Introduction to the Charging System
The charging system converts mechanical energy from the engine into electrical energy in order to charge the battery. The charging system supplies current in order to operate the electrical systems of the machine.
AC and DC Charging Circuits
The charging system recharges the battery and generates current during operation. There are two kinds of charging circuits:
- DC charging circuits that use generators.
- AC charging circuits that use alternators.
Both circuits generate an alternating current (AC). The difference in the circuits are in the way the circuits rectify the AC current to direct current (DC). DC charging circuits have a generator and a regulator.
The generator supplies the electrical power. The generator rectifies the current mechanically by using commutators and brushes.
The regulator has three functions:
- The regulator opens the charging circuit. The regulator closes the charging circuit.
- The regulator prevents the battery from overcharging.
- The regulator limits the generators output to safe rates.
AC charging circuits include an alternator and a regulator. The alternator is really an AC generator. The alternator produces AC current, like the generator, but rectifies the current by using diodes. Alternators are generally more compact than generators of equal output. Alternators supply a higher current at low engine speeds.
The regulator in AC charging circuits limits the alternator voltage to a safe preset level. Transistorized models are used in many of the modern charging circuits.
Charging Circuit Operation
Charging circuits operate in three stages:
- During starting, the battery supplies all the load current
- During peak operation, the battery helps the generator (or alternator) supply current
- During normal operation, the generator (or alternator) supplies all current and recharges the battery
In both charging circuits, the battery starts the circuit when the battery supplies current to the starting motor in order to start the engine (Illustration 2, top diagram). The engine then drives the generator (or alternator). This produces current to take over the operation of the ignition, lights, and accessory loads in the whole system.
The center diagram (Illustration 2) shows that the battery supplies current during peak operation when the electrical loads are too high for the generator (or alternator).
Once the engine is started, the generator (or alternator) provides the current to the machine electrical systems (Illustration 2, bottom diagram). The generator supplies current as long as the engine is running above the idle speed. When the engine is at idle or stops, the battery takes over part or all of the load. However, an alternator will continue to supply current during engine idling.
Generators in DC charging circuits will be covered briefly. The generator is still found on some older machines. To service this equipment, you should have a working knowledge of how the charging system functions. The majority of this lesson will focus on AC charging circuits, which have replaced DC charging circuits in the late model machines.
The generator produces electrical energy by using electromagnetic induction. Electromagnetic induction is used to generate electricity in the charging system. Electromagnetic induction occurs when there is relative movement between a conductor and a magnetic field. As the conductor cuts through the field a voltage is induced in the conductor. This voltage causes current flow when the conductor is connected to a circuit. The amount of output depends on the strength of the magnetic field, the speed at which the magnetic field is cut, and the number of conductors cutting the field.
The basic generator has two components:
Armature - a rotating wire loop (conductor)
Magnetic poles - stationary magnetic field
As the armature rotates through the magnetic field of the poles, voltage is generated. The ends of the armature loop are connected to a split ring that is called a commutator. Brushes contact the commutator and wires connect the brushes to a load. Current will flow since the circuit is complete. To ensure a strong current and proper flow, wires are wound around the magnetic poles and the wires are attached to the brushes. The wiring is called the field circuit of the generator.
The basic generator produces an alternating current when the armature rotates. The armature reverses the polarity of the current on each side of the loop.
During the first half of the revolution, the top of the armature side (A) cuts through the magnetic field first, while the bottom of side (B) is first to cut through the field. Current flows toward side (A) and away from side (B). The conventional theory (+ to -) gives the polarities shown (+) for (A) and (-) for (B) .
During the second half of the revolution, the top of side (B) is the leading edge, while the bottom of side (A) is leading. Now (B) is (+) while (A) is (-). The armature loop ends reverse polarity during each revolution. The result is alternating current.
The commutator and brushes allow the AC current to flow to the load in the same direction. Twice during each rotation, the armature is vertical to the magnetic field. The armature loop is not passing through the field and no voltage is generated at this point. This is the static neutral point.
