Tuesday 7-23-96
A generator is essentially a motor operating in reverse. A motor usually converts electrical energy (or some other form of energy) into mechanical energy; a generator converts mechanical energy into electrical energy.
An AC (alternating current) generator utilizes Faraday's law of induction, spinning a coil at a constant rate in a magnetic field to induce an oscillating emf. The coil area and the magnetic field are kept constant, so, by Faraday's law, the induced emf is given by:
If the loop spins at a constant rate, theta = wt, where w is the Greek letter omega, representing the angular frequency. w is related to the frequency by the relation w = 2 pi f .Using calculus, and taking the derivative of the cosine to get a sine (as well as bringing out a factor of -w). The emf can then be expressed as:
The combination N B A w is the maximum value of the generated voltage (i.e., emf) and can be represented by a single variable. This reduces the expression for the emf to:
A coil turning in a magnetic field can also be used to generate DC power. A DC generator uses the same kind of split-ring commutator used in a DC motor. Unlike the AC generator, the polarity of the voltage generated by a DC generator is always the same. In a very simple DC generator with a single rotating loop, the voltage level would constantly fluctuate. The voltage from many loops (out of synch with each other) is usually added together to obtain a relatively steady voltage.
Rather than using a spinning coil in a constant magnetic field, another way to utilize electromagnetic induction is to keep the coil stationary and to spin permanent magnets (providing the magnetic field and flux) around the coil. A good example of this is the way power is generated, such as at a hydro-electric power plant. The energy of falling water is used to spin permanent magnets around a fixed loop, producing AC power.