Transcript DC Motors

CHAPTER
8
DC Motors
Instructor Name: (Your Name)
Copyright © 2014 Delmar, Cengage Learning
Learning Objectives
• List the components of a typical starting
(cranking) motor
• Describe how interacting magnetic fields
cause the armature in an electric motor to
rotate
• Explain why a starter motor draws less
current as motor speed increases
• List the advantages and disadvantages of
a gear reduction starter motor
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Learning Objectives
• Measure cranking circuit resistance using
the voltage drop method
• Troubleshoot the cause of a no-crank
problem
• Disassemble a starter motor, test the
internal components, and reassemble
• Perform a rapid assessment of a trucks
electrical system
• Explain how rotational direction is
reversed with a permanent magnet motor
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Introduction
• Electric motors are used extensively on
modern trucks, windshield wipers, heating
and A/C, some hydraulic ABS systems
• The starting or cranking motor is the largest
• Electric motors convert electrical energy into
mechanical energy
• Almost all motors used on trucks uses
brushes to contact the rotating elements,
hence the name brushed DC motors
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Important Facts
Two magnetic fields that interact with
each other combine to form a single
magnetic field. If the arrows on the
magnetic lines of force of both magnetic
fields are pointing in the same direction,
the resulting magnetic field is
strengthened. If the arrows on the
magnetic lines of force are pointing in
opposite direction, the resulting magnetic
field is weakened.
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Interaction of Current Carrying Conductor in
a Magnetic Field
Figure 8-3 (A) Current-carrying
conductor placed in magnetic
field causes an interaction
between magnetic fields;
conductor is compelled to move
from strong magnetic field to
weak field. (B) Current-carrying
conductor formed into a loop is
compelled to rotate around its
axis to move from strong field to
weak field.
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Important Facts
Conductors that are carrying current are
compelled (want) to move out of a
stronger magnetic field into a weaker
magnetic field.
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Components of a Simple Electric Motor
• Armature – The conductive loop that
rotates inside an electric motor
• Split Ring Commutator – Provides
connection to both ends of the armature
loop through brushes and allow it to rotate.
• Pole Shoes – Electromagnets that
surround the armature.
• Field Coils – Copper wire wrapped around
pole shoes that create the electromagnet
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Brushed DC Motor
Figure 8-4 Brushed DC
motor; current flow
through armature reverses
directions every 180
degrees of rotation.
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Magnetic Field Developed By Pole Shoes
Figure 8-5 Magnetic field developed by pole shoes.
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Armature Windings and
Commutator Segments
Figure 8-6 Armature windings and commutator segments.
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Cutaway View of Starter Motor
Figure 8-8 Cutaway view of a starter motor.
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Four Insulated Field Coils with Brushes
Figure 8-11 Four insulated
field coils with brushes.
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Pole Shoes and Field Coil Inside
Iron Housing
Figure 8-12 Pole shoes and field coils installed in iron housing.
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Interaction of Magnetic Fields
Results in Armature Rotation
Figure 8-13
Interaction of
magnetic
fields results
in armature
rotation.
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Series Wound Motor
Figure 8-14
Series-wound
motor.
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Shunt Wound Motor
Figure 8-15 Shuntwound motor.
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Compound Wound Motor
Figure 8-16
Compound motor.
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Starter Drive Components
• Pinion Gear – Small diameter gear that
acts as the starter output gear
• Ring Gear – Part of the engine fly wheel,
pinion gear engages the ring gear to rotate
the engine
• One Way or Over Riding Clutch –
Prevents destruction of the armature due
to rapid acceleration by ring gear
• Solenoid – An electromechanical device
used to engage the pinion gear to the ring
gear
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Solenoid
Figure 8-17 Solenoid with coil not energized.
Figure 8-18 Solenoid with coil energized.
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Shift Lever Type Drive
Figure 8-19 Shift-lever-type drive.
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Drive Mechanism
Figure 8-20
Drive mechanism.
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Roller Clutch Permits One-Way Drive
Figure 8-21
Roller clutch permits
one-way drive.
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Crank Inhibit Circuit
Figure 8-22 Crank
inhibit circuit.
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Gear Reduction Starter Motor
Cut-Away
Figure 8-23 Gear-reduction starter motor cutaway.
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Testing Cranking System Resistance
• Connect carbon pile resistor across starter
B+ and ground terminal
• Connect DMM across battery terminals
• Briefly load carbon pile to 500A, note
battery terminal voltage
• Connect DMM across the starter B+ and
ground terminal. Do not connect to carbon
pile clamps.
• Briefly load carbon pile to 500A, note
starter terminal voltage
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Testing Cranking System
Resistance (continued)
• Subtract the loaded starter terminal
voltage from the loaded battery terminal
voltage. The result is the amount of
voltage that is dropped on the positive and
negative cranking circuit battery cables.
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Measuring Cranking System Resistance
Figure 8-24
Measuring
cranking circuit
resistance by
loading to
500A and
measuring
voltage drop
on circuit.
