Introduction to Transformers

Question 1

By what principle(s) is voltage induced in a single phase transformer?

Answer 1

Mutual induction.

Self induction.

Question 2

What is transformer action?

Answer 2

Mutual induction.

When changing current in one circuit causes voltage to be produced in another magnetically linked circuit.

Question 3

What is the primary coil of a transformer?

Answer 3

The coil which is connected to a source of voltage.

Question 4

If the turns on a secondary coil are increased, how is secondary voltage affected?

Answer 4

It would increase as well.

Question 5

Is the reluctance of iron high or low?

Answer 5

Low.

Iron has a high permeability and magnetic fields are easily established.

Question 6

Which lamination method has the least amount of reluctance?

Answer 6

A long iron sheet that is spiralled around the core.

Question 7

What are the three basic types of transformer construction? Which has the most leakage flux?

Answer 7

Core, Shell and H type.

Core type has the most leakage flux and is the least efficient.

Question 8

Where are H type transformers commonly found?

Answer 8

High voltage distribution.

Question 9

Which losses are roughly static? Which vary with loading?

Answer 9

Hysteresis losses, as they are proportional to frequency.

Since a transformer has a net flux which is nearly constant, eddy current loss should be static as well.

Copper losses, because the load changes the current which will change I²R.

Question 10

What is typically the efficiency range of transformers?

Answer 10

95-98%.

Question 11

The VA rating of a transformer is always the __________?

Answer 11

Output capacity.

Question 12

What properties affect %IZ of a transformer? How is it determined?

Answer 12

• Amount and type of core material.
• Winding wire size.
• # of winding turns.
• Degree of magnetic coupling.

A short circuit test is performed with a very low primary voltage. The voltage is increased until rated secondary current flows. Impedance voltage is a percentage of rated voltage, and that percentage is %IZ.

Question 13

What are the three main functions of transformers?

Answer 13

1. To step up Voltage.
2. To step down Voltage.
3. To Isolate a circuit from ground.

Question 14

The H markings on a transformer always refer to the ________winding.

Answer 14

High voltage.

Question 15

What will happen if a transformer is overvoltaged (>110% of rated V)?

Answer 15

Primary current will increase to dangerous levels and burn out the coil.

Question 16

Is the transformer turns ratio the same as the current ratio?

Answer 16

No, the turns ratio is the same as the voltage ratio, the current ratio is the inverse of those two.

Question 17

Which ratio is always 1:1 across the primary and secondary coils?

Answer 17

The volts/turn ratio.

Question 18

When looking at a transformer from the low voltage side ANSI standards dictate the H1 terminal goes where?

Answer 18

The upper left hand corner.

Question 19

In most transformers, which winding is wrapped around the other?

Answer 19

The high voltage winding is usually wrapped around the low voltage winding.

Question 20

How is relative polarity indicated in transformer windings?

Answer 20

It is indicated through subscript.

At any instant and odd numbers will have the same polarity, and any even numbers will have the same polarity.

Question 21

When H₁ is directly opposite X₁ the trasformer is said to have _________.

Answer 21

Subtractive polarity.

Question 22

Why would a primary voltage tap be used?

Answer 22

To maintain rated secondary voltage, It would be used if the supply voltage was higher or lower than rated primary voltage.

Question 23

When referring to taps what does RCBN mean?

Answer 23

Reduced capacity below normal.

Question 24

Would a transformer with full capacity taps be larger or smaller than one with reduced capacity taps?

Answer 24

Larger.

This is because extra heat would need to be dissipated.