400 Hz

Paul Koning paulkoning at comcast.net
Wed May 5 10:37:38 CDT 2021

> On May 5, 2021, at 11:25 AM, Andrew Back via cctalk <cctalk at classiccmp.org> wrote:
> On 05/05/2021 16:07, Grant Taylor via cctalk wrote:
>> Were the higher frequencies used because it directly effected the amount
>> of time / duration in (fractions of) seconds between peaks of rectified
>> (but not yet smoothed) power?
> Haven't read the rest of the thread and so at the risk of being
> profoundly wrong... Benefit of 400Hz mains is that transformers can be
> much smaller. Think of switching power supplies that rectify to DC and
> then switch up into kHz, which are then able to use far smaller
> transformer cores than an old linear PSU. At least this is a key
> motivation with 115V/400Hz 3-phase aviation power AFAIK.
> By coincidence we've just built a big 28VDC power supply, so that we can
> run a vintage 400Hz aircraft rotary inverter, which will then be used to
> power up old mil surplus kit that wants this. 

Incidentally, a way to get three phase power at a frequency of your choice is to use a "variable frequency drive".  That's basically a high power solid state inverter intended to drive three-phase motors with a chosen frequency resulting in the RPM of your choice.  Depending on the model, those can go up to 120 Hz or so, or all the way to somewhere around 400 Hz.  I have a very cheap one at home that runs on single phase 220 volt power, producing up to 3 kW or so at anywhere up to 120 Hz.  (Made by a company named Teco, amusingly.)  Most of these and especially the larger ones want three phase mains input, though I'm told that even for those you can typically just connect them to single phase power (between two of the three inputs, leaving the third unconnected) at reduced power ratings.  These devices are surprisingly cheap, in particular they tend to be cheaper than "rotary phase converters" which is how machine shops traditionally produce three phase power when their mains is just single phase.


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