Alternator Charging Systems

Transcription

1 Alternator Charging Systems Acquiring a detailed working knowledge of how electrical components work and being technically familiar with all aspects of the electrical systems on your race car will be of benefit to the owner driver / mechanic. Furthermore being competent and comfortable with practical electrical skills will be of advantage too. Fortunately most vehicle electrical systems (particularly those of a single seater) are fairly straight forward and simple to understand. This document is not intended to teach you all there is to know about vehicle electrics, it is merely a guide to how various key pieces of equipment operate and how they should be installed and maintained. The Alternator The fitment of a functional alternator of minimum 240 watts was made mandatory when the Zetec engine was originally phased in. Modern motor vehicles work on a 12 volt dc negative earth type system and a Formula Ford Zetec is no exception. A battery is used to start and power the car whilst the alternator is not turning. A large amount of electrical energy is stored in the battery, however it is the alternator that powers the car and keeps the battery charged. Since the alternator can be powered by either the transmission or engine, we should consider the fuel as the ultimate source of this energy. We understand that electricity in a direct current (DC) application flows from a source of positive voltage through a conductor to a load and back to a negative voltage and as it does so a circuit is made. The term negative earth simply means that the chassis structure is connected to the negative battery terminal and is used as the return path for most electrical components. Principle of Operation If you took a strong magnet and moved it back and forth over a conductive wire then you would generate a voltage within the wire. This voltage would be present only as the magnet moved relative to the wire and would be positive in one direction, then negative as the magnet is pulled back in the other. If you were to loop the wire many times over to form a coil then repeat then experiment a significantly greater voltage could be generated. These principles were first properly discovered by Michael Faraday in the mid 1830 s and explain that as an electrical conductor moves through a magnetic field, a voltage is generated within it. The size of this voltage depends upon three main factors; the strength of the magnetic field, the length of the cutting wire and the speed at which they move relative to one another. Inside the alternator there is a magnetic rotor and stator. The cutting wire, it is arranged into coils to increase its length. These stationary coils form the stator and are positioned around the outside of a rotating magnet called the rotor. As the magnet sweeps past the stator coils a voltage is generated between the two ends of the wire. This voltage switches polarity each time the magnet moves through 180 degrees. One complete rotation of the magnet results in a voltage sine wave as first the north pole of the magnet generates a positive voltage and then the south pole generates a negative voltage.

2 Alternating Voltage Time How Voltage Alternates According to Magnet Position (Single Phase) The Rotor The magnetic rotor is turned by a pulley, which may be driven by either the engine or transmission. In reality the rotor is actually an electromagnet, consisting of a wire coil encased inside a soft steel or iron body, when a direct current is passed through this wire a magnetic field is created. If we look again at the factors governing the generation of voltage we understand that if we can control the strength of this magnetic field, we can control the output voltage of the alternator. A voltage regulator controls the amount of current being drawn by the rotor coil (and hence the strength of it magnetic field) to regulate the output between 13.5 and 14.5 volts. This is how the alternator manages to achieve a nearly constant output voltage over a wide range of rotational speed and is able to charge the battery even as the engine idles. Most alternators have what is known as a self excited rotor. A set of three diodes provide current directly from the stator windings to the rotor slip rings, so the alternator actually produces all of the power necessary to produce the electromagnetic field, hence the term self excited. However, before the alternator starts turning there is not always enough residual magnetism in the rotor for generation to begin, so power must be taken initially from the battery via the ignition switch.

3 Alternating Voltage The Stator & Rectification A single phase stator consists of a pair of coils positioned directly opposite each other around the outside of the rotor. As we can see from the graph on page two the voltage across these coils is constantly changing polarity. Any current drawn from this circuit would also change direction as current always flows from a negative voltage towards a positive one. This is known as alternating current or AC. The frequency at which it alternates is equal to the number of complete magnet revolutions per second and is measured in Hertz (Hz). The problem is we need direct current (DC) to charge our race car s battery and run all of the vehicles systems. Fortunately we can convert AC into DC with an arrangement of diodes in what is known as full wave form rectification. Diode Bridge Arrangement Diode Bridge Arrangement Stator Coil Stator Coil How a Diode Bridge Arrangement Steers the Alternating Current to Make Direct Current. A blocking diode allows power to flow in one direction only, it acts like a non return valve for electricity. Looking at the diagram above we can see how diodes can be arranged in such as way that regardless of the direction current flows from the alternator, it always flows out of the diode bridge to the battery in same direction. This diode arrangement shows that alternating current can be turned into direct current quite simply, but as you can see from the graph below the resulting DC voltage is very choppy and the average voltage is only 63% of its peak Average DC voltage 63% of peak voltage Single Phase Full Wave Rectification By adding more coils to the stator we can increase the amount of electrical power produced per revolution. In most applications three pairs of coils are positioned 120 degrees apart around the outside of the rotor to form a three phase stator winding.

