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

[Chris]

On 02/14/16 19:58, Johnny Billquist 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.
>

Sorry, but that's incorrect. Ac squirrel cage induction motors,
single or multiphase, have slip which varies with load. The only
exception is synchronous ac motors, which are used where constant
speed is required. They do tend to be less powerful than similarly
sized induction motors though. First year electrical eng stuff
really.

 From memory, many dec drives (R80, RA**) used induction ac motors for
spindle power, (they are cheap and reliable) but may have had
thyristor or other solid state control to stabilise the speed. RLxx 
series also used ac motors. Most vendors would try to avoid any
motor design with brushes, as they are very electrically noisy and
are not very reliable...

Regards

Chris