GB234822A - Improvements in or relating to the control of alternating current motors, particularly induction motors - Google Patents

Abstract

234,822. Rushen, P. C., (Hazeltine Research Corporation). July 4, 1924. Systems for alternating-current motors only. -Induction motors for traction purposes are supplied with polyphase current of adjustable frequency which is the sum of a frequency proportional to the motor speed and an independently controlled frequency. The polyphase current is obtained from high voltage direct current by two valve converters one of which gives a moderately high frequency and the other converts to a variable low frequency alternating current. The speed control is wholly effected by adjustments of the control electrodes of the valves. The supply taken from the directcument trolley wire 300, Fig. 1, is by the valve system 301 and associated transformer 302, converted into high frequency current, and then by the valve system 303 converted into low variable frequency current for supply to the motors 305. Current and voltage harmonics are absorbed respectively by condensers Cy and by the leakage reactance of the transformer. The control for valve system 301 is shown in Fig. 4<a>. The valve structure is of the magnetically guided, electrostatically controlled, thermionic type, and the grids G<1> &c. are rapidly charged from a reservoir condenser C<1>r through a control valve 401. A commutator 421 rotating counter clockwise is shown in the position for connecting through brushes 461, 464 the grids G1, G1<SP>1>, and G4, G4<SP>1> to the negative and positive sides respectively of the condenser C<1>r through the valve 401, which has just closed through its grid being connected bv another commutator 411, rotating synchronously with the commutator 421, to a positive nectifier 451. As the commutator 421 rotates, the brushes 461, 464 span the insulation between adjacent segments, thereby connecting the condenser C<1>r for recharging to a rectifier 431, the valve 401 still being closed. Thereafter, contact on commutator 411 is broken and the valve 401 opens, thereby isolating the condenser C<1>r until the cycle is repeated with the next pair of grids of the valve system 301. The grids G1, G3, G5, are given a positive bias by means of the rectifiers B1, B3, B5 and the grids G<1>1, G<1>3, G<1>5 are given a negative bias by means of the rectifiers B<1>1 B<1>3, B<1>5. A similar control arrangement is used for the valve system 303. The output frequency of the system 301 is the frequency of its control commutator, whilst the output frequency of the system 303 is the difference between the frequencies of the control commutators for the two systems. The two control commutators can be combined and one set of brushes revolved at a speed corresponding to the desired output frequency. If the relative speeds of the commutators is invariable with the load on the induction motor 305, then they will have a speed-torque characteristic similar to a direct current shunt motor, whereas if one set of brushes is positively driven by the retor of the motor, and the other set of brushes is rotated slowly, then the motor will have a characteristic similar to a direct-current series motor. Specification 218,675, [Class 39 (i), Electric lamps, Arc. &c.], is referred to.