<P> The earliest dynamos used permanent magnets to create the magnetic field . These were referred to as "magneto - electric machines" or magnetos . However, researchers found that stronger magnetic fields, and so more power, could be produced by using electromagnets (field coils) on the stator . These were called "dynamo - electric machines" or dynamos . The field coils of the stator were originally separately excited by a separate, smaller, dynamo or magneto . An important development by Wilde and Siemens was the discovery (by 1866) that a dynamo could also bootstrap itself to be self - excited, using current generated by the dynamo itself . This allowed the growth of a much more powerful field, thus far greater output power . </P> <P> Self - excited direct current dynamos commonly have a combination of series and parallel (shunt) field windings which are directly supplied power by the rotor through the commutator in a regenerative manner . They are started and operated in a manner similar to modern portable alternating current electric generators, which are not used with other generators on an electric grid . </P> <P> There is a weak residual magnetic field that persists in the metal frame of the device when it is not operating, which has been imprinted onto the metal by the field windings . The dynamo begins rotating while not connected to an external load . The residual magnetic field induces a very small electrical current into the rotor windings as they begin to rotate . Without an external load attached, this small current is then fully supplied to the field windings, which in combination with the residual field, cause the rotor to produce more current . In this manner the self - exciting dynamo builds up its internal magnetic fields until it reaches its normal operating voltage . When it is able to produce sufficient current to sustain both its internal fields and an external load, it is ready to be used . </P> <P> A self - excited dynamo with insufficient residual magnetic field in the metal frame will not be able to produce any current in the rotor, regardless of what speed the rotor spins . This situation can also occur in modern self - excited portable generators, and is resolved for both types of generators in a similar manner, by applying a brief direct current battery charge to the output terminals of the stopped generator . The battery energizes the windings just enough to imprint the residual field, to enable building up the current . This is referred to as flashing the field . </P>

Elucidate the construction and working of dynamo and alternator