3.15 Transformers

Question 1

What is a transformer?

Answer 1

An electrical device that is used in AC circuits to change the voltage and current supplied to a circuit or device

Question 2

What does a step-up transformer do?

Answer 2

Increases the voltage and decreases the current

Question 3

What does a step-down transformer do?

Answer 3

Decrease the voltage and increases the current

Question 4

How do transformers transfer energy?

Answer 4

Via a magnetic circuit with no electrical connection between input and output terminals

Question 5

What is the general build up of a transformer?

Answer 5

Two coils and an iron core

Question 6

In a transformer what is the input coil called?

Answer 6

Primary winding

Question 7

In a transformer what is the output coil called?

Answer 7

The secondary winding

Question 8

What makes up the iron core in a transformer?

Answer 8

Isolated thin iron layers (laminated core)

Question 9

Why is it a laminated core in a transformer instead of a solid iron core?

Answer 9

To prevent eddy currents which waste power by heating the iron core

Question 10

Does frequency change in a transformer?

Answer 10

No it doesn’t

Question 11

The voltage phase relationship between the primary circuit and secondary circuit can be either?

Answer 11

0˚ or 180˚

Question 12

In a transformer what determines whether its 0˚ or 180˚ ?

Answer 12

0˚ or 180˚how the secondary is ‘wound’ relative to the primary

Question 13

What indicates whether a transformer is 0˚ or 180˚?

Answer 13

It is marked with phasing dots

Question 14

On a drawing of a transformer what does it mean when the phase dots are in line?

Answer 14

It’s in phase (0˚)

Question 15

On a drawing of a transformer what does it mean when the phase dots are not in line with each other?

Answer 15

It is out of phase (180˚)

Question 16

What is the efficiency range of a small power transformer ?

Answer 16

80-90%

Question 17

What is the efficiency range of a large commercial power line transformer?

Answer 17

98%

Question 18

What are the two types of loss you can have in a transformer?

Answer 18

-copper
-iron

Question 19

What is copper loss in a transformer?

Answer 19

Loss caused by direct current resistance in the primary and secondary windings

Question 20

What is iron loss in a transformer?

Answer 20

Loss that’s caused by eddy currents and hysteresis loss (lag)

Question 21

When a transformer losses some energy what is it converted into?

Answer 21

Heat energy

Question 22

In copper loss how much power dissipated is directly proportional to?

Answer 22

-resistance of wire
-the square of the current (size of wire)

Question 23

What is hysteresis loss?

Answer 23

When a conductor is passed through a magnetic field inside the conductor the poles in the core must align themselves however they can lag behind causing heat loss

Question 24

When load is applied on the secondary winding of a transformer what happens?

Answer 24

A current flows in the winding which creates a magnetic field

Question 25

Once the secondary current has a load and creates a magnetic field, what happens to the first and second magnetic field?

Answer 25

Because they’re opposite in direction it reduces the overall magnetic strength

Question 26

In a transformer if the voltage increases what must happen to the current?

Answer 26

It must decrease

Question 27

In a transformer if the current increases what must happen to the voltage?

Answer 27

It must decrease

Question 28

What does a transformers ratio depend on?

Answer 28

-number of turns in the primary and secondary windings -voltage across each winding -current through the windings

Question 29

When is a transformer under no load?

Answer 29

When there is no electrical load connected to its secondary winding and therefore no current flowing

Question 30

Why is the input power always the same as the output power in a transformer ?

Answer 30

Because a transformer can only change voltage and current not power

Question 31

Why does a transformer not suffer from friction or windage losses?

Answer 31

Because it has no moving parts

Question 32

How can hysteresis losses be reduced?

Answer 32

By making the core from special steel alloys

Question 33

If the primary voltage and load are connected in the same way, what happens to the phase?

Answer 33

No phase difference can be seen

Question 34

In a transformer what does it mean if the primary winding has a red dot and black dot on the secondary winding?

Answer 34

It’s out of phase

Question 35

In a transformer what does it mean if the primary and secondary winding both have a red dot?

Answer 35

It’s in phase

Question 36

What is commonly used instead of a single phase transformer?

Answer 36

A three limbed transformer

Question 37

What does a three phase transformer consist of?

Answer 37

-core -core insulation -primary winding -insulation -secondary winding

Question 38

In a three phase transformer what is the first layer of insulation used for?

Answer 38

To protect the primary winding from damage by the metal core

Question 39

What are the primary and secondary windings made from in a three phase transformer?

Answer 39

Insulated copper or aluminium

Question 40

In a three phase transformer what is the insulation between primary and secondary made from?

Answer 40

Treated paper or cloth

Question 41

Why is the insulation in a three phase transformer treated?

Answer 41

To reduce the possibility of fire

Question 42

In a three phase transformer what is known as ‘tight coupling’?

Answer 42

When the secondary winding is wound on top of the primary

Question 43

Why is tight coupling used?

Answer 43

It’s said to be 99.9% efficient

Question 44

What are the three types of couplings used in a three phase transformer?

Answer 44

-tight -medium -loose

Question 45

What is the benefit o using a star/Y connection?

Answer 45

You can have multiple voltages

Question 46

What is the main benefit of a delta connection?

Answer 46

Higher level of reliability (if one fails open the other two can still maintain voltage)

Question 47

How many windings does a three phase transformer have?

Answer 47

6 (3 primary, 3 secondary)

Question 48

In a star connection you have Red, Yellow, Blue. What is the fourth connection?

Answer 48

Neutral

Question 49

When labelling a three phase transformer what is indicated by capital A,B,C?

Answer 49

The primary windings

Question 50

In a three phase transformer what is indicated by lower-case a,b,c?

Answer 50

The secondary windings

Question 51

If you have a large power transformers and its unit are too high what do the unis turn into?

Answer 51

KVA (kilo volt-amperes) or MVA (Mega volt-amperes)

Question 52

What is active power used for?

Answer 52

To generate real work

Question 53

In a transformer, the power in must be the same as?

Answer 53

The power going out

Question 54

Having different amount of coils on a side of a transformer causes what?

Answer 54

A step up or down transformer

Question 55

How do you create a step up transformer?

Answer 55

You have more turns on the secondary winding

Question 56

How do you create a step down transformer?

Answer 56

You have more turns on your first winding

Question 57

How many coils does a auto transformer have?

Answer 57

One

Question 58

How do the primary and secondary windings work in an auto transformer if it only has one coil?

Answer 58

They share a part of the winding

Question 59

What is the major advantage of a auto transformer?

Answer 59

It’s small design

Question 60

What is the main disadvantage of a auto transformer?

Answer 60

It doesn’t have the primary to secondary winding isolation of a normal transformer

Question 61

What is a variable transformer known as?

Answer 61

Variac

Question 62

What is a variable transformer?

Answer 62

The same as a auto transformer but with a slider arm to vary the output

Question 63

What are current transformers?

Answer 63

Transformers with a built in AC current measurement so you don’t have to break the current carrying leads

Question 64

What must current transformers always have?

Answer 64

A load