Power P=1xA

March 1, 1998

Power P = 1 x A

By Jim Sparks

March 1998

Electronics as applied to aviation" is probably one of the simplest definitions of "avionics." Intimidating as it may be, electronics are still nothing more than the application of electrical principals. With the onslaught of "Next Generation" aircraft utilizing computerized systems such as engines, air-conditioning and pressurization systems, fuel systems, wheel brake ,and steering— not to mention auto pilot and flight deck displays — Computer technology offers numerous advantages over systems of yesteryear.

Size and weight are considered an important advantage in aviation, plus electronic systems have few moving parts, so reliability can be significantly higher. Another benefit of computer technology is "BITE" (built-in test equipment). That is, if a fault is detected, the computer can advise the flight crew and retain the fault in memory until a technician can retrieve it. This self-diagnostic capability, when properly used, can save hours in troubleshooting.

One thing all computerized systems have in common is the need for electrical power. Airframe manufacturers have different specifications on the type and amount of energy needed. General aviation aircraft most frequently use direct current (DC) for primary power while commercial aircraft use alternating current (AC). Each type has benefits as well as drawbacks. Problems with a power source can have dramatic effects on sensitive electronic equipment.

Power may be best described as "the ability to do." Electrical power is measured in wattage (watts). In a DC system, wattage is determined by multiplying the electrical potential or voltage times the amount of electrical current flow or amperage:

P = I x E

A similar principle applies to AC systems; however, current flow and electrical potential are rarely in phase, so an approximation can be made by multiplying voltage times amperage and using 70 percent of the answer as approximate power. Exact calculations require factoring in circuit impedance and determining the phase relationship between circuit potential and flow.

P = I x E x .7

Advisory Circular 43.13-1A dictates that in an aircraft, the size of wire utilized in a circuit is directly proportional to the amount of current flow and the overall distance. Some other factors include whether the wire is in a bundle or free air and if the amperage draw of the circuit is continuous or intermittent.

For example, a No. 18 copper wire in a bundle can carry 10 amps and have a resistance of 6.44 ohms per 1,000 feet. The weight of 1,000 feet of this wire would be 8.4 pounds. In an aircraft using an 115-volt AC power system, the amperage or current flow through a conductor is substantially lower than an aircraft using a 28-volt DC system. A typical, electrically-operated fuel boost pump might require 300 watts of power for operation. An aircraft using 115 volts would require around three amps, while a 28-volt DC system would need about 11 amps to achieve the same result. Although an AC motor will differ internally from a DC motor, the amount of power used to perform a specific task is still similar. Aircraft operating on AC can have a significant weight savings in wire over a DC-powered counterpart.

Turbine-powered aircraft found in general aviation will frequently utilize a combined starter generator. This device can use energy stored in aircraft batteries to produce the necessary torque and speed to get the engine started. Rather than going along for the ride, its internal components are used to produce power to replenish the battery and provide for the electrical system.

The mechanical means of producing electrical power requires a magnetic field, coils of wire, and relative motion between the two. The relative motion is supplied by the mechanical connection between the generator and the engine. Coils are incorporated in the generator case and the commutator (spinning section). The power requirements of various aircraft dictate that generators be able to produce a significant output, in some cases as much as 400 amps with the voltage output remaining relatively constant.

This is accomplished by regulating the strength of the magnetic field within the generator. A separate generator control unit (GCU) or a voltage regulator is used for this task. Electrical connections of most starter generators are similar. There is either four or five terminal posts on the connection block and they are labeled A,B,D,E, and in some units C. In a short field-type starter generator or stand-alone generator, terminals B and E are large terminal posts with B connected to the aircraft bus and E connected to earth (aircraft ground).

Terminals A and D (the small ones) are used for control. Terminal C is used to supply a separate start winding in generators with five terminal posts. The GCU will supply a regulated voltage (frequently between 3 to 14 volts DC) to the generator terminal A. The result is a precisely-controlled magnetic field within the generator. Voltage on terminal A will have to be adjusted anytime engine speed changes or electrical load is altered.

Short field type starter/generator.

With a constant electrical load on the generator by increasing the mechanical, excitation (increasing engine RPM) will increase the electrical output. So anytime speed is increased electrical excitation needs to be decreased. A similar situation exists when an engine is operating at a constant RPM and electrical circuits are energized or deenergized. By drawing more amperage from a generator, the electric potential or voltage would decrease. So as more systems are switched ON, the voltage on terminal A increases. Terminal D is an output from the generator to the GCU and is used to make GCU aware of the amperage flowing through the generator.

In the case of the starter/generators manufactured by Auxilec, the relationship of current flow to D wire voltage is 300 to 1. That is, if the generator is producing 300 amps, there will be about 1 volt DC at terminal D. If the generator is only delivering 150 amps the D lead will have about 0.5 volts. For a combined short field starter generator, this D lead voltage will also be used in the starting phase to control field weakening which regulates starting torque and speed. This is not used in devices utilizing a terminal C.

