Transformers Flashcards
Transformers stuff
Induction is described as the property of a circuit that _____
Opposes change in current
Mutual Induction occurs when a change of _____ in one circuit causes _____ to be induced into another circuit
Current, voltage
Mutual induction is also called
Transformer action
The coil of a transformer which is connected to a source of potential difference is the _____ (Primary or Secondary)
Primary
Once primary current in a DC connected transformer reaches a steady value, what happens in the secondary coil?
No voltage is induced.
A self or mutually induced voltage only occurs when the current is _____.
Changing (as with AC)
What does reluctance do? What does it have to do with transformers?
Opposes magnetic flux on a magnetic circuit.
The soft iron core is low-relunctance.
Is the efficiency of a transformer high or low?
High. 95-98%
Which type of core has the most leakage flux? And the least?
Core type - most
H TYPE - least
An H type center is commonly found on what type of transformers?
High voltage distribution
Hysteresis losses are _____ to the frequency of the source voltage
Proportional
Power losses due to eddy currents and hysteresis losses are also called _____
Core losses
How do you determine %Z of a transformer?
A short circuit test
What is a common use for a step up transformer?
To transfer power over long distances, such as power station switchyard for transmission.
What happens to a transformer beyond 110% of rated voltage?
Primary current increase and transformer burns out.
If the secondary voltage on a transformer doubles, what does the current do?
Reduces to half
If the current on a secondary winding is halved, what happens to the voltage?
Doubles
Transformer polarity is the relative direction of _____ voltage to the direction of _____
Primary voltage, Secondary voltage
With power or distribution transformer, polarity is important when _____
Paralleled
A tap labelled -5% would go on the primary if the supply voltage is _____
5% lower than rated
A transformer whose capacity is reduced in proportion to the reduction of primary voltage has a _____
Reduced capacity tap
A full capacity tap transformer has higher overall capacity so that ____
Rated kVA is available at any tap setting
FCBN taps would be used when primary supply voltage is _____
5% lower than rated
How does an automatic tap changer work?
Feedback control circuit that changes taps when secondary V is above or below a preset value.
What is % Voltage Regulation?
Change of the secondary output Voltage from no-load to full-load.
Formula for % Voltage Regulation
%VR = “v(secondary no load) - v(secondary full load) “/ v(secondary full load) x 100
When E(s) is reduced, the secondary output voltage V(s) is (reduced or increased).
Reduced
When E(p) is reduced, secondary induced voltage E(s) is (reduced or increased).
Reduced
Volt drops from secondary current, R(s) and XL(s) of the secondary will (reduce or increase) secondary output voltage
Reduce
Secondary output voltage V(s) at full load is (less or more) than at no load.
Less
Core losses in a transformer can be determined by _____.
Open circuit test
Copper losses are proportional to the _____ and vary from no to full load conditions (true or false)
Square of the currents.
True
If a transformer’s load is decreased, what happens to the %efficiency?
Also decreases
Increased power factor means (increase, decrease) %efficiency
Increase
Increased copper and core losses means (increase, decrease) %efficiency
Decrease
Short-circuit current is limited by the _____ of the transformer as well as ____ of the remainder of the circuit.
Impedances
Impedance Voltage of a transformer is the primary voltage required to circulate rated full-load _____ current in a transformer when the _____ is _____ at the terminals
Secondary, secondary, short circuited
Copper losses in a transformer are determined by _____
Short circuit test
Core losses are are dependant on applied voltage (true or false)
True
With constant rated voltage applied, core losses remain constant from no to full load (true or false)
True
%Z =
(Impedance Voltage / Primary rated voltage) x 100
Impedance Voltage =
Primary rated voltage x ( %Z/100 )
When transformers are connected in parallel, the impedance (increases, decreases) and what issue can this cause?
Decreases. The available short-circuit current can increase considerably.
When paralleling transformers, the no-load secondary voltages must be _____
Equal
Should you use different tap settings on paralleled transformers? Why or why not?
No. Leads to different secondary voltages.
Transformers in parallel should have a %Z within _____ of each other
5%
What happens if one paralleled transformer is disconnected from the source but the other is not?
The disconnected transformer backfeeds (induces) voltage into the primary due to the connection to the transformer that is still connected to source.
What happens if an emergency generator is connected to a home’s electrical system before disconnecting the utility?
Induced voltage in generator backfeeds to utility transformer
Formula for %Z
%Z = Impedance Voltage / E (p)
Transformer efficiency formula
(P(out) / P(in) + copper & core losses) x 100
What two pieces of info are obtained by an Open Circuit test?
Exciting current, core loss
How do core losses change in reference to a change of supply voltage?
Exponentially
What is a harmonic? What is a third harmonic in a transformer?
Integer product of supply frequency. 3 x 60hz = 180hz, can induce a current which causes excessive heat.
A Wye/Wye connection without a primary neutral is normal, true or false?
False. It is rare because it makes it hard to maintain balanced output voltage.
What should a voltmeter read in a properly connected Delta open corner test?
0V
An advantage to the Wye/Delta connection is _____.
Primary winding conductors do not need to be insulated for full three-phase line voltage.
Are the primary and secondary line voltages of a Delta/Wye connection in phase with each other?
No
If the polarity of one of the secondary windings in a Delta/Wye bank is reversed, will the correct three phase line voltages be available?
No
The new neutral on the secondary of a step down transformer bank is typically called the _____
XO point
3, 5 and 7th Harmonics (low, odd) are the worst because _____. What does it cause regarding voltage and current?
They are in phase and stack on the sine waveform. Can cause extremely high dangerous voltages and current can flow in neutral.
If you remove one transformer of a 3 phase Delta Delta system, how much of the power capacity will you have between the remaining two transformers?
57.7% of the original VA
The primary and secondary line voltages of a Delta/Wye 3 phase transformer are in phase with each other, true or false?
False. 30 degrees separate
What are the coordinating closed contacts for each phase when using a voltmeter transfer switch? And for the lines?
Phase A: 2, 3
Phase B: 2, 7
Phase C: 2, 11
Line AB: 3, 6
Line BC:: 6, 11
Line CA: 3, 10
