Low voltage DC systems

LOW VOLTAGE DC SYSTEMS

Electrical Definitions

Direct Current:

Direct current is a form of electricity often supplied by batteries. DC has a steady voltage in a constant direction (polarity).

Volt:

Volt is the unit of electrical pressure. It can be compared to the pressure from a pump in a piped water circuit.

Ampere:

Ampere or amps are the unit of electric current. Amps can be compared to the water flowing through a water pipe.

Ohm:

Ohm is the unit of electrical Resistance. Resistance opposes the electric current (flow) resulting from a voltage (pressure). In a water circuit, resistance is like a restricting jet, or an uphill pipe.

All electric circuits and components have some resistance. Voltage is dropped across the resistance, just as water pressure is lost pushing water uphill.

Watt:

Watt is a unit of electric power, and is equal to Volts multiplied by Amps. For example:

a 12 volt light globe which draws 5 Amperes is rated as 60 Watts (12 x 5 = 60)

Conductor:

An electrical conductor is a material which will carry electric current. Most metals, sea-water, the earth, and your body are all conductors. The term is often used for a wire in an electric circuit. Wire conductors must be large enough to carry the circuit current without overheating.

Insulator:

An electrical insulator is a material which will not carry electric current. Plastics, rubber, glass, and dry air are examples. A wire is a conductor which will carry current, wrapped in an insulator to prevent it touching other wires or earth/frame. Wire insulation must be tough enough for it’s purpose and environment. The terms conductor and insulator may also be used to show a materials ability to carry or block heat, that is, thermal conductors/insulators.

Figure 1 Simple Electric Circuit

Open Circuit:

Is an electric circuit which is broken by a broken component, disconnected wire, etc. Electric current will not flow in an open circuit. In Fig 1, the switch in the off position is an open circuit.

Short Circuit:

A short-circuit (short), is a circuit connection where there should not be one, for example, insulation rubbed through on a metal bulkhead. A short circuit bypasses the normal circuit and usually stops it working. A short circuit normally reduces resistance and increases current flow, perhaps to the point where damage is done. Fuses and circuit breakers are usually placed in electric circuits to ‘blow’ and break the current if a short circuit occurs.

If the switch in Fig 1 was ‘On’, and there was an accidental short-circuit connection between the lamp wires–the lamp would not light and the resistance would be reduced, the current would increase, and the fuse would ‘blow’ to break the circuit.

Series Circuit:

A series circuit is one which has the components connected in a series, that is, one after the other. Figure 1 is a series circuit.

Figure 2 Simple Parallel Circuit

Parallel Circuit:

A parallel circuit has the components connected in parallel, one beside the other, with the same wires going to each. Fig 2 is a parallel circuit.

Earth / Earth Return Circuit:

Some systems do not use a return wire, but use another conductor instead. The earth’s soil is a conductor and this can be used (hence the name). Metal car bodies are also used as a return wire for automotive circuits, and there are also called ‘earth-return’ circuits. If the negative supply and one side of each lamp were connected to a metal chassis, Figure 2 would be an ‘earth-return’ circuit. Earth-return circuits are prohibited for vessels.

Electrical Components in Marine Circuits

Figure 3 Batteries in Series

Figure 4 Batteries in Parallel

Figure 5 Batteries in Series- Parallel

Batteries:

Batteries are made up of groups of electric cells connected together.

They use chemical reactions between two different metals and an ‘electrolyte’ to produce electricity.

They may be connected in ‘series’ (negative to positive as shown in Figure 3). This increases the supply voltage only.

Batteries connected in parallel (neg. to neg., pos. to pos. as shown in Figure 4) keep the supply voltage the same, but storage capacity is larger, to deliver more current if needed.

Batteries may also be connected in series-parallel combinations (Figure 5) to increase both voltage and current capacity if necessary.

The polarity of a DC electrical system is normally identified as negative or positive earth, that is, either the negative or positive side of the battery is taken as the earth side of the system. The polarity is usually determined by the maker of the charging system.

An easy way to identify the polarity of a system is to look at the main battery wires to the engine. The earth pole of the battery connects onto the engine block. The non-earth side of the battery is normally connected to the ships circuits via fuses or circuit breakers. The earth side is not fused.

Alternators, Generators, and Regulators:

Alternators or generators driven by the main engine, may be used to recharge vessel batteries.

Regulators are used to control alternator/generator outputs to prevent overcharging a fully-charged battery.

