Here is a 3 phase generator output voltage formula to calculate the three phase generator output voltage. To calculate the voltage output, multiply the number of turns with the area of coil on the armature. On the other end multiply 2.pi times the armature rotation speed, then divide the resultant value by 60. Then multiply the DC magnetic field with the square root of 3. Multiply the obtained value with step two. Now multiply the resultant value with step one.