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Flashcards in Transformer Operation 1 Φ Deck (20)
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What is transformer action?

Electromagnetic (mutual) Induction.


Why is an equivalent circuit of a transformer useful?

It allows us to see clearly, the volt drops which affect a fully loaded transformer's output.


Is current flowing in an open secondary transformer, with it's primary connected to source?

Yes, there will be a small exciting current.


Does secondary current flux enhance the exciting flux?

No, it opposes exciting flux.


What property is the net flux of a loaded transformer most closely related to?

The exciting flux.


What is Voltage regulation?

The change in a transformer's output voltage from no load to full load.


Who is under the table?

The fully loaded guy.


How are core losses of a transformer determined?

An open circuit test, using a wattmeter in the primary.

I2R losses can be ignored since exciting current is so low.


Do core losses increase as load is added to a transformer?

No, they are (roughly) constant with a consistent applied voltage.


Do copper losses vary with load?

Yes, they are proportional to the square of the current.


How can Full load copper losses be determined?

With a short circuit test, and a wattmeter in the primary.

Applied V is low so core losses are minimal and may be ignored.


What are the copper losses at one half load?

They are one quarter of full load losses.


How is efficiency in a transformer calculated?

It is the ratio of useful power output to power input.


Which factors affect a transformer's efficiency?

  • Load capacity. As load is decreased, efficiency is as well
  • Power factor. As pf increases, efficiency does as well.
  • Losses. As losses increase, efficiency decreases.


What is impedance voltage?

The primary voltage required to circulate rated secondary current in a short circuited winding.


How is available fault current calculated?

It will be the full load secondary current divided by the percent imdedance.


Why would transformers be paralleled?

To increase the capacity and reliability of a system.


Which precaustions should be observed when paralleling transformers?


VS must be equal. (Unnecessary heating);

Same tap settings. (Different VS);

%IZ within 5% of eachother. (Disproportionate loading);

Must be connected in phase [observe polarity]. (Fault condition).


What is a backfeed?

When one transformer coil induces voltage onto a paralleled, presumably "disconnected" coil.

To avoid this, always ensure the transformer is disconnected from all circuitry.


If a transformer has 1 coil on the primary and 2 coils on the secondary and it is feeding a 120/240 V load, how is it connected?

The secondary coils are in series, with a tapped neutral at the mid point. 

The max load allowable is a series load.