WELCOME TO PERIOD 23
Homework Exercise #22 is due today.
CAUTION:
Keep your phones, tablets,
calculators, watches, etc. off of the tables.
PHYSICS 1103 – PERIOD 23
•How do alternating current electric motors
work?
•What is an induction motor?
•How can you make a buzzer?
•Put away your phones and electronics. The
large magnets will damage them.
Electric motors
Motors convert electrical energy into kinetic energy
using the forces between magnetic fields to spin a
rotor.
Motors require two magnetic fields. At least one
must be a changing magnetic field (the poles reverse)
A changing current is used to produce a changing
magnetic field.
St. Louis direct current motor
The rotor is an electromagnet. Permanent magnets provide
the second magnetic field.
A split ring commutator provides a changing current. As the
commutator rotates, the current reverses direction.
When the current reverses direction, the poles of the
electromagnet reverse.
Permanent
magnets
Commutator
Rotor coils (electromagnets)
Rotor
DC motor with slit ring commutator
Alternating current (AC) motors
Alternating Current (AC) reverses direction 120 times per
second.
When the current reverses direction, the poles of the
magnetic field around the current reverse.
The reversal of the magnetic poles produces the changing
magnetic field required in a motor.
This changing magnetic field can be used to spin the rotor
of an AC motor.
Synchronous AC motors
Make a synchronous motor from a solenoid and a
magnaprobe. The permanent magnet in the mangaprobe
acts as a rotor.
Synchronous motors use the current reversals of
alternating current to provide a changing magnetic field to
spin the rotor.
Thus, synchronous motors do not need a commutator to
reverse the current.
Since the rotors of synchronous motors spin in synch
with the changing AC current, synchronous motors have a
fixed rotational speed.
What is the major limitation of a synchronous motor?
What is a common use of a synchronous motor?
Universal: AC motor with a commutator
Universal motors are used when various motor speeds are
desired, as in an electric mixer.
A DC motor can be converted into an AC universal motor
by replacing the permanent magnets with electromagnets.
The amount of current through the electromagnets can be
varied to produce different rotational speeds.
The alternating current through each electromagnet
reverses direction at the same time. Therefore, the poles
of each electromagnet reverse at the same time.
A commutator is used to reverse the current direction in
the rotor coils at the proper time to spin the rotor.
Rotating disc watt hour meter
How does a watt hour meter measure current?
1) Current flowing through wires in the meter induces a
changing magnetic field.
2) This changing magnetic field induces a current in the
disc.
3) The current in the disc is surrounded by a second
magnetic field.
4) The two magnetic fields interact, causing the disc to
spin.
This is the basis of an induction motor!
How do induction motors work?
An induction motor uses magnetic fields from two
electromagnets. (No permanent magnets used.)
1) The changing AC current in one electromagnet induces a
changing magnetic field. This changing magnetic field
induces a changing current in the rotor’s shell.
2) The induced current in the rotor shell induces a second
magnetic field around the rotor.
3) The two magnetic fields repel, causing the rotor to spin.
A changing
magnetic field
induces a
current in the
rotor shell.
The induced
current in the
shell induces a
magnetic field in
the shell.
The two magnetic
fields repel,
causing the rotor
shell to spin.
Induction motor
• In this induction motor, the electromagnet is in the outer
shell. The rotor with an induced current and magnetic
field is in the center of the motor.
Building a buzzer
• Follow the directions in the activity sheet and refer to the
buzzer model on your table.
How does the buzzer work?
The motion of the
paper clip opens and
closes the circuit.
The second paper
clip is a conducting
pathway.
The steel paper clip is attracted
to the nail when the nail
becomes an electromagnet.
Wire stripped of
insulation provides
an electrical
connection to the
paper clip.
When current flows
through the wire, the
nail becomes an
electromagnet.
BEFORE THE NEXT CLASS…
Read textbook chapter 24
Complete Homework Exercise 23
Bring a blank Activity Sheet 24 to class.