PD = voltage.
This is an AC generator, as you say. AC reverses direction periodically. So asking how to reverse the direction of the voltage is meaningless, as it reverses periodically on it's own.
You can increase the voltage many ways, by more turns on the coils, by a stronger magnetic field, by adding more coils of wire, by using a better core material, by enhancing the angles of the coils, and by increasing the rotation rate.
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