CHARGING CIRCUITS

IN THIS SECTION, YOU WILL FIND THE FOLLOWING SUBSECTIONS:

GENERATORS
ALTERNATORS
MAGNETO TYPE ALTERNATORS
THE CHARGING SYSTEM
RECTIFIERS
REGULATORS
GENERATOR CIRCUIT
LOW CURRENT ALTERNATOR CIRCUITS
HIGH CURRENT ALTERNATOR CIRCUITS
CHARGING CIRCUITS
THE 1.25 A AND 3 A UNREGULATED SYSTEM
THE 30 A CHARGING SYSTEM

GENERATORS

When current flows through conductor, magnetic field created around conductor. When conductor passes through magnetic field, electricity generated. Placing wire into form coil intensifies either magnetic field created by electricity passing through conductor or amount of electricity generated when conductor passed through magnetic field. Coil passed through magnetic field generates more electricity than straight conductor. If you double turns in coil, double electricity produced; if you triple turns in coil, triple amount of electricity.

It’s on these principles that generator and alternator create electricity. This electricity primarily used to charge battery, operate accessories on outdoor power equipment. For years, generators were widely used charging system devices. Alternators have replaced generators because generator’s output voltage at low engine rpm very low. Voltage, current flow from higher voltage potentials to lower voltage potentials, so generator’s output must be greater than voltage of battery for charging.

Alternators produce higher voltage at lower rpm. We’ll refer to generator as device that produces DC voltage, alternator as device that produces AC voltage. Consider basic generator. Armature or rotor of generator consists of loops copper wire. Loops connected to two copper strips that serve as commutator. Commutator is connection point between armature and brushes. Armature is all of conductive coils in which electricity produced when coils rotate through magnetic field. Brushes make contact with rotating commutator, transfer generated electricity to device being powered. Brushes made of soft conductive material such as carbon graphite. This allows brushes to rub against commutator while its rotating without damaging. Armature shaft supported by bearings at each end. Since many coils wire in armature, commutator requires many segments of copper on which brushes will ride. In general, output voltage generator isn’t pure smooth DC; is pulsating.

Pulsating voltage doesn’t influence operation battery or accessories; at low rpm, may notice flickering of headlights. The three main problems with generators are: (1) worn brushes, (2) dirty, burnt, or pitted commutator segments, (3) mechanical problems such as bent shaft, slipping belt, or failed bearings.

ALTERNATORS

Alternators produce AC rather than DC. So require output voltage to be rectified or converted to DC before it’s placed on battery or accessory circuits.

In generator, magnetic field stationary, armature rotates in magnetic field. In alternators used in small engine applications, armature stationary and magnetic field rotates around it. When armature doesn’t move, known as stator. Rotating portion of alternator produces magnetic field by means permanent magnets.

Two types alternators used on outdoor power equipment. Most uses alternator that’s part of magneto system. Flywheel contains up to six magnets just for charging system. Magnets rotate over stator mounted under flywheel. Magnets placed away from electronic ignition coil and its magnets. As flywheel turns, so do magnets. This provides moving magnetic field to generate voltage in stator windings.

Second type alternator is stand alone unit belt driven off crankshaft by means pulleys and belt. Belt driven alternator produces more current than magneto type.

Whether or not dynamo or alternator effectively charging can be easily tested. Select multimeter scale for +12 volt readings and check voltage coming from battery. Then start engine, run it up to 2,000 rpm, and read battery voltage a second time. If alternator charging properly, new reading will be from 1 to 2.5 volts higher than first reading, the reading taken when engine was idle.

MAGNETO TYPE ALTERNATORS

Consider two small stator assemblies for 1.25 A and 3 A charging systems. These usually found under flywheel. Stators have six coils used solely for electricity for accessories such as lights and horn. The 1.25 A stator has two poles used for charging battery. The 3 A stator has four poles for charging battery. Flywheel contains two magnets on inside surface.

