Without details on the motor make and model, I'd speculate that the 400V is DC phase to phase which is around 230V AC.
No, 400 V three-phase is genuine 400 VAC phase to phase, or 400/sqrt(3) = 230 V phase to neutral. [ Edit: but motors never use a neutral connection; I mention this to point out why 380/400/415 V is a common 3-phase voltage level; it corresponds to 220/230/240 V phase to neutral). ]
Yes, 400 VAC is a little high for EVs; you do need around 400 x sqrt(2) = 565 V on the DC bus, which would normally imply a pack voltage that high. I've been involved with a 720 V nominal conversion
, so it can be done. We used a custom industrial motor to get 22/26 kW continuous (@ 50/60 Hz) into a 132 frame (~ 10.4" diameter).
It's occasionally possible to find an off-the-shelf motor wound for about 400 V when configured in Y, so you can re-configure it for 230 V delta (as we did), but it is rare. Generally, you want to over-voltage the motor, so you can get more than ~3000/3600 RPM from it (50/60 Hz), so that it better suits a typical EV drivetrain. That allows you to get a lot more peak power then the noninal (continuous) power of the motor, but that still requires a high DC pack voltage.
You can occasionally find motors designed for Japanese mains, which are 100 V phase to neutral, or wound for other voltages. It is also possible to rewire a motor for lower voltage, but again you have to be very lucky to find one that is suitable (see http://forums.aeva.asn.au/forums/changing-an-induction-motor-voltage_topic1237.html
). Finally, it is possible to have any motor rewound for whatever voltage you want; this involves burning out the existing wiring and starting again.
Unfortunately, most of these options take away from the main attraction of using off-the-shelf 3-phase induction motors: they are plentiful and can be found cheaply. As a result, such conversions are rare.