4 Stator Voltage Regulator Field Diodes Slip Rings Rotor Ignition Switch Alternator Casing Negative Diodes Positive Diodes Typical Self Exciting Three Phase Alternator The stator coils need to be connected in a specific way, (usually a star arrangement) in order to bring the power out. Six diodes are used this time in order to convert the alternating current into DC. The three field diodes are necessary to provide power to the rotor. Load Direction of Current Flow Altinator G Chassis Charging System Schematic Since all three coils are equally spaced around the rotor they will each produce an identical sine wave, but each one will be delayed by 120 degrees as the rotor turns, causing them to be out of sequence. The result of three phase full wave form rectification can be seen on the following graphs. Average voltage output is very close to peak and ripple is greatly reduced. This produces a suitable direct current necessary to power the car and charge the battery.

5 Alternating Voltage Alternating Voltage Alternating Voltage Unrectified Three Phase Sine Wave Full Wave Three Phase Rectification Average DC voltage very close to peak voltage Full Wave Three Phase Rectification

6 Charging System Wiring Modern alternators come as complete units with all of the electronic parts necessary for voltage regulation and rectification and built into the casing. There are only two connections that need to be made to the race cars wiring; the output and the excitation power. The output terminal should be connected directly to the battery or to the battery side of the main isolator. A relay can be fitted to isolate the alternator when the car is shut down although this is not absolutely necessary. The excitation terminal needs to be connect to a switched power source coming from the ignition. The alternator casing provides the earth and must be securely bolted to the chassis with steel bolts. Suitable wiring and connectors MUST be used. Where a 250 watt alternator is fitted the maximum current that may be supplied at 12 volts is amps. This is actually quite a lot and a cable of adequate cross sectional area must be fitted. Aux. Power Plug 12vdc 14Ahr Main Batt. Isolator Ignition Switch Starter Push Button Starter Motor M Alternator G Earths Direct into Chassis Earths Direct into Chassis Typical Start / Charge Wiring Diagram Electrical Terminals Alternator & Wiring

7 Charging System Failure The Zetec engine was the first power plant featuring full Electronic Fuel Injection (EFI) to be used in Formula Ford. The system differs from a traditional carburettor / distributor engine where electrical power consumption is relatively low. On such engines maintaining battery voltage isn t critical, however with the fuel injected engine a fault in the charging system can causes a loss of engine power, particularly at high speeds. The fuel rail is supplied under pressure by an electrically driven DC pump and is controlled by the fuel pressure regulator, which bypasses excess fuel back to the tank. The injectors spray fuel for precise length of time known as the injector pulse width. Controlling fuel pressure relative to inlet manifold pressure ensures fuel is delivered at an known and exact rate. The injector pulse width is a very precise value calculated by the ECU to achieve peak engine performance. Because the fuel pump is run by a DC motor, the pump speed and hence discharge pressure is relative to the voltage applied to it. When the alternator is turning and the charging circuit is operating as expected the battery voltage will be between 13.5 and 14.5 volts, ensuring a good pump speed and a plentiful supply of fuel to the injectors. If however the charging system were to stop working, the battery voltage would settle out at 12 volts, then begin to drop as power is consumed. Following two or three engine starts and the onset of a 20 minute race the voltage could easily be down to 11.0 or even 10.5 volts. The motor driving the pump might have slowed enough to drop the fuel pressure below what the regulator can cope with and the amount of fuel injected per pulse may be less than intended, resulting in a loss of power. For this situation, some modern fuel injection systems have a fuel pressure sensor which compensates by making slight changes to the fuel map calculations to increase the time each injector is opened to ensure the correct amount of fuel is injected. However, being one of the first EFI engines Ford produced this level of sophistication was not available on the Zetec engine. N S Alternator Rotor and Stator Windings Alternator Removal The alternator is a mandatory feature of the Formula Ford regulations, however, if you are competing in F4 Class A or Mono 1800 it may be removed. There is defiantly a weight saving and depending on where it is located a centre of gravity saving also. But the theory that an electrical generator absorbs power from the engine / drive train is perhaps unfounded. Not accounting for friction, if a 240 watt generator is fitted and supplies all the power it can this would only equate to only 0.32 horse power.

8 Technical articles should be taken as advice only, if in any doubt consult a qualified mechanic or the parts supplier/manufacture. Author: James Chapman