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Alternators and Generators

Their job is keeping the battery charged

By Steve W. Ells
Firewall-mounted voltage regulator.
Belt-driven 12-volt, 60-amp Ford-type installation on a Continental engine. A red insulator typically identifies an alternator output terminal.
Alternators are capable of producing full rated output at low engine rpm.

With the advent of affordable integrated circuitry, today's light-airplane avionics are capable of showing the pilot a real-time display of weather conditions, pointing out convective activity, navigating in three dimensions with reference to satellite signals, and saving a database of engine operating parameters. This is wonderful, but without a reliable source of electrical power — a generator or an alternator — those wonders of the electronic age are worth little. This month we'll take a look at charging systems.

In the same way that starting woes are most often traceable to system problems, changing an alternator or voltage regulator when charging stops is usually shortsighted. A well-educated mechanic or owner determines the cause of the problem first. Since there are still generator-equipped airplanes flying, let's start there.

A generator looks a lot like a black two-pound coffee can with wires connected to posts on one end and a round wheel (pulley) on the other end. A continuous belt conducts rotation from a bigger pulley on the engine to the generator pulley. Gear-driven alternators are bolted directly onto the accessory case of the engine.

Generators and alternators are rated in volts (12 or 24) and amps. Common sizes for 12-volt systems are 12, 15, 25, 38, 50, or 60 amps while 24-volt alternator ratings are typically 60 or 95 amps. The first airplane that I owned, a 1947 Piper Super Cruiser, had a 12-volt, 15-amp Delco Remy belt-driven generator. My second airplane, a 1966 Cessna 182, supplied electrical power with a 12-volt, 60-amp alternator adapted from a Ford automotive unit.

Generators: dead at idle

Generators and alternators produce electrical energy by moving wires (conductors) through strong electrical fields or vice versa. In the generator, the conductors are copper wires that are wound around an armature that is bolted to the drive pulley. (As the armature rotates, the copper wires move through a magnetic field that is produced by permanent magnets.) Electrical power is induced in the wires and terminates in a part of the armature called the commutator. This power is then transferred from the spinning commutator to stationary carbon brushes that are held against the commutator segments by spring pressure.

Generators don't produce rated output until engine rpm is up in the midrange of operation — typically above 1,400 rpm. This liability can be a real pain in the pilot seat.

Pilots who have experienced the rapid dimming of a landing light as they reduce engine rpm on short final will understand one of the drawbacks of a generator-powered system.

There are other drawbacks to generators. Compared to alternators, they're heavy, the amperage ratings are lower, and because the full electrical output of a generator is conducted across a carbon brush commutator copper segment junction dirt and arcing often cause electrical noise and static that radiate to other avionics. Generators require more maintenance than alternators. There is carbon dust to deal with, commutators to smooth and polish, and bearings to grease and clean. Generators aren't all bad; they do have two big advantages over alternators — they're not sensitive to errant electrical spikes or reversed polarity, conditions that can render an alternator inoperative in a New York minute, and they can produce electrical power even if the battery is dead.

Alternators: powerful but sensitive

Alternators are capable of producing full rated output at low engine rpm. That's important for two reasons — because today's GA airplanes are stuffed with avionics that require electrical power from the beginning of every flight and because systems are changing.

Twenty-first-century airplanes, such as the Lancair 350, Cirrus SR22, and Liberty XL–2, have replaced their vacuum systems with electrically driven instruments. This can be done in part because of the reliability of today's alternator systems, and because the installation of a second, independent electrical system is becoming easier. Light-aircraft alternator systems weren't always so dependable.

An alternator can be thought of as a current multiplier because a small amount of current (typically 1 to 4 amps) is fed into an alternator through the field terminal, and, after the magic happens, electrical power up to the alternator rating is available at the output terminal.

The strength of the magnetic field in alternators and generators is automatically varied by an excitement power from the voltage regulator (VR). A generator, produces electrical power when the aircraft battery is completely discharged, because a generator creates a portion of its output (because of residual magnetism) from the wire-through-magnetic-field interaction that produces power. Alternators don't have permanent magnets so when the aircraft battery is completely discharged, the alternator will not charge. Does a generator ever lose its residual magnetism? Sort of. Occasionally a generator will need polarizing, especially after inactivity. Flashing the field restores function. Service manuals detail this procedure. Alternators should never be flashed.

Alternator feedback loop

If the alternator-charged system has a healthy battery and resistance-free connections, the VR senses the aircraft electrical system voltage and varies the excitement current flow to maintain a charging-system voltage between 13.8 and 14.2 volts in a 12-volt system and 27.1 to 28.4 volts in a 24-volt system. The electrical system voltages are higher than the battery ratings to ensure that the battery gets fully charged. Sounds good. But alternators have their own set of problems.

When a VR malfunctions and feeds too many amps into the alternator field circuit, the voltage output skyrockets almost instantaneously.

If this happens, things, especially those high-cost avionics-style things, signal their displeasure by producing acrid clouds of smoke before turning out their lights and taking an expensive nap. As electricians often say, once the smoke has escaped from those expensive boxes, the party's over.

To prevent costly overvoltage system meltdowns, an overvoltage relay (OVR) guards against runaway voltage outputs. Cessna started installing alternators in the mid-1960s, and OVRs in its single-engine line around 1970. Fortunately for owners of airplanes that came out of the factory without overvoltage protection, virtually all of today's retrofit VRs have overvoltage protection incorporated inside the units. VRs featuring built-in overvoltage protection are alternator control units (ACU). The OVR works like this — whenever system voltage goes above 16 volts (for 12-volt systems) or 32 volts (for 24-volt systems), the circuit between the bus and the ACU is automatically opened by the OVR. This cuts off the excitation current and the alternator output drops to zero. Since transient, intermittent high-voltage spikes occasionally fool the VR or ACU into thinking the system is in meltdown, a pilot should attempt to reset the system by temporarily turning off the alternator switch and then turning it back on. If the system won't reset after one or two attempts, then the pilot must shed electrical load and evaluate his options.

