PHEP23 Power Transformers

Question 1

A power transformer transfers electric power from one circuit at a certain frequency and voltage, an to a second circuit at:

Answer 1

A different voltage and same frequency.

Question 2

Transformer polarity markings show:

Answer 2

The ends of the windings at which the induced voltages have the same instantaneous polarity.

Question 3

The efficiency of a well designed transformer is:

Answer 3

An average of 98 percent.

Question 4

A single phase transformer may have a/an:

Answer 4

Air core

Question 5

Compared to the secondary of a loaded step-down transformer, the primary has:

Answer 5

Higher voltage and lower current.

Question 6

Potential transformers are instrument transformers used to:

Answer 6

Step voltage down.

Question 7

The secondary of a potential transformer should never be:

Answer 7

Short-circuited to remove the device it operates from service.

Question 8

Current transformers are instrument transformers used to:

Answer 8

Step current down.

Question 9

Current transformers are used with:

Answer 9

Ammeters

Watt meters

Protective relays

Question 10

When the number of turns or voltage on the secondary of a transformer is greater than that of the primary, it is known as a:

Answer 10

Step-up transformer.

Question 11

Transformers are classed as static AC machines.

Answer 11

True.

Question 12

Transformers having only one winding are called:

Answer 12

Autotransformer.

Question 13

In an energized transformer iron losses are present even if ther is no load on the secondary.

Answer 13

True.

Question 14

An auto transformer has:

Answer 14

One winding which is composed of both the primary and secondary winding.

Question 15

The core of a transformer is designed:

Answer 15

To carry the magnetic field for the induction of voltages between the primary and secondary windings.

Question 16

The secondary of a current transformer should never be:

Answer 16

Opened to remove the device it operates from service.

Question 17

Eddy currents are circulating currents that cause:

Answer 17

A loss of energy.

Question 18

In a core-type transformer:

Answer 18

A section of the primary and secondary windings are wound around each leg of the core.

Question 19

Hysteresis and eddy currents are known as:

Answer 19

Iron losses.

Question 20

Hysteresis can be explained as:

Answer 20

Friction of the molecules due to alternating current.

Question 21

Increasing the rate of cooling on a transformer will increase its capacity.

Answer 21

True.

Question 22

The rating of a transformer is usually given in:

Answer 22

VA (volt-amperes)
KVA (kilovolt-amperes)
MVA (megavolt-amperes)

Question 23

Transformer oil will lose its dielectric strength rapidly when:

Answer 23

Exposed to air or moisture.

Question 24

Bushings are used on transformers to:

Answer 24

Serve as entrances into the transformer for the main leads.

Question 25

Taps on a transformer allow the:

Answer 25

Output voltage to be varied.

Question 26

The condition(s) that must be met when transformers are operated in parallel are:

Answer 26

Ratio of transformation approximately the same

Voltage rating approximately the same

Polarity must be the same

Question 27

The conservator system is used on transformers to:

Answer 27

Preserve the dielectric strength of the transformer oil.

Question 28

A series street lighting system requires what type of transformer?

Answer 28

A constant current transformer.

Question 29

In a transformer the winding which receives the energy inductively is called the:

Answer 29

Secondary winding.

Question 30

In a transformer, excluding the small amount of losses, the induce voltage per turn in the secondary winding is:

Answer 30

Equal to the voltage across each turn of the primary winding.

Question 31

In a transformer a ______________________ must always exist between the primary and the secondary fields to maintain the required primary current to supply the transformer losses and secondary load.

Answer 31

Balanced condition

Question 32

Transformers permit the transfer of electricity between two circuits which are:

Answer 32

Completely insulated from each other.

Question 33

A transformer with more turns on the secondary than on the primary is called a/an:

Answer 33

Step-up transformer.

Question 34

When a transformer’s windings are connected delta:

Answer 34

The line current will be greater than the phase current.

Question 35

When a transformer’s windings are connected wye:

Answer 35

The line current will be the same as the phase current.

Question 36

In a transformer, both the primary to secondary voltage and turn ratio will vary inversely as the primary:

Answer 36

To secondary current ratio.