Why the alternator don't charge until the RPM reach around 1200 to 1500?
An alternator converts rotary energy into electrical energy by moving a magnetic field (the armature) thru the coils in the housing. The magnetic field MUST be present for the alternator to generate electricity. It is initially created by the excitation current that is supplied thru the charging warning light and the parallel resistor (mounted on the back of the voltage gauge). If the excitation current is too weak (bad resistor, bad bulb, bad connection), but present, the alternator will start charging ONLY at higher RPM levels. If the excitation current is not present at all, the alternator will never charge.
Once the alternator starts charging, it supplies its own field current. Since there is now voltage on both sides of the light/resistor combo, the light goes off. Once the alternator starts charging, the light/resistor combo has no effect.
If the charging warning light is on, there is voltage on one side of the bulb and not the other, and there is a problem, usually in the alternator.
The alternator coils produce AC voltage, which is not useful for our purposes. The alternator includes six charging diodes (one-way check valves for electricity) that convert the AC voltage to slightly-pulsing DC voltage. If one diode is failed open, the charging amperage will be reduced. If one diode is failed shorted, there will be AC ripple in the output voltage, and you will get whine in the radio. As more diodes fail, the conditions will worsen.