CHARGING SYTEM (CIRCUIT)

 CHARGING CIRCUIT

Learning Objective: Identify charging-circuit components, their functions, and

maintenance procedures.

The charging system performs several functions, which are as follows:

·       It recharges the battery after engine cranking or after the use of electrical accessories with the engine turned off.

·       It supplies all the electricity for the vehicle when the engine is running.

·       It must change output to meet different electrical loads.

·       It provides a voltage output that is slightly higher than battery voltage.

 

 charging circuit.

 

 

A TYPICAL CHARGING CIRCUIT CONSISTS OF THE FOLLOWING:

 

BATTERY- provides current to energize or excite the alternator and assists in

stabilizing initial alternator output.

ALTERNATOR or GENERATOR- uses mechanical (engine) power to produce

electricity.

ALTERNATOR BELT- links the engine crankshaft pulley with alternator/ generator

pulley to drive the alternator/ generator.

VOLTAGE REGULATOR- ammeter, voltmeter, or warning light to inform the

operator of charging system condition.

ALTERNATORS

The alternator  has replaced the dc generator because of its improved

efficiency. It is smaller, lighter, and more dependable than the dc generator. The

alternator also produces more output during idle which makes it ideal for late model

vehicles.

The alternator has a spinning magnetic field. The output windings (stator) are

stationary. As the magnetic field rotates, it induces current in the output windings.

Alternator Construction

Knowledge of the construction of an alternator is required before you can understand

the proper operation, testing procedures, and repair procedures applicable to an

alternator.

The primary components of an alternator are as follows:

ROTOR ASSEMBLY (rotor shaft, slip rings, claw poles, and field windings)

STATOR ASSEMBLY (three stator windings or coils, output wires, and stator core)

RECTIFIER ASSEMBLY (heat sink, diodes, diode plate, and electrical terminals)

ROTOR ASSEMBLY (fig. 2-22).- The rotor consists of field windings (wire wound

into a coil placed over an iron core) mounted on the rotor shaft. Two claw-shaped pole

pieces surround the field windings to increase the magnetic field

STATOR ASSEMBLY (fig. 2-24).- The stator produces the electrical output of the

alternator. The stator, which is part of the alternator frame when assembled, consists of

three groups of windings or coils which produce three separate ac currents. This is

known as three-phase output. One end of the windings is connected to the stator

assembly and the other is connected to a rectifier assembly. The windings are wrapped

around a soft laminated iron core that concentrates and strengthen the magnetic field

around the stator windings. There are two types of stators-

·       Y -type stator and

·       Delta type stator.

Figure

Figure 2-24.- Stator assembly.

The Y-type stator (fig. 2-25) has the wire ends from the stator windings connected to a

neutral junction. The circuit looks like the letter Y. The Y-type stator provides good

current output at low engine speeds.

 

 

Figure 2-25.- Electrical diagram indicating a Y-type stator

 

The delta-type stator (fig. 2-26) has the stator wires connected end-to-end. With no

neutral junction, two circuit paths are formed between the diodes. A delta-type stator is

used in high output alternators.

 

 

Figure 2-26.- Electrical diagram indicating a delta-type stator.

 

RECTIFIER ASSEMBLY.

The rectifier assembly, also known as a diode assembly, consists of six diodes used to

convert stator ac output into dc current. The current flowing from the winding is

allowed to pass through an insulated diode. As the current reverses direction, it flows

to ground through a grounded diode. The insulated and grounded diodes prevent the

reversal of current from the rest of the charging system. By this switching action and

the number of pulses created by motion between the windings of the stator and rotor, a

fairly even flow of current is supplied to the battery terminal of the alternator.

The rectifier diodes are mounted in a heat sink (metal mount for removing excess heat

from electronic parts) or diode bridge. Three positive diodes are press-fit in an

insulated frame. Three negative diodes are mounted into an uninsulated or grounded

frame.

 

 

When an alternator is producing current, the insulated diodes pass only outflowing

current to the battery. The diodes provide a block, preventing reverse current flow

from the alternator. Figure 2-27 shows the flow of current from the stator to the

battery.

A cross-sectional view of a typical diode is shown in figure 2-28. Note that the figure

also shows the diode symbol used in wiring diagrams. The arrow in this symbol

Indicates the only direction that current will flow. The diode is sealed to keep moisture out. 

