> On Mar 10, 2023, at 6:26 PM, Sytse van Slooten via cctalk
> <cctalk@classiccmp.org> wrote:
>
>>
>> On Fri, 10 Mar 2023, Kevin Anderson via cctalk wrote:
>>>
>>
>> Which machines needed 3-phase?
>
> None that I am aware of - even the very power hungry 9021 'mainframes' I
> mentioned earlier could be powered from a single phase, if you wanted to. The
> power company might have expressed a view, but that's a different subject. 3
> phase is relevant to circular motion, not to the amount of power you need -
> although lots of electrical equipment tends to come in 3 phase versions,
> especially if the amount of power goes up.
True. I remember that the house where I grew up in Holland had 3 phase service
because we were one of the (then unusual) households to have an electric
cookstove. It was fed off 380 volt 3 phase power.
But apart from circular motion (i.e., feeding motors) there is a second reason
to use three phase power: reduced ripple. Full wave rectification of n Hz
mains power gives you a 2n Hz ripple; full wave rectification of 3-phase n Hz
mains produces a 6 Hz ripple, and tha amplitude is a lot less as well. So the
filters are much smaller, which is a non-trivial matter if you need to filter
several hundred amps. That of course assumes your power supplies are actually
fed off the three phase mains, rather than being individual single phase
supplies spread over the phases. The former was certainly the case with the
CDC 6600, you can see it clearly in the schematics.
paul
