Generator Operation: Principles, Construction, and Testing

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2. OBJECTIVES: After studying Chapter 41, the reader should be able to: • Describe how a generator works. • Discuss the various generator test procedures. • Explain how to disassemble a generator and test its component parts. • Discuss how to check the wiring from the generator to the battery.

3. KEY TERMS: AC voltage • brushescharging voltage delta-connected stator • delta winding • diodes • diode check • diode trio • drive-end (DE) housing • duty cycle Electrical power management (EPM)fusible link generator output test Continued

4. KEY TERMS: overrunning alternator dampener (OAD) • overrunning alternator pulley (OAP) polesrectifier bridge • rotors sine wave • slip ring end (SRE) • stators • stator tap temperature compensation • thermistor wye-connected stator

5. All vehicles operate electrical components by takingcurrent from the battery. It is the purpose and functionof the charging system to keep the battery fully charged.The Society of Automotive Engineers (SAE) term forthe unit that generates electricity is the generator.The term alternator is also commonly used, especiallyin service manuals before 1993 when the SAE termwas adopted by most vehicle manufacturers.

6. PRINCIPLES OF GENERATOR OPERATION All electrical generators use the principle of electromagnetic induction to convert mechanical energy into electrical energy.Electromagnetic induction involves the generation of an electrical current in a conductor when the conductor is moved through a magnetic field. The amount of current generated can be increased by the following factors: • Increasing the speed of the conductor through the magnetic field • Increasing the number of conductors passing through the magnetic field • Increasing the strength of the magnetic field

7. ALTERNATING CURRENT GENERATORS (ALTERNATORS) An AC generator generates an alternating current when the current changes polarity during the generator’s rotation.A battery cannot “store” alternating current; therefore, this alternating current is changed to direct current (DC) by diodes inside the generator.Diodes are one-way electrical check valves that permit current to flow in only one direction. Most manufacturers call an AC generator an alternator. Continued

8. GENERATOR CONSTRUCTION A generator is constructed of a two-piece cast-aluminum housing.Aluminum is used because of its lightweight, nonmagnetic properties and heat transfer properties needed to help keep the generator cool.A front ball bearing is pressed into the front housing (called the drive-end [DE] housing) to provide the support and friction reduction necessary for the belt-driven rotor assembly.The rear housing, or the slip ring end (SRE), usually contains a roller-bearing support for the rotor and mounting for the brushes, diodes, and internal voltage regulator (if the generator is so equipped). See Figures 41–1 and 41–2. Continued

9. Figure 41–1 A typical generator (alternator) on a Chevrolet V-8 engine. Figure 41–2The end frame toward the drive belt is called the drive-end housing and the rear section is called the slip-ring-end housing. Continued

10. Many technicians are asked how much power certain accessories require. A 100-ampere generator requires about 2 horsepower from the engine. One horsepower is equal to 746 watts. Watts are calculated by multiplying amperes times volts: Generator Horsepower and EngineOperation - Part 1 Power in watts 100A x 14.5V = 1450 1hp = 746 W 1450 watts is about 2 horsepower and the generator uses about 2 hp to generate 100 A. Allowing about 20% for mechanical and electrical losses adds another 0.4 horsepower. When someone asks how much power it takes to produce 100 amperes from a generator, the answer is 2.4 hp.

11. Generator Horsepower and EngineOperation - Part 2 Power in watts 100A x 14.5V = 1450 1hp = 746 W Many generators delay output to prevent the engine from stumbling when a heavy electrical load is applied. The voltage regulator or vehicle computer is capable of gradually increasing the generator output over a period of up to several minutes. Though it does not sound like much, a sudden demand for 2 horsepower from an idling engine can cause the engine to run rough or stall. The difference in part numbers of various generators is often an indication of the time interval over which the load is applied. Therefore, the use of the wrong replacement generator could cause the engine to stall!

12. ALTERNATOR OVERUNNING PULLEY Purpose and Function Many generators are equipped with an overrunning alternator pulley (OAP), called a clutch pulley.This eliminates noise and vibration in the accessory drive belt system, especially when at idle, when engine impulses are transmitted to the alternator through the accessory drive belt.The mass of the rotor of the alternator tends to want to keep spinning, but the engine crankshaft speeds up and slows down slightly due to the power impulses.Using a one-way clutch in the alternator pulley allows the belt to apply power to the alternator in only one direction thereby reducing the fluctuations in the belt. See Figure 41–3. Continued