Will be semiconductor device called diode in one of stator’s leads. Diode converts AC current from charging stator to DC current for battery. Lighting and accessories operate on AC output. Diode acts as one-way street for electricity. When stator applies positive cycle voltage and current to diode, diode conducts and passes positive cycle to battery. When negative cycle AC applied to diode, diode blocks voltage and current from passing to battery. This known as rectification.

Consider magneto mounted stator for 15 A charging system. Stator will possess large number of poles. This type stator receives magnetism by six ceramic magnets mounted to flywheel. Ceramic magnet is permanent magnet made from ceramic and magnetic powders. Mixture pressed and fired in kiln.

Consider 25 A stator and 30 A stator. In these stators, all stator poles wound rather than blank poles like in 15 A stator. 30 A stator has two sets output leads.

Other type alternator used on larger equipment belt driven. Consider internal rotor of separately mounted alternator. Rotor energized with electricity to create rotating magnetic field. Rotor consists of shaft on which series of coils pressed. Coils connect to voltage source by slip rings on which brushes ride. Two slip rings are continuous bands of copper insulated from shaft by mica or plastic slip ring mounts. When brushes pass electricity to rotor coils, magnetic field created around pole pieces.

Consider typical set stator windings. Windings typically pressed into alternator’s case assembly. Windings connected at one common point, exit as separate leads (three in diagram). Rotor turned by belt drive, creates spinning magnetic field in stator windings. Once per revolution of rotor, each stator winding creates electricity. Once per revolution, stator produces three AC pulses electricity. AC output then rectified, sent on to battery.

THE CHARGING SYSTEM

RECTIFIERS

Rectification is conversion AC to DC. Rectification changes voltage that alternates between positive and negative to voltage either all positive or all negative. DC generator designed so output voltage always positive. Voltage produced by DC generator dependent on speed at which generator turns. More quickly generator turns, greater the voltage.

Alternator produces alternating voltage, one that’s sometimes negative, sometimes positive. Alternator designed to produce higher voltage at low rpm, so can power electric system even if engine not running at full power. Voltage from alternator needs to be modified so can be supplied to circuit as always positive. Modification made by device known as rectifier. Rectifier uses diode to convert AC to DC. Half wave rectifier prevents negative portion AC signal from being supplied to circuit.

Full wave rectifier uses both positive and negative portions AC voltage. Diodes connected electrically so can supply negative portion of AC as positive voltage pulse. Full wave rectifier supplies peak voltage twice as often as half wave rectifier, so more efficient.

REGULATORS

Generators and alternators produce electricity to recharge battery after engine has started, provide power for horn, lights, other accessories. Small charging systems such as 1.25 A and 3 A magneto mounted alternators require only simple diode to rectify AC to DC for battery charging. Larger alternators require both rectifier and regulator in charging circuits. Rectifier provides same function as diode: to convert AC to DC. Regulator provides separate function.

If high current charging system connected to battery, battery will overcharge. Can create excessive heat on plates of battery. Once plates get hot enough, electrolyte becomes warm, begins to bubble. This termed “boiling” a battery. If process continues, electrolyte evaporates, leaving battery completely dry and dead. Regulator used to monitor battery voltage, disconnects charging system from battery once battery of sufficient voltage. If battery voltage drops during operation of equipment, regulator again connects charging system.

GENERATOR CIRCUIT

Consider older voltage regulator system. System employs relays. Relays are electromechanical devices. This means that electricity is applied to their coils, causing mechanical motion of metal armature. Contact sets attached to armature. If coil energized and armature attracted to coil, contact sets either open or close a circuit.

This regulator makes use three relays: (1) a voltage regulator relay, (2) a current regulator relay, (3) a cutout relay. Voltage and current regulator relays monitor and control output of generator. Cutout relay opens generator circuit if generator turning slowly. If generator turning slowly, output voltage low. If output voltage lower than voltage battery, and generator remains connected to battery, battery will discharge through generator. Voltage and current flow from higher voltage potential (the battery) to lower voltage potential (the generator).

