International Islamic University Malaysia

Charging System

Dr. Md. Ataur Rahman

Department of Mechanical Engineering, Kulliyyah of Engineering, IIUM

Automotive Charging System

§  The charging system converts mechanical energy into electrical energy –  IC engine powered car battery charges when engine runs. –  Electrical car battery charges when the charger plug-in with 220

volt ac power. §  The main function of automobile charging system is to

control and regulate the charge in the battery of a vehicle. –  When the charging system voltage is more than the vehicle

electrical load needed, it sends to the battery for charging.

§  Automobile charging system generally generates a voltage between 13.5 and 14.4 volts when the engine is working.

§  It produces electrical current for operating automobile lights, music systems, heater, engine electrical system and other electrical components.

§  Charging Voltage – The main consideration for the charging voltage

is the battery terminal voltage when fully charged.

–  If the charging voltage is set to this fully charged value then there is no risk of overcharging the battery. § This is known as constant voltage charging technique

–  The 14.18 to 14.22 is the accepted charging voltage for a 12V battery

§  The charging system objective to meet the following criteria (when the engine is running:

§  Supply the current demands made by all loads §  Supply whatever charge current the battery demands §  Operate at idle speed §  Supply constant voltage under all conditions §  Have an efficient power-to-weight ratio §  Provide an indication of correct operation.

§  The main components of the charging system is the alternator-with the exception of its associated wiring.

§  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. –  Alternator produces AC voltage which needs to convert to DC

voltage to charge the battery. –  The output of the alternator must be a constant voltage regardsless

of the engine speed and current load.

§  The alternator generates AC but must produce DC at its output terminal.

–  DC can be used to charge the battery and run electronic circuits.

–  The output of the alternator must be a constant voltage regardless of engine speed and current load.

Magnetic lines of force cutting across a conductor induce a voltage and current in the conductor.

§  Rectification of AC to DC –  A vehicle electrical circuits and battery operate with direct current

(DC). The AC current produced by the alternator needs to be converted to a DC voltage. This process is called rectification. §  Rectification process is made with the sets of diodes §  The simplest kind of semiconductor is called diode.

–  It is made of one layer of P-type material and one of N type material. –  The point where two types materials joint is called junction. –  The word of diode means having two electrodes

§  The (+ive) is anode and (- ive) is cathode. –  It can function as a switch.

§  It is used in electronic circuit as a sort of one-way check valve that will conduct electricity in one direction (forward) and block it in other direction (reverse).

§  In a DC generator, the split ring commutator is used to rectify AC voltage to DC voltage

§  In an AC generator, the AC voltage is rectified by diodes that act as an one-way check valves, allowing current to flow in one direction only.

–  In a simple circuit, if AC voltage passes through a positively biased diode, the diode will block the negative portion of sine wave.

–  To charge the battery and run vehicle components §  It must be converted from alternating current (AC) to direct current (DC). §  The common most suitable for this task is the silicon diode.

§  Rectifier circuit –  If single phase AC are passed of an alternator to DC output

§  Single diode rectifier circuit –  Its output will be half-wave rectification –  The diode will allow the positive half cycle to be conducted towards

the positive of the battery –  While, the negative cycle are blocked.

§  A four-diode bridge RC –  Its output will be full-wave rectification. –  The process of converting a complete waveform to a positive DC

waveform is called full-wave rectification.

–  Three phase AC are passed of an alternator to a DC output §  Series of six diodes rectifier circuit

–  One pair for each stator windings –  Three of the diodes are positive biased and mounted in a heat sink –  Three remaining diodes are negative biased and attached directly

to the alternator rear housing which also act like a heat sink

§  Alternator and Charging Circuits –  Three phase alternator

§  Electromagnetic induction is caused by a rotating magnet inside a stationary loop or loops of wire.

§  The stationary loop of wire are known as the stator and consist of three separate phases, each with a number of windings.

§  The windings are mechanically spaced on a laminated core (to reduce eddy currents).

§  Must be matched number of poles on the rotor §  The three-phase windings of the stator can be connected in two ways:

–  Star –  Delta

§  Star connection can be though of as a type of series connection of the phases –  The output voltage across any two phases will be the vector sum of the

phase voltages –  Current output will be the same as the phase current –  Star-wound stators therefore, produce a higher voltage, whereas delta-wound

stators produce a higher current

§  Full-wave rectification in a delta wound is accomplished with the same diode rectification of wye (star) wound stator.

–  Difference – current flows from each winding all the time and this increase the alternator output current compared to the wye wound alternator.

§  While a good quality diode will block reverse flow up to a pressure of about 400V

–  It will require only a small voltage pressure of about 0.6V to conduct in the forward direction.

- 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.

Delta-connected stator winding.

§  A star connection can be thought of as a type of series connection of the phases –  If any of the phases is absent

§  The output voltage across any two phases will be the vector sum of the phases voltage.

