# How much electrical power does one generator produce on a large turbofan?

From my related question, now I want to know just how much power does the generator produce? In other words, what is the max rating of the generator? It looks so small and that surprised me, but since it spins very fast it could get high power in a small package.

The generator from any large turbofan is fine. The answer will also help us figure how much electrical power a large commercial airliner could need at any given moment.

• This question is really too broad and dependent upon the engine in question. – Carlo Felicione Sep 5 '17 at 5:33
• Keep in mind there is a close correspondence between copper mass and kVA, and a close inverse correspondence between iron (magnetic) rotor mass and frequency. Hence the high frequencies of aircraft really reduce the iron mass. Nothing can reduce the copper mass. Meanwhile railways love 16.67 and 25 Hz because weight translates into traction, so they don't mind the iron mass. – Harper - Reinstate Monica Sep 5 '17 at 16:00

This is one of the four Variable Frequency Generator (VFG) of the Airbus A380. It's apparent power is 150 kVA.

Rotor, source: Safran

The total power available from the engines is 600 kVA.

Seen from the other side:

Stator, source: Thales

The A380 is a one of these "more electrical" planes, where hydraulics tends to be replaced by electric devices, and the electric circuits have been optimized by electronics and variable frequency. There are secondary sources for electricity: APU (auxiliary power unit) and RAT (ram air turbine). Overall view from the book Civil Avionics Systems (Moir, ‎Seabridge, ‎Jukes):

Source: Civil Avionics Systems

In case of total failure, the ultimate RAT backup is totally electric-based contrary to conventional aircraft where some hydraulics is also provided by the RAT to move the control surfaces.

• Interesting. Why do they measure it in apparent power? 150 kVA should be equivalent to 150 kW, since power is just volts times amps. – DrZ214 Sep 5 '17 at 5:28
• Power and apparent power are only equivalent when the load is a pure resistor. When it's a general impedance, that is when there is a capacity or inductance component, then voltage and current don't vary in phase, one is delayed (phase difference $\phi$), and instantaneous $u \times i$ is therefore different than when in phase (there is a real and an imaginary part appearing in the product). See Wikipedia. Actually the relationship is $P=U \times I \times cos \space \phi$ – mins Sep 5 '17 at 6:41
• @DrZ214 Or harmonics, or other non-linearity. Power factor is rarely equal to one in AC systems. – Someone Somewhere Sep 5 '17 at 8:09
• @aroth so do electricians. Unfortunately, lag and harmonics crashed the party, and all of us have to deal with it. If you draw 150kW from your 150kVA generator, expect magic smoke. – Harper - Reinstate Monica Sep 5 '17 at 16:04
• @DrZ214 because the actual limits of pretty much any kind of generating equipment are going to come at some number of VA. If you give it a rating in W, it has to come with a big fat asterisk, but if you rate it in VA, it will deliver rated VA under specified conditions. – hobbs Sep 5 '17 at 16:42

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
• Frequency:400±5Hz(3)
• Voltage:115/200V
• Size:375L×460W×330Hmm
• Weight:54.0kgf(dry)

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.

A380

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.

And

At least two engine generators (or APU generators) are necessary to supply the entire network.

B787

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.

• Three such IGTs make up the typical AC power capability of a typical single isle aircraft, such as the A320. Three generators for a twin-engine A320? Where does the 3rd generator go? – DrZ214 Sep 5 '17 at 8:51
• The third one is powered by the APU. – Koyovis Sep 5 '17 at 10:10

Variable Frequency Starter Generator

The variable frequency starter generator is a six-pole machine within an aluminum housing driven directly from the main engine gearbox. The generator is a brushless, three-phase, alternating current, and variable frequency synchronous machine. It has a nominal rating of 235 volts alternating current (VAC), 250 kVA, three phases, and 360–800 Hz output.

The 'all-electrical' Boeing 787 has two of these in each engine, for a total of four. Each is rated at 250 kVA. (For comparison, on an early 737 it was 45 kVA.)

The 787 has four times the potential electric generation of the 777 – 1.45 megawatts. The generators (four at 250 kVA, two per engine, and two at 225 kVA on the APU) produce 235 VAC for the large users on the bus – otherwise the traditional 115 VAC and 28 VDC are operational. Seventeen Remote Power Distribution Units (RPDU) power about 900 loads through the aircraft. The power distribution system is in the aft belly along with the Power Electronics Cooling System (PECS). It’s liquid cooled for the large motors, along with an Integrated Cooling System (ICS) for use by the galley carts, cabin air and IFE.

Total power generation potential is 1.45 MW.

Sources:

• and that 1.45 MW is enough for 150 homes running quite a range of electrical appliances including air conditioners – Hanky Panky Sep 5 '17 at 8:32