Generators produce DC output, and are compatible with the DC output of the battery.

Alternators produce Alternating Current (AC–the supply polarity alternates or reverses rapidly). The alternator AC output is changed to DC by internal ‘diodes’, so the alternator is compatible with the battery. Alternators are more efficient, more reliable, and longer lasting.

Fuses / Circuit Breakers:

Fuses are thin pieces of special wire (often mounted in insulating tubes) which are placed in an electric circuit to protect against short-circuits. A fuse is effectively an electrical weak link–it will burn and break the circuit before other damage is done. Insulating ceramic or glass tubes around fuses reduce the risk of fires started by electrical flashes.

Circuit-breakers are special switches which are triggered by the heating or magnetic effect of excessive currents. When they trigger, they switch off the circuit current. The circuit breaker can be simply reset, but a fuse must be replaced.

Caution! Before replacing a fuse or resetting a circuit breaker, the short-circuit must first be found and fixed.

Switches:

Switches are a convenient way of connecting and disconnecting circuit power as required. They are rated for the voltage and current they can handle, and must be matched to the circuit they will be used in. Circuits should all be equipped with a fuse and a switch.

Lamps:

Lamps are rated according to their operating voltage and wattage. The term ‘candlepower’ is sometimes incorrectly used in place of Watts. The correct lamp must be selected for each purpose.

Ignition Systems:

Petrol engines use electrical ignition circuits to produce sparks at the spark plugs for ignition. All ignition systems use the link between electricity and magnetism. (If a magnetic field moves or varies around a coil of wire, a voltage is produced in the wire–If an electric current flows in a coil of wire an electro-magnetic field is produced.)

Magneto Ignition

Magneto Ignition is used in many outboards and small petrol engines.

A magnet on the flywheel spins past a coil of wire, and this generates a voltage in the coil (see Figure 38). A current flows in the coil, and a strong electro-magnetic field is built up around it.

At the end of the engine compression stroke, a cam opens a switch (‘breaker points’) in the coil circuit. The current stops and the magnetic field collapses.

The collapsing magnetic field cuts a secondary winding of many thousands of turns of wire, and this produces a ‘High Tension’ voltage of several thousands volts. This voltage is applied to the spark plug to ignite the fuel/air in the cylinder. A capacitor or ‘condenser’ across the points reduces electrical arcing and burning of the points. If the spark is weak or absent, and the breaker points are blackened and burned, the capacitor may be faulty.

After ignition, the breaker points close again, to prepare the ignition circuit for the next cycle. If the points are not kept clean and properly adjusted, the ignition system will fail.

Figure 6 Construction of a Flywheel Magneto

Many new outboard engines have ‘electronic ignition’ circuits. These are more effective and very reliable as there are no ‘points’ to wear out and maintain. However, if they do give trouble, there is little an operator can do to make repairs at sea.

Battery Operation and Safety

Batteries are made up of ‘electric cells’ in a housing, wired in series or parallel to produce the needed voltage and capacity.

Cells may be Primary (disposable), Secondary (rechargeable), Wet (uses a liquid electrolyte), or dry (uses a damp paste electrolyte.

Common batteries in use are:

Automotive type batteries	Secondary wet battery using lead & sulphuric acid
High Power NIFE	Secondary wet cells using nickel, cadmium, alkaline
Torch Cells (disposable)	Primary dry cells using carbon, zinc & sal ammoniac
Torch Cells (long life)	Primary dry cells using an alkaline electrolyte
Torch Cells (rechargeable)	Secondary dry cells using nickel & cadmium

Correct Battery Installation and Maintenance

Marine batteries must be strongly held down in an acid proof, leak proof container at least 100mm deep
Be placed in a cool, well ventilated place

Have the terminals protected against short circuits and damage

Be provided with isolating switches. (Don’t isolate the battery when the alternator / generator is charging. This may destroy the charging system.)

· Make sure that battery polarity is correct. Incorrect polarity may cause severe electrical system damage, and injury.

· Regularly check the operation of regulators (overcharging will produce explosive hydrogen)

Learn how to interpret the meaning of ammeter and voltmeter readings. Keep batteries and battery terminals clean (flush the terminals and top of the battery clean with hot water)

· Maintain the electrolyte at the correct level (9mm above the plates), using distilled water

· Regularly check the state of charge of the battery with a voltmeter or hydrometer (Wear gloves and safety goggles if using a hydrometer)

Don’t leave batteries in an uncharged state, as ‘sulphating’ of the plates will ruin the battery.