The commutator is split into two parts with the open areas matching the neutral point of the armature. This means that there is an air gap as the commutator passes the brushes. The other half of the commutator contacts the brushes past this point. Since the coil is in the same relative position as during the preceding one half revolution, current flow to the brush stays in the same direction. This results in direct current.
Direct current systems will automatically provide more field current as generator output increases. This increase in field current will result in an increase in generator output. If the current is left unregulated, the continuous increase will result in current and voltage levels that will destroy the generator, other electrical circuits, and the battery.
The generator cannot control the amount of voltage that is produced. Therefore, an external unit that is called a voltage regulator is used in the field circuit. A voltage regulator has a shunt coil and contact points to control the strength of the magnetic field. The result is limiting the voltage that is generated.
An alternator operates on the same principle as a generator. An alternator converts mechanical energy into electrical energy. The alternator could be called an AC generator. The difference between the generator and alternator is the way the alternator rectifies AC current to DC current. The alternator rectifies current electronically by using diodes.
Alternators are generally more compact than generators. Alternators can supply a higher current at low engine speeds. Since late model machines include many electrical accessories, the alternator can best supply the current output for the increased electrical loads.
In the alternator, the magnetic field rotates inside the wire loop. This rotating magnetic field is generated by a rotor. The wire loop, which is stationary, is the conductor.
Magnetic lines of force move across the conductors. Magnetic lines induce current flow in the lines. Since the conductors are stationary, the conductors can be directly connected instead of using brushes. This reduces heat and wear.
Voltage will be induced in a conductor when a magnetic field is moved across the conductor. For example, consider a bar magnet with the magnetic field rotating inside a loop of wire. When the magnet is rotating, and with the (S) pole of the magnet is directly under the top portion of the loop and the (N) pole is directly over the bottom portion, the induced voltage will cause current to flow in the circuit in the direction shown. Since current flows from positive to negative through the external or load circuit, the end of the loop of wire that is marked (A) will be positive polarity and the end that is marked (B) will be negative polarity.
After the bar magnet has moved through one half revolution, the (N) pole will have moved directly under the top conductor and the (S) pole will have moved directly over the bottom conductor. The induced voltage will now cause current to flow in the opposite direction. The end of the loop wire that is marked (A) will become negative polarity, and the end that is marked (B) will become positive polarity. The polarity of the ends of the wire has changed. After a second one half revolution, the bar magnet will be back at the starting point where (A) is positive and (B) is negative.
Consequently, current will flow through the load or through the external circuit first in one direction and then in the other direction. This is an alternating current, which is developed internally by an alternator.
How Voltage is Induced
Very little voltage and current are produced with a bar magnet that is rotating inside a single loop of wire. When the loop of wire and the magnet are placed inside an iron frame, a conducting path for the magnetic lines of force is created. Since iron conducts magnetism very easily, adding the iron frame greatly increases the number of lines of force between the (N) pole and the (S) pole.
A large number of magnetic lines of force are at the center of the tip of the magnet. Therefore, a strong magnetic field exists at the center of the magnet and a weak magnetic field exists at the leading and trailing edges. This condition results when the air gap between the magnet and field frame is greater at the leading and trailing edges than at the center of the magnet.
The amount of voltage that is induced in a conductor is proportional to the number of lines of force which cut across the conductor in a given length of time. The voltage will also increase if the bar magnet turns faster because the lines of force cut across the wire in a shorter time period.
The rotating magnet in an alternator is called the rotor and the loop of wire and frame assembly is called the stator.
In Illustration 9 the single loop of wire acting as a stator winding and the bar magnet acting as a rotor illustrate how an AC voltage is produced in a basic alternator. When two more separate loops of wire, spaced 120 degrees apart, are added to the basic alternator, two more separate voltages will be produced.
When the (S) pole of the rotor is directly under the (A) conductor, the voltage at (A) will be maximum in magnitude and positive in polarity.
After the rotor has turned through 120 degrees, the (S) pole will be directly under the (B) conductor and the voltage at (B) will be maximum positive. Also 120 degrees later, the voltage at (C) will be maximum positive. The peak positive voltages at (A) , (B), (C) in each loop of wire occur 120 degrees apart. These loop voltages are also shown in Illustration 9.
