AFAIK, the alternator would not produce 150 amps as soon as it spins.
The alty produces as per the load.
For example - If you are just driving the car, the alty would produce say 20 amps for the engine, another 10 amps for battery charging. Later if you turn on A/C or head lamps it would start producing according to the load.
In swift or any modern cars, as soon as the load is high the engine rmp would slightly increase so that the alty can produce the amount of power required.