Halderman ch047 lecture

MAGNETISM AND ELECTROMAGNETISM 47

Objectives The student should be able to: Prepare for ASE Electrical/Electronic Systems (A6) certification test content area “A” (General Electrical/Electronic Systems). Explain magnetism. Describe how magnetism and voltage are related. Describe how an ignition coil works. Explain how an electromagnet works.

Fundamentals of Magnetism Definition Magnetism: a form of energy caused by motion of electrons in some materials Recognized by attraction exerted on other materials

Fundamentals of Magnetism Definition Magnetism cannot be seen Magnetite is naturally occurring magnet

Figure 47-1 A freely suspended natural magnet (lodestone) will point toward the magnetic north pole.

Fundamentals of Magnetism Many materials can be artificially magnetized. Soft iron is easily magnetized.

Fundamentals of Magnetism Lines of Force Lines of force around magnet caused by way in which atoms are aligned in magnet

Fundamentals of Magnetism Lines of Force Magnetic force does not flow like electrical current flows Magnetic force lines come out north end and enter south end

Figure 47-2 If a magnet breaks or is cracked, it becomes two weaker magnets.

Figure 47-3 Magnetic lines of force leave the north pole and return to the south pole of a bar magnet.

Fundamentals of Magnetism Lines of Force Opposite ends of magnet are north and south poles. Magnetic poles related to Earth’s North Pole and South Pole. The more lines of force, the stronger the magnet.

Fundamentals of Magnetism Lines of Force Magnetic lines of force also called magnetic flux or flux lines. Magnetic field, lines of force, flux, and flux lines are interchangeable. Flux density refers to number of flux lines.

Fundamentals of Magnetism Lines of Force Magnetic field measured by Gauss gauge. Magnetic lines can be seen by spreading fine iron filings on paper laid on top of magnet. Compass is thin magnetized iron needle on pivot.

Figure 47-4 Iron filings and a compass can be used to observe the magnetic lines of force.

Fundamentals of Magnetism Magnetic Induction Iron or steel placed in magnetic field becomes magnetized. Process called magnetic induction. Metal removed from magnetic field retains residual magnetism.

Fundamentals of Magnetism Attracting or Repelling Magnetic force exits from north pole and enters south pole Force lines equal at each pole

Fundamentals of Magnetism Attracting or Repelling Flux lines concentrated at poles Magnetic force strongest at poles

Fundamentals of Magnetism Attracting or Repelling Opposite poles of two magnets draw magnets together Like poles of two magnets push magnets apart

Figure 47-5 Magnetic poles behave like electrically charged particles—unlike poles attract and like poles repel.

Fundamentals of Magnetism Permeability Flux lines cannot be insulated. No known material blocks magnetic force. Some materials allow magnetic force to pass through more easily than others.

Fundamentals of Magnetism Permeability Degree of passage is permeability. Example: reluctor wheel in magnetic-type camshaft position and crankshaft position sensors. Increase and decrease of force creates AC voltage signal.

Figure 47-6 A crankshaft position sensor and reluctor (notched wheel).

Fundamentals of Magnetism Reluctance Certain materials resist passage of magnetic force Compares to resistance in electrical circuit

Fundamentals of Magnetism Reluctance Air doesn’t allow easy passage; it has high reluctance Flux lines concentrate in permeable materials and avoid materials with high reluctance Magnetic force follows path of least resistance

Electromagnetism Definition Current-carrying conductors are surrounded by magnetic field These fields can be many times stronger than in conventional magnets

Electromagnetism Definition Magnetic field around conductor can be controlled by changing current Higher current, more flux lines Lower current, flux lines contract

Electromagnetism Definition Interaction and relationship between magnetism and electricity is electromagnetism

Electromagnetism Creating an Electromagnet Wrap nail in 20 turns of insulated wire. Connect wire to 1.5 volt dry cell battery. Nail becomes magnet.

Electromagnetism Straight Conductor Magnetic field around current-carrying conductor has several concentric cylinders of flux. Amount of amperes determines number of flux lines (cylinders). Amperes also determine how far out flux lines extend.

Figure 47-7 A magnetic field surrounds a straight, current-carrying conductor.

