400Hz ?
Joined: Apr 2002
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From: IO83VI
Size and Weight, for the same power 400hz motors and generators and motors are physically smaller and therefore lighter. 400hz is a very convenient frequency to generate, going to make a bigger idiot of myself now but IIRC it's an eight pole generator @ 6000 rpm - easy.
Joined: Oct 2000
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From: Berkshire, UK
<<Do any of you extremely clever folk know why aircraft use 400Hz ac? It must be nearly DC at that frequency>>
On the contrary, surely? It would be almost DC if it was 1 or 2Hz.... or is my aged brain playing tricks with my radio theory?
On the contrary, surely? It would be almost DC if it was 1 or 2Hz.... or is my aged brain playing tricks with my radio theory?
Joined: Jun 2001
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From: In da north country
in a non-technical explanation to MANAGP, if you were to touch a 400 HZ live wire, it would feel more like DC, because the current reversal would be so fast you wouldn't feel the "buzz" like you would for a 60 or 50 HZ circuit.
Its all about power to weight ratio!
Its all about power to weight ratio!
Joined: Aug 2005
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From: fairly close to the colonial capitol
Much lighter power transformers in electronic equipment can be used with 400hz AC as opposed to 50/60 cycles.
Pre-1970, before electronic (transistorized) inverters were common, aircraft used mechanical inverters - A DC motor directly driving a 400hz generator.
Pre-1970, before electronic (transistorized) inverters were common, aircraft used mechanical inverters - A DC motor directly driving a 400hz generator.
Joined: Aug 2005
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From: UK
Here is a reason
1 Some equipment require a 3 phase supply ie motors
2 some use only one ie windscreen heaters
if you had single phase generators you would need 3 for 3 phase and then you would require a very complicated timing circuit to make sure the phases were the correct angle apart.You would also need to duplicate this system or triplicate it to get an airframe certified -the weight would be large .
So why not put a 3 phase genny on two engines and the apu that can supply all the power required and attach them to a constant speed drive that rotates the gen at a speed that gives you 400Hz depending on how many poles your choice of gen has because Freq= RPM x pairs of Poles
--------------------------
60
So know we know how to gear the constant speed drive to get 400 Hz whilst varying engine RPM ( still not said why 400Hz stay with me)
Now that we have gone to the effort of making 400Hz we can do a couple of things .
We tell everyone that's the standard so all equip is made to operate at 400 Hz and when they ask Why? we can say it's so we can use 3 phase generators to save weight and simplify power production because 400 Hz means a complete cycle occur's every 1/400 th of a second therefore a pole passes a coil of the gennny and generates max voltage every 1/1200 second (remeber 3 phase so 3 coils.The less the coils again the greater the weight saving but three is the minimum ) we therefore get the 3 phases 120 degrees apart because the same pole passes the next coil another 1/1200 of a sec later and then the third and all our electrical need's right down to Coffee is done on each 360 degree cycle of the genny or 400 times a second.
1 Some equipment require a 3 phase supply ie motors
2 some use only one ie windscreen heaters
if you had single phase generators you would need 3 for 3 phase and then you would require a very complicated timing circuit to make sure the phases were the correct angle apart.You would also need to duplicate this system or triplicate it to get an airframe certified -the weight would be large .
So why not put a 3 phase genny on two engines and the apu that can supply all the power required and attach them to a constant speed drive that rotates the gen at a speed that gives you 400Hz depending on how many poles your choice of gen has because Freq= RPM x pairs of Poles
--------------------------
60
So know we know how to gear the constant speed drive to get 400 Hz whilst varying engine RPM ( still not said why 400Hz stay with me)
Now that we have gone to the effort of making 400Hz we can do a couple of things .
We tell everyone that's the standard so all equip is made to operate at 400 Hz and when they ask Why? we can say it's so we can use 3 phase generators to save weight and simplify power production because 400 Hz means a complete cycle occur's every 1/400 th of a second therefore a pole passes a coil of the gennny and generates max voltage every 1/1200 second (remeber 3 phase so 3 coils.The less the coils again the greater the weight saving but three is the minimum ) we therefore get the 3 phases 120 degrees apart because the same pole passes the next coil another 1/1200 of a sec later and then the third and all our electrical need's right down to Coffee is done on each 360 degree cycle of the genny or 400 times a second.