 

 

Alternator Operation

The operation of an alternator is somewhat different than the dc generator. An

alternator has a rotating magnet (rotor) which causes the magnetic lines of force to

rotate with it. These lines of force are cut by the stationary (stator) windings in the

alternator frame, as the rotor turns with the magnet rotating the N and S poles to keep

changing positions. When S is up and N is down, current flows in one direction, but

when N is up and S is down, current flows in the opposite direction. This is called

alternating current as it changes direction twice for each complete revolution. If the

rotor speed were increased to 60 revolutions per second, it would produce 60-cycle

alternating current.

Figure

Figure 2-27.- Current flow from the stator to the battery.

ALTERNATOR OUTPUT CONTROL

A voltage regulator controls alternator output by changing the amount of current flow

through the rotor windings. Any change in rotor winding current changes the strength

of the magnetic field acting on the stator windings. In this way, the voltage regulator

can maintain a preset charging voltage.

The three basic types of voltage regulators are

as follows:

1.     Contact point voltage regulator, mounted away from the alternator in the engine compartment

      2.  Electronic voltage regulator, mounted away from the alternator in the engine compartment

       3.  Electronic voltage regulator, mounted on the back or inside the alternator

The contact point voltage regulator uses a coil, set of points, and resistors that limits

system voltage. The electronic or solid-state regulators have replaced this older type.

For operation, refer to the "Regulation of Generator Output" section of this chapter.

 

The electronic voltage regulators use an electronic circuit to control rotor field strength

and alternator output. It is a sealed unit and is not repairable. The electronic circuit

must be sealed to prevent damage from moisture, excessive heat, and vibration. A

rubber like gel surrounds the circuit for protection.

An integral voltage regulator is mounted inside or on the rear of the alternator. This is

the most common type used on modern vehicles. It is small, efficient, dependable, and

composed of integrated circuits.

An electronic voltage regulator performs the same operation as a contact point

regulator, except that it uses transistors, diodes, resistors, and capacitors to regulate

voltage in the system. To increase alternator output, the electronic voltage regulator

allows more current into the rotor windings, thereby strengthen the magnetic field

around the rotor. More current is then induced into the stator windings and out of the

alternator.

To reduce alternator output, the electronic regulator increases the resistance between

the battery and the rotor windings. The magnetic field decreases and less current is

induced into the stator windings.

Alternator speed and load determines whether the regulator increases or decreases

charging output. If the load is high or rotor speed is low (engine at idle), the regulator

senses a drop in system voltage. The regulator then increases the rotors magnetic field

current until a preset output voltage is obtained. If the load drops or rotor speed

increases, the opposite occurs.

Alternator Maintenance

Alternator testing and service call for special precautions since the alternator output

terminal is connected to the battery at all times. Use care to avoid reversing polarity

when performing battery service of any kind. A surge of current in the opposite

direction could bum the alternator diodes.

·       Do not purposely or accidentally "short" or "ground" the system when disconnecting wires or connecting test leads to terminals of the alternator or regulator. For example, grounding of the field terminal at either alternator or regulator will damage the regulator. Grounding of the alternator output terminal will damage the alternator and possibly other portions of the charging system.

·       Never operate an alternator on an open circuit. With no battery or electrical load in the circuit, alternators are capable of building high voltage (50 to over 110 volts) which may damage diodes and endanger anyone who touches the alternator output terminal.

Alternator maintenance is minimized by the use of pre lubricated bearings and longer

lasting brushes. If a problem exists in the charging circuit, check for a complete field

circuit by placing a large screwdriver on the alternator rear-bearing surface. If the field

circuit is complete, there will be a strong magnetic pull on the blade of the

screwdriver, which indicates that the field is energized. If there is no field current, the

alternator will not charge because it is excited by battery voltage.

Should you suspect troubles within the charging system after checking the wiring

connections and battery, connect a voltmeter across the battery terminals. If the

voltage reading, with the engine speed increased, is within the manufacturer's

recommended specification, the charging system is functioning properly. Should the

alternator tests fail, the alternator should be removed for repairs or replacement. Do

NOT forget, you must ALWAYS disconnect the cables from the battery first.

ALTERNATOR TESTING

To determine what component( s) has caused the problem, you will be required to

disassemble and test the alternator.

ROTOR TESTING.- To test the rotor for grounds, shorts, and opens, perform the

following:

To check for grounds, connect a test lamp or ohmmeter from one of the slip rings to

the rotor shaft (fig. 2-29). A low ohmmeter reading or the lighting of the test lamp

indicates that the rotor winding is grounded.

