This might be a basic question, but I’m wondering: is it possible to run brushless AC/DC motors directly from a diesel-electric generator without using batteries?
No, you need a controller (sometimes called an inverter, depending on the kind of motor, but it's not the same as a DC-AC house inverter) to operate the motor. A brushless motor can't be run directly off a generator (or battery).
A brushed motor could be, but you would have no control over it's speed or torque, it would just be full on, or full off, and it would take a contactor or relay or other switch that can handle the full surge current under maximum load, that can also break that current under maximum load without damage to itself, to operate it with any certainty of turning it on and off as required. It's likely the switch would be as big, or nearly so, as the controller to run it, and possibly in the same cost range.
If the generator is capable of supplying the entire load's power demands with the right "resistance"*** then it could run it without batteries. However, the system must be completely regen-disabled, and have no way to create a voltage feedback into the generator under any conditions, unless the generator is designed to absorb this (I don't know of any rating for it to check a spec sheet for, so that's something you'd have to ask the designers of the generator (not the sales people), or that you would have to design into it.
***Because there are rapidly changing load conditions, the equivalent resistance of the generator must be as low as a battery should be, or there will be wildly fluctuating voltages on the controller input, which it may not be able to tolerate--spikes of voltage above component limits could instantly destroy the controller, for instance, dips in voltage below LVC could shut the system down just when you need power to avoid traffic conditions, etc.
For example, could a 250kVA generator provide enough power to charge or run something like a Tesla?
How much power does the vehicle require, at peak? The generator must be able to supply this without stress.
How much power does the vehicle require, continuously? The generator must be able to supply this without stress.
How much voltage does the vehicle require? The generator must be able to supply this without stress under the highest current draw the vehicle will have.
How much voltage / current can the vehicle generate under any form of regenerative conditions? The generator must be able to absorb this without stress on it, or the voltage may spike and destroy the controller and anything else on hte vehicle that is on that power bus.
Etc.
The generator must be able to do what the battery does under all conditions the battery has to do them under. If it can, then it would work without a battery. Otherwise, you will require a battery on the same bus that can take care of all those conditions.
Also, how do you calculate the power requirements of a motor?
That depends on the specific conditions you are using it in. You can use online simulators and calculators such as those at ebikes.ca (which has custom inputs for stuff not listed in the dropdowns, and instructions on how to use it below the chart).
If I have four 10kW AC hub motors, could I run them directly off a 40kVA diesel generator, or would I still need batteries, regulators, capacitors, or other components in the system?
That's answered in the sections above.
