DC generators are widely used to produce a DC voltage. The amount of voltage produced depends on a variety of factors. A basic DC generator has four basic parts: a magnetic field; a single conductor, or loop; a commutator; and brushes. The magnetic field may be supplied by either a permanent magnet or an electromagnet. For our purposes here, we will use a permanent magnet to describe a basic DC generator.
A single conductor, shaped in the form of a loop, is positioned between the magnetic poles. As long as the loop is stationary, the magnetic field has no effect (no relative motion). If we rotate the loop, the loop cuts through the magnetic field, and an EMF (voltage) is induced into the loop.
When we have relative motion between a magnetic field and a conductor in that magnetic field, and the direction of rotation is such that the conductor cuts the lines of flux, an EMF is induced into the conductor. The magnitude of the induced EMF depends on the field strength and the rate at which the flux lines are cut. The stronger the field or the more flux lines cut for a given period of time, the larger the induced EMF.
A DC generator contains four ratings.
Voltage: Voltage rating of a machine is based on the insulation type and design of the machine.
Current: The current rating is based on the size of the conductor and the amount of heat that can be dissipated in the generator.
Power: The power rating is based on the mechanical limitations of the device that is used to turn the generator and on the thermal limits of conductors, bearings, and other components of the generator.
Speed: Speed rating, at the upper limit, is determined by the speed at which mechanical damage is done to the machine. The lower speed rating is based on the limit for field current (as speed increases, a higher field current is necessary to produce the same voltage).