When the ends of the loops of wire marked (A1), (B1) and (C1) are connected to the ends marked (B), (C), and (A ) respectively, a basic three-phase delta wound stator is formed (Illustration 10). The three AC voltages ( (BA), (CB) and (AC) ) available from the delta wound stator are identical to the three voltages previously discussed.
When the ends of the loops of wire marked (A1), (B1) and (C1) are connected together, a basic three-phase Y wound stator is formed (Illustration 11). Each of these voltages consist of the voltages in two loops of wire that are added together. Three AC voltages that are spaced 120 degrees apart are available from the Y stator.
In delta windings, each of the individual windings is connected to the end of another winding (Illustration 10). This creates parallel connections in the delta stator, which generally allows for higher current output than the Y wound stator. In the Y wound stator the windings are connected in order to form pairs of series connections (Illustration 11). The series connections generally provide higher voltages but lower current output than the delta wound stators.
The following modifications are used to increase the output of the alternator:
- Increase the number of conductors in each of the phase windings.
- Increase the strength of the magnetic fields.
- Increase the speed of rotation.
- Magnetic field generation
Current Rectification Using Y or Delta Wound Stators
Even though the alternator seems complete, the current that is being generated from the alternator is still alternating. The electrical system requires direct current. In order for the output of the alternator to be of any value, the alternator must be converted from AC to DC.
The ideal device for this task is the diode. The diode is compact. The diode will conduct current in one direction only. The diode can be easily installed in the alternator housing.
Diodes are normally used in the alternator in two groups of three. Since there are three phases or windings in the alternator, three positive diodes and three negative diodes are required. In systems that require higher output, more diodes may be required.
A battery that is connected to the DC output terminal will have the energy restored as the alternator provides charging current. The blocking action of the diodes prevents the battery from discharging directly through the rectifier.
The three AC voltage curves that are provided by the Y type stator have been divided into six periods. These periods are shown in Illustration 13. Each period represents one sixth of a rotor revolution, or 60 degrees.
During period 1, the maximum voltage that is being induced appears across stator terminals (BA). This means the current flows from (B) to (A) in the stator winding during this period, and through the diodes as illustrated in Illustration 14.
The peak phase voltage that is developed from (B) to (A) is 16 volts. This means that the potential at (B) is 0 volts and the potential at (A) is 16 volts. From the voltage curves, the phase voltage from (C) to (B) at this instant is negative 8 volts. This means that the potential at (C) is 8 volts, since (C) to (B), or 8 to zero, represents a negative 8 volts. At this same time, the phase voltage from (A) to C is also negative 8 volts since (A) to (C), or 16 to 8, represents a negative 8 volts. The voltage potentials are shown in Illustration 13.
Only two of the diodes will conduct current, since these are the only diodes in which current can flow in the forward direction. The other diodes will not conduct current because these are reverse biased. The voltages that exist at the rectifier and the biasing of the diodes determine the current flow directions. These voltages are represented by the phase voltage curves in Illustration 13, which are the voltages that actually appear at the rectifier diodes. Following the same procedure for periods 2 through 6, the current flows can be determined.
The voltage that is obtained from the stator-rectifier combination when connected to a battery is not perfectly flat, but is so smooth that the output may be considered to be a nonvarying DC voltage. Illustration 15 shows the voltage that is obtained from the phase voltage curves.
A delta type stator that is wound to provide the same output as a Y stator will also provide a smooth voltage and current output when the stator is connected to a six diode rectifier. For explanation purposes, the three-phase voltage curves that are obtained from the basic delta connection for one rotor revolution are reproduced here, and the curves are divided into six periods.
During period 1 (Illustration 17), the maximum voltage that is being developed in the Delta stator is in phase (BA). The current flow through the rectifier is exactly the same as for the Y stator since the voltage potentials on the diodes are identical. The difference between the Delta stator and the Y stator is that the Y stator conducts current through only two windings throughout one period. The delta stator conducts current through all three windings.
Phase (BA) is in parallel with phase (BC) and (CA). Since the voltage from (B ) to (A) is 16, the voltage from (B) to (C) to (A) also must be 16 because 8 volts is developed in each of these two phases ( (B) to (C) and (C) to (A). Following the same procedure for periods 2-6, the current flows can be determined.