Electromagnetism Left-Hand and Right-Hand Rules Magnetic flux cylinders have directions like magnets Left-hand rule identifies direction

Electromagnetism Left-Hand and Right-Hand Rules Grasp conductor with left hand with thumb pointing in direction of electron flow (– to +) Your fingers curl around wire in direction of magnetic flux lines

Figure 47-8 The left-hand rule for magnetic field direction is used with the electron flow theory.

Electromagnetism Left-Hand and Right-Hand Rules Automotive circuits use conventional theory of current (– to +). Right-hand rule determines direction of magnetic flux lines.

Figure 47-9 The right-hand rule for magnetic field direction is used with the conventional theory of electron flow.

Electromagnetism Field Interaction Cylinders of flux surrounding current-carrying conductors interact with other magnetic fields

Electromagnetism Field Interaction Symbols in illustrations: + means current moves inward (•) indicates current moves outward

Electromagnetism Field Interaction If two conductors carry current in opposite directions; magnetic fields carry current in opposite directions

Electromagnetism Field Interaction Current-carrying conductors move from strong field to weak field The conductors move away from each other

Figure 47-10 Conductors with opposing magnetic fields will move apart into weaker fields.

Electromagnetism Field Interaction Placed side-by-side, opposing flux lines create strong magnet If conductors carry current in same direction, flux lines cancel each other Conductors move toward each other

Electromagnetism Motor Principle Electric motors use magnetic field interaction to convert electrical energy into mechanical energy

Electromagnetism Motor Principle Two conductors carrying current in opposite directions placed between strong north and south poles

Electromagnetism Motor Principle Magnetic field of conductor interacts with magnetic fields of poles These forces cause center of the motor to turn clockwise

Figure 47-11 Electric motors use the interaction of magnetic fields to produce mechanical energy.

Electromagnetism Coil Conductor Wire looped into coil strengthens magnetic flux density. Flux lines around coil same as flux lines around bar magnet.

Figure 47-12 The magnetic lines of flux surrounding a coil look similar to those surrounding a bar magnet.

Electromagnetism Coil Conductor Flux lines from coil exit from north pole and enter at south pole. Use left-hand rule to determine north pole of coil.

Figure 47-13 The left-hand rule for coils is shown.

Electromagnetism Electromagnetic Strength Magnetic field around conductor can be strengthened in three ways Place soft iron core in center of coil Increase number of turns of wire in coil

Electromagnetism Electromagnetic Strength Magnetic field around conductor can be strengthened in three ways Increase current flow through coil

Electromagnetism Electromagnetic Strength Iron is highly permeable Flux lines concentrate in iron and avoid less permeable air

Electromagnetism Electromagnetic Strength Adding turns to coil or increasing current results in greater field strength Magnetic field strength is called ampere-turns Coils with iron cores are electromagnets

Figure 47-14 An iron core concentrates the magnetic lines of force surrounding a coil.

Uses of Electromagnetism Relays Relay is control device Small amount of current controls large amount of current in another circuit

Uses of Electromagnetism Relays A simple relay Simple relay has electromagnetic coil in series with battery and switch Near electromagnet is movable flat arm (armature) of material attracted by magnetic field

Figure 47-15 An electromagnetic switch that has a movable arm is referred to as a relay.

Uses of Electromagnetism Relays A simple relay Armature pivots and is held short distance away from electromagnet by a spring

Uses of Electromagnetism Relays A simple relay Contact point attached to free end of armature

Uses of Electromagnetism Relays A simple relay Another contact point is fixed a short distance away

Uses of Electromagnetism Relays A simple relay The contact points are wired in a series

Uses of Electromagnetism Relays A simple relay When switch is closed Current travels from battery through coil, creating electromagnet

Uses of Electromagnetism Relays A simple relay When switch is closed Magnetic field attracts armature, pulling it until contact points close

Uses of Electromagnetism Relays A simple relay When switch is closed Closing contacts allows current in heavy current circuit from battery to load

Uses of Electromagnetism Relays A simple relay When switch is open Electromagnet loses magnetism

Uses of Electromagnetism Relays A simple relay When switch is open Spring pressure lifts arm back up

Uses of Electromagnetism Relays A simple relay When switch is open Heavy current circuit is broken when contact points open

Uses of Electromagnetism Solenoid Solenoid is example of electromagnetic switch Uses movable core rather than movable arm

Uses of Electromagnetism Solenoid Generally used in higher-amperage applications Solenoid can be separate unit or attached to starter

Figure 47-16 (a) A starter with attached solenoid. All of the current needed by the starter flows through the two large terminals of the solenoid and through the solenoid contacts inside.

