trns 220 Flashcards
What do transformers change values of? and whats the one thing it cant change?
-voltage
-current
-impedance
-CANT CHANGE FREQUENCY
All values of voltage,current and impedance in a transformer are ________________ to the turns ratio
proportional
The primary winding of a transformer is connected to the __________________. The secondary is connected to the __________
power source
load
A transformer that has a lower secindary voltage than primary voltage is called a _________
step down tranny
A transformer that has a higher secondary voltage than primary voltage is called a ___________ transformer
step up
An ______________ transformer has its primary and secondary windings electronically and mechanically seperated from eachother
isolation
what is self induction
when a coil induces voltage into itself
what is mutual induction
when a coil induces voltage into another coil
isolation transformers help filter _______ and ____________ spikes between its primary and secondary side
voltage
current
polarity dots are often added to schematic diagrams to indicate the _____________
tranny spolarity
What 2 ways can transformers be connected as
-additibe polarity
-subtractive polarioty
what causes magnetomotive force
(mmf) created by moving electrical charges
the amt of mmf produces by eacg turn of a coil is ________________ to the current it carries
proportional
The magnetizing force of a coil is equal to the _____________________
coil current in amps times the number of turns in a coil
The path through which the field passes is called a _____________
magnetic cct
______________ is the magnetic equivalent of mechanical friction or electrical resistance
reluctance
equation for calculating current
emf divided by resistance
equation for calculating flux
mmf divided by reluctance
The term _____________ is used to decribe the material placed inside a coil to define the magnetic cct
core
An open coil has an _________ core
air
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
-ferrite
-iron
-magnetic steel
A continuous loop of ________________ material provides the strongest field
low reluctance
Openings in the core cct are called ___________
air gaps
the amount of emf is directly proportional to the ___________at which the magnetic flux is changing and the number of turns in each coil.
what does the polarity of the emf depend on
polarity of:
-field relative to orientation of the coil
-increasing or decreasing
what is a flux line called
maxwell
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
what does self inductance do
-oposes an increase or decrease in current
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
what happens when theres a high voltage spike
can cause damage to insulation and voltage sensitive electronic components
what force is responsible for all magnetic fields
mmf, magnomotive force
what changes in a magnetic material when it becomes magnetized
-domains are aligned along the samw axis (clusters)
how much mmf is generated by a coil with 100 turns when its carrying a 15 A current
1500mmf
(turns x amps)
what property is the magnetic equivalient of electrical resistance
reluctance
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.
whats faradays law of electromagnetic induction
-any change in magnetic flux passing through a coil will induce a emf into the coil
when current in a coil stops rising and starts to fall what happens to the polarity of the cemf
reverses
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
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
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
in a transformer cct what electrical quanity is a measure of field strength?
-induced voltage
what is a tap in a transformer
-connection made at somepoint in the winding
whats the primary in a transformer
-input side of the transfromer
whats the secondary in a transformer
-output side of the transformer
how can you calculate the volts/turns of a transformer winding?
V/T= total voltage across windings divided by the number of turns
how can you calculate the ampere-turns of a transformer winding?
A/T= winding current x # of turns
an ampere-turn is a unit of _____________–
magnomotive force (mmf)
what are the most common construction arrangements in a transformer
-shell
-core
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
What is a core-type transformer
- core iron is a single loop and the windings encircle the core legs.
what material can transformer coils be made from
-copper
-aluminum
in large shell type transformers how are the windings aranged
-in concentric layers or pancake coils
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
what is vaccum pressure impregnation used for
improve varnish protection
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
Taps with a higher voltage rating then the nominal are called ____________ taps
above normal
Taps rated for less than the nominal are called ____________- taps
below normal
What happens to other taps when one is energized
-when one is all are
where are the taps on small dry type transformers
individual leads for the main and tap connections
where are the taps on larger dry type transformers
- seperate tap connections accessible right on the winding assembly.
when are shielded transformers used
when neccesary to provide total isolation
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
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
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
what precautions must be taken with tap switches?
- denergize the transformer before switching off bc it can create an arc blast capsulating magnetic form
what is the purpose of the typical on-line tap changer?
