Transformers

Question 1

What is a transformer?

Answer 1

It is a magnetically operated machine that can change values of voltage, current, and impedance without changing frequency.

Question 2

What are the three classifications of transformers?

Answer 2

Isolation transformer, autotransformer, and current transformer

Question 3

All values of a transformer are proportional to its ________

Answer 3

Turn ratio

Question 4

How do you calculate the turns ratio of a transformer?

Answer 4

Divide the number of turns of the primary wire by the number of turns of the secondary wire

Question 5

What is primary winding?

Answer 5

The winding that is connected to the incoming power supply

Question 5

What is secondary wiring?

Answer 5

It is the winding that is connected to the driven load.

Question 6

What are isolation transformers?

Answer 6

Transformers that have secondary wiring that is physically and electrically separated from primary wiring.

Question 7

Why is isolation desirable in a transformer?

Answer 7

It reduces voltage spikes that originate on the supply side.

Question 8

What is the excitation current?

Answer 8

The amount of current required to magnetize the core of a transformer. It remains constant from no load to full load.

Question 9

How is mutual induction formed?

Answer 9

The magnetic field produced by the primary winding cuts the windings of the secondary which then produces a voltage.

Question 9

What is mutual induction?

Answer 9

The ability of one coil to induce a voltage in another coil.

Question 10

The amount of voltage induced in the secondary is determined by _________

Answer 10

The ratio of the number of turns of wire in the secondary to those in the primary.

Question 10

Multiple tapped windings

Answer 10

Isolation transformers that have more than one set of lead wires connected to the primary and secondary.

Question 10

What is a distribution transformer?

Answer 10

Type of transformer that changes the high voltage of a power company to the standard 240/120 volts that runs most homes and businesses.

Question 11

If you measured the voltage across the entire secondary of a 240/120-volt line, what would the voltage be? What would it be if you measured each line to the center tap?

Answer 11

240 volts. 120 volts.

Question 12

What is a control transformer?

Answer 12

A transformer that is used to reduce the line voltage to the value needed to operate control circuits.

Question 13

The most common type of control circuit contains ________ and __________.

Answer 13

Two primary windings and one secondary winding

Question 14

Why are laminated cores preferred in transformers?

Answer 14

A thin layer of oxide forms on the surface of each lamination which insulates the core and reduces the formation of eddy currents.

Question 15

The magnetic coupling between the windings and the efficiency of the transformer is determined by the ________ and _______ of the core

Answer 15

Type and shape

Question 16

How are the windings arranged in a core-type transformer?

Answer 16

Windings are placed around each end of the core material. Low voltage winding is closest to the core and high voltage winding is over the low voltage winding.

Question 17

How are the windings arranged in a shell-type transformer?

Answer 17

Primary and secondary windings are wound around a centerpiece of the core (there is a window in the core).

Question 18

How are the windings arranged in a H-type transformer?

Answer 18

Similar to shell-type core in which windings are wound around a centerpiece; however, the core surrounds the windings on four sides instead of two.

Question 19

How are the windings arranged in a tape-wound core transformer?

Answer 19

Winding is one long continuous steel tape that is wound into a spiral. Flux leakage is kept to a minimum with this design.

Question 20

What is inrush current?

Answer 20

The amount of current that flows when power is initially applied to the circuit.

Question 21

What is the difference between reactors and transformers when it comes to inrush current?

Answer 21

Reactors are used to prevent inrush currents while transformers produce high inrush currents.

Question 22

What are autotransformers?

Answer 22

Transformers with one winding

Question 23

What are the pros and cons for autotransformers?

Answer 23

Pros: High efficiency and cheaper than an isolation transformer

Cons: Does not provide line isolation

Question 24

How do you determine volts per turn of a transformer?

Answer 24

Divide the applied voltage by the number of turns of the primary winding.

Question 25

How can you determine the secondary voltage if the volts per turn is known?

Answer 25

Multiply the number of secondary turns by volts per turn

Question 26

What is a Variac or Powerstat transformer?

Answer 26

A transformer with a sliding tap that allows the turns ratio to be adjusted.

Question 27

How is polarity of a transformer indicated on a schematic?

Answer 27

Dots on the end of a winding

Question 28

How can the polarity of transformer windings be determined?

Answer 28

By connecting one lead of the secondary to one lead of the primary and measuring the voltage across both windings.

If the voltmeter indicates a sum of the two voltages, the windings are connected in additive (boost). If the voltmeter indicates a difference between the two voltages, the windings are connected subtractive (buck).

Question 29

What are the eight things that are required on a transformer nameplate?

Answer 29

1. Name of a manufacturer
2. Rated Kilovolt ampere
3. Frequency
4. Primary and secondary voltage
5. Impedance of transformers rated 25 kVA and larger
6. Required clearances of transformers with ventilating openings
7. Amount and kind of insulating liquid when used
8. The temperature class of the insulating system of dry-tape transformers

Question 30

What factors determine transformer impedance?

Answer 30

• Amount/type of core material
• Wire size used to construct windings
• Number of turns
• Degree of magnetic coupling between windings

Question 31

How is impedance expressed and measured?

Answer 31

It is expressed as a percentage. It is measured by connecting a short circuit across the low-voltage winding and connecting a variable voltage source to the high-voltage winding. The impedance is determined by calculating the variable voltage as compared to the rated voltage of the high-voltage winding.

Question 32

What is a constant current transformer?

Answer 32

They are a special type of isolation transformer that delivers a constant output current under varying load conditions.