Generally speaking, you will need an alternator that is at least 5 x the peak requirement for the drive. This is not specific and in some cases the multiple can be as high as 7 x the drive size. This should be calculated as a multiple of the Input Current (not motor kW). The alternator should be capable of 5 to 7 x the drive Input KVA.
For a DC drive - Input Current is DC Current x 0.82 = AC Current for the armature. The over-sizing is necessary because alternators are a soft supply and rely on the alternator electronics to automatically regulate the Voltage.
The supply to the Inverter, Soft Start or DC Thyristor Drive controller needs to have better than 10% impedance (see note 1) to avoid a situation where the Voltage waveform becomes notched and out of shape. The electronic regulator in the Alternator only looks at the peak Voltage and assumes the Voltage curve is sinusoidal, if the waveform is distorted it can produce unusual Voltages that can damage the drive, the alternator regulation components and other equipment.
The best way forward is to ask the Alternator supplier to provide a suitable unit for the ‘Inverter, soft Starter or Thyristor Drive’ load. If you approach a reputable manufacturer like Newage, they will know how to over-size the Alternator to suit this type of load.
Only the Alternator needs to be oversized to make it a stiff supply, the prime-mover does not need to be oversized of course.
Note 1: 10% supply impedance is taken to mean the supply to the drive terminals has the capacity to produce 10 x full load current as a minimum fault level. Many Alternators have such a high impedance, they are capable of making only 200% to 300% fault Current.