why three phase system is not preferred in aircrafts power circuits....

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------------- Ah, but you have to generate and your prime mover may be a high speed turbine. There is then an advantage to 400Hz on aircraft in that the coupling of the generator to a high speed turbine is easier and lighter as a lot of gearing is avoided.
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Don Kelly snipped-for-privacy@shawcross.ca
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Don Kelly wrote:

If I'm not mistaken, turbine speed on aircraft are anything but constant. So driving a 400Hz generator directly from them (or through any fixed gearing) isn't practical. But I'm not sure how they are driven on aircraft. A separate 'auxlilary power unit' (APU)??
On naval ships, they were often driven from 60Hz motors and variable-slip couplings. On subs, that have an abundance of DC systems anyway, DC motors were used to drive the 400Hz motor-generator sets.
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Thanks- I wasn't sure. However, there are some advantages to high speed motors as the weight/HP/volume ratio is better than for slower motors. Of course there are more reactance problems- both L and C. On land or on ships- there is less need to save weight. However, I do recall a compressor which was driven at these higher speeds -in a refinery. The details are now pretty hazy.
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VIDHYA wrote:

Who told you that? Three phase (400 Hz) is used extensively onboard aircraft.
28 VDC is used for systems where battery backup is provided.
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Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
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