Figure 2-29.- Testing rotor for grounds.

To check the rotor for shorts and opens, connect the ohmmeter to both slip rings, as

shown in figure 2-30. An ohmmeter reading below the manufacturer's specified

resistance value indicates a short. A reading above the specified resistance value

indicates an open. If a test lamp does not light when connected to both slip rings, the

winding is open.

         

Figure 2-30.- Testing the rotor for opens and shorts.

 

STATOR TESTING.- The stator winding can be tested for opens and grounds after it

has been disconnected from the alternator end frame.

If the ohmmeter reading is low or the test lamp lights when connected between each

pair of stator leads (fig. 2-31), the stator winding is electrically good.

A high ohmmeter reading or failure of the test lamp to light when connected from any

one of the leads to the stator frame (fig. 2-32) indicates the windings are not grounded.

It is not practical to test the stator for shorts due to the very low resistance of the

Winding

Figure 2-31.- Testing a stator for opens.

Figure 2-32.- Testing a stator for grounds.

 

DIODE TESTING.- With the stator windings disconnected, each diode may be tested

with an ohmmeter or with a test light. To perform the test with an ohmmeter, proceed

as follows:

Connect one ohmmeter test lead to the diode lead and the other to the diode case (fig.

2-33). Note the reading. Then reverse the ohmmeters leads to the diode and again note

the reading. If both readings are very low or very high, the diode is defective. A good

diode will give one low and one high reading.

An alternate method of testing each diode is to use a test lamp with a 12-volt battery.

To perform a test with a test lamp, proceed as follows:

Connect one of the test leads to the diode lead and the other test lead as shown in

figure 2-34. Then reverse the lead connections. If the lamp lights in both checks, the

diode is defective. Or, if the lamp fails to light in either direction, the diode is

defective. When a good diode is being tested, the lamp will light in only one of the two

checks

 

\

Figure 2-34.- Testing diodes with a test lamp.

CHARGING SYSTEM TEST

Charging system tests should be performed when problems point to low alternator

voltage and current. These tests will quickly determine the operating condition of the

charging system. Common charging system tests are as follows:

Charging system output test-measures current and voltage output of the charging

system.

Regulator voltage test- measures charging system voltage under low output, low load

conditions.

Regulator bypass test- connects full battery voltage to the alternator field, leaving the

regulator out of the circuit.

Circuit resistance tests- measures resistance in insulted and grounded circuits of the

charging system.

Charging system tests are performed in two ways- by using a load tester or by using a

volt-ohm-millimeter (VOM/ multimeter). The load tester provides the accurate method

for testing a charging system by measuring both system current and voltage.

Charging System Output Test

The charging system output test measures system voltage and current under maximum

load. To check output with a load tester, connect tester leads as described by the

manufacturer, as you may have either an inductive (clip-on) amp pickup type or a no inductive

type tester. Testing procedures for an inductive type tester are as follows:

With the load tester controls set as prescribed by the manufacturer, turn the ignition

switch to the RUN position. Note the ammeter reading.

Start the engine and adjust the idle speed to test specifications (approximately 200

rpm).

Adjust the load control on the tester until the ammeter reads specified current output.

Do not let voltage drop below specifications (about 12 volts). Note the ammeter

reading.

Rotate the control knob to the OFF position. Evaluate the readings.

To calculate charging system output, add the two ammeter readings. This will give you

total charging system output in amps. Compare this figure to the specifications within

the manufacturer's manual.

Current output specifications will depend on the size (rating) of the alternator. A

vehicle with few electrical accessories may have an alternator rated at 35 amps,

whereas a larger vehicle with more electrical requirements could have an alternator

rated from 40 to 80 amps. Always check the manufacturer's service manual for exact

values.

If the charging system output current tested low, perform a regulator voltage test and a

regulator bypass test to determine whether the alternator, regulator, or circuit wiring is

at fault.

Regulator Voltage Test

A regulator voltage test checks the calibration of the voltage regulator and detects a

low or high setting. Most voltage regulators are designed to operate between 13.5 to

14.5 volt range. This range is stated for normal temperatures with the battery fully'

charged. Regulator voltage test procedure is as follows:

Set the load tester selector to the correct position using the manufacturer's manual.

