Why is 115 VAC at 400Hz use for airplane circuits?

[Billy Mayo]

So are they using a Battery also? or no battery?

 

[MikeMl]

Notice the direction the propellors on these two aircraft are designed to turn. The left is British; the right is American:
View attachment 82085View attachment 82086
One must be be aware of that difference as it causes different flight characteristics, particularly during takeoff and descent.
...

My deceased father-in-law flew P51s and Spitfires interchangeably in 1944 and 1945. He remarked that he had to have a little talk with himself about which way the P-factor was going to pull the nose of the airplane prior to each take off...

 

[MikeMl]

So are they using a Battery also? or no battery?

They use a starting battery, just like a car. It takes about 450A of cranking current to start the 470 cu. in. Continental in the 182.

 

[Billy Mayo]

So do u know why the 115VAC at 400hz? for aircraft is used for?

 

[MikeMl]

So do u know why the 115VAC at 400hz? for aircraft is used for?

Mostly for de-icing equipment, like heated props, heated windscreens, deicing boots (pumps). Those are all huge power hogs.

The total load in my non-deiced single-engine 14V Cessna is about 45A at night, which covers lighting, heated pitot and stall vane, engine instruments and avionics. It has a 60A alternator.

 

[Billy Mayo]

So Aircrafts have a start up battery connected to an alternator which outputs 115VAC at 400hz right? to power all those things?

What is your start up battery DC voltage at? to power your alternator

 

[MikeMl]

So Aircrafts have a start up battery connected to an alternator which outputs 115VAC at 400hz right? to power all those things?

What is your start up battery DC voltage at? to power your alternator

Turbine-powered aircraft have starting batteries that are usually 24V or higher. Big Jet transports have a small jet-engine called an APU, which is started with a battery. It turns a bigger AC alternator which is used to start the big turbines. It also runs the heating/Air-conditioning systems.

The system voltage in my Cessna is determined by a nominal 6cell lead-acid battery, so 12.6V with engine stopped, and 14.2V with the alternator charging, just like a car. Newer civilian aircraft have a 12-cell battery, so the voltages are just double what I quoted above.

 

[Billy Mayo]

So the 400hz is really the alternators speed or frequency? I'm guessing they wanted 400hz to get the RPM speed to output a high current amps

Because a car has an alternator which is 12VDC at some HZ which i don't know

 

[ChrisP58]

So the 400hz is really the alternators speed or frequency? I'm guessing they wanted 400hz to get the RPM speed to output a high current amps

Because a car has an alternator which is 12VDC at some HZ which i don't know

There is no direct correlation between the frequency and the output current.

The frequency for your car alternator will be tied to the rpm of the engine speed. But, since the alternator has internal rectifiers, the output is DC.

 

[Billy Mayo]

So what is the reason for aircrafts to be at 400hz than?

The 400hz is the frequency or RPM speed of the alternator/generator?

And it needs to be at 400hz because why?

The only thing I can think of is that 400hz outputs more current/amps right?

 

[KeepItSimpleStupid]

I gave you the BIG explanation including some of the design reasons for selecting 400 Hz. The bigger reason is as the frequencies increase, the weight goes down.

400 Hz is easier to filter into DC. 400 Hz 3 phase is even easier to filter into DC. Not sure if you have ever seen a transformer with a 3 phase primary and a single secondary. Try getting 20 Kw of 15,000 V out of a 240 V single phase and now try doing it with a 3 phase input primary.

Yea, look here for fun: https://en.wikipedia.org/wiki/Auxiliary_power_unit

These APU's may be run to a commercial airliner after the aircraft has landed, so all power is coming from the ground rather than consuming precious fuel.

 

[Billy Mayo]

Why is it so easy to filter 400hz?

 

[KeepItSimpleStupid]

What I really mean is the capacitor value needed is smaller and thus the whole supply is smaller as well.

 

[ChrisP58]

Why is it so easy to filter 400hz?

Because the peaks of the sinewave are closer together, you need less capacitance to hold the voltage up between the peaks.

 

[Billy Mayo]

Yes I understand

but do u get more output current because its at 400hz than 60hz?

 

[ChrisP58]

Yes I understand

but do u get more output current because its at 400hz than 60hz?

No. The power, voltage and current will be the same. Nothing changes. None of the ohms law equations have a frequency term in them.

But as has been said many times. The magnetic elements that are part of a 400 Hz system will be smaller and lighter than the 60 Hz equivalent of the same power rating.

 

[Styx]

So what are they using than inside an airplane or aircraft? a AC generator or an AC alternator? a car uses a 12VDC battery with an alternator , do planes have a battery connected to the alternator?

The electrical network of a typical single-isle or wide-body jet is such:

Each engine will typically have two generators.
Pre-2000 these generators were coupled to the engine via a constant-speed gearbox giving an almost constant rotor speed == supply freq
Post-2000 these EXTREMELY heavy gearbox was removed for a simpler, lighter gearbox and thus the supply frequency varies from about 360Hz to 800Hz across a flight as the rev's of the jet changes.

All these generators go via distribution panels so one set of equipment will be fed from one and others from another...
They can be switched in and out at will depending on loading and engine performance.


There is also a 28Vdc bus fed from a battery. This powers the Avionics and will also fire the APU in the event of a significant powerloss.
This battery is continuously tricklecharged from the main generators.

That battery, along with the RAT is actually capable of landing an aircraft will full generator failure - assuming immediate divergance to the closest airport. This is why atlantic flights, esp dual engine, fly nearer to greenland



And why not 400Hz and not say 1kHz or 1MHz since increase freq results in smaller magnetics?
Engineering tradeoff. Increase freq increases the impedance of a cable so lower voltage seen at the loads and reduced ability to regulate the PCC
Decrease frequency means larger magnetics but tighter PCC regulation

 

[Ironchef]

Notice the direction the propellors on these two aircraft are designed to turn. The left is British; the right is American:
View attachment 82085View attachment 82086

One must be be aware of that difference as it causes different flight characteristics, particularly during takeoff and descent.

The reason is probably like I mentioned, "it worked." The most common exception is in twin-engine aircraft in which one of the engines may be counter-rotated relative to the other. The American P-38 is one of the more famous/notorious examples of that.

John

What about the large commercial jet planes. Is this the same convention?