Thyratron Motor Control

The control of d-c motors was one of the first electronic-control projects. Recent developments, particularly the fully automatic and co-ordinated control of the armature power circuit and the field power circuit, have greatly increased the acceptability of these controls for industrial use. This paper describes the control functions associated with an all-electronic d-c motor drive, in which grid-controlled thyratrontube rectifiers automatically perform the variable voltage functions associated with the d-c generator, exciter, and field rheostats of a conventional motor-generator drive. In addition, the grid-control action of the thyratron-tube rectifier dispenses with the usual magnetic starting and accelerating control devices and gives superior motor performance in the matter of almost idealized constant-current acceleration and flat preset speed-control characteristics resulting from IR-drop compensation and regulated field excitation. Reversing is accomplished by magnetic reversing switches which reverse the armature terminals relative to the rectifier. Contact duty during switching is minimized by the supplementary action of the thyratron tubes which themselves serve to interrupt and initiate the armature current. Proper grid-control sequence in these same tubes causes them to act as inverters during the decelerating portion of the reversing cycle so that the rotational energy of the armature is actually pumped back into the a-c system. Stopping is normally by means of a dynamic braking resistor, although the motor may be decelerated to a stop by utilizing the inverter action associated with reversing to regenerate power back into the a-c system.