On 28-Feb-16 14:40, Paul Koning wrote: >> 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. > > All true. I should have qualified my comment to magnetic disk drives of this era, which do tend to be related line frequency. There isn't much load once the disk is up to speed - just friction losses. The disks of this era are quite massive; inertia takes over.
I don't know about the RD10. The RP drives, which came a bit later, nominally require 3 phase power, but use 1 phase, 1HP motors. Here are some semi-random examples: IBM 350 - 1200 RPM IBM 1405 - 1200 RPM IBM 1301 - 1800 RPM IBM 1311 - 1500 RPM BUR IA2 - 1500 RPM BUR IC3 - 1200 RPM BUR IC4 - 750 RPM BUR IIB2 - 860 RPM BUR IIB4 - 500 RPM BUR IIB6 - 1500 RPM RP01 - 2400 RPM RP02 - 2400 RPM RP03 - 2400 RPM RP06 - 3600 RPM RP07 - 3633 RPM RS04 - 3600 RPM RM02 - 2400 RPM RM03 - 3600 RPM RM80 - 3600 RPM Of these 19, 15 are even multiples of 50 and 60. I have one reference, which is of uncertain quality, that claims the RD10 is 1800 RPM. I still think this is the most likely, and that the "34 ms" is simply imprecision in the manual. In any case, the more compelling argument is the second one: the sectors/track density numbers don't make sense.
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