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

[Johnny Billquist]

On 2016-02-14 23:30, Jan-Erik Soderholm wrote:
> Den 2016-02-14 kl. 23:10, skrev Scott Dorsey:
>> Johnny Billquist  <bqt at softjar.se> wrote:
>>> On 2016-02-14 14:31, Scott Dorsey wrote:
>>>> 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.
>>>
>>> "Slip"? An AC motor cannot have slip. It is controlled by the phases of
>>> power. It has to follow the phases.
>>
>> Look at the nameplate on a typical induction motor, you will see that it
>> is probably rated for around 2% slip.  A fan motor will have much higher.
>> A synchronous motor won't have any.
>>
>> Because of that, a synchronous motor has an additional winding (or set
>> of windings) in order to get it up to speed, or else it needs to be
>> spun up by a second motor or a hand crank.
>>
>> If you watch Cinema Paradiso, for instance, the projectionist is using
>> a Prevost projector with a synchronous motor and no start winding.  On
>> the first film cue, he pushes in a crank, spins the motor up, turns the
>> motor on so it can lock in on speed, then waits for the second film cue
>> which is spaced long enough for the motor to be at speed and hits the
>> douser to change over to the projector.
>>
>>>> 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.
>>>
>>> Sounds like it could have a lot of loss involved in conversions. But I'm
>>> not got at batteries.
>>
>> Yes, there is a good bit of conversion loss, although these days we have
>> pwm waveform control which gives a lot better efficiency than previous
>> generations of inverters.
>> --scott
>>
>
> The wood work machinery in my grandfathes shipyard had 3-phase AC
> motors. It took several minutes to rev up some of them to speed.
> You put the switch first to the "Y" position and when the speed
> of the machinery was stable switched to "D". Seems to be called
> "star-delta (YΔ) starting" in English...
>
> So I do not understand 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.
>
> https://en.wikipedia.org/wiki/AC_motor
> seems to have the detailes...

I am obviously not a big expert on these things, and am most likely not 
using correct terminology either.

Going through the Wikipedia article, what I'm talking about are 
synchronous AC motors.

Maybe that will clear things up a bit?

But the RP06, as well as all the other spindle motors of all DEC drives 
I've ever seen, are not AC motors, unless I'm more confused. I just 
checked the RP06 manuals to make sure, and they definitely show a DC 
feed for the motor.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol