Unit 30 DC Motors
Objectives:
• Discuss the operation of direct-current
motors.
• Discuss different types of DC motors.
• Review schematic diagrams of different
types of DC motors.
Unit 30 DC Motors
Construction
• A motor is a device used to convert
electrical energy into mechanical energy.
• A DC motor and a DC generator use the
same magnetic principles of attraction and
repulsion.
• The basic construction of both devices is
very similar.
Unit 30 DC Motors
Basic one-loop motor.
Unit 30 DC Motors
Construction
• The turning force or torque of a motor is
caused by the interaction of the magnetic
fields surrounding the wire loops and the
pole pieces.
• Torque is determined by two factors: the
magnetic strength of the pole pieces, and
the magnetic strength of the armature.
Unit 30 DC Motors
Torque is caused by interaction of
magnetic fields.
Unit 30 DC Motors
Construction
• The commutator is a mechanical rectifier
converting AC into DC.
• The commutator also provides pathways
and connections to the external circuit.
• DC motors can use schematics similar to
the ones used for DC generators.
Unit 30 DC Motors
Increasing the number of loops and turns
increases output torque.
Unit 30 DC Motors
Construction
• The armature is the rotating part of a DC
motor.
• Three classes of DC motors are:
1. shunt motors
2. series motors
3. compound motors
Unit 30 DC Motors
Shunt motor schematic.
Unit 30 DC Motors
Series motor schematic.
Unit 30 DC Motors
Compound motor schematics.
Unit 30 DC Motors
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•
•
Construction
Speed regulation is the amount that
speed decreases as mechanical load is
increased.
Speed regulation is proportional to the
resistance of the armature.
Lower armature resistance means better
speed regulation.
Unit 30 DC Motors
Comparison speed / current curves.
Unit 30 DC Motors
Comparison torque / current curves.
Unit 30 DC Motors
•
Direction of Rotation
The direction of rotation of a DC motor
can be reversed by changing the
connections of the armature leads or the
field leads.
Unit 30 DC Motors
Changing rotation of series motors.
Unit 30 DC Motors
Changing rotation of shunt motors.
Unit 30 DC Motors
Changing rotation of compound motors.
Unit 30 DC Motors
Reversing rotation of shunt motor with a switch.
Unit 30 DC Motors
Forward-reversing control schematic.
Unit 30 DC Motors
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•
Horsepower
James Watt determined that the average
horse could do work at a rate of
550 foot/pounds per second. This was
standardized as one horsepower.
Electrical power can be compared to
mechanical power using this formula:
1 horsepower = 746 watts
Unit 30 DC Motors
Horsepower
These are some more standard conversion
formulas:
• 1 watt = 3.42 BTUs per hour
• 1055 watts = 1 BTU per second
• 4.18 watts = 1 calorie per second
• 1.36 watts = 1 foot/pound per second
Unit 30 DC Motors
Motor Types
There are many different ways of designing
DC motors to enhance particular
characteristics.
• brushless DC motors
• permanent magnet motors
• servomotors and ServoDisc® motors
Unit 30 DC Motors
Brushless DC motor cutaway view.
Unit 30 DC Motors
Basic ServoDisc® motor.
Unit 30 DC Motors
Right-Hand Motor Rule
The fingers of the right hand can be used to
determine the direction of rotation of the
armature when the magnetic field polarity
of the pole pieces and the direction of
current flow through the armature are
known.
Unit 30 DC Motors
Right-hand motor rule.
Unit 30 DC Motors
Review:
1. A motor is a machine that converts
electrical energy into mechanical energy.
2. Direct-current motors operate on the
principle of attraction and repulsion or
magnetism.
3. DC motors can use schematics similar to
the ones used for DC generators.
Unit 30 DC Motors
Review:
4. Two factors that determine the torque
produced by a motor are:
a. the strength of the magnetic field of the
pole pieces.
b. the strength of the magnetic field of the
armature.
Unit 30 DC Motors
Review:
5. Three basic types of DC motors are the
series, shunt, and compound.
6. Brushless DC motors do not contain a
wound armature, commutator, or brushes.
Unit 30 DC Motors
Review:
7. When full voltage is applied to both the
armature and shunt field, the motor will
operate at base speed.
8. When full voltage is applied to the field
and reduced voltage is applied to the
armature, the motor will operate below
base speed.
Unit 30 DC Motors
Review:
9. When full voltage is applied to the
armature and reduced voltage is applied
to the shunt field, the motor will operate
above base speed.
10. The direction of rotation of a direct current
motor can be changed by reversing the
connection of either the armature or the
field leads.
Unit 30 DC Motors
Review:
11. It is common practice to reverse the
connection of the armature leads to
prevent changing a compound motor from
a cumulative to a differential compound.
12. A shunt field relay is used to disconnect
power to the armature if shunt field current
drops below a certain level.