LOW CURRENT ALTERNATOR CIRCUITS

When small engine contains magneto mounted alternator up to 3 A, no need for regulator. Simple diode rectifier (or full wave rectifier) used to convert AC to DC. Low current is constantly applied to battery for recharging. Lights and horns powered by separate winding of stator. Diode rectifier normally placed in series with one of alternator’s stator wires.

HIGH CURRENT ALTERNATOR CIRCUITS

Once alternator capable producing more than 3 A charging current, possibility of overcharging or boiling battery. Charging circuit must contain rectifier and regulator to control stator’s output.

Rectifier regulator assembly with various stator assemblies which are electronic devices containing diodes, silicon controlled rectifiers (SCRs), and transistors. These devices usually molded and sealed in plastic case, so little opportunity for components replacement. These rectifier regulator assemblies typically mounted on blower assembly to keep airflow circulating over the devices to keep them cool.

Rectifier portion of device normally a full bridge rectifier. Bridge rectifier uses four diodes to rectify AC voltage. When full wave rectifier used, both positive and negative half cycles of AC waveform rectified into pulsating DC waveform. Full wave rectifier more efficient in passing electricity from stator to battery due to more continuous peaks full voltage.

Regulator portion of circuit monitors battery voltage. If battery voltage level falls below preset limit, regulator closes circuit, allowing rectifier voltage to be passed to battery for charging. If battery voltage higher than preset limit, regulator opens circuit to battery, stopping flow charging current.

CHARGING CIRCUITS

Primary purpose charging system to maintain proper electric charge in battery. System designed to maintain battery, not power any other accessories. This system rated at 1.25 A output at 12 volts DC. Is a full wave rectifier but no regulator in circuit. Electricity generated by two magnets embedded in flywheel. As magnets pass stator coils, AC voltage generated. This voltage passes into full wave bridge rectifier.

Output of rectifier connected to keyswitch. When keyswitch turned to right, battery’s electricity used to energize heavy duty relay called a solenoid. The coil in solenoid energized, closing solenoid’s contacts. These contacts connected between battery positive terminal and electric motor called a starter. Holding keyswitch in this position causes starter to turn, starting engine.

Once engine started, keyswitch released. Internal spring allows switch to return to center or run position. Output of charging system’s rectifier connected through fuse and meter to battery. Fuse protects circuit from excessive currents that may develop due to short or voltage surge. Meter is DC ammeter displaying charging or discharging current.

THE 1.25 A AND 3 A UNREGULATED SYSTEM

Consider slightly more complex version 1.25 A unregulated charging system. System contains simple diode half wave rectifier. Diode located in wiring harness from stator. Circuit operates through keyswitch, however stator of alternator also contains extra poles and windings to power accessory lighting system. Another addition to circuit is oil sentry switch. Are two different switches used in oil monitoring system. First is oil pressure switch that’s open as long as there’s enough oil pressure in system. If oil pressure drops, switch closes. A ground connected to kill terminal of electronic ignition system stops engine. Second switch places ground on side of light bulb. Since alternator’s output connected to top side light bulb, bulb illuminates if crankcase oil pressure drops below preset level. This system can have additional stator for powering lights. 15 A and 25 A charging circuits similar, but use different type stator and rectifier regulator.

THE 30 A CHARGING CIRCUIT

30 A regulated charging system different from previous charging circuits. Look at alternator. One red output lead from stator attached to ground. To black wires provide AC output to rectifier. Final wire from stator, also red, connects to regulator section of stator. Regulator winding energized by magnets in flywheel, as are charging windings. Regulator winding provides signal to solid state regulator to allow regulator to switch charging windings in and out of battery circuit. Second major difference use self contained solenoid within starter. Accessory circuits (not shown) connected to battery through switches on dashboard of equipment.