Patent GB-814458-A (GB)
Title: Improvements in or relating to adjustable voltage drives for electric motors
IPC: H02P7/29
Filed: 1955-11-09
Abstract: 814,458. Automatic speed-control systems. IGRANIC ELECTRIC CO. Ltd. Nov. 9, 1955 [Nov. 22, 1954], No. 32033/55. Class 38 (4). [Also in Groups XXXVII and XXXIX] A reversible D.C. motor has its armature A connected in a loop circuit with the output terminals 10, 11 of a magnetic amplifier 12, its field winding sections F1, F2 being supplied from magnetic amplifiers 16,19, through bridge rectifier 21. For forward operation, contacts 66a, 67b are closed and contacts 67a, 66b are opened so that reference windings 58, 59 are energized to turn amplifier 16 full on and to render the amplifier 19 ineffective. Current then flows from terminal 15, winding F1, rectifier 28, regulator winding 26 of amplifier 12 and rectifier 30 to terminal 23. The output voltage of amplifier 12 is thereby increased and the motor accelerates to its basic speed, the ampere-turns developed in winding 56 opposing those of the reference winding 58. An increase of motor speed above its basic value, due to a change of load, increases the excitation of winding 56 and reduces the output voltage of the amplifier 16 whereby the energization of filed winding F1 and regulator winding 26 are both decreased. Although the motor field flux is reduced, the reduced output of amplifier 12 is sufficient to return the motor speed to its basic value before any appreciable increase can occur. The system also operates to prevent appreciable deviation from the basic speed when the load on the motor is suddenly increased. The motor is reversed by opening contacts 66a, 67b and closing contacts 66b, 67a to complete a reverse circuit for reference windings 59, 58, so that the amplifiers 19, 16 are respectively rendered effective and ineffective. Current now flows through regulator winding 26 and motor field winding F2 to cause the motor back-electromotive force to reverse its polarity with relation to the applied voltage. A heavy current develops in the armature loop circuit, the motor being braked to a standstill and then accelerated in the reverse direction. Rectifier 63 prevents reverse current flow through control windings 56, 57, during the deceleration period. With motors of large horse power, the braking current is automatically limited during the reversing operation, the magnetic amplifiers 100, 101 (corresponding to amplifiers 16, 19) being provided with current-limiting windings 102, 103, for this purpose. When the motor is running at the basic speed and switches 66, 67, are operated to reverse the motor, a heavy regenerative current passes through rectifiers 116, 111 and rectifiers 117, 115 in armature amplifier 106. Amplifier 133 then energizes current-limiting winding 103 whereby the output of amplifier 101 is prevented from rising too rapidly. Consequently, the reverse field flux is limited as well as the excitation of the control winding 124 of the amplifier 106 so that the braking current is kept within safe limits. Under such conditions, amplifier 137 energizes relay DB so that resistor 108 is included in the armature circuit. When the motor approaches zero speed, the amplifiers 133, 137 are rendered ineffective and the output of amplifier 106 is increased to accelerate the motor in the reverse direction. If the armature current again becomes excessive the amplifier 129 energizes current-limiting winding 126 of amplifier 106. In another arrangement, the dynamic braking resistor 108 and relay DB are dispensed with and are replaced by a brake of the eddy-current or magnetic powder type which is mechanically coupled to the motor and has a winding connected to the output terminals of the amplifier 137, Fig. 4 (not shown). In a further modification, the magnetic amplifiers 133, 129, 137 are replaced by a number of electromagnetic relays which perform a similar function to the said amplifiers, Fig. 5 (not shown).