reducing input voltage to motor
If the rated speed at 100v is 4400rpm, then the nominal speed at 12v would be 4400*12/100 = 528rpm. In practice, it might be a little quicker or slower than that, depending on how the losses vary with speed.

The rated current at full power is 1500W/100v = 15Amps. It is this current that heats your windings.

With a motor that large, it's possible you could get 15A or more to flow through the windings with 12v applied. However, if the motor is self-cooled by a fan on the shaft, or simply by stirring air around in the casing, then the cooling air available would be much less than at rated speed, so you would have to reduce the current significantly below 15A to avoid the risk of it overheating.

If the motor has a cooling duct driven by a separate fan, and if you can run this cooling fan at full speed, you can cool it properly and it should be safe up to 15Amps.