[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 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

-- 
"C'est un Nagra. C'est suisse, et tres, tres precis."