Alternator Drive and Control Systems

Alternators are key components in an aircraft that help power the electronics onboard. Rather than working independently, most alternators function alongside different control systems that ensure that the device is working effectively. All of these devices, including the constant speed drive (CSD), hydraulic transmission, bus power control unit (BPCU), and the generator control unit (GCU), will be discussed in this blog along with a general description of the alternators they support.

What Is an Alternator?

In gasoline or diesel-powered vehicles, the charging system consists of three major components: the battery, the voltage regulator, and an alternator. The alternator works with the battery to generate power for all the electronic accessories onboard. It is driven by a crankshaft which converts the pistons’ up-and-down movement into circular motion. On airplanes and cars alike, the rotational movement harnessed by the alternator is used to generate electricity by way of a magneto, or electromagnetic device that creates its own electrical power. In essence, the alternator converts the engine’s mechanical motion into electricity to power the aircraft electrical systems.

Alternator Drive Systems

All alternators must rotate at a specific rpm to keep the frequency of AC voltage within the proper limit (about 400 Hz). A special unit called a constant speed drive (CSD) is used to ensure that the alternator rotates at the correct speed. It can either be independent or mounted in the alternator housing. When the two devices are contained in one assembly, it is called an integrated drive generator (IDG).

The CSD is a hydraulic unit similar to the automatic transmission in modern cars. Just as how the engine can change its rpm while the speed of the car is constant, the same process occurs in the AC alternator of an aircraft. Even if the plane changes speed, the alternator rpm will remain constant. This process can be controlled either electronically or mechanically, but in modern planes, it is always electronic. The constant speed drive allows the alternator to produce the same frequency at just above idle as it does at maximum engine output.

Hydraulic oil or engine oil is used to operate the transmission, which creates the constant output speed to drive the alternator. Whereas the input drive shaft is powered by the aircraft gear case, the output drive shaft, on the opposite side of the transmission, engages the drive shaft of the alternator. Overall, the CSD employs a hydraulic pump assembly, a mechanical speed control, and a hydraulic drive. Engine rpm drives the hydraulic pump that turns the alternator while the speed control unit adjusts hydraulic pressure to control constant output speed.

Alternator Control Systems

Most modern aircraft use computerized control units with their alternators. As AC alternators are found on large commercial aircraft, their control systems always have redundant computers that provide safety in case of system failure. The role of alternator control systems is to ensure that output frequency stays within limits. If the output frequency varies from 400 Hz, damage to the system will occur. All control units regulate frequency and voltage as well as monitor for system failure. The most common units used are the bus power control unit (BPCU) and generator control unit (GCU). The GCU is the main computer that controls alternator functions whereas the BPCU controls distribution of AC power to the power distributor buses located throughout the aircraft. On aircraft, there is generally one GCU used to monitor and control each AC alternator, and one or more BPCUs to route power distribution.


All these instruments work in conjunction to generate the power used in aircraft electronics. As such, each component is vastly important to the overall effectiveness of a plane and should therefore be monitored often and replaced as needed. On Internet For Aviation, you can find a wide array of quality-tested aircraft alternator parts and other electrical components. To find a specific part or browse our various offered items, you may use our easy-to-navigate online catalog and search tools. We invite you to take advantage of our 24/7x365 RFQ service to receive quotes for your comparison on any item of interest within just 15 minutes of receiving your completed form.


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