The three types of
control schemes for an electric drive are manual control, semiautomatic control
and fully automatic control. In a fan, manual control is employed where a
starter and resistance is used for setting required speed. In semi-automatic
control, some commands are manually controlled and some commands automatically.
Automatic control electric drive consists of a control gear.
Electric drives are
divided into four categories based on speed control. They are first group,
second group, third group and fourth group. The first group drives provide
uncontrolled constant speed of rotation during normal operating
conditions. The electric drives used in
pumps, compressors and ventilators are first group drives. The second group
electric drives provide step changes in speed during normal operating
conditions. In each step, constant speed is maintained here. This type of drive
is used in machine tools. The third group electric drive is used in electric
locomotive where smooth speed control of the fraction motor is required. The
third group provides smooth, narrow step speed control within certain ranges.
Lifting devices use fourth group electric drives where drives are capable of
providing speed control for fixed positions of the load.
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