The voltage generated by an alternator is directly proportional to the rotational speed of the rotor. Therefore, if the speed of the alternator drops from 1500 RPM to 1000 RPM, the generated voltage per phase will decrease proportionally.

The relationship between the generated voltage and the rotational speed of an alternator is given by the following equation:

V = K * N * A * Phi

Where

V is the generated voltage per phase

K is a constant that depends on the design of the alternator
N is the rotational speed of the rotor in revolutions per minute (RPM),
A is the number of turns of wire in the stator winding, and Phi is the magnetic flux per pole.

Assuming that the value of the constant K is constant, the voltage generated per phase at 1500 RPM can be expressed as:

V1 = K * 1500 * A * π

Similarly, the voltage generated per phase at 1000 RPM can be expressed as:

V2 = K * 1000 * A * π

To find the generated voltage per phase at 1000 RPM, we can use the ratio of the two equations above:

V2 = V1 * (N2/N1)

Where N1 is the initial speed of the alternator (1500 RPM) and N2 is the new speed of the alternator (1000 RPM).

Plugging in the values, we get:

V2 = 210 * (1000/1500) = 140 volts per phase

Therefore, the generated voltage per phase will drop to 140 volts per phase when the speed of the alternator drops to 1000 RPM.