Although we can find many applications of AC in industry, we still find that there are many industrial processes that require DC for their operation.
Some of these applications include battery charging, emergency lighting, welding, electroplating, metal refining, metal reduction, communications and many others.
Unlike AC generators, the speed of DC generators are not so critical. Therefore, small DC generators can be belt-driven, gear-driven or direct driven by prime movers. Large DC generators are always direct driven. Some of the prime movers used are AC motors, petrol or diesel engines, water or steam turbines.
Small DC generators are driven from 300 to 1800 rpm, while larger generators are driven from 60 to 250 rpm. Because of line voltage drops, the direct current produced are only transmitted over short distances.
For convenience, many DC applications uses rectifiers to convert AC supply to DC supply. These rectifiers usually make use of solid-state semiconductors because they have no moving parts and are very robust in construction. When such rectifiers are used, some sort of wave filters need to be installed to smoothen out the pulsating DC supply obtained through rectification.
For short term applications, batteries are used to supply the necessary DC supply. These have to be recharged during and after use by DC generators or rectifiers. The DC supply obtained from batteries are smooth.
Many people do not realize it, but the generation of AC power requires the use of a DC supply at its field coil to maintain a constant magnetic field. This source of DC supply is obtained from either a DC generator or a rectifier unit.
In modern brushless AC generators, the rectifier unit together with the field coil are actually fixed to the rotating rotor and moves with the rotor.
DC motors have the advantage of excellent starting torque and wide ranges of speed control. They are therefore used to drive machines that are difficult to start under load or are required to run at varying speeds. Another advantage of DC motors is the fact that they can be reversed very easily. By just varying the polarity of the field coil supply, the motor can be made to run in the reversed direction.
Some of the applications of DC motors are in large steel rolling mills, electric railways, elevators, hoists and cranes. All these applications require that the machines can be easily started from standstill on heavy loads and then proceed to full speeds in a very short time. The speeds of rotation of these machines are also required to be controlled very accurately.
Electronic controls are used to vary the speeds of DC shunt motors by adjusting the armature voltages. Variable voltage for the armatures are supplied by silicon controlled rectifiers (SCR) circuits.
Can you imagine how life will be like without these DC machines? For tall buildings, the occupants will have to endure erractic elevator movements whenever the number of passengers changes. Isn't that annoying?
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