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Transformer
SONNY P. DE LEON
MT 1
Electromagnets and the Electric Motor
 Electromagnets are magnets that
are created when electric current
flows in a coil of wire.
 A simple electromagnet is a coil
of wire wrapped around a rod of
iron or steel.
 Because iron is magnetic, it
concentrates and amplifies the
magnetic field created by the
current in the coil.
The Principle of the Electric Motor
 An electric motor uses electromagnets to convert
electrical energy into mechanical energy.
Electric Motors

Electric motors are very common.

All types of electric motors have three key
components:
1. A rotating element (rotor) with magnets.
2. A stationary magnet that surrounds the rotor.
3. A commutator that switches the electromagnets
from north to south at the right place to keep the
rotor spinning.
Electric motors
 The rotating part of the
motor, including the
electromagnets, is called
the armature.
 This diagram shows a
small battery-powered
electric motor and what it
looks like inside with one
end of the motor case
removed.
Induction and the Electric Generator
Key Question:
How does a generator
produce electricity?
Induction and the Electric Generator
 If you move a magnet near a coil of wire, a
current will be produced.
 This process is called electromagnetic
induction, because a moving magnet induces
electric current to flow.
 Moving electric charge creates magnetism and
conversely, changing magnetic fields also can
cause electric charge to move.
Induction
 Current is only produced if
the magnet is moving
because a changing
magnetic field is what creates
current.
 If the magnetic field does not
change, such as when the
magnet is stationary, the
current is zero.
Induction
 If the magnetic field is increasing, the induced current
is in one direction.
 If the field is decreasing, the induced current is in the
opposite direction.
Magnetic flux
 A moving magnet
induces current in
a coil only if the
magnetic field of
the magnet
passes through
the coil.
Generators
 A generator is a device that uses induction to
convert mechanical energy into electrical energy.
Transformers
 The transformer uses
electromagnetic
induction, similar to a
generator.
Transformers
 The induced emf in the primary and secondary
coil is given by
EMF P = -Np Δ(BA) / Δt ( primary coil)
EMF S = -Ns Δ(BA) / Δt ( secondary coil)
 Transformer efficiency is the ratio of the power
output to the power input.
E = power output / power input
= Ve S Is / Ve P I p
 Step-up transformer – transformer that produces
a larger output voltage.
 Step-down transformer - gives a lower output
voltage.
Sample Problems
P1. The primary coil of a transformer has 150
turns. It is connected to a 120-V source.
Calculate the number of turns on the
secondary coil needed to supply these
voltages; (a) 625 V, (b) 35 V (c) 6.0V
 A step-up transformer has 80 turns on its primary
coil. It has 1200 turns on its secondary coil. The
primary circuit is supplied with an alternating
current at 120 V.
a. What voltage is across the secondary circuit?
b. The current in the secondary circuit is 2.0 A.
What current is in the primary circuit?
c. What are the power input and output of the
transformer?
Solve:
1. A friend being back from Europe a device that she
claims to be the world’s greatest coffee
maker.Unfortunately, it was designed to to operate
from 240 V line to obtain the 960 W of power that it
needs.
a. What can she do to operate it at 120-V?
b. What current will the coffee maker draw from 120-V
line?
2. An ac generator that delivers 20 A at 6000 V is connected
to a step-up transformer . What is the output current at
120 000 V if the trnsformer efficiency is 100%?
3. A step-up transformer has 400 secondary turns and only
100 primary turns. A 120V alternating voltage is connected
to primary coil.What is the output voltage?
Application: Trains that Float by
Magnetic Levitation