[Info-vax] OT: the Daily WTF for today is a VAX/VMS story

[Scott Dorsey]

Johnny Billquist  <bqt at softjar.se> wrote:
>However, no hard drive uses a 3-phase AC motor to drive the disk. Anyone 
>with half a brain should realize this. 3 phase AC motors always follows 
>the phases of the supply like slaves. There is no spinup time. You go 
>from standstill to full speed immediately, always. That is not how you 
>want to spin up a disk drive.

If the motor is designed to have some slip, you can bring it up to speed
without too much trouble.  Synchronous motors in general always follow
the supply like slaves, no matter how many phases they have.

>The RP06 uses two phases actually, but it does it to extract more power. 
>The actual motor is still a DC thing. And yes, it spins at 3000 rpm (I 
>think that is correct) both in 50 Hz and 60 Hz countries.

I don't know of any DEC drives that use a motor attached directly to the
AC line.  That's not to say that they don't exist, but I have never used
one.  IBM had some.

>I do not know for sure how they dealt with battery backup for such 
>drives. Maybe someone else can tell?

Same way you deal with battery backup for everything, you have a big UPS
in the basement.  Power goes into the UPS, it's turned into DC, and then
the DC is regenerated into three phases by an oscillator and three big
electronic switches.

If you're lucky you have a changeover switch on the input so that once
you are on battery you can disconnect from the ac line and shift to a 
generator, so the battery only needs to last long enough for you to get
the generator running.

It's entirely possible to miswire all of these things so that phases are
swapped and three-phase motors run in reverse.  If you were to go directly
from one to the other, though, the back-current from the motors would
blow some fuses pretty fast, though.

We have fuses on everything specifically to prevent the sort of failure
described.
--scott
-- 
"C'est un Nagra. C'est suisse, et tres, tres precis."