> On Feb 28, 2016, at 11:29 AM, Timothe Litt <[email protected]> wrote:
>
> On 27-Feb-16 14:19, Bob Supnik wrote:
>> Thanks, Tim. Burroughs made a lot of fixed head disks at the time. I
>> can't identify the model, but the IA2 (see page 7-4 of the B6700
>> Hardware Handbook, on bitsavers) seems like a possibility. It has 7552
>> sectors per surface vs 8000, but Burroughs sectors were longer than
>> DEC sectors (180 x 8b = 1440b vs 32 x 36b = 1152b), so perhaps DEC
>> format had more sectors per track.
>>
> Maybe. But I don't think it's the IA2. Probably the previous
> generation Burroughs disk.
>
> The RC10 manual (DEC-10-I5AA-D) states that the disk makes one
> revolution in approximately 34ms.
> (17 ms access time). That corresponds to 1765 RPM, an unlikely number
> since motors tend
> to be a multiple of the line frequency (50/60 Hz). 1800 RPM is 33.33
> ms/rev. That's "approximately
> 34 ms", and my guess at the correct speed.
A hair under 1800 rpm is quite plausible. Synchronous motors turn at line
frequency or an integer fraction of it, because that't the rate at which the
flux pattern turns around the motor poles.
But induction motors, which are the most common power machinery motors until
you get to very large sizes, turn a hair slower, and slow down slightly more
under load. This is called "slip"; it comes from the fact that the induced
field in the rotor is not fixed to the rotor. For example, a randomly picked 3
phase motor (Leeson 1.5 hp) has a spec of 1725 rpm.
paul
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