Joined: Aug 2005
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From: fairly close to the colonial capitol
1 Some equipment require a 3 phase supply ie motors
Freq= RPM x pairs of Poles
FREQ = RPM x Pole Pairs / 60
It would be almost DC if it was 1 or 2Hz.... or is my aged brain playing tricks with my radio theory?
Joined: Aug 2005
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From: UK
vapilot2004 you are quite correct many motors use one or two phase but if you want a motor that moves a load at a constant speed then it's the good old squirrel cage and that uses 3 phase also A TRU will use 3 phase AC to convert to DC to feed the DC Bus bars perhaps this is a better example. If anyone wants to understand the magic of this subject in greater depth may I suggest Aircraft Electrical Systems by EHJ Pallett he also does good books on Instruments and Autopilots.
Joined: Jan 2005
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From: north
3 phase fuel pumps and cooling fans are common.
146 reasons. You make the case for a.c., 3 phase and explain a.c. generator theory but, why 400Hz. Is it just an historic link to engine RPM and a convenient genny size/number of pole pairs?
Anyway 400hz makes a comforting whine in the headset.
146 reasons. You make the case for a.c., 3 phase and explain a.c. generator theory but, why 400Hz. Is it just an historic link to engine RPM and a convenient genny size/number of pole pairs?
Anyway 400hz makes a comforting whine in the headset.
Joined: Feb 2005
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From: flyover country USA
400 Hz requires much less iron in the magnetic flux path of transformers and motors, compared to 50/60 Hz.
800 Hz was even lighter, and was used for a while in several military aircraft of the 50s & 60s. Never really caught on though.
But solid state inverters / DC-DC converters use much higher frequencies (several KHz) to achieve much lighter weight. The power supply in your computer is such a device.
800 Hz was even lighter, and was used for a while in several military aircraft of the 50s & 60s. Never really caught on though.
But solid state inverters / DC-DC converters use much higher frequencies (several KHz) to achieve much lighter weight. The power supply in your computer is such a device.
Joined: Aug 2005
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From: UK
LPeacock Cooling fan motors are a classic example you want it to switch on run at a constant speed so yes Brake fan motors and Hyd pump cooling fans and also fuel pumps are good examples.
Everything in an areoplane is a comprimise the aim being since the day's of Wilbur & Orville to get the maximum power for the minimum weight and from the minimum expended energy-switching on the genny means some Avtur gets used running it.
So 400Hz is the best fit at present as it gives us 3 phase 120 degrees apart with the least amount of iron and copper as barfit1 says. It also gives us the best Power (and I dread to go here because sums aint my strong point) but you must remember for a sine wave that we deal with rms because its impossible to measure max power like we would in a dc circuit where P=IV.
3 phase= 3 sine waves 120 degrees apart ( the next bit you cant do but I remember it from collage for the simple reason I spent a lot of time trying to work out what the F was going on) If you take a sine wave and sample the power at 6 points during 1 cycle square each value add them all together and divide by 6 then take the square root of the answer you get the rms or average power.
1.414 times rms gives you Peak power so do the sums for each phase for the total power of the gen in 1 revolution and that is the best fit for todays technoligy however with the 380 we may move on and good for us but will uncle sam certify it if he didn't invent it ?-now there's a whole new topic.
sorry for the misspelling barit1 to late to go back now
Everything in an areoplane is a comprimise the aim being since the day's of Wilbur & Orville to get the maximum power for the minimum weight and from the minimum expended energy-switching on the genny means some Avtur gets used running it.
So 400Hz is the best fit at present as it gives us 3 phase 120 degrees apart with the least amount of iron and copper as barfit1 says. It also gives us the best Power (and I dread to go here because sums aint my strong point) but you must remember for a sine wave that we deal with rms because its impossible to measure max power like we would in a dc circuit where P=IV.
3 phase= 3 sine waves 120 degrees apart ( the next bit you cant do but I remember it from collage for the simple reason I spent a lot of time trying to work out what the F was going on) If you take a sine wave and sample the power at 6 points during 1 cycle square each value add them all together and divide by 6 then take the square root of the answer you get the rms or average power.
1.414 times rms gives you Peak power so do the sums for each phase for the total power of the gen in 1 revolution and that is the best fit for todays technoligy however with the 380 we may move on and good for us but will uncle sam certify it if he didn't invent it ?-now there's a whole new topic.
sorry for the misspelling barit1 to late to go back now
