Year 2 I2 Flashcards
Mutual induction
is the action by which a change in the current in one conductor induces a voltage in a neighboring conductor
Induced voltage
whenever a conductor cuts or is cut by magnetic flux, a voltage is induced in the conductor
Practical transformer efficiency is
about 96 to 99%
Voltage ratio and turns ratio
the voltage induced in the two coils is directly proportional to the rate at which the flux cuts the coils. Since the same changing flux cuts both coils, and the length of each turn of the two coils is aprrox the same, the volt-per-turn induced in the two coils must be equal.
This means that the voltage ratio is directly proportional to the turns ratio.
Voltage ratio and turns ratio and current ratio
Np/Ns = Ep/Es = Is/Ip
classification of transformers
Cooling method: air, oil, natural convection, forced air, forced oil
Insulation between the windings
Number of phases
Type of service: power, distribution, instrument, control
Type of winding: isolated or autotransformer windings
Three major parts of a transformers are:
High-voltage winding
Low-voltage windings
Core
Transformer efficiency
n = Pout/Pin or Pout/(Pout + Plosses)
kVA ratings
the power ratings of the high side equal the power ratings of the low side
subtractive polarity
standard for power transformers
if the X1 terminal is adjacent to the H1 terminal
Additive polarity
standard for distribution transformers
if the X1 terminal is diagonally across form the H1 terminal
Instrument transformers have
subtractive polarity and are marked with dots these terminals have the same instantaneous polarity
describe the AC voltmeter test
Identify high low windings. High has thinner wire
Identify H1 it is on the left when facing low terminals
Attach jumper from H1 to X. apply suitable low AC voltage
If voltmeter reads difference of tag voltage it is subtractive
If voltmeter reads sum of tag voltage it is additive
The DC inductive kick test
Steps 1 and 2 same as AC test
With H1 connected to the positive the moment switch is closed if the meter kicks up-scale identify the red lead of the voltmeter as X1
Conditions that must be met when connecting transformers in parallel
same primary and secondary voltage ratings
observe the terminal polarity of the transformers
all the transformers must have the same percent impedance
Test closure voltage
before making the final secondary connection, you just do a simple voltmeter test. If the voltmeter reads 0v proper polarity has been observed.
Classify energy losses in transformers
core losses: eddy-currents and hysteresis
copper losses: I2R losses in the primary and secondary
What two tests can be done for losses in transformers
open-cct test for core losses page 58 I2
short-cct test for copper losses
Significance of percent impedance
(%Z)
to calculate available fault current Ifulload/%Z
Determine whether two transformers are suitable for paralleling
Percent voltage regulation =
((Vno load - Vfull load)/Vfull load) x 100%
why does the terminal voltage equal the secondary induced voltage when the secondary of the transformer has no-load
when no current is flowing there can’t be any I2Z drops or IZ drops
Tap changers
No-load
On-load
Large fault currents
to help control damage that might occur during a fault, a section of the winding are removed from the centre of the winding
Autotransformers do have a
electrically conductive link between primary and secondary because some wire turns are shared by both windings
Applications for autotransformers
small increase or decrease
reduced-voltage starting for squirrel-cage induction motors
power supplies where multiple voltages are required
automatic voltage regulators to compensate for line drop as load changes occur
Danger of opening the secondary of a CT
in CTs secondary current is determined by current flowing in the primary if flow is interrupted:
transformer core is driven hard into saturation
as the core flux collapses in a short period a very high voltage is induced
Never open the secondary of a CT on a live cct.
List types of CT
wound
window
bar
Potential transformer (PT)
connected in parallel to step down high voltages to a safer level for monitoring and control