Starter/generator with a separate start winding (note extra power leads).

In the generating mode, the D wire is used to maintain load sharing in aircraft with multiple on-line generators. Should a generator control unit sense its generator with a lower D wire voltage than an adjoining generator, the result is that the GCU of the low generator boosts the A wire voltage until the D leads are operating at the same potential. Corroded or dirty connections on terminal D can lead to problems in parallel operation.

Aircraft using multiple generators to supply a common electric system often experience difficulties having generators maintain equal loads. Pilots sometimes add to this difficulty by describing a specific generator as weak. Should a small amount of corrosion exist on a D wire electrical connection, it will cause a small voltage drop. When this occurs, the GCU will see a somewhat lower input from the D lead and the GCU will respond by increasing field excitation. The result, at least from the cockpit, is the generator with the greater excitation will be delivering more amperage than its partner. Rather than having a problem with the "weak" generator the problem could very well be with the "healthy" unit. Before making any adjustments on a GCU to correct for load splits, it is worth while to inspect for signs of corrosion and even clean the electric connections.

Rectified alternator installed next to a pneumatic starter (alternator is gold color).
Typical starter generator brush set.

Voltage fluctuations or spikes are a genuine concern. A frequent cause of this condition is brush bounce. This is where an abnormality in the commutator surface can cause the brush to momentarily open its electrical circuit. In this situation, arcing can occur creating further deformation to the commutator surface and rapid brush wear.

As the brushes wear, the tension on the retaining spring tends to decrease, which results in aggravation of the problem. Many generator manufacturers require inspection of brushes that require removal. The disturbance of the brush spring during this removal also has the potential of reducing spring tension. It is advisable to include a good visual check of the commutator during brush inspection.

In several installations the generator manufacturer has installed the brushes so they trail the commutator. In this case, if the generator drive shaft is rotated opposite the normal direction, brush damage can occur. Many operators elect to replace their own brushes in the field. This is usually not a problem as long as brush spring tensions are tested and adequate brush seating techniques are used.

One nonrecommended technique used for brush seating is after installation of two brush sets (180 degrees apart) in a starter generator to complete two successive ground power starts. After the second start, the brushes are seated. Unfortunately, implementing this technique results in excessive arcing between the brushes and the commutator during the first start. The reason the brushes are so well seated after the second start is the arcing causes the commutator surface to resemble a rough cut file. So much for extended brush life. Attention should always be paid to the manufacturer's recommendations prior to conducting any maintenance on a starter generator.

Aircraft using separate starters can incorporate brushless generators or rectified alternators. There are several distinct differences in the internal operation of these devices as compared with a unit using brushes. One primary difference is that a brushless unit will include a permanent magnet generator (PMG). Anytime the shaft of this device is rotating, the PMG has relative movement with a series of coils. This means that even if the cockpit switch is selected "off" there will still be excitation power available when the engine is in operation. Frequently, three phase AC power is supplied from this PMG to a generator control unit. Here the excitation power is converted to DC and is then metered into the main excitor winding within the generator case.

The magnetic field produced by this stationary excitor winding works in conjunction with (usually three) main power coils installed on the rotor. Output from each power coil is AC and is directed through a diode circuit to change the AC into a rippled DC. Feeder cables are used to connect this output into the aircraft electrical distribution system. The generator control unit will also monitor this output and adjust the regulation system according to voltage deficiencies or surges.

Numerous avionic/airframe system problems arise from anomalies in the power systems. Most autoflight systems, as well as electronic flight instrument systems (EFIS), are voltage sensitive. In the event of voltage drop below a specific threshold, the autopilot may disconnect or the EFIS could black out.

In one situation, an aircraft had gone in for a complete interior refurbishment and new avionics. Afterwards, on extended duration flights, the autopilot would periodically trip off but would always reset. Numerous technicians replaced various components in the autopilot circuit. Nothing solved the problem. Eventually, a technician flew with the aircraft on an overseas mission and happened to notice that when the flight attendant switched on the new oven in the galley, the autopilot tripped.

The technician was able to duplicate this condition time after time. The current draw associated with turning on the oven was causing a momentary voltage drop on the distribution system resulting in the autopilot trip. Relocating the oven power supply to a bus that was supplied by more than one generator solved the problem.

In other cases, the ripple produced by brush bounce or regulation malfunctions can cause various computers to see voltages that might be out of tolerance for a computer sensor. The computer then signals the flight crew that it has sensed a failure. Ground power units (GPU) are also not exempt from spiking electrical power systems. In many aircraft the battery(ies) can be brought on line with the GPU and serve as a filter. When using a questionable GPU on an electrically sensitive aircraft, connecting a lead acid battery in parallel with the GPU will provide some protection. It is a good idea to periodically test each GPU for excessive voltage ripple.

When troubleshooting electronic problems, even those associated with self diagnosing systems, consider the power source before replacing too many components.