During the nineteenth century, when power supply was dc, dc motors were used extensively to draw power direct from the dc source. The motor speed could be varied by adjusting field current by a rheostat. That was an open loop control. Most of the drives were constant speed and the characteristics could not be matched with a job requirement. A vast development in the dc drives system took place when the ward Leonard Control System was introduced in the 1980s. The system was motor-generator system to deliver power to the drive motor. The supply power available was still dc and dc motor was used to drive the dc generator set at a more or less constant speed. Afterwards when the ac power system came into existence and became popular, ac motors were developed and became attractive owing to their constructional simplicity, ruggedness and lower initial as well as maintenance cost. Machine requiring variable speed drives use the ward Leonard System employing ac motors driving dc motors at a constant speed. In the 1950s electronic came into existence and brought about remarkable improvement in the speed control system. The open- loop manual control system was replaced by close loop feedback control, which resulted in improved response and better accuracy. Initially, gas diodes and ignitrons were developed and ac to dc converters were used to control dc motors.
The advent of thyristors capable of handling large current has revolutionized the field of electric power control. Thyratrons, ignitrons, mercury arc rectifiers, magnetic amplifiers and motor generator sets have all been replaced by solid state circuits employing semi-conductor diodes and thyristors. Thyristor controlled drives employing both ac and dc motors find wide applications in industry as variable speed drives. In the 1960s ac power was converted into dc power for direct control of drive motors with solid state devices (high power silicon diodes and silicon controlled rectifiers). Initially saturable reactors were employed in conjunction with power silicon rectifiers for dc drives.
Of late solid state circuits using semi- conductor diodes and thyristors are becoming popular for controlling the speed of ac and dc motors and are progressively replacing the traditional electric power control circuit based on thyratrons, ignitrons, mercury arc rectifiers, magnetic amplifiers, motor-generator sets, etc as compared to the electric and electro-mechanical systems of speed
control. The electronic system has higher accuracy, greater reliability, and quick response and also has higher efficiency as there is no I2R losses and moving parts. Moreover four-quadrant speed control is possible to meet precise high standards. All electronic circuits control the speed of the motor by controlling either, ➢ The voltage applied to the motor armature or
➢ The field current or
➢ Both of the above
DC motors can be run from dc supply if available or from ac supply, after it has been converted to dc supply with the help of rectifiers which can be either half wave or full wave and either controlled ( by varying the conduction angle of the thyristors used) or uncontrolled. AC motors can be run on the ac supply or from dc supply, after it has been converted into ac supply with the help of inverters (opposite of rectifiers). As stated above, the average output voltage of a thyristors controlled rectifiers by changing its conduction angle and hence the armature voltage can be adjusted to control its speed. When run on a dc supply, the armature dc voltage can be changed with the help of uncontrolled rectifiers (using only diodes and not thyristors). The dc voltages so obtained can be then chopped with the help of a thyristors chopper circuit.
In this method of speed control of a dc motor, available ac supply is first rectified into dc supply using uncontrolled rectifiers. The supply is...