13. Figure 41–3 An OAP on a generator on a Chevrolet Corvette.

14. A conventional drive pulley attaches to the alternator (rotor) shaft with a nut and lock washer.In the overrunning clutch pulley, the inner race of the clutch acts as the nut and screws onto the shaft. Special tools are required to remove and install this type of pulley.Another type of alternator pulley uses a dampener spring inside plus a one-way clutch. This unit is called an overrunning alternator dampener (OAD).An OAD is larger than an OAP and is used on the Pontiac G8 and other vehicles.See Figure 41–4. Continued

15. Figure 41–4 An overrunning alternator dampener (OAD) disassembled, showing all of its internal parts.

16. Diagnosis and Service Overrunning alternator pulleys and alternator dampeners can fail, with the most common factor the one-way clutch.If it fails, it can freewheel and not power the alternator or it can lock up and not provide dampening as designed. If the charging system is not working, the OAP or OAD could be the cause rather than a fault in the alternator itself.In most cases, the entire alternator assembly will be replaced because each OAP or OAD is unique for each application and they require special tools to remove and replace.

17. No. An alternator needs to be equipped with the proper shaft to allow the installation of an OAP or OAD. This also means a conventional pulley cannot be used to replace a defective overrunning alternator pulley or dampener. Can I Install an OAP or OAD to My Alternator?

18. ROTORS The rotor creates the magnetic field of the generator and produces a current by electromagnetic induction in the stationary stator windings.The generator rotor is constructed of many turns of copper wire coated with a varnish insulation wound over an iron core. The iron core is attached to the rotor shaft. At both ends of the rotor windings are heavy-gauge metal plates bent over the windings with triangular fingers called poles.These pole fingers do not touch, but rather they alternate or interlace as shown in Figure 41–5. Continued

19. Figure 41–5 A cutaway of a General Motors CS-130D generator showing the rotor and cooling fans that are used to force air through the unit to remove the heat created when it is charging the battery and supplying electrical power for the vehicle. Continued

20. If current flows through the rotor windings, the metal pole pieces at each end of the rotor become electromagnets. Whether a north or a south pole magnet is created depends on the direction in which the wire coil is wound.Because the pole pieces are attached to each end of the rotor, one pole piece will be a north pole magnet. The other pole piece is on the opposite end of the rotor and therefore is viewed as being wound in the opposite direction, creating a south pole. The rotor fingers are alternating north and south magnetic poles.The magnetic fields are created between the alternating pole piece fingers. These individual magnetic fields produce a current by electromagnetic induction in the stationary stator windings.See Figure 41–6. Continued

21. Figure 41–6 Rotor assembly of a typical alternator (AC generator). Current through the slip rings causes the “fingers” of the rotor to become alternating north and south magnetic poles. As the rotor revolves, these magnetic lines of force induce a current in the stator windings. The current necessary for the field (rotor) windings is conducted through carbon brushes to the slip rings. Maximum-rated generator output in amperes is largely dependent on the number and gauge of the windingsof the rotor. Substituting rotors from one generator into another can greatly affect maximum output either positive or negative.

22. GENERATOR BRUSHES The current for the field is controlled by the voltage regulator and is conducted to the slip rings through carbon brushes.The brushes conduct only the field current (approximately 2 to 5 amps), and therefore tend to last longer than the brushes used on a DC generator, where all the current generated in the generator must flow through the brushes.

23. STATORS Supported between the two halves of the generator housing are three copper wire windings wound on a laminated metal core.See Figure 41–7.As the rotor revolves, its moving magnetic field induces a current in the windings of the stator.See Figure 41–8. Continued

24. Figure 41–7 A cutaway view of a typical AC generator (alternator) Continued

25. Figure 41–8 An exploded view of a typical generator (alternator) showing all of its internal parts.

26. Whenever checking for the root cause of a generator failure, one of the first things that a technician should do is to sniff (smell) the generator! If the generator smells like a dead rat (rancid), the stator windings have been overheated by trying to charge a discharged or defective battery.If the battery voltage is continuously low, the voltage regulator will continue supplying full field current to the generator. The voltage regulator is designed to cycle on and off to maintain a narrow charging system voltage range.If battery voltage is continually below the cutoff point of the voltage regulator, the generator is continually producing current in the stator windings. This constant charging can often overheat the stator and burn the insulating varnish covering the stator windings. If the generator fails the sniff test, the tech should replace the stator and other generator components found to be defective and replace or recharge and test the battery. The Sniff Test

27. DIODES Diodes are constructed of a semiconductor material (usually silicon) and operate as a one-way electrical check valve that permits the current to flow in only one direction.AC generators use six diodes (one positive and one negative diode for each of the three stator windings) to convert alternating current to direct current. The symbol for a diode is shown here. Figure 41–9 A diode symbol.

