OBJECTIVES

2. OBJECTIVES After studying Chapter 13, the reader 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.

3. FUNDAMENTALS OF MAGNETISM • Magnetism is a form of energy that is caused by the motion of electrons in some materials. • It is recognized by the attraction it exerts on other materials. FIGURE 13-1 A freely suspended natural magnet will point toward the magnetic north pole.

4. A CRACKED MAGNET BECOMES TWO MAGNETS • Magnets are commonly used in vehicle crankshaft, camshaft, and wheel speed sensors. • A typical problem occurs when a magnetic crankshaft sensor becomes cracked, resulting in a no-start condition. FIGURE 13-2 If a magnet breaks or is cracked, it becomes two weaker magnets.

5. FUNDAMENTALS OF MAGNETISMLines of Force • The lines that create a field of force around a magnet are believed to be caused by the way groups of atoms are aligned in the magnetic material. FIGURE 13-3 Magnetic lines of force leave the north pole and return to the south pole of a bar magnet.

6. FUNDAMENTALS OF MAGNETISMLines of Force • The more lines of force that exist, the stronger the magnet. • The magnetic lines of force, also called magnetic flux or flux lines, form a magnetic field. • Flux density refers to the number of flux lines per unit of area.

7. FUNDAMENTALS OF MAGNETISMLines of Force FIGURE 13-4 Iron filings on a compass can be used to observe the magnetic lines of force.

8. FUNDAMENTALS OF MAGNETISMMagnetic Induction • If a piece of iron or steel is placed in a magnetic field, it will also become magnetized. • This process of creating a magnet by using a magnet field is called magnetic induction.

9. FUNDAMENTALS OF MAGNETISMAttracting or Repelling • The poles of a magnet are called north (N) and south (S) because, when a magnet is suspended freely, the poles tend to point toward the North and South poles of the Earth. FIGURE 13-5 Magnetic poles behave like electrically charged particles—unlike poles attract and like poles repel.

10. FUNDAMENTALS OF MAGNETISMPermeability • Magnetic flux lines cannot be insulated. • There is no known material through which magnetic force does not pass, if the force is strong enough. • However, some materials allow the force to pass through more easily than others. • This degree of passage is called permeability.

11. FUNDAMENTALS OF MAGNETISMReluctance • Although there is no absolute insulation for magnetism, certain materials resist the passage of magnetic force. • This can be compared to resistance without an electrical circuit. • Air does not allow easy passage, so air has a high reluctance.

12. ELECTROMAGNETISM • The interaction and relationship between magnetism and electricity is known as electromagnetism.

13. ELECTROMAGNETISMCreating an Electromagnet • A magnet can be created by magnetizing a piece of iron or steel or by using electricity to make an electromagnet.

14. ELECTROMAGNETISMStraight Conductor • The magnetic field surrounding a straight, current-carrying conductor consists of several concentric cylinders of flux that are the length of the wire. FIGURE 13-6 A magnetic field surrounds a straight current-carrying conductor.

15. ELECTROMAGNETISMLeft and Right Hand Rules • Magnetic flux cylinders have direction, just as the flux lines surrounding a bar magnet have direction. • The left-hand rule is a simple way to determine this direction. • When you grasp a conductor with your left hand so that your thumb points in the direction of electron flow (- to +) through the conductor, your fingers curl around the wire in the direction of the magnetic flux lines.

16. ELECTROMAGNETISMLeft and Right Hand Rules FIGURE 13-7 The left-hand rule for magnetic field direction is used with the electron flow theory.

17. ELECTROMAGNETISMLeft and Right Hand Rules • Most automotive circuits use the conventional theory of current (+ to -) and, therefore, the right-hand rule is used to determine the direction of the magnetic flux lines. FIGURE 13-8 The right-hand rule for magnetic field direction is used with the conventional theory of electron flow.

18. ELECTROMAGNETISMField Interaction • The cylinders of flux surrounding current-carrying conductors interact with other magnetic fields. FIGURE 13-9 Conductors with opposing magnetic fields will move apart into weaker fields.

19. ELECTROMAGNETISMMotor Principle • Electric motors, such as automobile starter motors, use this field interaction to convert electrical energy into mechanical energy. FIGURE 13-10 Electric motors use the interaction of magnetic fields to produce mechanical energy.

