11.5 AC Generation: Brushless Alternators, CSDs, IDGs & Paralleling

11.5 AC Generation: Brushless Alternators, CSDs, IDGs & Paralleling

Quick Answer: Large aircraft generate three-phase alternating current at a nominal 400 Hz, and "if the frequency strays more than 10 percent from this value, the electrical systems do not operate correctly." Since alternator frequency is set by rotational speed and engine speed varies continuously, "a unit called a constant-speed drive (CSD) is used to ensure the alternator rotates at the correct speed to ensure a 400-Hz frequency." The CSD is a hydraulic transmission mounted between the engine gear case and the alternator, and "when the CSD and the alternator are contained within one unit, the assembly is known as an integrated drive generator (IDG)." The alternator itself is normally brushless, containing three generators in one case: a pilot exciter, an exciter, and the main AC alternator. An APU-driven alternator usually needs no CSD, because the APU runs at constant rpm and its speed control holds the frequency.

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Why a Powerplant Technician Owns This

The Powerplant ACS engine electrical subject lists AC generation systems, CSD and IDG systems and components, and the purpose and procedure for paralleling a dual-generator electrical system alongside the DC generators and alternators of Section 11.1. The reason is physical: the CSD or IDG bolts to the engine accessory gearbox, it is lubricated and cooled by engine or drive oil, and its disconnect is a flight-deck-commanded engine action. It is powerplant hardware that happens to make electricity.

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Why 400 Hz, and Why Speed Must Be Held Constant

Aircraft use 400 Hz rather than the 50 or 60 Hz of ground power for one dominant reason: transformer and motor iron mass falls as frequency rises. A 400 Hz transformer is a fraction of the weight of a 60 Hz transformer of the same rating.

The cost is a tight speed requirement. In any alternator, output frequency is fixed by rotor speed and pole count:

$f = \frac{P \times N}{120}$

where f is frequency in hertz, P is the number of poles, and N is rotor speed in rpm. Pole count is built in, so frequency tracks speed directly. A turbofan's N2 spool varies widely from idle to takeoff, and an alternator geared straight to it would swing far outside the ±10 percent tolerance the handbook cites.

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The Constant-Speed Drive

"The CSD is a hydraulic unit similar to an automatic transmission found in a modern automobile. The engine of the automobile can change rpm while the speed of the car remains constant. This is the same process that occurs for an aircraft AC alternator. If the aircraft engine changes speed, the alternator speed remains constant."

Construction and Operation

• "The hydraulic transmission is mounted between the AC alternator and the aircraft engine. Hydraulic oil or engine oil is used to operate the hydraulic transmission, which creates a constant output speed to drive the alternator."
• "The input drive shaft is powered by the aircraft engine gear case. The output drive shaft, on the opposite end of the transmission, engages the drive shaft of the alternator."
• "The CSD employs a hydraulic pump assembly, a mechanical speed control, and a hydraulic drive. Engine rpm drives the hydraulic pump, the hydraulic drive turns the alternator. The speed control unit is made up of a wobble plate that adjusts hydraulic pressure to control output speed."
• "The constant-speed drive enables the alternator to produce the same frequency at slightly above engine idle rpm as it does at maximum engine rpm."

The Control Loop

"The alternator input speed is monitored by a tachometer (tach) generator. The tach generator signal is rectified and sent to the valve assembly. The valve assembly contains three electromagnetic coils that operate the valve. The AC alternator output is sent through a control circuit that also feeds the hydraulic valve assembly. By balancing the force created by the three electromagnets, the valve assembly controls the flow of fluid through the automatic transmission and controls the speed of the AC alternator."

The control loop is therefore driven by two signals at once — a speed signal from the tach generator and an output signal from the alternator itself — which is why a CSD problem can present as either a frequency excursion or a voltage excursion.

Oil: a Powerplant Servicing Item

"Some alternators are cooled by circulating oil through the internal components of the alternator. The oil used for cooling is supplied from the constant speed drive assembly and often cooled by an external oil cooler assembly. Located in the flange connecting the generator and drive assemblies, ports make oil flow between the constant speed drive and the generator possible. This oil level is critical and typically checked on a routine basis."

Practical consequences the exam and the line both care about:

• The drive oil level is a scheduled check, not a "when it looks low" item, and the check is specified at a particular time after shutdown because level reads differently hot and cold.
• The drive has its own oil cooler, which means an air path and a cooler that can foul or leak independently of the engine oil cooler.
• Rising drive oil temperature is the classic CSD distress signal and is normally annunciated on the flight deck.
• A thermal plug and a disconnect solenoid are fitted so the crew can mechanically disengage a failing drive in flight. A disconnected CSD cannot be re-engaged in flight; it is reset on the ground per the maintenance manual.

Integrated Drive Generator

"The CSD can be an independent unit or mounted within the alternator housing. When the CSD and the alternator are contained within one unit, the assembly is known as an integrated drive generator (IDG)."

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The Brushless AC Alternator: Three Machines in One Case

"The modern AC alternator does not utilize brushes or slip rings and is often referred to as a brushless AC alternator. This brushless design is extremely reliable and requires very little maintenance. In a brushless alternator, energy to or from the alternator's rotor is transferred using magnetic energy."

"The brushless alternator actually contains three generators: the exciter generator (armature and permanent magnet field), the pilot exciter generator (armature and fields windings), and the main AC alternator (armature winding and field windings). The need for brushes is eliminated by using a combination of these three distinct generators."