28. HOW A GENERATOR WORKS A rotor inside a generator is turned by a belt and drive pulley turned by the engine. The magnetic field of the rotor generatesa current in the windings of the stator by electromagnetic induction. Field current flowing through the slip rings to the rotor creates alternating north & south poles on the rotor, with a magnetic fieldbetween each finger ofthe rotor. Figure 41–10 Magnetic lines of force cutting across a conductor induce a voltage and current in the conductor. Continued

29. The induced current in the stator windings is an alternating current because of the alternating magnetic field of the rotor.The induced current starts to increase as the magnetic field starts to induce current in each winding of the stator.The current then peaks when the magnetic field is the strongest and starts to decrease as the magnetic field moves away from the stator winding.The current generated is described as being of a sine wave pattern See Figure 41–11. Continued

30. Figure 41–11 Since wave voltage curve created by one revolution of a winding rotating in a magnetic field. As the rotor continues to rotate, this sine wave current is induced in each of the three windings of the stator. Continued

31. Figure 41–12 When three windings (A, B, and C) are present in a stator, the resulting current generation is represented by the three sine waves. The voltages are 120° out of phase. The connection of the individual phases produces a three-phase alternating voltage. Because each of the three windings generates a sine wave current, the resulting currents combine to form a three-phase voltage output. AC generators contain six diodes, one pair of positive and a negative for each of the three stator windings.

32. WYE-CONNECTED STATORS The Y (pronounced “wye” and generally so written) type or star pattern is the most commonly used generator stator winding connection. Figure 41–13 Wye-connected stator winding. Continued

33. The output current with a wye-type stator connection is constant over a broad generator speed range.Current is induced in each winding by electromagnetic induction from the rotating magnetic fields of the rotor. In a wye-type stator connection, the currents must combine because two windings are always connected in series. See Figure 41–14.The current produced in each winding is added to the other’s current and flows through the diodes to the generator output terminal. One-half of the current produced is available at the neutral junction (usually labeled “STA” for stator).Voltage at this center point is used by some generator manufacturers (especially Ford) to control the charge indicator light or is used by the voltage regulator to control the rotor field current. Continued

34. Figure 41–14 As the magnetic field, created in the rotor, cuts across the windings of the stator, a current is induced. Notice that the current path includes passing through one positive () diode on the way to the battery and one negative () diode as a complete circuit is completed through the rectifier and stator.

35. Figure 41–15 Delta-connected stator winding. DELTA-CONNECTED STATORS The delta winding is connected in a triangular shape, as shown in here. (Delta is a Greek letter shaped like a triangle.) Continued

36. Current induced in each winding flows to the diodes in a parallel circuit. More current can flow through two parallel circuits than can flow through a series circuit (as in a wye-type stator connection).Delta-connected stators are used on generators where high output at high-generator rpm is required. The delta-connected generator can produce 73% more current than the same generator with wye-type stator connections. If a generator with a wye-connected stator can produce 55 A, the same generator with delta-connected stator windings can produce 73% more current, or 95 A (55  1.73 = 95).The delta-connected generator, however, produces lower currentat low speed and must be operated at high speed to produce its maximum output. Continued

37. GENERATOR OUTPUT FACTORS Output voltage and current of a generator depend on: • The Speed of rotation Generator output is increased with generator rotational speed up to the generator’s maximum possible ampere output. Generators normally rotate at a speed two to three times faster than engine speed, depending on relative pulley sizes used for the belt drive. • The Number of conductors. A high-output generator contains more turns of wire in the stator windings. Stator winding connections (whether wye or delta) also affect the maximum generator output. See 41-16. Continued

38. Figure 41–16 A stator assembly with six, rather than the normal three, windings. Here is an example of a stator that has six rather than three windings, which greatly increases the amperage output of the generator (alternator). Continued

39. The Strength of the magnetic field. If the magnetic field is strong, a high output is possible because the current generated by electromagnetic induction is dependent on the number of magnetic lines of force that are cut. • The strength of the magnetic field can be increased by increasing the number of turns of conductor wire wound on the rotor. A higher-output generator has more turns of wire than a generator with a low-rated output. • The strength of the magnetic field also depends on the current through the field coil (rotor). Because magnetic field strength is measured in ampere-turns, the greater theamperage or the number of turns, or both, the greater the generator output.

