Instructor Name Your Name 8 CHAPTER Learning Objectives List the components of a typical starting cranking motor Describe how interacting magnetic fields cause the armature in an electric motor to ID: 230476
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DC Motors
Instructor Name: (Your Name)
8
CHAPTERSlide2
Learning Objectives
List the components of a typical starting (cranking) motorDescribe how interacting magnetic fields cause the armature in an electric motor to rotateExplain why a starter motor draws less current as motor speed increasesList the advantages and disadvantages of a gear reduction starter motorSlide3
Learning Objectives
Measure cranking circuit resistance using the voltage drop methodTroubleshoot the cause of a no-crank problemDisassemble a starter motor, test the internal components, and reassemblePerform a rapid assessment of a trucks electrical systemExplain how rotational direction is reversed with a permanent magnet motorSlide4
IntroductionElectric motors are used extensively on modern trucks, windshield wipers, heating and A/C, some hydraulic ABS
systemsThe starting or cranking motor is the largestElectric motors convert electrical energy into mechanical energyAlmost all motors used on trucks uses brushes to contact the rotating elements, hence the name brushed DC motorsSlide5
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. Slide6
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.Slide7
Important Facts
Conductors that are carrying current are compelled (want) to move out of a stronger magnetic field into a weaker magnetic field. Slide8
Components of a Simple Electric Motor
Armature – The conductive loop that rotates inside an electric motorSplit 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 electromagnetSlide9
Brushed DC Motor
Figure 8-4 Brushed DC motor; current flow through armature reverses directions every 180 degrees of rotation.Slide10
Magnetic Field Developed By Pole Shoes
Figure 8-5 Magnetic field developed by pole shoes.Slide11
Armature Windings and Commutator Segments
Figure 8-6 Armature windings and commutator segments.Slide12
Cutaway View of Starter Motor
Figure 8-8 Cutaway view of a starter motor.Slide13
Four Insulated Field Coils with Brushes
Figure 8-11 Four insulated field coils with brushes.Slide14
Pole Shoes and Field Coil Inside Iron Housing
Figure 8-12 Pole shoes and field coils installed in iron housing.Slide15
Interaction of Magnetic Fields Results in Armature Rotation
Figure 8-13 Interaction of magnetic fields results in armature rotation.Slide16
Series Wound Motor
Figure 8-14 Series-wound motor.Slide17
Shunt Wound Motor
Figure 8-15 Shunt-wound motor.Slide18
Compound Wound Motor
Figure 8-16 Compound motor.Slide19
Starter Drive Components
Pinion Gear – Small diameter gear that acts as the starter output gearRing Gear – Part of the engine fly wheel, pinion gear engages the ring gear to rotate the engineOne Way or Over Riding Clutch – Prevents destruction of the armature due to rapid acceleration by ring gearSolenoid – An electromechanical device used to engage the pinion gear to the ring
gearSlide20
Figure 8-17
Solenoid with coil not energized.
Solenoid
Figure 8-18 Solenoid with coil energized.Slide21
Shift Lever Type Drive
Figure 8-19 Shift-lever-type drive.Slide22
Drive Mechanism
Figure 8-20 Drive mechanism.Slide23
Roller Clutch Permits One-Way Drive
Figure 8-21 Roller clutch permits one-way drive.Slide24
Crank Inhibit Circuit
Figure 8-22 Crank inhibit circuit.Slide25
Gear Reduction Starter Motor Cut-Away
Figure 8-23 Gear-reduction starter motor cutaway.Slide26
Testing Cranking System Resistance
Connect carbon pile resistor across starter B+ and ground terminalConnect DMM across battery terminalsBriefly load carbon pile to 500A, note battery terminal voltageConnect DMM across the starter B+ and ground terminal. Do not connect to carbon pile clamps.Briefly load carbon pile to 500A, note starter terminal voltageSlide27
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.Slide28
Measuring Cranking System Resistance
Figure 8-24 Measuring cranking circuit resistance by loading to 500A and measuring voltage drop on circuit.Slide29
Determining the Source of Excessive Voltage Drop in a Cranking Circuit
Connect carbon pile resistor across starter motor B+ and ground terminalConnect 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 dropSlide30
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 dropThe positive and negative circuit voltage drops should each be about half the maximum allowable voltage drop or 0.25VSlide31
Finding Source of High Cranking System Resistance
Figure 8-25 Finding the source of the high cranking circuit resistance.Slide32
Basic Electrical/Electronic Diagnostic Procedure Flowchart
Figure 8-29 Diagnostic flowchart.Slide33
Cranking Circuit Diagram
Figure 8-30 Cranking circuit diagram.Slide34
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.Slide35
Voltage Measurements at Neutral Start Switch
Figure 8-32 Voltage measurements at neutral start switch.Slide36
Starter No-Load Bench-Test Setup
Figure 8-33 Starter no-load bench-test setup.Slide37
Testing For Open Field Coils
Figure 8-35 Testing for open field coils.Slide38
Testing For Shorted-to-Ground Field Coils
Figure 8-36 Testing for a shorted-to-ground field coil.Slide39
Testing Armature For Shorts-to-Ground
Figure 8-39 Testing armature for shorted-to-ground windings.Slide40
Testing Armature For Open Circuits
Figure 8-40 Testing armature for open circuits.Slide41
Energizing the Starter to Measuring Pinion Clearance
Figure 8-42 Energizing the starter motor solenoid to measure pinion gear clearance.Slide42
View Looking into a Single Loop Armature
Figure 8-46 View looking into a single-loop armature.Slide43
Armature Rotating Due to Magnetic Field Interaction and Commutation
Figure 8-47 Armature rotating due to magnetic field interactions and commutation.Slide44
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.Slide45
Starter Solenoid Pull-In and Hold-In Windings
Figure 8-50 Starter solenoid hold-in and pull-in windings.Slide46
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.Slide47
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.Slide48
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.Slide49
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.Slide50
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. Slide51
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.Slide52
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.Slide53
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.