Inspecting battery installations in vessels.

What voltage is used for the vessel circuits?

Are two separate battery systems fitted? Why/why not? Discuss this with others.

Find the fuse panel, and find out how to replace fuses.

Identify the types and current ratings of fuses.

Could the flash from a blown fuse ignite fuel vapours in the vessel, or are the fuse elements closed and ‘flash-proof’?

Is distilled water, and water for electrolyte burns available in the battery room?

Are extinguishers for electrical fires available in the battery room?

Marine Battery Safety

Heavy metals and the chemicals used in batteries are dangerous. Mishandling batteries can cause explosions, poisoning, burns, and loss of eyesight. The following precautions should be carefully followed:

· No smoking, welding, or naked flames when charging or servicing batteries, as explosive hydrogen gas is present. Do not leave cell caps off.

· Battery storage spaces must be well ventilated to remove explosive gases.

· Don’t let tools fall across battery terminals, or between the live cases of alkaline batteries. Beware if you wear rings or a metal watch band). Explosions and/or burns may result.

· Do not touch the materials used in wet or dry cells. They are poisonous. Battery electrolyte can cause severe burns to the skin, and total loss of eyesight. Use rubber gloves and safety goggles if there is any risk of contacting the electrolyte.

If battery chemicals contact the skin, flush the skin immediately for at least 10 minutes with clean, fresh water. The electrolyte from alkaline batteries is dangerously corrosive. Wash immediately with clean water. (Use vinegar, lemon juice, or boric acid wash to help neutralise the chemical. But don’t delay a water wash by looking for these remedies - they should be kept on hand if using alkaline batteries.) Use rubber gloves to avoid contact.

If battery chemicals get into the eyes, immediately flush them gently with clean, fresh, water. Continue gently flushing for at least 20 minutes, and seek urgent medical aid.
Total loss of the eyes can result, particularly from alkaline batteries. Don’t allow chemicals to splash, drip or flick from hydrometers. Use goggles to prevent electrolyte getting into the eyes.

· Wear protective clothing (rubber or plastic gloves, rubber or plastic footwear, rubber or plastic apron, and safety goggles) if handling large volumes of electrolyte. Add acid to water and stir with a non-corrosive rod if mixing electrolyte. Do not add water to the acid.

· If the electrolytes of lead-acid cells and alkaline cells are mixed, an explosion can result. Do not use the same hydrometer for lead-acid and alkaline cells.

Re-charging Batteries

Generators or alternators are used to re-charge the batteries while the main engine is running. Generators and alternators are usually driven by vee belts from the main engine.

Generators and alternators both spin coils of wire through magnetic fields to produce electric power. This is fed back into the batteries to recharge them.

Both alternators and generators produce different voltages depending on the speed of the engine, so regulators are needed to control the output.

Generators

Generators use fixed field coils (see Figure 7), while the output windings rotate with the armature. To connect the power from the rotating coils to the generator output, carbon ‘brushes’ sweep across copper strips on a ‘commutator’ at the end of the armature. The commutator segments connect each coil of the generator in turn to the output terminals to maintain a DC output with a constant polarity.

The brushes, commutator, and shaft supporting bush are all subject to wear. These must be periodically replaced.

Figure 7 Construction of a typical (Lucas) Generator

A generator can be connected into a circuit of either polarity and will automatically charge in the right direction.

The regulator consists of circuits to regulate the generator voltage, and current. The voltage regulator stops the generator overcharging the battery and damaging vessel circuits when the generator is running at high speed. The current regulator stops the generator from burning out by producing to much current.

A ‘cut-out’ is also wired into the regulator to open the generator circuit when the main engine is not running. Without the cut-out, the generator would draw current from the battery when the engine is stopped.

Figure 8 shows a typical mechanical regulator assembly used for a generator (These units are normally protected by a metal cover).

Alternators may use either a mechanical regulator consisting only of a voltage regulator section, or a ‘black box’ electronic voltage regulator.

Figure 8 Mechanical Regulator Unit for a Generator

Alternators

Brushes are used with slip rings to provide a small amount of current into a rotating field winding (see Figure 9). The output windings are fixed and hard wired out to the output through ‘diodes’ which convert the output to DC.

Alternators produce more current and last longer because:

· The short rotating armature shaft has a ball bearing each end–there is no ‘bush’ to wear out.

