Download Unit 31 Direct Current (DC) Motors

Unit 31 DC Motors
Unit 31
DC Motors
Basic one-loop motor.
Unit 31 DC Motors
Unit 31 DC Motors
Objectives:
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Discuss the operation of direct-current motors.
Discuss different types of DC motors.
Review schematic diagrams of different types
of DC motors.
Unit 31 DC Motors
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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 31 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.
Torque is caused by interaction of magnetic fields.
Unit 31 DC Motors
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Unit 31 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.
Shunt motor schematic.
Unit 31 DC Motors
Increasing the number of loops and turns
increases output torque.
Unit 31 DC Motors
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Unit 31 DC Motors
Series motor schematic.
Unit 31 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
Compound motor schematics.
Unit 31 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.
Unit 31 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.
Lower armature resistance means better speed
regulation.
Unit 31 DC Motors
Comparison speed / current curves.
Unit 31 DC Motors
Comparison torque / current curves.
Unit 31 DC Motors
Changing rotation of series motors.
Unit 31 DC Motors
Changing rotation of shunt motors.
Unit 31 DC Motors
Unit 31 DC Motors
Horsepower
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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
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Changing rotation of compound motors.
Unit 31 DC Motors
Unit 31 DC Motors
Horsepower
These are some more standard conversion
formulas:
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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
Reversing rotation of shunt motor with a switch.
Unit 31 DC Motors
Unit 31 DC Motors
Motor Types
There are many different ways of designing
DC motors to enhance particular
characteristics.
Forward-reversing control schematic.
– brushless DC motors
– permanent magnet motors
– servomotors and ServoDisc® motors
Unit 31 DC Motors
Unit 31 DC Motors
Right-hand motor rule.
Brushless DC motor cutaway view.
Unit 31 DC Motors
Unit 31 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.
Basic ServoDisc® motor.
Unit 31 DC Motors
Unit 31 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.
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 31 DC Motors
Unit 31 DC Motors
Review:
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 31 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 31 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.
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.