To keep a constant frequency of 400 Hz, most aircraft have a CSD (Constant Speed Drive) associated to the AC generator. On most types (e.g. A320, A330, B737, B747, B777) an Integrated Drive Generator (IDG) is used. This unit is simply a CSD and an oil cooled generator inside the same case that provides this fixed frequency output.

Sometimes we even use a variable-speed constant-frequency (VSCF) generator, that provides AC power at constant frequency using a tap converter.

The A380 and A350 have variable-frequency electrical generator, with no CSD or IDG. How can they work without a fixed AC frequency? If the fixed AC frequency is not needed, why it is needed in all the other aircraft types?

  • $\begingroup$ If you worry about the frequency not being constant, first ask the question: why is the frequency important at all? $\endgroup$
    – Agent_L
    Commented Apr 10, 2017 at 10:32
  • $\begingroup$ 400 Hz systems typically rely on transformers to produce all required voltages. I guess A380 designers were none too keen on installing an extra ton of soft iron in their airplane. $\endgroup$ Commented Apr 10, 2017 at 10:46

3 Answers 3


According to Civil Avionics Systems, such system is found on the Airbus A350, A380, Boeing 787, and new business jets. But only certain systems use the variable frequency (VF) AC. Namely motors such as in the fuel and hydraulic systems. They are designed to operate at different speeds with tolerances allowed for the slowest modes of operation.

In short, the motors, heaters, etc., are designed to operate on different loads reliably.

From the 787 manual, items such as cockpit displays and instruments use DC power instead. DC fuel pumps, brakes, and igniters, are also used on the 787.

The reason for it not being implemented decades ago is that motors weren't easily made reliable, especially DC motors. Another reason is the cooling requirements for the motor controllers, on the 787 for example, liquid cooling is used. Solid-state relays replaced most of the traditional circuit breakers, enabling automatic load shedding. One reason for that is to further limit the generated heat.

enter image description here
Snippet from the 787 manual.

The above VF system is by far the lightest, and is less complicated. Between it and the IDG system, there was the short-lived variable-speed constant-frequency (VSCF) used on the McDonnell Douglas MD-90. In its early days it proved troublesome to maintain but eventually it achieved high dispatch reliability. As with the VF, the engine generators outputted VF, but then tap converters (a mid-80's invention) were used to produce constant frequency. Below are the MD-90's air-scoops used for the cooling of this system which is placed in unpressurized compartments:

enter image description here
Sources: airliners.net and an MD-90 paper

Related: Why are the 737's variable-speed generator drives so unreliable?

  • 2
    $\begingroup$ @mins the answer skips a step but is sound: If you want constant speed with a variable frequency supply, converting to DC and running a DC motor is one way to do it. But a reliable DC motor is a prerequisite. Brushless DC motors remove one major failure mode, but have to include a suitable drive circuit -- the coils don't run on DC. Good brushless motors are a result of improvements in power conversion electronics, also needed for the rectifiers, and various other parts of the DC system $\endgroup$
    – Chris H
    Commented Apr 10, 2017 at 8:49
  • $\begingroup$ @ymb1 It does not completely answer the question: if it is more reliable and lighter, why the previous airplane did need a system with constant frequency? Why this change only now? $\endgroup$ Commented Apr 13, 2017 at 10:02
  • $\begingroup$ Hi @GianniAlessandro - please check fourth paragraph. I'll try and add more info. $\endgroup$
    – user14897
    Commented Apr 13, 2017 at 16:40

Not specific to aviation but general electric equipment: AC with a consistent frequency is important whenever you transmit power through solenoids directly fed from mains voltage. Now, traditionally, this included almost everything except primitive devices based on resistive heating (including light bulbs). Motors obviously need to generate alternating electromagnetic fields to push their rotors, and most electronic devices can't just operate with the given supply but need some specific voltage levels: the simplest way to achieve that is a transformer. In either case, reliable AC frequency is needed if you want it to work efficiently.

But these electromagnetic devices have their problems – they're big/heavy (more so in 50/60 Hz than in 400 Hz) and inflexible. If you need to change the speed of an AC motor, you need to either use an asynchronous model which does not have the precision of a synchronous one, employ inefficient step-down techniques or even mechanical gearing. Similarly, changing voltage in an LF transformer (or keeping it constant when the input fluctuates) is only possible with tricky-to-operate coil taps, or with wasteful resistive power-burning.

Over the last decades we have gained powerful, efficient, compact HF switching solid-state circuitry which can largely solve all of these issues, and doesn't depend on any fixed AC input (neither frequency- nor voltage-wise). It basically uses always DC (first rectifying the AC, if necessary) and then internally generates AC of whichever frequency is optimal for the application. This allows quickly stepping motors at any speed and precision to degrees, as well as feeding sensitive electronics with just the right voltage, without significant losses or input-fluctuation sensitivity.

So when none of the consumers really cares for 400 Hz anymore, you might as well allow that frequency to change too, if it means the necessary power can then be generated more efficiently.

  • $\begingroup$ @leftaroundabout In a nutshell, now we can use variable frequency because we have better solid state rectifiers and users less sensible to feeding frequency. Am I wrong? $\endgroup$ Commented Apr 13, 2017 at 10:05
  • $\begingroup$ @GianniAlessandro right, that's what I think is the dominant reason. $\endgroup$ Commented Apr 13, 2017 at 10:07
  • 5
    $\begingroup$ Rectifiers are a small part of the puzzle. The important part is that modern power electronics are much better than they used to be. Modern power converters can efficiently connect your variable frequency variable voltage generator to a fixed voltage DC bus. Some of them can even move power bidirectionally so your generator can double up as a starter. $\endgroup$ Commented Feb 8, 2018 at 21:42

A VFG on a A350 (Variable Freq Generator) ranges from 360Hz to 800Hz (idle to takeoff) but it weighs less than a traditional IDG or CDS due to the fact that it has less components inside since it does not need to convert the freq to a fixed 400Hz. Most components that use AC convert the variable frequency to a fixed 400Hz anyway.

The output of the generator is actually 230VAC , so you can go with a higher gauge wiring for even more weight reduction.

At the end of the day its all about weight reduction. The A350 also has a 5000 psi Hydraulic system because by upping the pressure you can use a smaller diameter of titanium tubing.


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