Figure 47-16 (b) A relay is designed to carry lower current compared to a solenoid and uses a movable arm.

Electromagnetic Induction Principles Involved Electricity can be produced by relative movement of electrical conductor and magnetic field

Electromagnetic Induction Principles Involved Three items needed to produce electricity (voltage) Electrical conductor (usually coil of wire)

Electromagnetic Induction Principles Involved Three items needed to produce electricity (voltage) Magnetic field

Electromagnetic Induction Principles Involved Three items needed to produce electricity (voltage) Movement of either the conductor or the magnetic field

Electromagnetic Induction Principles Involved Therefore: Electricity creates magnetism Magnetism can create electricity

Electromagnetic Induction Principles Involved Magnetic flux creates electromotive force, or voltage, if flux lines or conductor is moving

Electromagnetic Induction Principles Involved The movement is called relative motion The process is called induction

Electromagnetic Induction Principles Involved Resulting electromotive force is called induced voltage Creation of voltage in conductor by moving magnetic field is electromagnetic induction

Figure 47-17 Voltage can be induced by the relative motion between a conductor and magnetic lines of force.

Electromagnetic Induction Voltage Intensity Voltage is induced when a conductor cuts across magnetic flux lines Amount of voltage depends on rate at which flux lines are broken

Electromagnetic Induction Voltage Intensity The more flux lines broken per unit of time, the greater the voltage 1 million flux lines broken in one second induces 1 volt

Electromagnetic Induction Voltage Intensity Four ways to increase induced voltage Increase strength of magnetic field

Electromagnetic Induction Voltage Intensity Four ways to increase induced voltage Increase numbers of conductors

Electromagnetic Induction Voltage Intensity Four ways to increase induced voltage Increase speed of relative motion between conductor and flux lines

Electromagnetic Induction Voltage Intensity Four ways to increase induced voltage Increase angle between flux lines and conductor up to 90 degrees

Electromagnetic Induction Voltage Intensity Maximum voltage induced if conductors break flux lines at 90 degrees

Figure 47-18 Maximum voltage is induced when conductors cut across the magnetic lines of force (flux lines) at a 90-degree angle.

Electromagnetic Induction Voltage Intensity Voltage induced electromagnetically can be measured Induced voltage creates current

Electromagnetic Induction Voltage Intensity Direction of induced voltage is polarity and depends on direction of flux lines

Electromagnetic Induction Lenz’s Law Induced current moves so its magnetic field opposes the motion that induced the field

Electromagnetic Induction Self-Induction When current flows through a coil, flux lines expand As current continues to increase, flux line continue to expand

Electromagnetic Induction Self-Induction Flux lines cut across wires of coil and induce another voltage within coil The self-induced voltage opposes the current that produces it

Electromagnetic Induction Self-Induction If current continues to increase, second voltage opposes the increase If current stabilizes, opposing voltage no longer induced

Electromagnetic Induction Self-Induction When current is shut off, collapsing flux lines self-induce voltage that tries to maintain current

Electromagnetic Induction Self-Induction Self-induced voltage opposes and slows decrease in current Self-induced voltage that opposes changes in current flow is counter electromotive force (CEMF)

Electromagnetic Induction Mutual Induction When two coils are close together, energy may be transferred Expansion or collapse of magnetic field around one coil induces voltage in the second

Electromagnetic Induction Mutual Induction Expansion or collapse of the magnetic field around one coil induces a voltage in the second coil

Ignition Coils Ignition Coil Windings Ignition coils use two windings around same iron core Primary winding connects to battery through a switch

Ignition Coils Ignition Coil Windings Ignition coils use two windings around same iron core Secondary winding connects to external circuit

Ignition Coils Ignition Coil Windings When switch is open, there is no current in primary winding There is no magnetic field or voltage in secondary winding

Ignition Coils Ignition Coil Windings When switch is closed current builds magnetic field around both windings Primary winding changes electrical energy from battery into magnetic energy

Ignition Coils Ignition Coil Windings As field expands, it cuts across secondary winding and induces voltage

Figure 47-19 Mutual induction occurs when the expansion or collapse of a magnetic field around one coil induces a voltage in a second coil.