· The output coils are stationary and hard wired to the output–the brushes do not have to carry the heavy output current, but only a small field current

· The slip rings maintain a constant slipping connection–there is no switching commutator segments to arc and spark as the armature rotates.

Occasionally an alternator diode will break down. This fault will usually show up as a low charging rate.

Figure 9 Construction of a typical Alternator

The alternator only needs a voltage regulator, which may be mechanical or electronic. The current regulator and cut-out are not needed due to the diodes in the alternator.

Caution! An alternator is polarity conscious and can only charge in one direction. If an alternator is connected incorrectly, the diodes may be destroyed.

Ammeter

Figure 10 Ammeter

An ammeter or a charge warning light is included on the engine control panel.

The centre zero ammeter will show if the battery is charging (+) or discharging (-).

The meter should indicate that the system is charging if the battery charge is low (‘flat’ battery), but it will only show a slight charge during normal operation.

It should only indicate a discharge when heavy electrical loads are being drawn from the battery.

Electric Starting Systems

An electric starter motor (see Figure 11) produces very high starting power at minimum speed, but draws a very heavy starting current (Over 100 Amps) from the battery. Heavy batteries, cables, switches, and starters must be in good condition to handle these currents. For electric starters, the heavy duty main switch may be operated manually in some applications, or electrically by using a light-weight control switch to operate the main ‘solenoid’ switch.

Starter drives may be

· Inertia engaged (the drive pinion gear winds back and forth on a coarse screw thread)

· Pre-engaged (uses an electric solenoid to move the gears into mesh before the starting current is switched on)

Figure 11 Typical Electric Starters showing Inertia and Pre-Engaged Drives

Heavy engine starters may be axial types in which the complete engine shaft moves to engage with the ring gear, or co-axial types in which only the pinion gear moves. In these starters, power is partially applied to allow the drive to mesh before full starting power.

If you need more information on alternators, generators, starters, batteries and electric circuits, the text Marine Engine Driving gives more information. Machinery Handbooks may also be useful.

Troubleshooting

Identifying Faults in Electrical Systems

Short Circuits

Short-circuits (shorts) usually result in a blown fuse or tripped circuit breaker. Where partial shorts occur, high currents may flow, and the charge meter may show a discharge (–). Lights may dim or flicker. Electrical crackling noises may be heard in radio equipment.

Beware: A short circuit resulting from a tool dropped across a battery may cause a fire or explosion, as there is no fuse protection at the terminals.

Look closely for shorts where there is exposed wiring , or where wiring may be cut or damaged as it passes through metal casings, bulkheads etc.

Earths

Earths are leakages of current perhaps caused by a short circuit onto the ship’s hull or machinery. A serious earth may be a full short circuit. Partial earths may only show up as confusing faults on some equipment–lights half on etc.

Earths are shorts to the body of the vessel, so look closely where there is exposed wiring , or where wiring may be cut or damaged as it passes through metal casings, bulkheads etc.

Open Circuits

These are usually caused by broken wires, loose or corroded connections, or ‘dry’ soldered joints. Open circuits usually show up when a piece of equipment fails. Open circuits can only be found by working through the circuit wiring.

Hint: First check the most likely causes of open circuits:

Fuses and Circuit Breakers

Plugs and Sockets

Flexible Cords and Cables

Exposed connections

Charging Systems - Low charging rate

Check generator/alternator drive belts first.

Check connections and wiring

Check brushes gummed up or worn?

Alternator diodes or regulator may be faulty

Ignition Systems

Once over checks for quickly checking the ignition system are:

· Check for signs of water or moisture–Liquid moisture displacers are useful

· Check wiring and connections, push in HT and spark plug connections

If these do not show a problem, try this sequence:

· Remove a spark plug and check it quickly for carbon deposits and gap.

· Earth the plug to the engine with the HT wire connected and spin the engine. There should be an adequate spark?

· Check the points–they should be clean with the correct gap when the cam passes. They may need cleaning and readjustment, or replacement

· If the points are blackened and burned the capacitor may be at fault.

Starting Systems - Fail to crank

Try cranking the engine with the starter. If the engine won’t turn, the most likely causes are:

Flat battery

Check and recharge
Loose, damaged or dirty terminals– check and clean
Worn or gummed up brushes or jammed drive mechanism.

For an emergency start, try tapping the starter solidly and turning the engine slightly by hand, then try again. Pre-engaging starter mechanisms may also gum up and prevent the power being applied.