Transformer Operation

Question 1

Describe a transformer operating at no load

Answer 1

1. Exciting current flows in the primary
2. Exciting current produces a magnetic flux in the core
3. Exciting flux follows the core and cuts across both the primary and secondary windings
4. Voltages are induced in both the primary and secondary windings
5. The voltage induced in the primary opposes source voltage
   6.magnitude of primary induced voltage is less than the primary supply voltage
6. Because primary supply voltage is greater than the primary induced an exciting current continues to flow under no load conditions
7. Main component that restricts current in the primary winding is the primary induced voltage

Question 2

Describe a transformer operating with load

Answer 2

1. Secondary current flows because of the induced voltage in the secondary winding and the connection to the load
2. Current in the secondary winding produces a magnetic flux
3. Secondary current flux opposes exciting flux
4. The difference between the secondary flux and the exciting flux is now the resultant flux
5. The lower value of resultant flux produces a new induced voltage that is less than what it was under no load
   6.primary current increases
6. The increase in primary current produces a third flux, called primary current flux
7. Primary current flux and secondary current flux are in opposite directions so they basically cancel each other out

Question 3

What are the 2 losses that make up core losses

Answer 3

1. Hysteresis losses - when the magnetic domains in the core are forced to flip back and forth
2. Eddy currents - circulating current to in the iron core itself due to voltage induced by the changing flux

Question 4

How can you determine losses in the core

Answer 4

Open circuit test

Question 5

How can you determine the copper losses in a transformer

Answer 5

Short circuit test

Question 6

What is impedance voltage

Answer 6

1. The primary voltage required to circulate the rated full load secondary current in a transformer

Question 7

What is the benefit of paralleling single phase transformers

Answer 7

1. Increase capacity of the system
2. Improve system reliability

Question 8

What are some possible hazards of paralleling single phase transformers

Answer 8

1. Increases available short circuit current on the secondary
2. Impedance drops