Ignition Coils Ignition Coil Windings When magnetic field is fully expanded, it remains steady There is no relative motion and no voltage in secondary winding

Ignition Coils Ignition Coil Windings When switch is opened, primary current stops and field collapses Flux lines now cut across secondary winding in opposite direction

Ignition Coils Ignition Coil Windings Movement induces secondary voltage with current in opposite direction Mutual induction is used in ignition coils

Ignition Coils Ignition Coil Windings Low-voltage primary current induces high secondary voltage Because voltage is increased, ignition coil is a step-up transformer

Ignition Coils Ignition Coil Windings Electrically connected windings Many ignition coils have two separate but electrically connected windings of copper wire

Ignition Coils Ignition Coil Windings Electrically connected windings This type coil is a “married” type

Ignition Coils Ignition Coil Windings Electrically insulated windings True transformer in which primary and secondary windings are not electrically connected

Ignition Coils Ignition Coil Windings Electrically insulated windings This type coil is called “divorced” type

Ignition Coils Ignition Coil Construction Center of ignition coil contains core of laminated soft iron Core increases magnetic strength of coil

Ignition Coils Ignition Coil Construction About 20,000 turns of fine wire are wound around core These windings are secondary windings

Ignition Coils Ignition Coil Construction Surrounding secondary windings are about 150 turns of heavy wire These windings are primary windings

Ignition Coils Ignition Coil Construction Secondary winding has 100 times more turns than primary winding Turn ratio is 100 : 1

Ignition Coils Ignition Coil Construction Windings often surrounded by thin metal shield and insulating paper Everything fits in a metal container

Ignition Coils Ignition Coil Construction Primary and secondary windings produce heat from electrical resistance Many coils have oil to help cool coil

Ignition Coils Ignition Coil Construction Other coil designs: Air-cooled, epoxy-sealed E coil Wire turns wrapped around E-shaped iron core

Figure 47-21 A GM waste-spark ignition coil showing the section of laminations that is shaped like the letter E . These mild steel laminations improve the efficiency of the coil.

Ignition Coils Ignition Coil Construction Other coil designs: Spool design. Windings are around nylon or plastic spool.

Figure 47-22 The coil-on-plug (COP) design typically uses a bobbin-type coil.

Ignition Coils Ignition Coil Operation Negative terminal attached to ignition control module (ICM, or igniter) ICM closes primary ignition circuit

Ignition Coils Ignition Coil Operation With ignition switch on, positive and negative terminals have current Spark is created in this sequence Magnetic field is created in primary winding

Ignition Coils Ignition Coil Operation Spark is created in this sequence When ICM opens ground circuit, stored magnetic field collapses

Ignition Coils Ignition Coil Operation Spark is created in this sequence Collapse of magnetic field creates high voltage (up to 40,000 volts) in secondary winding

Ignition Coils Ignition Coil Operation Spark is created in this sequence High-voltage pulse flows to spark plug and creates spark

Electromagnetic Interference Definition Electromagnetic Interference (EMI) problem is mainly one of radio-frequency interference (RFI)

Electromagnetic Interference Definition RFI caused by use of secondary ignition cables Spark plug wires made of high-resistance, nonmetallic core mostly solved RFI

Electromagnetic Interference Definition RFI is part of EMI All electronic devices used in vehicles affected by EMI/RFI

Electromagnetic Interference How EMI Is Created When there is current in a conductor, electromagnetic field is created When current stops and starts, field strength changes

Electromagnetic Interference How EMI Is Created This process creates electromagnetic signal waves Rapid fluctuation of field strength creates high-frequency waves (EMI)