40. GENERATOR VOLTAGE REGULATION An automotive generator must be able to produce electrical pressure (voltage) higher than battery voltage to charge the battery.Excessive high voltage can damage battery, electrical components, and the lights of a vehicle. If no current (zero amperes) was flowing in the field coil of the generator (rotor), generator output would be zero because without field current a magnetic field does not exist. The field current required by most automotive generators is less than 3 amps. It is the control of the field current that controls the output of the generator. See Figure 41–17. Continued

41. Figure 41–17 Typical voltage regulator voltage range. Current for the rotor flows from the battery through the brushes to the slip rings. After generator output begins, the voltage regulator controls the current flow through the rotor.

42. CHARGING VOLTAGE CONTROL If an automotive battery is discharged, its voltage will be lower than the voltage of a fully charged battery. The generator will supply charging current, but it may not reach maximum charging voltage.A good but discharged battery should be able to convert into chemical energy all the current the generator can produce. As long as generator voltage is higher than battery voltage, current will flow from the generator (high pressure, high voltage) to the battery (lower pressure, lower voltage). The condition and voltage of the battery do determine the charging rate of the generator. If a discharged battery is used during charging system testing, tests could mistakenly indicate a defective generator and/or voltage regulator. Continued

43. Temperature CompensationAll voltage regulators (mechanical or electronic) provide a method for increasing the charging voltage slightly at low temperatures and for lowering the charging voltage at high temperatures.A battery requires a higher charging voltage at low temperatures because of the resistance to chemical reaction changes. However, the battery would be overcharged if the charging voltage were not reduced during warm weather.Electronic voltage regulators use a temperature-sensitive resistor, called a thermistor in the regulator circuit, and it provides lower resistance as temperature increases.A thermistor is used to control charging voltage over a wide range of under-the-hood temperatures. See Figure 41–18. Continued

44. Figure 41–18 A typical electronic voltage regulator showing the connections and the circuits involved. NOTE:Voltmeter test results may vary according to temperature. Charging voltage tested at 32°F (0°C) will be higher than for the same vehicle tested at 80°F (27°C) because of the temperature-compensation factors built into voltage regulators.

45. DIODE TRIO GENERATORS Some generators (alternators) use a diode trio to provide current to the rotor (field) and turn off the charge indicator light on the dash.A diode trio is three diodes connected in parallel with the anode ends connected to the three stator winding connectors at the rectifier bridge. The single output terminal of the diode trio is applied to the ignition feed terminal. NOTE:If one of the three diodes is open, only 8 volts will be applied against the 12 volts from the ignition. The result is a dim generator charge indicator light. While this light may be a customer concern, the generator will produce normal output.

46. When the ignition is on, 12 volts is applied through the charge light bulb and is grounded through the rotor winding.This current is blocked from flowing through the diode trio because the diodes are reversed biased. When the stator and the generator start to generate voltage, the 12-volt output of the diode trio opposes the 12 volts from the ignition through the charge light causing the charge light to go out.

47. ELECTRONIC VOLTAGE REGULATORS The electronic circuit of the voltage regulator cycles between 10 and 7,000 times per second as needed to accurately control the field current through the rotor, and therefore control the generator output.The control of the field current is accomplished by opening and closing the ground side of the field circuit through the rotor of the generator. Electronic voltage regulators also use many resistors to help reduce the current through the regulator, and the resulting heat must be dissipated into the air to prevent damage to the diodes and transistors.Whether mounted inside the generator or externally under the hood, electronic voltage regulators are mounted where normal airflow can keep the electronic components cool. Continued

48. The zener diode is a major electronic component that makes voltage regulation possible. Generator voltage from the stator and diodes is first sent through a thermistor, which changes resistance with temperature, and then to a zener diode.Whenever the upper-limit voltage is reached, the zener diode conducts current to a transistor, which then opens the field (rotor) circuit. All the current stops flowing through the generator’s brushes, slip rings, and rotor, and no magnetic field is formed. Without a magnetic field, a generator does not produce current in the stator windings. Continued

49. When no voltage is applied to the zener diode, current flow stops and the base of the transistor is turned off, closing the field circuit. The magnetic field is thus restored in the rotor.The rotating magnetic fields of the rotor induce a current in the stator, which is again controlled if the output voltage exceeds the designed limit as determined by the zener diode breakdown voltage.Depending on the generator rpm, vehicle electrical load, and state of charge of the battery, this controlled switching on and off can occur between 10 and 7,000 times per second.See Figure 41–19. Continued

50. Figure 41–19 Typical General Motors SI-style AC generator. Full Caption next slide.