§  Current output will be same as the phase current. §  A star-wound alternator produces a higher voltage than a delta-wound stator. §  But, a delta-wound alternator produces a higher current than a star-wound

stator. –  For a star-wound stator,

p

p

IIVV

=

= 3

–  A delta connection can be thought of as a type of parallel circuit. §  The output voltage will be the same of the phase voltage, but the

output current is the vector sum of the phase current. –  For a delta-wound stator,

where, V =output voltage, Vp=phase voltage, I=output current, Ip=phase current

§  Heavy vehicle are used the delta connected/winding alternator for the advantages of higher current output

–  But, most vehicle uses star winding alternator. §  The frequency of an alternator output can be calculated as;

( )rpminspeedalternatorn

pairspolehasrotorclawpairpoleofnopHzinFrequencyfwhere

pnf

=

==

=

,612.,,60

3p

p

II

VV

=

=

Charging System-Inducing EMF (Voltage)

§  Improving the Voltage Induction: –  A magnetic field and a conductor or conductors are required. –  Conductor must be movable (rotation-able) –  Lines of force must be cut – parallel movement has no effect. –  Increasing the speed of cutting increases the induce of EMF or voltage –  Increasing the strength increases the induced EMF –  Increasing the number of conductors increases the induced EMF.

Charging System –Field Diodes

§  Field Diodes and Rectification –  The rotor field must be energized (excited) to enable the alternator

to operate. §  It could be permanent magnet or electro-magnet

–  The current can be obtained from the battery for electro-magnet or from the alternator itself due to the permanent magnet.

–  When current from the battery is used, the alternator is referred to being externally excited.

–  While, when the current from the alternator stator is used, then the alternator is self excited.

–  Self excited alternators use three extra diodes, making nine diodes in all. These three diodes are known as field diodes or excited diodes, and other six diodes are known as power diodes.

–  Field diodes are used to rectify the un-rectified AC voltage of the power diodes.

§  Field Diodes

§  When a star-wound startor is used, the addition of the voltage at the neutral point of the star is, in theory 0 volt.

§  But, in practice due to the slight inaccuracies in the construction of the stator and rotor, a potential voltage develop at this point.

–  This potential is known as the third harmonic. §  Its frequency is 3 times the fundamental frequency of the phase windings. §  By employing two extra diodes, one positive and one negative connected to the

star point, the energy can be collected. Photo §  This can increase the power output of an alternator up to 15%.

Alternator Performance

§  Field Diodes: increase alternator performance by 10% §  Third Harmony: Increases alternator performance by

5-15%

Charging System-Voltage Regulator §  Voltage Regulators

–  Alternators are fitted with a regulator to control their voltage. –  A voltage regulator is needed because the alternator produces a rising

voltage. –  The voltage is related to alternator speed –  the faster the rotor spins, the higher the voltage. –  Regulation is achieved by varying the strength of the rotor of the field. –  Transistor and integrated circuit are the voltage regulator of the

alternator.

§  Vehicle Electrical Loads on Alternator –  The loads placed on an alternator can be considered

as : §  Continuous load: used continuously and the charging

system will have to keep up. §  Prolonged load: keep it on until turn off. §  Intermittent load: used infrequently. Such as the heated

rear windows and seat heaters.

–  The charging system of a modern vehicle has to cope with high demands under many varied conditions. § Output that may be required power used by the individual

components and add this total to power required to charge battery

§  The current draw about 0.5A at 14 and 28V.

Continuous Load Power (W)

Current (amp)

14V 28V

30 2.0 1.0

Fuel Injection 70 5.0 2.5

Fuel Pump 70 5.0 2.5

Instruments 10 1.0 0.5

Total 180 13.0 6.5

Side and tail lights 30 2.0 1.0

Number plate lights 10 1.0 0.5

Headlight mean beam 200 15.0 7.0

Headlight dip beam 160 12.0 6.0

Dashboard light 25 2.0 1.0

Radio/CD 15 1.0 0.5

Total 260 19.5 9.5

Heater 50 3.5 2.0

Indicator 50 3.5 2.0

Brake light 40 3.0 1.5

Front Wippers 80 6.0 3.0

Horn 40 3.5 1.5

and etc 1440 106 53.5

1700

§  Charging system regulation –  A vehicle’s electrical system and battery must be protected from

unstable or excessive charging system voltage.

–  Computer sensitive system is the best one to control the alternator output for both in charging and vehicle load demand

.

§  Adjust the alternator for varying the load demands §  While at the same time keeping the battery fully charged.

§  It is noted that –  Battery will gradually run down over time if the charging

current is low – While, the vehicle electrical circuit or the battery

component will be damaged if the charging current is higher than needed.

–  A figure of V is used for all 12 V battery charging system

2.02.14 ±

§  Charging circuits –  The charging circuit is one of the simplest on the vehicle.

§  The main output is connected to the battery via a suitable sized cable

§  The warning light is connected to an ignition supply on one side and to the alternator terminal at the other.

–  The output of the alternator is often connected to the starter main supply simply for the convenience of wiring.

–  If the wire are kept as short as possible, reduce the voltage drop across the main supply wire.

§  Testing procedure –  Connecting voltmeter across the battery and an ammeter in series

§  Battery must be 70% charge (i.e., 8.4 volt) §  Regulated voltage (ammeter reading 10A or less) :14.2 0.2 Volt §  Circuit voltage drop 0.5V maximum.

§  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.

±

Typical voltage regulator voltage range.

General Motors CS generator.

Notice the use of zener diodes in the rectifier to help control any high-voltage surges that could affect delicate computer circuits.

If a high-voltage surge does occur, the zener diode(s) will be reverse biased and the potentially harmful voltage will be safely conducted to ground.

Voltage must be preset at the L terminal to allow the generator to start producing current.

The alternator field (rotor) current is controlled by the computer.

SMEC stands for single module engine controller.