Electromagnetic Interference How EMI Is Created EMI is also produced by static electric charges from other sources Friction of tires Engine drive belts

Electromagnetic Interference How EMI Is Created EMI is also produced by static electric charges from other sources Drive axles Driveshafts

Electromagnetic Interference How EMI Is Created Four ways of transmitting EMI in vehicle Conductive coupling Capacitive coupling

Electromagnetic Interference How EMI Is Created Four ways of transmitting EMI in vehicle Inductive coupling Electromagnetic radiation

Electromagnetic Interference EMI Suppression Devices Four ways to reduce EMI Resistance suppression Adding resistance to circuit suppresses RFI in high-voltage system

Electromagnetic Interference EMI Suppression Devices Four ways to reduce EMI Suppression capacitors and coils Capacitors absorb voltage fluctuations

Electromagnetic Interference EMI Suppression Devices Four ways to reduce EMI Suppression capacitors and coils Coils reduce fluctuations from self-induction

Electromagnetic Interference EMI Suppression Devices Four ways to reduce EMI Shielding Metal housings help protect onboard computers

Electromagnetic Interference EMI Suppression Devices Four ways to reduce EMI Ground wires or straps Provide low-resistance circuit ground path

Electromagnetic Interference EMI Suppression Devices Four ways to reduce EMI Ground wires or straps Strap has no purpose but to suppress EMI

Figure 47-23 To help prevent underhood electromagnetic devices from interfering with the antenna input, it is important that all ground wires, including the one from this power antenna, be properly grounded.

TECH TIP A Cracked Magnet Becomes Two Magnets Magnets are commonly used in vehicle crankshaft, camshaft, and wheel speed sensors. If a magnet is struck and cracks or breaks, the result is two smaller-strength magnets. Because the strength of the magnetic field is reduced, the sensor output voltage is also reduced. BACK TO PRESENTATION A typical problem occurs when a magnetic crankshaft sensor becomes cracked, resulting in a no-start condition. Sometimes the cracked sensor works well enough to start an engine that is cranking at normal speeds but will not work when the engine is cold. Figure 47-2 If a magnet breaks or is cracked, it becomes two weaker magnets.

TECH TIP Magnetize a Steel Needle A piece of steel can be magnetized by rubbing a magnet in one direction along the steel. This causes the atoms to line up in the steel, so it acts like a magnet. The steel often will not remain magnetized, whereas the true magnet is permanently magnetized. When soft iron or steel is used, such as a paper clip, it will lose its magnetism quickly. BACK TO PRESENTATION The atoms in a magnetized needle can be disturbed by heating it or by dropping the needle on a hard object, which would cause the needle to lose its magnetism. Soft iron is used inside ignition coils because it will not keep its magnetism.

TECH TIP Electricity and Magnetism Electricity and magnetism are closely related because any electrical current flowing through a conductor creates a magnetic field. Any conductor moving through a magnetic field creates an electrical current. BACK TO PRESENTATION This relationship can be summarized as follows: Electricity creates magnetism. Magnetism creates electricity. From a service technician’s point of view, this relationship is important because wires carrying current should always be routed as the factory intended to avoid causing interference with another circuit or electronic component. This is especially important when installing or servicing spark plug wires, which carry high voltages and can cause high electromagnetic interference.

TECH TIP Cell Phone Interference A cellular phone emits a weak signal if it is turned on, even though it is not being used. This signal is picked up and tracked by cell phone towers. When the cell phone is called, it emits a stronger signal to notify the tower that it is on and capable of receiving a phone call. It is this “handshake” signal that can cause interference in the vehicle. BACK TO PRESENTATION Often this signal causes some static in the radio speakers even though the radio is off, but it can also cause a false antilock brake (ABS) trouble code to set. These signals from the cell phone create a voltage that is induced in the wires of the vehicle. Because the cell phone usually leaves with the customer, the service technician is often unable to verify the customer concern. Remember, the interference occurs right before the cell phone rings. To fix the problem, connect an external antenna to the cell phone. This step will prevent the induction of a voltage in the wiring of the vehicle.

Halderman ch047 lecture

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