An alternator converts mechanical energy into electrical energy that varies sinusoidally, resulting in AC current. Meanwhile, a DC generator converts mechanical energy into electrical energy, which are DC pulses with the same polarity. The construction of a DC generator is similar to that of an alternator, except that the pair of slip rings is replaced by a single split ring, also called a commutator. The commutator functions like a periodic rotary switch; it changes the contacts with the brushes for each half rotation of the conducting loop, ensuring that the induced emf always has the same polarity.

The basic working principle of DC generators is based on Faraday's law of induction. The conducting loop is rotated inside the uniform magnetic field, and this rotation changes the magnetic flux passing through the loop periodically. When the conducting loop is connected to the external circuit, the changing magnetic flux induces an emf, which results in an induced current. The connection to the external circuit is envisioned using a commutator and a pair of brushes.

The magnitude of the induced emf is maximum when the magnetic flux through the conducting loop is zero and minimum when the magnetic flux through the conducting loop is either maximum or minimum. The magnetic flux has positive and negative values, but the induced emf has the same polarity and fluctuates between zero and maximum. The average of the induced emf is calculated by taking the average value of the sinusoidally varying component. The magnitude of the induced emf can be increased by increasing the applied uniform magnetic field, the conducting loop's area, and the rotation's angular velocity.