How can the speed of a DC generator rotor affect its output voltage?

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In a direct-current (DC) generator, the magnitude of the output voltage is directly proportional to the rotor speed and the magnetic flux.1

Because the voltage is induced by the rate of change of the magnetic field through the coils as the machine rotates, increasing the speed of the rotor increases the rate at which the magnetic flux changes, thereby increasing the induced voltage. Conversely, a decrease in rotor speed results in a lower output voltage.12

While the rotor speed is a primary factor in determining the magnitude of the voltage, control of the output voltage in practical applications is typically managed by adjusting the direct current in the field coils.1