When current is passed through the coil assembly, a magnetic field is created in each of the rotor pole pieces. One set of fingers will become north poles while the other set of fingers will become south poles.
Since the rotor fingers overlap each other, many individual flux loops will be formed between the alternator north and south poles. Instead of passing one magnetic field past each winding during one revolution of the rotor, many fields will pass the windings, which will increase the output of the stator.
Since the rotor must be supplied with current to create the magnetic field, the coil assembly inside the pole piece is connected to slip rings. These slip rings are provided so that brushes can be used to provide current to the moving field. Slip rings are pressed onto the shaft and insulated from the shaft. The coil conductors are soldered to the slip rings in order to form a complete circuit that is insulated from the shaft.
Because the rotor will be spinning at high speed, the rotor must be supported by bearings. The front end of the shaft has a bearing that is mounted in the drive end housing assembly (Illustration 18).
Note: Spacers are added to place the rotor in the correct position once the alternator is assembled. This will keep the fan from hitting the housing.
Since the generation of electricity creates heat, a fan is included in order to provide a flow of air through the assembly for cooling. A pulley is attached to the end of the rotor shaft. The pulley is driven by a belt.
The end housing supports the slip ring end of the rotor shaft. The end housing provides a mounting surface for the brushes, rectifier assembly, stator and regulator (if equipped). The drive end housing with the rotor and the slip ring end housing with the components are assembled as a unit. The stator is held in between. This assembly is held together with through capscrews.
The stator assembly is a laminated soft iron ring with three groups of coils or windings. One end of each stator winding is connected to a positive diode and a negative diode. The other ends of the stator windings can be connected in either a Y type stator configuration or a delta stator configuration.
The rectifier assembly converts the (AC) current to (DC) current. Three positive diodes and three negative diodes are mounted to the rectifier assembly.
The alternator is designed to provide minimal clearance between the rotor and stator, in order to maximize the effects of the magnetic field. The alternator is a compact assembly that is capable of generating high current flow in order to satisfy the needs of the electrical system.
The brushes are in contact with the copper slip rings in order to provide the necessary current for production of the magnetic field in the rotor. Since good contact is important for good conductivity, the brushes are held against the slip rings by small coil springs.
There are two brushes, which are usually contained in a brush holder assembly. This assembly can be easily attached to the slip ring end housing of the alternator.
Regulating the Alternator Output
If the alternator were allowed to operate uncontrolled, the alternator would produce voltages too high to be used in the machine. This would result in damage to the components. The regulator controls alternator output.
Current output is limited by the construction of the alternator. Current output is indicated as a maximum on the housing. For example, a housing may have a listing such as 12V 85A. This indicates that the maximum output is 85 amperes and the alternator is designed for a 12 volt system.
The regulation circuit controls the voltage output of the alternator by changing the strength of the magnetic field that is produced by the rotor. The regulator circuit does this by controlling the amount of current that flows through the brushes to the rotor coil.
The regulator is sensitive to the voltage of the battery. The regulator is sensitive to the electrical load that is being placed on the system. The regulator can then adjust the amount of current to the rotor in order to satisfy the demand.
If the battery voltage is low and the demand from electrical accessories is high, the voltage regulator will increase the output of the alternator to charge the battery. This provides sufficient current to operate accessories. When battery voltage is high and the electrical demands are low, the voltage regulator will reduce output from the alternator.
Alternator regulators can be of three different designs:
- Electro-mechanical (older machines)
- Electronic external regulators
- Electronic integral regulators
Electro-mechanical regulators can be found on some older systems. These regulators use relays to operate contact points. The double contact voltage regulator controls alternator output by regulating the amount of current flow to the rotor. Reducing current flow will reduce the strength of the magnetic field and result in lower output from the stator.
Electronic Voltage Regulators
Electronic voltage regulators perform the same function as the electro-mechanical regulators. In the electronic regulator, the field circuit is switched ON and OFF by electronic circuits. These circuits controll switching transistors. These electronic devices can be switched more quickly and carry more current than the contact points in the electro-mechanical regulators. Higher output from the alternator can be obtained because of greater current flow through the field circuit.
