# 400 Hz

Discussion in 'Electrical Engineering' started by Estimation, Dec 4, 2003.

1. ### EstimationGuest

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

Paul
Electrical Estimator

2. ### Spehro PefhanyGuest

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

Best regards,
Spehro Pefhany

3. ### Spehro PefhanyGuest

An injunction?

Best regards,
Spehro Pefhany

4. ### Spehro PefhanyGuest

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

5. ### B J ConnerGuest

The synchronous speed of a 400 hrz motor is 24,000 RPM. That will make a
good gyrocompass.

6. ### daestromGuest

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

7. ### artGuest

What was the application for them?
Art.

8. ### daestromGuest

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

daestrom

9. ### artGuest

I red that military some where ordered 4000A DC service, you explain one of
the possible application. But why DC driven?
Artur.

10. ### daestromGuest

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

11. ### A+Guest

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.

12. ### Andrew GabrielGuest

Used to work for a company which made computers for ships, and
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.

13. ### Jon ElsonGuest

Larger IBM 360 computers (1968-1972) used 5000 Hz, 120 V single-phase
power to supply all the computing circuitry. They used a "converter-
inverter" to turn 208 V 50/60 Hz 3 Phase mains power to DC, then inverted
it with a fast SCR inverter. The 5 KHz whine was almost earsplitting
when you walked behind the main CPU cabinet. All the modular power
supplies in the machine ran of the roughly regulated 5 KHz power.
They used very thin transformer laminations, and the magnetics and
filter caps were VERY small for the day. This would apply to the
360/50 and above.

Large 370 mainframes used 415 Hz 3-phase power from a GE motor-generator
set. The advantage of these was the rotary inertia in the MG set, that could
power the machine for a few seconds when line power failed. it was pretty
wierd to be near the machine when there was a short power blip. All the
fans in the machine would fall silent, but the lights would keep flickering
away on the console. They used some really inventive circuits in the
regulators, too. They called this the electronic capacitor. It was
a big inductor that circulated current during the line peaks. At
the time of the dips between peaks, a transistor turned off for a
moment, allowing the current in the inductor to be added to the
current flowing out of the filter caps. The currents in these things
were huge, several hundred amps from each supply at -2 and +3.3 V.
The logic was similar to standard ECL.

Jon

14. ### Jon ElsonGuest

No, 3600 RPM was fast enough for nearly all disk drives of that
date. If they wanted it a bit faster, it would be no big deal
to change the pulley diameters. The windage losses of 14"
disk platters were already huge at 3600 RPM, you really couldn't
spin a big stack of them at 10,000 RPM without reducing air pressure
or supplying process cooling or they'd literally burn up!

Jon