There are several max ratings of a particular generator, depending on the duty cycle:
- peak rating (5 sec);
- max. Continuous (no time limit);
- and often an intermediate value for 5 minutes is given.
The generator produces internal heat together with the electricity, which is not easy to remove by forced cooling and must dissipate through convection. The generator is rated in kVA, not kW, because the three phases consist of sine waves which are not in phase with each other. The kVA rating denotes the electrical power used, a kW rating is for the useful power output.
The Integrated Drive Units consist of a drive mechanism and the generator itself. The generator is a fast spinning device, rotating at a high, constant velocity. Since the rotational speed of the LP shaft is not constant, a constant speed mechanism is added between the generator and the LP axis. Classic technology is a hydraulic-mechanical solution with associated losses (like the automatic transmission in a motor car), newer technology such as presented here uses Constant Speed Variable Transmission.
The generator example in the picture is typical for single aisle and many twin aisle aircraft, and has the following power rating:
- Cont.Rating : 90kVA
- 5 min. Rating: 112.5kVA
- 5 sec. Rating : 150kVA
Three IGTs of this class make up the AC power capability of a typical single isle aircraft such as the A320, the third generator being driven off of the APU and normally not operating during cruise. The A380 uses six generators of this same rating.
Power demand for some systems scale up with aircraft size. This site gives electrical power demand of a typical classic wide-body jet (pre-bleedless-air):
- In-flight entertainment and galley demand scales up with passenger count (33%).
- Air conditioning scales up with cabin volume (19%).
- Fuel system demand scales up with engine size (10%).
- Wing anti-icing scales up with wing span (8%).
Other electricity users such as the avionics are a relatively constant factor as a function of aircraft size. The same site gives the power demand as a function of flight time for a single aisle aircraft, showing that one 90 kVA generator can drive essential loads plus utility loads. The dotted lines in the graph are for continuous rating.
The A380 has six AC generators on board: one driven by an engine each, and two APU generators. From the A380 FCOM:
Each engine has one generator. These engine-driven generators are the main source of electrical power. When an engine is running, its generator provides 115 V AC power at variable frequency. This frequency ranges from 360 Hz to 800 Hz, depending on the N3 rotation speed of the engine.
At least two engine generators (or APU generators) are necessary to supply the entire network.
The B787 has a different arrangement. The B787 has no engine bleed air, and systems normally powered by bleed are now powered by AC electric power. A bleedless aircraft requires considerable extra generating capacity in order to power Cabin Air Compressors (about 19% of total power load for a wide-body) and electrical wing anti-icing systems (about 8% of power load), and the Boeing 787 incorporates four engine-driven Variable Frequency Starter-Generators (VFSGs) of 250 kVA apiece in order to accommodate these loads.