Patent GB-847665-A (GB)
Title: Improvements in apparatus for controlling an electric motor
IPC: B66C13/26
Filed: 1957-06-03
Abstract: 847,665. Control of A.C. motors. HARNISCHFEGER CORPORATION. June 3, 1957 [June 15, 1956], No. 17582/57. Class 38 (3) [Also in Group XXXV] An induction motor 20 for driving a hoist or the like has reversing contactors actuated by coils 27, 28, which are alternatively energized in accordance with the position of a handle 33 for induction controller 29, the handle being associated with cam-operated switches 34, 35. The A.C. windings 53 of a saturable reactor are in shunt with portions of the rotor resistors 23, and a control winding 55 of the reactor is supplied through thyratron 56 the output of which is determined by the position of the handle 33. The controller 29 comprises a primary winding 102 and secondary windings 103, 104, the mutual inductance between the primary and secondary windings being varied by rotating a split ring armature core 105. The voltage developed across winding 103 is rectified and impressed across potentiometer 82 in the grid circuit of the thyratron 56. Negative bias voltage is developed across potentiometer 86 to oppose that derived from potentiometer 82, and an A.C. control voltage is applied to resistor 84, which is also in the grid circuit and is connected to phase-shift transformer 76. Advance of handle 33 to increase the mutual inductance between windings 102, 103, 104 increases the voltage across potentiometer 82 and causes earlier firing of the thyratron 56. Increased saturation of the reactor reduces the effective resistance in the secondary circuit of the induction motor and increases the speed. A feedback circuit for controlling the thyratron 56 comprises transformer 115 to which is applied the slip voltage of the motor. A discriminating circuit comprises reference potentiometer 122 and feedback load resistor 133, the arrangement being such that, for motor speeds between zero and 100% slip, the voltage across potentiometer 122 exceeds that across resistor 133. Current then passes through resistors 89, 137, and the voltage developed across resistor 89 tends to reduce the grid voltage. Thus, if the speed of the motor tends to increase above a desired value, the voltage drop across resistor 89 is increased for reduced excitation of the saturation winding 55, whereby the speed is brought back to the desired value, a reduction in speed having the opposite effect. When the slip exceeds 100%, during reversing, for example, the voltage across resistor 133 exceeds the reference voltage and the entire difference voltage is imposed upon resistor 89. Under these circumstances, the excitation of the winding 55 is reduced to zero and the entire rotor resistance is effective for protecting the motor against excessive current surges. An eddy-current brake 47 is coupled to the motor shaft, and has its winding 48 energized through a thyratron 148, which is controlled in a manner similar to the thyratron 56. The winding 104 of the controller 29 is connected to potentiometer 168 in the grid circuit of the valve 148. At low values of the mutual inductance between the primary and secondary windings of the induction controller, the winding 48 is supplied with a large current to impose a heavy drag on the motor 20, but, as the handle 33 is moved away from the neutral position in either direction, the effect of the brake is reduced. Feedback voltage derived from the secondary circuit of the motor is developed across resistor 185 and is compared with a reference voltage across potentiometer 187. When the slip is less than 100% the feedback voltage is less than the reference voltage and current passes through resistors 201, 178, so that the voltage across resistor 178 tends to increase the output of the thyratron 148. Thus, if the motor speed increases above a desired value, the voltage across resistor 178 increases and the drag-effect of the brake is varied to counteract the tendency to over-speeding. For higher values of slip, as when the motor is reversed, the resistor 178 is subjected to substantially the entire difference voltage. The thyratron will then deliver maximum output to impose a heavy drag on the motor-shaft. A potentiomter 176 in the grid circuit of the valve 148 is effective to reduce brake excitation at the standstill condition of the motor, to reduce heat losses in the brake. When the handle 33 is in the neutral position, switches 213, 214 are both closed to complete a circuit to potentiometer 176 so that the thyratron delivers a low output to the brake winding. As the controller is advanced in the lowering direction, switch 214 is opened to de-energize potentiometer 176. During hoisting operation, resistor 215 is included in circuit for partial energization of potentiometer 176, so reducing the overall average excitation of the brake for such operation. In a modification, the discriminating circuit comprising resistors 178, 201 is omitted, the feedback voltage being applied directly to the grid 152, Fig. 3 (not shown). In another arrangement, the feedback from the secondary circuit of the motor to the brake control circuit is completely eliminated, Fig. 4 (not shown). A differential relay, having series coil 219 and shunt coil 220, detects a shortcircuit or open-circuit fault in the brake winding 48 and is effective to de-energize the whole system under these circumstances. The induction controller has a cam 234 for operating the reversing switches 34, 35, and notches 235, 239, 240 co-operate with a spring- pressed ball for locating the controller in its neutral and " full on " positions. Primary winding 102 is wound on poles 243, 244, whilst secondary windings 103, 104 are wound on poles 245, 246, respectively. The rotor of the controller comprises armature sections 247, 248 separated by air gaps 249, 250, so that, in the neutral position of the rotor, the secondary windings have zero output. When the rotor is moved clockwise, the air gaps 249, 250 intervene in the flux paths 254, 256, but the flux on paths 253, 255 continue to pass through unbroken portions of the segments 247, 248. Thus a net flux passes through the secondary poles which will induce voltage in windings 103, 104. The rotor segments 247, 248, and air gaps 249, 250, may be asymmetrically disposed with respect to the handle 33 and the primary poles of the controller, Fig. 17 (not shown). This has the effect of reducing motor torque and increasing brake excitation for one direction of handle movement, and increasing motor torque and reducing brake excitation for opposite movement of the handle. An independent manual switch may be used to remove a portion of the rotor resistance of the motor in the event of overloading, Fig. 7 (not shown).