In a DC generator, an external prime mover supplies mechanical torque that rotates the armature through magnetic flux. The motion generates EMF, and the commutator presents a unidirectional voltage at the brushes.

Generated voltage

The generated EMF is approximately

$$E=k_e\Phi\omega.$$

Increasing magnetic flux or rotational speed therefore increases the internal generated voltage.

Supplying a load

When current $I_a$ is delivered to a load, the armature resistance causes an internal voltage drop. A simple generator relation is

$$V=E-I_aR_a,$$

so terminal voltage falls below the generated EMF as load current increases.

Opposing torque

The armature current interacts with the magnetic field and produces electromagnetic torque opposing the externally driven rotation.

This reaction torque is required by energy conservation: delivering more electrical power requires the prime mover to supply more mechanical power.

Excitation

The magnetic field may come from permanent magnets or field windings. When field current controls $\Phi$, changing excitation changes the generated voltage as long as magnetic saturation and other nonideal effects remain moderate.

Generator operation is therefore the same electromechanical coupling as motor operation with the net power flow reversed.