Electronic regulators use Zener diodes as part of the voltage sensing circuit. These special diodes allow current to flow in reverse of normal flow when a specific voltage across the diode is reached. When the current flows back through the Zener diode the field transistor is turned off and current flow is stopped in the field rotor. The electronic components can switch on and off several thousand times a second. This provides very smooth, accurate control of alternator output.
Most electronic regulators are not adjustable. If the regulators do not accurately control the output of the alternator, the regulator must be replaced.
Electronic Regulator Operation at Engine Start-Up
When the starter switch is turned ON, the circuit is completed (Illustration 22). Battery current flows to the starter solenoid and the starter key switch, as shown by the red lines. The key start switch directs current flow to the alternator indicator lamp and the regulator.
As the current flows into the regulator, different voltage values govern the course of the current. The voltage across resistors (R7) and (R8) is below the Zener diode critical or below the breakdown voltage. Therefore, the voltage that is felt at the base of (TR2), is the same as the voltage at the emitter. So the current cannot flow through (TR2) (as shown by the blue lines).
Thus the voltage difference in the emitter base circuit of (TR1) allows current to flow from the emitter through the base and collector. The collector current then goes on to excite the alternator field (vertical red line). At the same time, a slight amount of current flow travels to the alternator ground.
Regulator Operation During Engine Operation
Regulator operation at the beginning of engine operation (Illustration 23) is similar to the engine start-up period, except that as the engine speeds up the alternator field around the rotor generates voltage in order to supply electrical loads.
However, the voltage values are still the same and transistor (TR1) still conducts the current to the alternator field as shown by the vertical red line.
As the engine operates and load requirements begin to decrease, the alternator voltage increases (Illustration 24). This causes the voltage across the resistors to also increase. Then the voltage across (R7) and (R8) becomes greater than the Zener diode critical voltage. The Zener diode immediately breaks down allowing current to flow in the reverse direction. This turns on transistor (TR2) and current is able to flow through (TR2's) emitter, base and collector. When current flows through (TR2), the voltage at the base of (TR1) is equal to or greater than the emitter, which prevents current from flowing though (TR1) to the alternator field. The field collapses,alternator output is reduced and the circuit is protected.
The system voltage then drops below the critical voltage of the Zener diode. The Zener diode stops conducting. This turns off (TR2) and turns off (TR1). Current again flows to the alternator field. This operation is repeated many times a second. The two transistors act as switches that control the voltage and alternator output.
When (TR1) turns off, the alternator field current cannot drop immediately to zero, because the rotor windings cause the current to continue to flow. Before the current reaches zero, the system voltage and the regulator start current flow again. However, the decreasing field current flow induces a high voltage which can damage the transistor.
The field discharge diode that is shown in Illustration 24 prevents damage to transistor (TR1) .
Internal electronic regulators
Internal alternator regulators are mounted either inside or outside of the slip ring end housing of the alternator. This type of regulator eliminates the wiring harness between the alternator and regulator simplifying the system. This type of regulator is usually much smaller than other types and uses electronic circuits known as integrated circuits or ICs. ICs are miniaturized electronics with much of the circuit on one small chip. Integral regulators perform the same function as the external electronic regulators.
Some alternators with integral regulators have only one wire that is going to the regulator. This wire is the alternator output wire. The ground circuit is completed through the housing to the engine block. Current for the integral regulator is fed from the stator through a diode trio. The alternator starts charging by using the small amount of permanent magnetism in the rotor. This small amount of output is fed back into the field which increases the output. This continues until full output that is determined by the regulator is reached.
There are two basic field circuit connections for an alternator A circuit and B circuit.
An A type circuit alternator (Illustration 26) uses two insulated brushes in the alternator. One brush is connected directly to the battery, while the other brush is connected to ground with the regulator and ignition switch or relay in series. The regulator is located after the field, between the field and the alternator ground or negative diodes. Full alternator output is obtained by grounding the field windings. Some alternators have a tab in a test hole so that the field is grounded by placing a screwdriver against the tab end and the alternator frame. This type of circuit is used with integral regulators and some external electronic regulators.