- regulate voltage without denergizing
how are dual windings usually installed
- in series with a centre tap
transformers that are not liquid filled are _______
dry types
an ____________ type transformer has no enclosure
open
largre dry type transformers are usually _________ type
core
most highvoltage distribution transformers are filled with a ________________
dielectric (insulating) liquid
light mineral oil
acts like a coolant
what improves heat transfer
fins
tubes
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
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
do liquid or dry types have a longer service life
-liquid able to handle temperary overloads better
where are open type transformers normally found
-equipment enclosures
-smaller appliances
most small dry type transformers have a _________– coil and core construction
shell
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
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
how do you acess the connections for a typical mid-size dry type transformer
removable panels
why are oil filled transformers rarly used indoors
-fire hazard
-need a fire vault
what benifets does transformer oil provide?
-longer service life
-better at handling temperaty overloads
what is the purpose of a berm around a large high-voltage transformer
-contains the oil if the tank leaks
in transformer windings the lower the resistance the __________ turns
less
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
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
What does H2 indicate
tap with the lowest voltage rating, H3 is the next
X1 has __________ instantaneous polarity as H1
the same
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
the connection arangement ___________ affect the actual terminal polarity
does not
AC voltages and currents have a __________ polarity relative to one and another
instantaneous
what is instantaneous polarity
- polarity of voltages or the firection of the currents has to be compared at the same instant in time
X1 terminal must have the same instantanous polarity as ___________
H1
transformers are listed by _______ with the primary voltage first
voltage
At the instant current entres H1 supply from the supply, it exits _______ of the load
x1
On a transformer with dual secondary windings, which terminal has the same instantaneous polarity as X1?
X3
where should you look for information before connecting any equipment?
nameplate
would the instantaneous polraity of the X1 terminal vhange if a transformer were converted from additive to subtractive connection arrangement?
no
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
The start and finish of high voltage windings are identified as:
H1 H2
CEC requires the secondary winding to be ___________
grounded, for 3 wire systems its always applied to the centre connection
Why does changing taps change the output voltage?
different amount of turns
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
A transformer is rated 600:120/240V. How should the secondary windings be connected to supply a 120V load?
in parallel
What is the purpose of turns ratio compenstation?
-without, output voltage would be low
-with, the full load is close to nameplate rating
What effect will turn ratio compensation have if a step-down transformer is connected for a step-up operation?
- work backwards
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
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
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
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
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
when are potential transformers used
-when voltages are too high
-reasons for isolation
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
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
why are instrument transformers not used in DC systems?
-dont work on dc
-not changing magnetic fields
Typical PTs have secondary windings rated at what voltages?
120V
typical CTs have secondary windings rated at what cuurent?
5A
A PT transformer is normally connected to the power cct through a set of _____________________
-protective fuses
the primary of the CT is always connected in _______________- with the load
series
The secondary windings of a CT must be connected to a _____________- load such as an armature or shorted with a jumper
low impedance
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
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
The primary of a strap type CT is protected by the _________________________
power cct fuses
The secondary windings of a CT is grounded where
at one end
How much voltage does a ungrounded terminal have
almost no voltage to ground
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
What is likely to happen if the ground is removed from the secondary of an instrument transformer?
-high voltages
What are the letter and number markings at the polarity marked terminals of a typical PT?
H1, X1 and sometimes a dot
Sould the H1 terminal of a PT be connected to the power cct or the instruments?
-power cct
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
when its safe to open the short-circuting device provided in the secondary cct of a CT?
when its enrgized
What meter would be used to measure tge output of a PT, and how should it be connected?
- voltmeter in paralell
What meter would be used to measure the output of a CT, and how should it be connected
amp meter in series
what is a capacitive coupling?
-if not grounded voltages can add together
volts/turn= _________________
volts/turn
The ____________ of a transformer is connected of supply voltage
primary
The ___________ of a transformer is connected to the load
secondary
The primary voltage of a step up transformer will be ________ than the secondary voltage
lower
The primary voltage terminals of a step down transformer will be _________ than the secondary voltage
higher
The higher voltage terminals of a transformers always marked with the letter ___________
H
The lower voltage terminals are marked with the letter _________
X
The primary terminals of a step up transformer would be marked with the letter ___________
X
What is the relationship between H1 and X1
they have the same intaneous polarity
Whats the code rules for impedance and short ccts
Section 14
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
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