        BRUSHLESS AC ALTERNATOR — ENERGY PATH TO THE ROTATING FIELD

   [ PILOT EXCITER ]  permanent-magnet machine, needs no external power
            |  output rectified and regulated
            v
   [ GENERATOR CONTROL UNIT ]  sets exciter field current  <-- voltage sensing
            |
            v
   [ EXCITER FIELD ]  stationary electromagnet
            |  magnetic coupling across the air gap (no brushes)
            v
   [ EXCITER ARMATURE ]  rotating
            |
            v
   [ ROTATING 3-PHASE BRIDGE RECTIFIER ]  turns with the shaft
            |  DC
            v
   [ MAIN ALTERNATOR FIELD ]  rotating
            |  induces output in the STATIONARY main armature
            v
   [ 3-PHASE OUTPUT, 115 V, 400 Hz, phases 120 degrees apart ]

"AC alternators produce a three-phase AC output. For each revolution of the alternator, the unit produces three separate voltages. The sine waves for these voltages are separated by 120°." Because the heavy output winding is stationary and only the field rotates, there are no brushes, no slip rings, and no commutator — the wear items that limit a DC generator's service life and cause its altitude arcing problems.

Voltage control still works the same way. As Section 11.2 established, output is governed by controlling field current. In a brushless machine the GCU controls the exciter field, which controls the current delivered by the rotating rectifier to the main field. The zener diode remains the voltage-sensitive reference element, and overvoltage protection "is used to open the relay that controls the field excitation current."

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Paralleling: DC Versus AC

DC Paralleling

"Since two alternators (or generators) are used on twin engine aircraft, it becomes vital to ensure both alternators share the electrical load equally. This process of equalizing alternator outputs is often called paralleling."

For DC systems the rule is simple: "If both alternators are connected to the same load bus and both alternators produce the same output voltage, the alternators share the load equally. Therefore, the paralleling systems must ensure both power producers maintain system voltage within a few tenths of a volt. For most twin-engine aircraft, the voltage would be between 26.5-volt and 28-volt DC with the alternators operating."

The mechanism: "both left and right voltage regulators contain a paralleling coil connected to the output of each alternator. This paralleling coil works in conjunction with the voltage coil of the regulator to ensure proper alternator output. The paralleling coils are wired in series between the output terminals of both alternators. Therefore, if the two alternators provide equal voltages, the paralleling coil has no effect. If one alternator has a higher voltage output, the paralleling coils create the appropriate magnetic force to open/close the contact points, controlling field current and control alternator output."

Overexcitation protection covers the failure case: "When one generator in a paralleled system fails, one of the generators can become overexcited and tends to carry more than its share of the load, if not all of the loads... this condition causes the generator to produce too much current. If this condition is sensed, the overexcited generator must be brought back within limits, or damage occurs." The circuit "often works in conjunction with the overvoltage circuit."

AC Paralleling Adds Two More Conditions

"Unlike DC systems, AC systems must ensure that the output frequency of the alternator stays within limits. If the frequency of an alternator varies from 400 Hz, or if two or more alternators connected to the same bus are out of phase, damage occurs to the system. All AC alternator control units contain circuitry that regulates both voltage and frequency."

The physical reason for the split in the fourth row: in an AC machine, real (kilowatt) load follows torque, so the control unit trims the CSD speed setting to shift real load between machines; reactive (kilovolt-ampere reactive) load follows excitation, so the control unit trims field current to shift reactive load. That is why an AC system needs the CSD in the control loop and a DC system does not.

Reverse current still matters. Section 11.2 covered the reverse-current cutout for DC generators, and the same logic appears here: "if a generator fails, it becomes a load to the other operating generators or the battery. The defective generator must be removed from the bus. The reverse current sensing function monitors the system for a reverse current... If this occurs, the system opens the generator relay and disconnects the generator from the bus."

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Where AC Power Comes From When There Is No Engine-Driven Alternator

• Inverters. "A modern inverter is a solid-state device that converts DC power into AC power... Many inverters supply both 26-volt AC, as well as 115-volt AC. The aircraft can be designed to use either voltage or both simultaneously. If both voltages are used, the power must be distributed on separate 26- and 115-volt AC buses."
• APU-driven alternator. "On many aircraft, the auxiliary power unit operates at a constant rpm. AC alternators driven by these APUs are typically driven directly by the engine, and there is no CSD required. For these units, the APU engine controls monitor the alternator output frequency. If the alternator output frequency varies from 400 Hz, the APU speed control adjusts the engine rpm accordingly to keep the alternator output within limits." This ties directly to Section 4.4's point that an APU is governed at a constant 100 percent rpm.
• Ram air turbine (RAT). Transport aircraft "also have at least one more AC backup power source, such as an AC inverter or a small AC alternator driven by a ram-air turbine (RAT)."
• Hydraulic motor-driven generator (HMDG). "The HMDG is a servo-controlled variable displacement motor integrated with an AC generator... designed to maintain a desired output frequency of 400 Hz. In case of an electrical failure, the HMDG could provide an alternative source of electrical power."

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Independent Prep Note

Independent FAA AMT Powerplant prep by OpenExamPrep. Not sponsored by or affiliated with the Federal Aviation Administration (FAA). Technical data compiled from FAA-H-8083-32B, FAA AC 43.13-1B, and 14 CFR Parts 43 and 65.