20. ELECTROMAGNETISMCoil Conductor • If several loops of wire are made into a coil, then the magnetic flux density is strengthened. • Flux lines around a coil are the same as the flux lines around a bar magnet. FIGURE 13-11 The magnetic lines of flux surrounding a coil look similar to those surrounding a bar magnet.

21. ELECTROMAGNETISMCoil Conductor • They exit from the north pole and enter at the south pole. • Use the left-hand thread rule to determine the north pole of a coil. FIGURE 13-12 The left-hand rule for coils is shown.

22. ELECTROMAGNETISMElectromagnet Strength • The magnetic field surrounding a current-carrying conductor can be strengthened (increased) three ways. • Place a soft iron core in the center of the coil. • Increase the number of turns of wire in the coil. • Increase the current flow through the coil windings.

23. ELECTROMAGNETISMElectromagnet Strength • The magnetic field strength is often expressed in the units called ampere-turns. • Coils with an iron core are called electromagnets. FIGURE 13-13 An iron core concentrates the magnetic lines of force surrounding a coil.

24. ELECTROMAGNETISMRelays • A relay is a control device which allows a small amount of current to control a large amount of current in another circuit. • A simple relay contains an electromagnetic coil in series with a battery and a switch. FIGURE 13-14 An electromagnetic relay.

25. ELECTROMAGNETISMRelays • A contact point, made of a good conductor, is attached to the free end of the armature. • Another contact point is fixed a small distance away. • The two contact points are wired in series with an electrical load and the battery.

26. ELECTROMAGNETISMRelays • When the switch is closed, the following occurs. • Current travels from the battery through the electromagnet. • The magnetic field created by the current attracts the armature, pulling it down until the contact points meet. • Closing the contacts allows current in the second circuit from the battery to the load.

27. ELECTROMAGNETISMRelays • When the switch is open, the following occurs. • The electromagnet loses its current and its magnetic field. • Spring pressure brings the armature back. • The second circuit is broken by the opening of the contact points.

28. ELECTROMAGNETISMRelays FIGURE 13-15 In this electromagnetic switch, a light current (low amperes) produces an electromagnet and causes the contact points to close. The contact points then conduct a heavy current (high amperes) to an electrical unit.

29. ELECTROMAGNETIC INDUCTION • Magnetic flux lines create an electromotive force, or voltage, in a conductor if either the flux lines or the conductor is moving. • This movement is called relative motion. • This process is called induction, and the resulting electromotive force is called induced voltage.

30. ELECTROMAGNETIC INDUCTION FIGURE 13-16 Voltage can be induced by the relative motion between a conductor and magnetic lines of force.

31. ELECTROMAGNETIC INDUCTIONVoltage Strength • Voltage is induced when a conductor cuts across magnetic flux lines. • There are four ways to increase induced voltage. • Increase the strength of the magnetic field, so there are more flux lines. • Increase the number of conductors that are breaking the flux lines. • Increase the speed of the relative motion between the conductor and the flux lines so that more lines are broken per time unit. • Increase the angle between the flux lines and the conductor to a maximum of 90 degrees.

32. ELECTROMAGNETIC INDUCTIONVoltage Strength FIGURE 13-17 No voltage is induced if the conductor is moved in the same direction as the magnetic lines of force (flux lines).

33. ELECTROMAGNETIC INDUCTIONVoltage Strength FIGURE 13-18 Maximum voltage is induced when conductors cut across the magnetic lines of force (flux lines) at a 90 degree angle.

34. ELECTROMAGNETIC INDUCTIONVoltage Strength • An induced current moves so that its magnetic field opposes the motion which induced the current. • This principle is called Lenz’s law.

35. ELECTROMAGNETIC INDUCTIONSelf Induction • When current begins to flow in a coil, the flux lines expand as the magnetic field forms and strengthens. • As current increases, the flux lines continue to expand, cutting across the wires of the coil and actually inducing another voltage within the same coil.

36. ELECTROMAGNETIC INDUCTIONMutual Induction • When two coils are close together, energy may be transferred from one to the other by magnetic coupling called mutual induction. • Mutual induction means that the expansion or collapse of the magnetic field around one coil induces a voltage in the second coil.

