8: Electrical Systems

Alternating Current Systems

Direct Current Systems

Batteries

Circuit Protection

Relays and Solenoids

Inverters, Wire Terminals and Lights

Alternating Current Systems

The AC generators used in airline-type aircraft must run at a constant voltage and frequency—usually 115 volts AC at 400 hertz (cycles per second). The advantage of using 115 volt AC circuits is that the high voltage and low current reduces wire size and weight, and AC motors are smaller and lighter than equivalent DC motors. The voltage is controlled by a voltage regulator. The frequency is determined by the generator RPM.

A Constant Speed Drive (CSD) is like an automatic transmission that connects the generator to the engine's mechanical drive shaft. It takes the varying RPM from the engine and converts it into a constant RPM for the generator. While maintaining this RPM, the CSD compensates for load variations on the generator. It also balances the load between generators when in parallel operation. The flight engineer panel usually does not have a generator RPM indicator but it does have a generator frequency meter. An AC generator's frequency can be used as a direct indication of generator speed.

A generator is rated in power output. Since a generator is designed to operate at a specified voltage, the rating is usually the number of amperes the generator can safely supply at its rated voltage. The term "Kilovolt-amps" (KVA) describes the rating of the generators found in airline-type aircraft. For example, a 90 KVA generator is capable of producing 90,000 volt-amperes.

The flight engineer's panel usually has KW (Kilowatt) and KVAR (Kilovolt Ampere Reactance) meters. KW measures the work being performed by the generator—such as operating radios, pumps, lights, etc. KVAR indicates how hard the generator is working to produce the power being used. This includes the energy used in magnetizing the iron cores of relays, transformers, motors and solenoids, and energy used in charging any capacitors in the circuit.

The AC generators used on large aircraft are self exciting. When the generator rotates, a permanent magnet produces a small residual voltage. This residual voltage energizes an electromagnet that creates the generator's main field. A voltage regulator controls the flow of the residual current that maintains the voltage output of the generator with the desired limits.

A Generator Control Relay (GCR) allows the flight engineer to control and monitor this excitation circuit. When the GCR is open, the only output of the generator will be residual voltage and the generator is deactivated. This condition will be indicated on the flight engineer's panel by the generator showing only residual voltage and no current flow (no amps). The same indications will be present if the generator field circuit breaker has tripped.

There are three protective functions provided by the GCR. These are protection against an open phase, under-excitation, and overvoltage. A Generator Breaker (GB) connects each generator to its power bus.

Transport aircraft are often equipped with a parallel bus system. In this system, all the generators connect to a common bus through their individual power busses. This allows generators to share loads and keeps all busses powered when a single generator fails. A Bus Tie Breaker (BTB) connects each generator to this common bus. A paralleling circuit increases or decreases the voltage of each generator powering the sync bus so that the load is shared equally between them.

Direct Current Systems

Aircraft Direct Current (DC) circuits power certain equipment and provide a partial backup if the AC generators fail or are not available. The unit of power used in DC circuits is the watt (volts x amps = watts).

The DC system is usually powered by the AC system. The main DC busses will be connected to a corresponding AC bus through a device called a transformer-rectifier (T/R). The transformer portion changes the voltage (usually 115 volts to 28 volts). The rectifier converts alternating current into direct current.

In those aircraft with DC generators, there must be some means of controlling the field voltage. A carbon-pile regulator does this by using a variable resistance element.

In a system where a DC generator directly charges the battery, there must be some means of disconnecting the battery from the generator when the generator voltage drops below the battery's voltage. This is usually done by using a reverse-current relay.

Batteries

The two types of batteries used in modern aircraft are the lead-acid and the nickel-cadmium (ni-cad). These types of batteries can be used interchangeably in modern aircraft.

Lead-acid batteries use an electrolyte composed of sulfuric acid diluted in water. Caution should be used when handling a battery, as the electrolyte will eat holes in clothes and burn the skin. Bicarbonate of soda will neutralize any spilled electrolyte. Ni-cad batteries use potassium hydroxide for electrolyte. This substance is much like household lye and will cause severe burns. It can be neutralized with a mildly acidic solution such as boric acid, vinegar or lemon juice.

Although the two types of batteries have different chemical characteristics, both release hydrogen and oxygen during charging. Because the mixture of the two is potentially explosive, care must be taken to ventilate the battery compartment. It is also important that all electrical loads and power sources be turned off before connecting or disconnecting a battery to protect against stray sparks.

