Control Data 841 disk drive's 3-phase power supply resurrection

Paul Koning paulkoning at
Sun Feb 18 12:24:06 CST 2018

> On Feb 18, 2018, at 12:34 PM, P Gebhardt via cctalk <cctalk at> wrote:
> Hello list,
> currently, I am in the process of trying to bring back to life a disk drive installation from Control Data known as "841 Multiple Disk Drive" ( MDD ). From the early '70s. It uses hydraulic disk head actuators! Pictures of the subsystem are here:
> I started with the power supply. Most of the electrolytic capacitors need to be reformed which is being done. 
> As far as I know, some computer installations used 400Hz 3-phase back in the days. Does anybody know, if that is the case for this type of drive systems? I couldn't find any indication so far, except for the input filter that supports up to 400Hz (written on it). 
> I've quite some experience with old linear power supplies, but never worked with three-phase supplies, yet.
> Has anybody experience with this? Anything particular to be considered?

CDC mainframe shops used 400 Hz three-phase power for the CPU and the display console (DD60).  I don't know if it was used in other places.  The advantage of 400 Hz power is smaller transformers and filters; in addition, it was generated by motor-generator units which produce good clean power even if the building receives crummy power from the utility.

> There is an operator's manual, but there don't seem to be manuals or schematics about this type of CDC drive nor on bitsavers, neither elsewhere on the net. How could help me in pointing out where to get these?
> A lot of questions, I know.... :)

There is a by-invitation list of CDC experts and fans, I included a BCC to the list owner on this reply.  There might be help available from that group.

There are two considerations for disk drive power.  Older disk drives often use AC induction motors, which tend to be three-phase motors.  With induction motors, the rotational speed is determined by the motor design and the power line frequency.  But I haven't heard of induction motors operating from 400 Hz power; those would spin quite rapidly indeed unless they had lots of poles.  50 or 60 Hz induction motors are fairly common, which is why you tend to see 3600 rpm nominal rotation specs for a lot of disk drives.

The other aspect is DC power supply design.  If a supply is designed specifically for 400 Hz input, it won't operate properly from 60 Hz mains.  The transformers will be too small, and the filtering inadequate.  But if the spec says 50-400 or 60-400 Hz, that means the transformer and filter sizing is done for mains power (50/60 Hz) and the transformer core is constructed to still have acceptable losses at 400 Hz.  That's not common but I could imagine CDC doing this because of their use of 400 Hz.  If you have such a wide-range device, just feed it mains power and all will be well.  But, say, a CDC 6600 central processor can't be powered by 50 or 60 Hz mains power.

As for 3-phase vs. 1-phase DC power supplies, that just means there are more rectifiers and smaller filters because the ripple is reduced by rectifying 3 phases.  You might be able to get away with feeding such a device 1-phaser power but it would probably be marginal, and 3 phase would be a better option if you can get it.  There are converters that do a decent job, and depending on where you live you might just get it from the power company.


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