With the load control OFF, run the engine at 2,000 rpm or specified test speed. Note

the voltmeter reading and compare it to the manufacturer's specifications.

If the voltmeter reading is steady and within manufacturer's specifications, then the

regulator setting is okay. However, if the volt reading is steady but too high or too low,

then the regulator needs adjustment or replacement. If the reading were not steady, this

would indicate a bad wiring connection, an alternator problem, or a defective

regulator, and further testing is required.

Regulator Bypass Test

A regulator bypass test is an easy and quick way of determining if the alternator,

regulator, or circuit is faulty. Procedures for the regulator bypass test is similar to the

charging system output test, except that the regulator be taken out of the circuit. Direct

battery voltage (unregulated voltage) is used to excite the rotor field. This should allow

the alternator to produce maximum voltage output.

Depending upon the system there are several ways to bypass the voltage regulator. The

most common ways are as follows:

Sorting a test tab to ground on the rear of the alternator (if equipped).

Placing a jumper wire across the battery and field terminals of the alternator.

With a remote regulator, unplug the wire from the regulator and place a jumper wire

across the battery and field terminals in the wires to the alternator.

 

CAUTION

Follow the manufacturer's directions to avoid damaging the circuit. You must NOT

Short or connect voltage to the wrong wires or the diodes or voltage regulator may be

Ruined.

When the regulator bypass test is being performed, charging voltage and current

INCREASE to normal levels. This indicates a bad regulator. If the charging voltage

And current REMAINS THE SAME, then you have a bad alternator.

 

CIRCUIT RESISTANCE TEST

A circuit resistance test is used to locate faulty wiring, loose connections, partially

Burnt wire, corroded terminals, or other similar types of problems.

There are two common circuit resistance tests- insulated resistance test and ground

Circuit resistance test.

 

INSULATED RESISTANCE TEST

To perform an insulated resistance test, connect the load tester as described by the

Manufacturer. A typical connection setup is shown in figure 2-35. Note how the

Voltmeter is connected across the alternator output terminal and positive battery

Terminal.

With the vehicle running at a fast idle, rotate the load control knob to obtain a 20-amp

Current flow at 15 volts or less. All accessories and lights are to be turned OFF. Read

the voltmeter. The voltmeter should NOT read over 0.7-volt drop (0.1 volt per

Electrical connection) for the circuit to be considered in good condition. However, if

The voltage drop is over 0.7 volt, circuit resistance is high and a poor electrical Connection exists.

 

GROUND CIRCUIT RESISTANCE TEST

With the ground circuit resistance test the voltmeter leads are placed across the

Negative battery terminal and alternator housing (fig. 2-36).

The voltmeter should NOT read over 0.1 volt per electrical connection. If the reading

is higher, this indicates such problems as loose or faulty connections, burnt plug

Sockets, or other similar malfunctions

 

6.6.2 Charge balance calculation

The charge balance or energy balance of a charging system is used to ensure that the alternator can cope with all the demands placed on it and still charge the battery. The following steps help to indicate the size of alternator required or to check if the one fitted to a vehicle is suitable.

As a worked example, the figures from Table 6.1

will be used. The calculations relate to a passenger car with a 12 V electrical system. A number of steps are involved.

1. Add the power used by all the continuous and prolonged loads.

2. Total continuous and prolonged power (P1) _440W.

3. Calculate the current at 14 V (I = W/V) _ 31.5 A.

4. Determine the intermittent power (factored by 0.1) (P2) _ 170W.

5. Total power (P1 +P2) =610W.

6. Total current =610/14 = 44 A.

 

Electrical component manufacturers provide tables to recommend the required alternator, calculated from the total power demand and the battery size. However, as a guide for 12 V passenger cars, the rated output should be about 1.5 times the total current demand (in this example 44 _ 1.5 _ 66 A).

Manufacturers produce machines of standard sizes, which in this case would probably mean an alternator rated at 70 A. In the case of vehicles with larger batteries and starters, such as for diesel-powered engines and commercial vehicles, a larger output alternator may be required. The final check is to ensure that the alternator output at idle is large enough to supply all continuous and prolonged loads (P1) and still charge the battery. Again the factor of 1.5 can be applied. In this example the alternator should be able to supply (31.5 _ 1.5) _ 47 A, at engine idle. On normal systems this relates to an alternator speed of about 2000 rev/min (or less). This can be checked against the characteristic curve of the alternator.