Batteries are rated in ampere-hours. An amp-hour consists of 1 amp delivered for 1 hour. For example, a 140 amp-hour battery delivering 15 amps will last for 9.33 hours (140 ÷ 15 = 9.33).

Each cell of a lead-acid battery can develop a nominal 2 volts. When cells are wired in series, the voltages of the cells are added to determine the battery voltage. For example, a 6-cell battery is rated at 12 volts. If two 12 volt batteries are in series, they have a voltage of 24 volts. Each cell of a ni-cad battery is rated at 1.28 volts. A 20-cell ni-cad battery develops just over 25 volts.

As a lead-acid battery discharges, the specific gravity of the electrolyte changes. For this reason, a hydrometer can be used to determine the state of charge of the battery. This is not true of ni-cad batteries, since the specific gravity of their electrolyte changes very little with use.

If a ni-cad battery becomes completely discharged, some of its cells may reach zero potential and then charge in the reverse direction. This condition, known as a cell imbalance, prevents the battery from charging to its full capacity. If this occurs, the battery should be fully discharged and each cell short-circuited before it is recharged.

If a ni-cad battery is overcharged, excess oxygen can combine with the cadmium and create heat in a process called thermal runaway. Unchecked, this can destroy the battery or even cause an explosion. Cockpit indications of a thermal runaway are continuous rising current and increasing battery temperature. Ni-cad batteries have cellophane as part of the separator between the positive and negative plates in each cell. This cellophane layer inhibits the flow of oxygen to the negative plate of the cell and so helps to prevent thermal runaways.

Circuit Protection

A bonding jumper is a short length of metal braid or a metal strip that electrically connects two parts of the aircraft that have no consistent connection. They reduce the possibility of lightning damage to aircraft parts such as control hinges, and prevent the build up of static electricity by providing a low resistance path between the two parts.

The purpose of static wicks and null field dischargers is to dissipate static charges from control surfaces into the air to prevent radio interference. Enclosing wires or electrical units in metal will also reduce radio interference.

Under the right conditions, there is a visible discharge of static electricity from the aircraft into the air. This phenomenon is called "Saint Elmo's Fire."

The purpose of a fuse is to protect against a short circuit situation and prevent damage to electrical equipment or overheating of the circuit wiring. A fuse is wired into the circuit in series so that all the current used must flow through it. If excessive current flows through the fuse, a metal strip will melt and break the circuit. Once a fuse has "blown" it must be replaced before the circuit can be used again.

Fuses are rated by their capacity in amperes. It is very important that a fuse of the correct capacity be used in each circuit. Using a fuse with too high an amp rating will not adequately protect the circuit. A fuse with too low a rating might blow unnecessarily with a normal current load for that circuit. Regulations require that transport category aircraft have a number of spare fuses for each rating equal to at least 50% of the number required for complete circuit protection. There must be at least 1 spare fuse for each rating.

Some circuits have a fuse-type current limiter installed. These permit short periods of overload before the fuse link melts and breaks the circuit.

The purpose of a circuit breaker (CB) is the same as a fuse: to prevent damage to electrical equipment or overheating of circuit wiring when there is an excessive current flow. The advantage of a CB over a fuse is that it can be reset after it trips.

Thermal circuit breakers trip when excessive current heats a bimetallic element in the switch, causing the contacts to open. A short cooling delay is required before the CB can be reset. An electromagnetic circuit breaker uses an electromagnet to pull an armature and trip the circuit when an overload exists. This type of CB can be reset immediately. Trip-free circuit breakers are used in aircraft applications. This type of CB holds a circuit open regardless of the position of CB control when an electrical fault exists. This feature is required by 14 CFR §25.1357. Automatic-reset-type circuit breakers exist, but they are not used on aircraft.

Relays and Solenoids

A relay is a magnetically operated switch. A fixed magnetic core moves an armature on a pivot to open or close a circuit. Relays allow control of remote, high-current equipment with a small switch.

A solenoid is also a magnetically operated switch, which has a moveable core. Solenoids usually operate mechanical devices rather than electrical switches.

Inverters, Wire Terminals and Lights

An inverter converts DC power to 115 volt AC, 400-Hz power. An inverter is often part of an emergency electrical system so that the battery can be used to power AC radios and instruments. There are rotary and static inverters, but they perform the same function.

All electrical wire connections are either swaged or crimped. Soldering is considered unsatisfactory for aircraft applications.

There is a surge of current when lights are first turned on. This is because the resistance of the filaments in incandescent lamps increases as they heat.