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