There a lot of discussion here https://retrocomputing.stackexchange.com/questions/7412/why-does-the-cray-2-use-400-hz-power-and-why-generate-that-from-motors ht

There a lot of discussion here https://retrocomputing.stackexchange.com/questions/7412/why-does-the-cray-2-use-400-hz-power-and-why-generate-that-from-motors https://retrocomputing.stackexchange.com/questions/7412/why-... but nothing seems conclusive.. I would wager the last answer, "IBM was using 400Hz", to be most directly causal reason. The motor-generator configuration might provide galvanic isolation and some immunity to spikes and transients as well?