The Mot-O-Trol (Westinghouse).

Author: E.E. Kimberly

Fig. 28-17. Mot-O-Trol Cabinet With Control Station

Fig. 28-17 shows the Mot-O-Trol unit complete with control station. The circuit for this controller is shown in Fig. 28-18. The armature power is obtained from the power transformer, and the current is rectified by the two thyratrons WL-672 producing full-wave rectification. The WL-672 tube is a gas-filled tetrode.

Fig. 28-18. Simplified Control Circuit for Mot-O-Trol

The KU-627 tubes are quite similar to the triode mercury-vapor thyratrons previously described. They act as full-wave rectifiers to provide the direct current for the motor shunt field. The time-delay relay in the field circuit prevents current flow in the plate circuits of the two KU-627 tubes until their filaments have reached a proper operating temperature in starting. Dynamic braking is accomplished by closing the armature circuit through LCR and RCR and the variable resistor between them.

The effective current through the armature, and hence the voltage across the armature and in turn the motor speed, is controlled by a phase-shift circuit applied to the WL-672 grid voltage.

A speed change of as much as 50 to 1 may be accomplished in this way, with the field excitation held constant. Additional speed change from 2 to 1 up to 4 to 1 (special motors) may be obtained by field-current control accomplished in the same manner by phase shift of grid voltage on the KU-627 tubes. In Fig. 28-17 both phase-shift potentiometers are controlled manually by the speed-control knob at the top of the operating control station.

Speed regulation of a motor whose armature is fed through electron tubes is poor, if no compensation is provided. This compensation is accomplished by means of the two-part (tapped) resistor just below the motor armature of Fig. 28-18. As the motor is loaded, it takes more armature current; and, so, less voltage appears across the armature and the speed-compensating two-part resistor in parallel with it. The voltage across the right-hand portion of this resistor is applied to the grid of a control tube in the control-circuit cabinet. As this voltage decreases with increase in load, the control tube acts to advance the firing angle of the WL-672 tubes and so to increase the armature voltage.