30.17:
DC Generator
A DC generator converts mechanical energy into DC electrical energy. The basic construction of the DC generator is similar to that of the simple alternator, except that the two slip rings are replaced with a single split ring.
As the conducting loop rotates in a uniform magnetic field, the magnetic flux through the loop changes sinusoidally, resulting in induced emf.
Here the split ring acts as a commutator. The commutator switches the contacts for each half rotation of the loop, ensuring the same polarity for the induced emf.
Similar to an alternator, the induced emf is zero when the magnetic flux is either maximum or minimum.
But because of the commutator, the induced emf is never negative and has a maximum when the magnetic flux is zero.
The magnitude of the average induced emf is estimated by replacing the sinusoidal term from the simple alternator with its average value.
The magnitude of the induced emf is proportional to the angular velocity for the given magnetic field and the area of the loop.
30.17:
DC Generator
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.
Suggested Reading
- Young, H.D and Freedman, R.A. (2012). University Physics with Modern Physics. San Francisco, CA: Pearson. Section 29.4, pages 964-965.