# 400 Hz

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Why do aircraft run on 400 Hz in stead of 60Hz?

Paul Electrical Estimator

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Smaller inductive widgets. Coils, servos, transformers etc work better at 400hz so they can be smaller.

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Magetic cores (in transformers, motors, generators) can be ~1/7 of the weight. Weight is really important on airplanes.

Best regards, Spehro Pefhany

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Correct. I can pick up a 100 kVA 400 Hz generator with one hand.

Question: What weights nothing but, when loaded onboard an airplane, can keep it from taking off?

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An injunction?

Best regards, Spehro Pefhany

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Doesn't an injunction carry the full weight of law?

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True enough. The full weight of the long arm of the law. I remember the right answer now, but I'll let Paul tell it.

Best regards, Spehro Pefhany

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Not *all* 400 hz machines are that fast. RPM = F*120/ (Poles). So a 26 pole 400 HZ motor has a sync speed of only about 1846 RPM

We had some M-G sets for 400hz that were this many poles, driven by DC motors.

daestrom

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To generate smooth 400 HZ for use in radar, sonar and fire-control systems. (i.e. military)

daestrom

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The particular application (submarine) already had a large, DC source. The DC busses are more reliable than even the 'vital' AC busses. And variable-speed DC drive motor is pretty simple (i.e. robust) technology in their day.

400HZ was also used in basic navigation equipment (like gyrocompass). The starboard DC bus that powered emergency propulsion, had DC diesel tie-in, battery-bus tie, 400HZ MG and emergency steering&diving hydraulics was called the 'go home' bus. As electrical operator, if I kept *that* one on, I knew, "well, at least we can go home."

I'm sure there are other applications that would prefer 60HZ -> 400HZ M-G sets instead. My brother (civilian) recounts story of large 400 HZ/12V m-g (60 -> 400) that was used for computer room power supply. Something about rectified 400HZ was smoother to filter before such wide spread use of switching power supplies. (these are *old* computer rooms that are long gone now ;-)

daestrom

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There was a story a few years back about an auto manufacturer considering spec.ing all their electrically-powered hand tools for

400Hz. It seems that they had a real problem with 60 Hz and pneumatic equipment walking off the property to employees' home workshops.
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I've seen 60 - 415 hz frequency converters used for the good old IBM Mainframes but those monsters are on their way of extinction. IBM justified their use by : Stable / lower power consumption... Certainly smaller and lighter than 60 Hz equivalent where in subs or planes it is crutial.

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I thought one reason for 400-415Hz on mainframes was to get high platter speeds using 3ø induction motors?

--s falke

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I/O was all 60hz. They used 208/240 3p 60 hz for the disk drive motors (typically 3/4HP 3600RPM)

400hz was only used for the big DC supplies in the processor complex which could be 300amps or more at 1.5 - 3v. That was before CMOS became the standard.
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Used to work for a company which made computers for ships, and they often also had 400Hz supplies. We had a large 50Hz-400Hz converter, 300kVA IIRC, but I can't remember what voltage the ship based kit used. Also had large 48VDC supplies for telephone exchange based equipment.

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Actually, the phase-controlled regulators worked well off 400Hz, for the obvious reasons. Switching regulators became the norm in the 308x, IIRC. Before that, the monstrosity phase-controlled regulators were at the core of these beasties.

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