The primary is usually referred to as the input; the secondary as the output.
PRINCIPLE OF OPERATION
An alternating current applied to the primary winding induces a alternating
magnetic flux in the iron core. Most of this flux stays with in the core
and only a small percentage of it travels through the air. The alternating
magnetic flux in the iron core then links the turns of the secondary
windings inducing a voltage. This all follows from Faraday's law of
induction. This explains why the primary has a voltage, and the secondary
has a voltage, yet there is no interconnection between them.
WHY WE USE TRANSFORMERS
TO DISTRIBUTE POWER AT HIGH VOLTAGE.
It is more cost effective to distribute power at higher voltages since
power dissipation (loss) in a resistive load is given by the square of the
current times the resistance of the wire. It is best to use the lowest
possible current and thus the largest potential difference (voltage). A
typical transformer will take in input of 480 or 600 volts and step the
voltage down to 240 volts for some motors, or 120 volts for other
applications like consumer products, lights, etc.. The overall result is
better voltage regulation, minimized line loss and reduced wiring costs.
TO ELIMINATE DOUBLE WIRING.
For maximum safety, 120 volt lighting and control circuits may be obtained from 240, 480, or 600 volt power circuits by installing transformers at the most convenient location to the load. This eliminates separate circuits and independent metering for power and often results in substantial savings.
Installing transformers provides a means of subdividing circuits to
accommodate independent demand. Connecting to a 3 phase, 480 volt circuit a
transformer can provide
TO INSULATE CIRCUITS.
Installing transformers provides a means of subdividing circuits to accommodate independent demand. Connecting to a 3 phase, 480 volt circuit a transformer can provide