PSU choose

I'm not really shure of how I should correctly choose my servo power supply.
Can someone tell me if I'm right or not ?

My 3 motors are rated at 72V - 38A (in peak) - 8.5A (continuous)

I don't knows how are labeled power supplies.
For example, www.kelinginc.net sell 72V - 20A PSU.
Is the transformer 50V and output equal 50x1.41 = 72 V
Or, is the transformer 72V and the output is 72 x 1.414 = 102 V

If I choose a transformer of 50V - 20A, my output will be at 50 x 1.414 =
72V
Is this ok for 72V motors ?
Or transformer should be less than motor maximum voltage ?

Then, capacitor should be 100V and C=(80000*20/50) ?
Should I replace 50V in this formula with 72V ?

Thanks,
GB