Stepping Motors and Associated Electronic Circuits for Reactor Control

Electrical stepping motors are now available which, because of their high torque and precise motion, are very suitable for use with reactor control rods. A typical motor will produce a torque of about 30 kg cm at 10 rev/min and make about 150 steps per revolution, so that a positional accuracy of 1 part in 10000 can be obtained on the control absorber. The motors may have three or four windings and are driven by d.c. pulse-generators specially designed for the particular reactor application. The low-frequency pulses may be produced in many ways, including: (a) rotating shutters moving over photo-transistors, (b) assemblies of standard computing elements, such as synchro-transmitters, and (c) circuits which cause frequency-reduction and squaring of the 50 c/s or 60 c/s electrical supply. It is advisable to arrange variation of the on/off ratio of pulses to obtain maximum torque or reduce energy losses in the motors. The usual electronic circuit consists of binary frequency-dividing circuits connected to Schmitt trigger circuits which switch a low-voltage d.c. supply to the motors via transistor amplifiers. A typical design will permit an absorber to be driven in steps over 60 cm in 5, 10, 20, 40, 80 or 100 min in the direction required. In the electronic circuits, separate channels are used for each speed and direction to give high reliability and solid-state components on portable cards are used throughout for ease of maintenance. Auto control may be applied, by using a two-phase motor to drive a rotary pulse-generator or by means of frequency- modulation about a chosen reference. It is convenient to measure position by displaying the number of pulses fed to the motor on a two-way counter. Various monitoring circuits may be used to verify that the motor has moved in the correct direction and the use of groups of three or more units permits redundancy principles to be applied. Six vertically-acting control rods with stepping motors will be installed in the high-flux, research reactor PLUTO in May 1965 and a report on operational experience will be presented. (author)