there are various types of d.c. drives. the most common type of d. drive is the semi-conductor drive. in this type of drive, a thyristor-based electronic inverter creates a variable voltage for the motor. the electronic inverter comprises a bridge of thyristors, with each thyristor connected to the power line through a series of diodes. the thyristors are usually turned on and off in a fixed time pattern to create a variable voltage. this voltage is applied to the motor armature through brushes or slip rings, and varies the motor speed. in the semi-conductor drive, the armature current is the direct current. the armature current is controlled by the thyristor firing angle, which varies the speed of the motor. the firing angle is controlled by the thyristor gate current, which varies the time delay between thyristor firing. 28 in some drives, the armature current is not direct current and the armature current is controlled by the converter firing angle. 29
in semi-conductor d.c. drives, the motor speed is controlled by varying the armature voltage. the armature voltage can be varied from full on to full off, or any amount in between. the firing angle of the thyristor determines the armature voltage. the thyristor firing angle is controlled by either a constant current or constant voltage circuit. (a constant current circuit controls the firing angle of the thyristor by varying the gate current while a constant voltage circuit varies the firing angle by varying the gate voltage. 30 ) the speed of the motor is controlled by the thyristor firing angle and by the load torque.