Transformers

Question 1

Primary winding

Answer 1

Winding connected to power source

Question 2

Secondary winding

Answer 2

Winding connected to load

Question 3

Core type transformer

Answer 3

Winding wrapped around two sides of the core

Question 4

Shell type transformer

Answer 4

Winding wrapped around the center leg

Question 5

Why are the primary and secondary windings wound on top of each other?

Answer 5

Much less leakage flux
Easier to insulate high voltage winding from core

Question 6

The ____ voltage winding is on top of the ____ voltage winding

Answer 6

Low
High

Question 7

What is the purpose of a unit transformer?

Answer 7

To step up the voltage at generation

Question 8

What is the purpose of substation transformer?

Answer 8

To step down voltage from transmission to distribution

Question 9

What is the purpose of distribution transformer?

Answer 9

Steps down distribution voltage to final voltage for usage

Question 10

What is the purpose of potential/voltage transformer?

Answer 10

Change high voltage to low voltage, but can handle very small current
(used for instrumentation)

Question 11

What is the purpose of current transformer?

Answer 11

Change the primary current to much smaller secondary current
(used for instrumentation)

Question 12

Emf per turn for an ideal transformer is the same (T/F)

Answer 12

True

Question 13

Mmf on each side for an ideal transformer is the same (T/F)

Answer 13

True

Question 14

What is magnetization current iM of transformer?

Answer 14

Current required to produce flux in core of transformer

Question 15

What is the core losses current (i h+e)of a transformer?

Answer 15

Current for hystersis and Eddy current losses

Question 16

Magnetization current ____ (leads/lags) applied voltage

Answer 16

Lags by 90 degrees

Question 17

Eddy currents are proportional to

Answer 17

Rate of change of flux

Question 18

What is leakage flux of a real transformer?

Answer 18

Flux which escapes the core and passes only through one transformer winding

Question 19

What is leakage flux modelled by

Answer 19

Primary and secondary inductances

Question 20

What is magnetization flux of real transformer?

Answer 20

Flux which links with both windings

Question 21

What assumptions do we make for idela transformer

Answer 21

No hystersis or eddy current losses in core
Ideal magnetization curve
Zero leakage flux (complete coupling)
Zero resistance of transformer winding

Question 22

What are the losses in a transformer?

Answer 22

Copper, leakage, Eddy current, hystersis

Question 23

Copper losses are proportional to

Answer 23

Square of current in the winding

Question 24

Leakage flux is modelled by

Answer 24

Self inductance Ls and Lp

Question 25

Eddy current losses are proportional to

Answer 25

Square of voltage applied to transformer

Question 26

Hysteresis losses are a function of

Answer 26

Nonlinear function of applied voltage

Question 27

What does Rc model

Answer 27

Eddy current and hysteresis losses (proportional to voltage and in phase)

Question 28

What does XM model

Answer 28

Magnetization current proportional to voltage and lagging by 90 degrees

Question 29

Why do we place XM and Rc inside after Lp and Rp

Answer 29

Voltage applied at core is input voltage minus internal voltage drop of winding

Question 30

Why do we move XM and Rc forward in the approximate circuit?

Answer 30

The excitation branch current is very low and the the voltage drop is negligible

Question 31

Why does output voltage vary with load even if input voltage is constant

Answer 31

Because of series impedance

Question 32

Voltage regulation of lagging load is

Answer 32

Large positive

Question 33

Voltage regulation of unity load is

Answer 33

Small positive

Question 34

Voltage regulation of leading load is

Answer 34

Negative

Question 35

Autotransformer is used to convert

Answer 35

Voltages of small ratio

Question 36

Common winding of autotransformer is

Answer 36

Where both primary and secondary voltage appear

Question 37

Series winding of transformer is

Answer 37

Where only one (primary or secondary) voltage appears and is in series with common

Question 38

Why can autotransformer handle more power

Answer 38

Because not all power from primary to secondary goes through winding

Question 39

Why can't we reconnect an ordinary transformer as autotransformer

Answer 39

Because insulation of low voltage may not be able to withstand full output voltage of autotransformer

Question 40

Series and common winding insulation of autotransformer is ____ and decided by

Answer 40

Same Higher voltage (SE + C)

Question 41

Autotransformers are used as

Answer 41

Low voltage tap variable transformer

Question 42

What is the primary disadvantage of autotransformer

Answer 42

Electrical isolation of two windings is lost

Question 43

How many taos are typically present

Answer 43

4 with spacing +- 2.5 % of full load

Question 44

Transformer with voltage sensor that automatically adjusts is called

Answer 44

Tap changing under load (TCUL) transformer