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Determining the Source of Excessive
Voltage Drop in a Cranking Circuit
• Connect carbon pile resistor across starter
motor B+ and ground terminal
• Connect DMM across battery positive and
starter positive terminals. Do not connect
to carbon pile clamps.
• Briefly load carbon pile to 500A and note
positive circuit voltage drop
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Determining the Source of Excessive
Voltage Drop in a Cranking Circuit
(continued)
• Connect DMM across battery negative and
starter negative terminals. Do not connect
to carbon pile clamps.
• Briefly load carbon pile to 500A and note
negative circuit voltage drop
• The positive and negative circuit voltage
drops should each be about half the
maximum allowable voltage drop or 0.25V
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Finding Source of High Cranking
System Resistance
Figure 8-25 Finding
the source of the
high cranking
circuit resistance.
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Basic Electrical/Electronic
Diagnostic Procedure Flowchart
Figure 8-29
Diagnostic
flowchart.
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Cranking Circuit Diagram
Figure 8-30
Cranking circuit
diagram.
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DMM Measuring High and Low Side of
Magnetic Switch During Cranking
Figure 8-31
DMM measuring
high and low
side of magnetic
switch coil
during crank.
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Voltage Measurements at Neutral Start Switch
Figure 8-32 Voltage
measurements at
neutral start switch.
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Starter No-Load Bench-Test Setup
Figure 8-33
Starter no-load
bench-test setup.
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Testing For Open Field Coils
Figure 8-35
Testing for open
field coils.
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Testing For Shorted-to-Ground Field Coils
Figure 8-36
Testing for a shorted-toground field coil.
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Testing Armature For Shorts-to-Ground
Figure 8-39 Testing armature for
shorted-to-ground windings.
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Testing Armature For Open Circuits
Figure 8-40 Testing
armature for open circuits.
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Energizing the Starter to Measuring
Pinion Clearance
Figure 8-42 Energizing the
starter motor solenoid to
measure pinion gear
clearance.
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View Looking into a Single Loop Armature
Figure 8-46 View looking into a single-loop armature.
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Armature Rotating Due to Magnetic
Field Interaction and Commutation
Figure 8-47 Armature rotating due to magnetic field interactions and commutation.
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CEMF in Stationary Motor and Motor
at Full Speed
Figure 8-48 CEMF
with motor
stationary (top)
and motor
rotating at full
speed (bottom)
and the effect on
current drawn by
the motor.
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Starter Solenoid Pull-In and
Hold-In Windings
Figure 8-50 Starter
solenoid hold-in and
pull-in windings.
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Summary
• Electric motors convert electric energy into
mechanical energy. Most electric motors
used on trucks are a brushed DC-type motor.
• A brushed DC motor has spring loaded
brushes that make contact with the
commutator segments. The commutator
segments are attached to loops of wire that
make up the armature assembly. The
armature is the rotating component of the
starter.
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Summary (continued)
• The pole shoes are the stationary
electromagnets bolted to the motor frame.
Field coils surround the pole shoes.
Current flow through the field coils causes
the pole shoes to be magnetized. This set
up a stationary magnetic field. The
stationary magnetic field interacts with the
magnetic field surrounding the armature
windings. The interaction causes areas of
weak and strong magnetic fields inside the
motor. The armature rotates to escape the
strong fields.
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Summary (continued)
• The commutation process describes the
reversal of current flow through the
armature winding at just the right time to
keep the armature in a location of strong
magnetic field. The current reversal
causes the armature to continually rotate
in an attempt to escape the strong
magnetic fields.
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Summary (continued)
• Counter-electromagnetic force (CEMF) is the
voltage that is induced in the armature
windings as they pass through the magnetic
fields set up by the pole shoes. The CEMF
acts as a series-opposing voltage to the
battery voltage. The CEMF increases as the
motor speed increases. This causes the
current drawn by the starter to decrease as
the motor speed increases. The highest level
of current draw is when the starter motor is
stationary.
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Summary (continued)
• The starter motor assembly contains the
pinion gear. The drive assembly assembly
causes the pinion gear to be meshed with
the engine ring gear when the motor
solenoid is energized. The drive assembly
contains a one-way clutch that permits the
starter motor to drive the engine but
prevent the engine from driving the starter
motor.
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Summary (continued)
• A positive engagement starter motor is
designed not to rotate until the pinion gear
is in full mesh with the ring gear. This
reduces the likelihood of ring gear milling.
• The starter motor causes the drive with
pinion gear to slide into mesh with ring
gear and also causes the high current
contacts for the starter motor to close.
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Summary (continued)
• Cranking circuit resistance is determined
by causing a steady known amount of
current flow through the battery cables
using a carbon pile resistor. The voltage
dropped on the cables with the known
current flowing is used to determine if
cranking circuit resistance is acceptable.
Low cranking circuit resistance is vital for
proper engine cranking speed.
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Summary (continued)
• Many smaller motors found on trucks are
permanent magnet motors. The term
permanent magnet refers to the pole
shoes, which are constructed of material
that has been magnetized. The direction of
these motors can be reversed by changing
the direction of current flow through the
armature windings through motor voltage
polarity reversal.
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