37. ELECTROMAGNETIC INDUCTIONMutual Induction FIGURE 13-19 Mutual induction occurs when the expansion or collapse of a magnetic field around one coil induces a voltage in a second coil.

38. IGNITION COILS • The heart of any ignition system is the ignition coil. • The coil creates a high-voltage spark by electromagnetic induction. FIGURE 13-20 Internal construction of an oil-cooled ignition coil. Notice that the primary winding is electrically connected to the secondary winding. The polarity (positive or negative) of a coil is determined by the direction in which the coil is wound.

39. IGNITION COILS FIGURE 13-21 Typical air-cooled epoxy-filled E coil. FIGURE 13-22 Cutaway of a General Motors Type II distributorless ignition coil. Note that the primary windings are inside of the secondary windings.

40. WHAT IS A “MARRIED”AND “DIVORCED”COIL DESIGN? FIGURE 13-23 A tapped (married) type of ignition coil where the primary winding is tapped (connected) to the secondary winding.

41. ELECTROMAGNETIC INTERFERENCE SUPPRESSION • Until the advent of the onboard computer, electromagnetic interference (EMI) was not a source of real concern to automotive engineers.

42. ELECTROMAGNETIC INTERFERENCE SUPPRESSION • There are four ways of transmitting EMI, all of which can be found in a vehicle. • Conductive coupling is actual physical contact through circuit conductors. • Capacitive coupling is the transfer of energy from one circuit to another through an electrostatic field between two conductors. • Inductive coupling is the transfer of energy from one circuit to another as the magnetic fields between two conductors form and collapse. • Electromagnetic radiation is the transfer of energy by the use of radio waves from one circuit or component to another.

43. ELECTROMAGNETIC INTERFERENCE SUPPRESSIONEMI Suppression Devices • There are four general ways in which EMI is reduced. • By the addition of resistance to conductors, which suppresses conductive transmission and radiation • By the use of capacitors and radio choke coil combinations to reduce capacitive and inductive coupling • By the use of metal or metalized plastic shielding, which reduces EMI radiation in addition to capacitive and inductive coupling • By an increased use of ground straps to reduce conductive transmission and radiation by bypassing the unwanted signals to ground

44. ELECTROMAGNETIC INTERFERENCE SUPPRESSIONSuppression Capacitors and Coils • Capacitors are installed across many circuits and switching points to absorb voltage fluctuations. • Among other applications, they are used across the following: • The primary circuit of some electronic ignition modules • The output terminal of most alternators • The armature circuit of electric motors

45. ELECTROMAGNETIC INTERFERENCE SUPPRESSIONGround Straps • Ground or bonding straps between the engine and chassis of an automobile help suppress EMI conduction and radiation by providing a low-resistance circuit ground path. • Such suppression ground straps are often installed between rubber-mounted components and body parts.

46. ELECTROMAGNETIC INTERFERENCE SUPPRESSIONGround Straps FIGURE 13-24 To help prevent under-hood electromagnetic devices from interfering with the antenna input, it is important that the hood be grounded to the body to form one continuous metal covering around the engine compartment. This is particularly important if the vehicle has a front fender-mounted antenna. This braided ground strap is standard equipment on this Dodge Caliber and helps eliminate radio interference.

47. SUMMARY • Most automotive electrical components use magnetism, the strength of which depends on both the amount of current (amperes) and the number of turns of wire of each electromagnet. • The strength of electromagnets is increased by using a soft-iron core. • Voltage can be induced from one circuit to another.

48. SUMMARY 4. Electricity creates magnetism and magnetism creates electricity. 5. Radio-frequency interference (RFI) is a part of electromagnetic interference (EMI).

49. REVIEW QUESTIONS • What is the relationship between electricity and magnetism? • What is the difference between mutual induction and selfinduction? • What is the result if a magnet cracks? • How can EMI be reduced or controlled?

50. CHAPTER QUIZ • Technician A says that magnetic lines of force can be seen by placing iron filings on a piece of paper and then holding them over a magnet. Technician B says that the effects of magnetic lines of force can be seen using a compass. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B