Why not just take the AC from the alternator to the motors? Why go AC to DC to AC again?
To change varying AC to fixed AC .for example the alternator that is attached to the engine will output varying AC voltage, whose frequency would be different from the operating frequency or rated values of the drive.so to get a rated values accordingly to drive AC-DC-AC converter are used.
To expand on this, controlling the speed of the motors requires feeding it AC and varying the frequency. The generator puts out AC at a fixed frequency. So you need to convert fixed-frequency AC to variable-frequency AC. The most straightforward way of doing that is to rectify the AC to DC, then invert it back to AC again.
A variable-frequency drive is the standard way of accomplishing this. Most take AC input into a rectifier, then have an inverter stage feeding the motor. Occasionally it makes more sense to have a single large rectifier creating a large DC bus, and then feeding several inverters off that single bus.