Patent GB-836000-A (GB)
Title: Improvements in and relating to electric motor control systems
Inventor: SHAW NORMAN HARRY
IPC: H02P7/34
Filed: 1955-08-26
Abstract: 836,000. Control of D.C. motors. ENGLISH ELECTRIC CO. Ltd. Aug. 17, 1956 [Aug. 26, 1955], No. 24573/55. Class 38 (3). [Also in Group XXXV] Motor 10 is supplied by generator 11, whose output is varied by a first control device 15 which is operated by magnetic amplifier 17 having winding 17a energized from speed reference voltage rheostat 23 and winding 17b energized by tacho-generator 33 via bias network 32. The second control device 13 varies motor field excitation and is operated by magnetic amplifier 14, which has four control windings such that current, proportional to armature voltage, has to flow in winding 14c to neutralize the biasing current in winding 14b before control of the amplifier output can be exerted by current in winding 14a from the reference voltage rheostat 22. The system operates such that when raising the motor speed the armature voltage is raised to maximum prior to decreasing the excitation; when decreasing motor speed the voltage and excitation are varied in opposite sequence. The speed regulator arm 18 controls two rheostats 22, 23; arm position a corresponds to zero speed, positions b and c to an intermediate or " base " speed in forward and reverse directions respectively and positions d and e to maximum speed in forward and reverse directions. Across terminals 24, 25 is a constant D.C. voltage and rheostat 22 is connected across this source with sections 22b, 22c in parallel, section 22a being a connecting strip; rheostat 23 is arranged with connecting strips 23b, 23c, in series with resistive section 23a. On moving the arm 18 towards base speed position b slider 20 has no effect, but through slider 21 amplifier winding 17a is energized resulting in output to control device 15 such that the generator supplies a rising armature voltage to motor 10 thus speeding it up to base speed. Motor excitation is arranged to be a maximum whilst amplifier 14 is inoperative. Tacho-generator 33 speeds up with the motor, but biasing network 32 prevents current flow through 17b until the base speed is exceeded, subsequent energization of this winding depending on the difference between actual and base speeds. Amplifier 17 saturates when maximum reference voltage is applied to winding 17a so that beyond base speed winding 17b exerts no further control on generator 11, i.e. motor armature voltage is a maximum at base speed. As arm 18 moves from position b to position d (or positions c to e in the reverse direction) current flows through amplifier 14 winding 14a which controls device 13 to reduce motor excitation so that the speed rises. It is a feature of the invention that winding 14a cannot control the motor excitation until the armature voltage is a maximum, at which time the biasing winding is neutralized by winding 14c. If the arm 18 is moved slowly from position d to position e the sequence of operations described will be reversed until the motor runs at full speed in the opposite direction. If, however, arm 18 is moved rapidly winding 14a is reenergized in the same sense before the motor has slowed to base speed and the motor then picks up again to maximum speed without reversing. This is because above base speed winding 17a has no control on the output of the saturated amplifier 17 and so motor armature voltage is neither reduced nor reversed. Winding 14d is included to overcome this drawback. Normally not energized due to opposing voltages in series across its bridge supply network, i.e. voltage across terminals 16 opposes that between slider 21 and point 23d, this winding becomes active when the polarity of the latter voltage is reversed by rapid movement of arm 18 between positions d and e. Winding 14d neutralizes the effect of winding 14a and the motor loses speed; below base speed winding 17a regains control of amplifier 17 and, with its reverse energization causes a reversal of motor armature voltage, which then builds up to the value corresponding with base speed in the reverse direction. The voltage across terminals 16 having reversed causes winding 14d to become inoperative once more so that winding 14a assumes control of amplifier 14 and varies the excitation for the motor to run up to full speed in the reverse direction. An alternative control system is described whereby the amplifiers 14 and 17 control variable-voltage rectifiers connected to supply respectively the motor field excitation and armature voltage, furthermore the amplifiers 14 and 17 could be of the electronic or rotating type. Although as described, winding 17b is only indirectly dependent on the motor field excitation, the system would still work if this dependence were more direct, e.g. winding 17b could be energized from terminals 13 of field control system 12.