Very large machines, certainly. There were "site preparation" documents that detail this.
I can quote from the 1975 edition of the STAR-100 hardware manual on bitsavers (PDF page 25): "Power for the basic computer consists of one 250 kva, 400 Hz motor generator set. The motor-generator set has the capability of providing power for the CPU, MCS, I/O and the MCU. The optional memory requires the addition of an 80 kva motor-generator set." Granted, the STAR was a monster of a machine, but similar setups were used for CDC 6000, 7000 and Cyber 70, 170 machines and others. That doesn't include the refrigeration necessary for the chilled water supply. I/O devices such as card readers and tape drives often used a "split" supply with the 400Hz supplying the electronics and normal 50/60 Hz AC supplying power for motors, etc. Small S/360 systems were operated from standard AC distribution. I don't know where the breakover was for IBM S/360; probably not for the model 30, 40, or 75. The 195, I suspect did use an MG set. The 7090 certainly used MG sets. From the "Power Supply and Distribution Manual" (http://bitsavers.org/pdf/ibm/7090/ce/7090%20Power%20Supply%20Control%20and%20Distribution%20223-6904.pdf, page 5): "The IBM 7608, a power converter or motor-generator set which converts incoming 60-cycle three-phase (3Ø), 208v power to regulated 400-cycle 3Ø 208v power." --Chuck
