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

[Scott Dorsey]

Johnny Billquist  <bqt at softjar.se> wrote:
>On 2016-02-14 21:28, JF Mezei wrote:
>> On 2016-02-14 04:48, already5chosen at yahoo.com wrote:
>>
>>> More I think about it, more the story of VAX hard drive with three-phase AC motor (in early 90s, non-the-less) appears apocryphal.
>>
>> RA82 drive motors were 1/3 horsepower motors. They were not 3 phase, but
>> required 3 wires in. There was one used to start the motor in the right
>> direction as I recall.  There was some capacitor that would discharge to
>> get motor going. I don't recall the exact details.
>
>RA81/RA82 do not, as far as I can recall, have any large capacitors to 
>get them running. And they draw a hell of a lot of power when spinning 
>up, which is why you have the power sequence cable on them, so that they 
>do not all try to spin up at the same time, as that would blow fuses. 

No, RA81 drives are single phase induction motors.  They have a capacitor
which provides phase shift to power a second winding, so that during
starting the motor is a "two-phase" motor with the second phase shifted
90 degrees.  That second winding is disconnected once the motor gets up
to speed.  There are other motors which use a "run winding" so that they
always are running with the second winding, but those motors tend to have
more slip.

>Once they are getting up to speed, the power requirement is much lower.

Indeed. 

>I can't remember how many wires went into the motor, but I think you are 
>right in that there was some trick to ensure that it would start 
>rotating in the right direction. The manuals are probably online if 
>someone wants to read up on the details.

If you swap the two windings, the motor will rotate in reverse.  You
can think of the start winding as pushing the rotor forward a little
bit after the main winding... if the two are swapped it pushes the 
rotor before the main winding and it motor runs backwards.
--scott
-- 
"C'est un Nagra. C'est suisse, et tres, tres precis."