Load shedding: what's that?

Since most GA airplanes don't have a backup charging system, an understanding of load shedding is important. Here's one way of looking at it. A fully charged battery is a bank with limited assets. Each electrical circuit is a drain on those assets. The point of load shedding is to turn off all unnecessary drains (circuits) in order to preserve, and best use, the battery's limited assets.

Pilots, especially those in trouble, need to keep their communication and navigation capabilities as long as possible — so think talking and tracking when load shedding. If the loss of charging is discovered right away, it's safe to say that an aircraft battery can power a nav/com radio and a transponder for at least an hour. Therefore, it's important to know which circuits are power gluttons. Any circuit that turns electrical power into heat (pitot heat) or light is a hungry circuit. An easy way to determine how much current a circuit draws is to look at the numbers etched on the circuit breakers. Locate the switches that control the circuits with the big numbers, and create a plan for flying situations that would be affected by a loss of electrical power. And remember that the regulations permit, even encourage, the pilot to deviate from normal procedures in abnormal situations. During an abnormal situation, such as flying instruments in the clag with no alternator, no one is going to fault the decision to turn off the position lights. The circuit breaker trick, plus research into the airplane service manual, enables each pilot to make informed load-shedding decisions if they ever become necessary.

Diodes :a rectification situation

Alternators produce alternating current (AC), which is useless in GA airplane electrical systems. To convert the AC to direct current (DC), three matched sets of silicon diodes are paired in a solid-state device called a rectifier bridge. The alternating current output of each leg (there are three legs) of an alternator starts at zero, climbs to a positive value, then falls through zero to a negative value, before again returning to zero. Thus, it's called alternating current. The rectifier removes the negative (or unusable) part of each leg's output, and combines the three positive outputs to produce a usable DC-like output. This is important because rectifier problems are sneaky. If one diode in a rectifier fails, the output (and bus) voltage will not be affected, but the amount of current being produced will drop off by approximately 20 percent. Alternators rated at 60 amps will become markedly less capable.

The loss of one diode may not be evident if the airplane isn't loaded with electrical equipment because even the smaller well of electrical power is sufficient to power all the circuits and keep the battery charged. But the pilot whose airplane has a full load of avionics, ice protection, and electrical instrumentation is in trouble because the compromised alternator can't produce enough amps for safe operation. One symptom of this malady is a battery that won't stay charged. Another tip that the rectifier isn't hitting on all six diodes is a high-pitched whine that varies with engine rpm — this can be heard in the radios, and if it's bad enough it may affect ADF pointer operations.

A common system problem

The alternator on/off switch is often overlooked in alternator system troubleshooting. Taking a few minutes to ensure that the switch is resistance-free solves all kinds of alternator system headaches. Why? Because all it takes is a little corrosion or wear in that inexpensive switch to completely throw the system voltage sensing function of the VR or ACU out of whack.

This example shows why. The electrical system charging circuit consists of the alternator, the VR or ACU, the alternator switch, and to a lesser degree, the aircraft ammeter and the overvoltage relay.

The loop that maintains equilibrium in the electrical system starts at the aircraft electrical bus. A bus (or buss) is simply a wire or a metal strip that the various circuits, such as position lights and landing-gear motor, tap into to get system power for operation. It's the central well of electrical power.

As we turn on more circuits, such as a landing light or pitot heater, each circuit's resistance is added to the bus. This increase in resistance, because of Ohm's law, lowers the bus (and system) voltage. Remember that voltage in a series circuit is inversely proportional to resistance (ohms). When resistance increases, voltage decreases. The VR or ACU receives its information on system voltage levels through the alternator switch.

Problems arise when the switch has internal resistance or is dirty. Both of these conditions cause resistance (increase in ohms) that drops the voltage across the switch. What happens? Even one ohm of resistance (that's not much) in the switch results in the VR or ACU seeing a lower voltage than is actually on the bus. This causes the VR or ACU to react to a low bus voltage reading by increasing the amperage flowing through the alternator field, which ups the alternator output voltage. As the bus voltage increases, the voltage value (after overcoming the resistance in the switch) sensed at the ACU rises above the "normal" voltage parameters and the ACU, sensing that the system voltage is too high, reduces the output of the alternator. The result is a constantly varying bus voltage that pilots first notice as pulsating instrument lights or a pulsing ammeter needle. The ACU is chasing its tail because the resistive switch is telling it lies.


One of the best tools for troubleshooting rectifier problems is an alternator ripple tester — maintenance shops that are savvy about charging systems often have one.

While this technical information is of limited value to the majority of pilots, every pilot should know the basics of his alternator system, and how to shed load in emergencies.

Many pilots with split master switches (Batt half and Alt half) have modified the owner's manual starting procedure by leaving the alternator half of the switch in the Off position until after the start sequence. After starting, but before turning on any other equipment such as radios or lights, the pilot turns on the alternator half of the switch and checks for positive movement of the ammeter needle. This verifies that the charging system is online. Engine starting is a time when the contactors that control large current flows are opening and closing. The potential for large voltage spikes rippling through the electrical system is very high during this brief moment. Since rectifiers, switches, and other solid-state devices are adversely affected by spikes, it's a good idea to isolate charging-system components during starting - unless you have a generator on your airplane.

There are two information sources that can help pilots gain general and specific knowledge regarding their electrical systems. Visit and The information from these two sources is invaluable for further understanding of electrical systems.