with the letter _________
X
142
The primary terminals of a step up transformer would be marked with the letter ___________
X
143
What is the relationship between H1 and X1
they have the same intaneous polarity
144
Whats the code rules for impedance and short ccts
Section 14
145
What to parallel transformers need
-identical voltage and frequency ratings
-impedance ratings within +-7.5%
-combines KVA that is equal or greater than the requirements of the load
146
If one transformer gas a lower impedance what happens to the other
-output voltage will stay higher as the load demand is increased
-transformer with the lowest impedance will tend to take a larger share of its total load
