Real Transformer

Question 1

The real voltage ratio is almost the same as the ideal voltage ratio (at no load) and diverges due to voltage drops, increases in resistance, or decreases in current?

Answer 1

Voltage drops

Question 2

The real current ratio is almost the same as the ideal current ratio (at no load) and diverges as the load decreases due to voltage drops, increase in resistance, or exciting current becoming more signnificant?

Answer 2

Exciting current becomes more significant.

Question 3

Exciting current is located in ONLY the primary, ONLY the secondary, or BOTH windings?

Answer 3

Only the primary

Question 4

Exciting current will be present and will not change (remain stable) because the system is energized and the system voltage is unstable. True or false?

Answer 4

False. As long as the system is energized and the system voltage is stable the exciting current will be present.

Question 5

Exciting current is dependent of the load (True/False)

Answer 5

It is independent of the load.

Question 6

What are the 4 things that can be done to reduce the exciting current?

Answer 6

1. Increase permeability
2. Reduce the air gaps
3. Insulation on the lamination
4. A lower flux density

Question 7

With regards to reducing exciting current, increasing permeability results in what?

Answer 7

Higher quality core steel

Question 8

With regards to reducing exciting current, reducing air gaps can be done in 2 ways. What are they?

Answer 8

1. Better quality of construction
2. Interleaving

Question 9

Insulation on the lamination reduces eddy current or voltage in the core?

Answer 9

Eddy current

Question 10

A lower flux density results in a higher exciting current. True/False?

Answer 10

False. It results in a lower exciting current.

Question 11

Small transformers below 2000VA may have an Iex in the range of 25%, 5%, or 10%

Answer 11

10%

Question 12

Small transformers between 2000VA and 100kVA may have an Iex in the range of 15%, 10%, or 5%?

Answer 12

5%

Question 13

Larger transformers at 1000kVA, the Iex% is 1%, 100%, 50%, or 35%?

Answer 13

1%

Question 14

For very large transformers that are oil filled what is the percentage Iex?

Answer 14

0.1%

Question 15

A transformer with a 1:1 ratio is rated at 120V and 15A. What would the %Iex be?

Answer 15

S = V * I = 120 * 15 = 1800VA, therefore %IEX ≈ 10% (from above)
IEX = ( %IEX * IRated ) / 100 = 10 * 15 / 100 = 1.5A

Question 16

What would the real current ratio be?
A transformer with a 1:1 ratio is rated at 120V with a 15A resistive load. What would the % exciting
current be, approximately?

Answer 16

S = V * I = 120 * 15 = 1800VA, therefore %IEX ≈ 10%
IEX = ( %IEX * IRated ) / 100 = 10 * 15 / 100 = 1.5A
IPRI = I1 + IEX = √(152 + 1.52 ) = 15.075A
ISEC = 15.00A
Real ratio: a = ISEC / IPRI = 15.00 / 15.075 = 0.995
Ideal ratio: a = I2 / I1 = 15.0 / 15.0 = 1.000

Question 17

For system losses, there are losses in the generator. What are these 3 losses?

Answer 17

Magnetic
Electrical
Frictional

Question 18

Transmission line losses from the generator to the transformer are known as ________ losses

Answer 18

Electrical

Question 19

Transformer losses are made up of _____ and ______

Answer 19

1. Core loss
2. Winding loss

Question 20

Winding loss is also known as _______

Answer 20

Load losses

Question 21

Core loss is also known as _____

Answer 21

No load losses

Question 22

Line losses from the transformer to the load are electrical, magnetic, or mechanical?

Answer 22

Electrical

Question 23

Is it possible for the load to have losses?

Answer 23

Yes. There may be losses due to lighting, heat, motors, etc.

Question 24

If we have 120V rated primary with 22 Turns, what is the secondary voltage if there are 5 turns?

Answer 24

a = N1 / N2 = 22 / 5 = 4.4
V2 = V1 / a = 120 / 4.4 = 27.27V

Question 25

Why must HV and LV turns must be correct?

Answer 25

The correct voltage will not appear at the terminals

Question 26

The CSA standards for voltage is 10%, 0.6%, 0.2% or 0.5%?