Year 2 I2

Question 1

Mutual induction

Answer 1

is the action by which a change in the current in one conductor induces a voltage in a neighboring conductor

Question 2

Induced voltage

Answer 2

whenever a conductor cuts or is cut by magnetic flux, a voltage is induced in the conductor

Question 3

Practical transformer efficiency is

Answer 3

about 96 to 99%

Question 4

Voltage ratio and turns ratio

Answer 4

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.

Question 5

Voltage ratio and turns ratio and current ratio

Answer 5

Np/Ns = Ep/Es = Is/Ip

Question 6

classification of transformers

Answer 6

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

Question 7

Three major parts of a transformers are:

Answer 7

High-voltage winding
Low-voltage windings
Core

Question 8

Transformer efficiency

Answer 8

n = Pout/Pin or Pout/(Pout + Plosses)

Question 9

kVA ratings

Answer 9

the power ratings of the high side equal the power ratings of the low side

Question 10

subtractive polarity

standard for power transformers

Answer 10

if the X1 terminal is adjacent to the H1 terminal

Question 11

Additive polarity

standard for distribution transformers

Answer 11

if the X1 terminal is diagonally across form the H1 terminal

Question 12

Instrument transformers have

Answer 12

subtractive polarity and are marked with dots these terminals have the same instantaneous polarity

Question 13

describe the AC voltmeter test

Answer 13

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

Question 14

The DC inductive kick test

Answer 14

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

Question 15

Conditions that must be met when connecting transformers in parallel

Answer 15

same primary and secondary voltage ratings
observe the terminal polarity of the transformers
all the transformers must have the same percent impedance

Question 16

Test closure voltage

Answer 16

before making the final secondary connection, you just do a simple voltmeter test. If the voltmeter reads 0v proper polarity has been observed.

Question 17

Classify energy losses in transformers

Answer 17

core losses: eddy-currents and hysteresis

copper losses: I2R losses in the primary and secondary

Question 18

What two tests can be done for losses in transformers

Answer 18

open-cct test for core losses page 58 I2

short-cct test for copper losses

Question 19

Significance of percent impedance

Answer 19

(%Z)
to calculate available fault current Ifulload/%Z
Determine whether two transformers are suitable for paralleling

Question 20

Percent voltage regulation =

Answer 20

((Vno load - Vfull load)/Vfull load) x 100%

Question 21

why does the terminal voltage equal the secondary induced voltage when the secondary of the transformer has no-load

Answer 21

when no current is flowing there can’t be any I2Z drops or IZ drops

Question 22

Tap changers

Answer 22

No-load

On-load

Question 23

Large fault currents

Answer 23

to help control damage that might occur during a fault, a section of the winding are removed from the centre of the winding

Question 24

Autotransformers do have a

Answer 24

electrically conductive link between primary and secondary because some wire turns are shared by both windings

Question 25

Applications for autotransformers

Answer 25

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

Question 26

Danger of opening the secondary of a CT

Answer 26

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.

Question 27

List types of CT

Answer 27

wound
window
bar

Question 28

Potential transformer (PT)

Answer 28

connected in parallel to step down high voltages to a safer level for monitoring and control