B type circuits use a brush that is grounded inside the alternator (Illustration 27). The other brush is connected to the battery in series with the regulator and the ignition switch or the relay. In a B circuit alternator the regulator is located before the field. The current flow is usually from the regulator terminal of the alternator to the regulator. After the regulator, the current flows to the field coil in the rotor, then to ground, and finally to the negative or return diode assembly. Full alternator output is obtained by connecting the field terminal to the regulator terminal or the output terminal.
Charge indicators may be an ammeter, a voltmeter or a charge indicator light. Ammeters may be installed in series if the ammeters are full current, shunt type, or in parallel if the ammeter is the nonshunt type.
Voltmeters are more commonly used because they more accurately indicate the operation of the system. Voltmeters can be easily installed in parallel with the charging system. Voltmeters provide information on both the operation of the charging system and condition of the battery.
Charge indicator lights show general system operation. Charge indicator lights will not indicate high alternator output or high voltage conditions, but will show low output.
Charging System Testing
Accurate testing of charging systems begins with an understanding of how the system functions. If your knowledge of the operation is complete, you can logically determine the fault through visual inspection and electrical testing.
Repair of the system may require replacement or repair of any of the items that are included in the system from the battery to the alternator.
All repairs should begin with a study or a review of the service manual for the machine upon which you are working.
When you test any electrical system a systematic approach will lead to quicker repairs. Charging systems require the same troubleshooting approach. Parts replacement without proper troubleshooting is not an acceptable method of finding and repairing system faults.
Verify the Complaint
Determine exactly what the complaint is, then verify that the fault is occurring. The following conditions are some of the common problems that occur in charging systems:
- The battery is discharged and the charging system is producing no charge or a low charge.
- The battery is charging and the charging system is overcharging.
- The alternator is noisy.
- The charge indicator light stays on or the light fails to come on.
Define the problem
Begin with a thorough visual inspection. Check for any of the following problems:
- Loose terminals or corroded battery terminals
- Loose connections or damaged ground connections at the engine and body
- Loose, dirty connections at the alternator and regulator
- Burnt fuse links or burnt wires
- Damaged, crimped, broken or cut wires
- Evidence of shorts or grounds
- Physical damage to the alternator or regulator
- Damage to belts and pulleys
- Odor of burnt electrical components
Determine whether the problem is electrical or mechanical. Alternators are belt driven. These drive belts must be inspected for tension, wear, and damage in order to make sure that the belts are doing the job. Inspect the belt for damage by checking the inside and outside surfaces for cracking, chipping, glazing or missing pieces.
Inspect the alternator pulley for wear and any other pulleys that the belt runs over. Premature belt failure is often caused by worn pulleys. Inspect all pulleys for alignment. Usually a visual inspection will show that the belts are not lined up correctly, but you may have to check with a straight edge against the pulley. Test the belt for proper tension. When you are adjusting belts or checking belt tension make sure that you are not over tightening or under tightening the belt. Incorrect tension will cause damage.
Noisy operation can be caused by worn belts, worn bearings, or internal problems. Some of these problems are, the rotor rubbing on the stator, the fan blades hitting the alternator, or defective diodes or stators.
Mechanical problems can be corrected by replacing the faulty components or repairing the defective unit as necessary. Electrical problems will require more detailed testing.
Continue your inspection by performing a complete battery service. Battery service and testing is covered in Lesson 1. A charging system will not function efficiently if the battery is defective.
Isolate the Problem
Once you have defined what the problem is, you must isolate the cause so that you can accurately make the necessary repairs. Mechanical faults can be located by inspecting or listening. Electrical faults require testing to locate the cause.
Charging System Tests
On machine charging system tests should be performed first in order to determine whether the alternator must be removed and tested further.
On machine tests include the following tests:
- Alternator output test
- Regulator test
Bench tests will determine if the alternator must be repaired or replaced.
Bench tests include include the following tests:
- Rotor field winding tests
- Stator tests
- Rectifier tests
- Brush tests