Patent GB-876726-A (GB)
Title: Control systems for electric motors
IPC: B66F9/075
Filed: 1959-05-04
Abstract: 876,726. Control of D.C. motors; controllers. CLARK EQUIPMENT CO. May 4, 1959 [May 21, 1958], No. 15096/59. Class 38 (3). An electric motor driving a fork lift truck is controlled by a carbon pile regulator 56 having an hydraulic actuator 110. An accelerator pedal 80 operates the piston 81 of a master hydraulic cylinder 82, a leaf spring 87 providing a limited lost-motion coupling between the pedal and piston. The transmission of pressure to the regulator may take place over hydraulic circuits 96 or 97, the former comprising an accumulator 101. The second circuit normally serves to operate the regulator and includes an adjustable needle valve 105, solenoid valve 106, and pressure switch 72. The actuator 110 has a piston rod 111 surrounded by a minimum pressure spring 112 and coupled to plate 113 engaging an insulator 114 for compressing the regulator. A one-way check valve 116 by-passes the elements 105, 106 and 72 and allows prompt return flow to the master cylinder when the accelerator pedal is released. The piston 122 of the accumulator 101 is linked with the plate 113 through lever 120 and leaf spring 123. The needle valve 105 provides a time-delay when compressing the carbon pile and so limits the rate of acceleration, whilst the valve 106 operates only when its solenoid is energized by the back E.M.F. of the motor during reversal, the circuit 97 being cut off and the flow of fluid transmitted to accumulator 101. Since the accumulator is directly connected with the master cylinder, the reverse braking torque is controlled in accordance with pedal operation. The solenoid valve 106, in conjunction with a micro-switch which is closed only when the accelerator pedal is released, prevents the application of full battery power to the motor in the reverse sense to the direction of rotation of its armature. The pressure switch 72 prevents short-circuiting of the carbon pile and a portion of the motor field winding when the accelerator pedal is fully depressed to close a second micro-switch, until a predetermined hydraulic pressure has been reached. This ensures full compression of the carbon pile before it is short-circuited. The check valve 116 ensures that hydraulic energy stored in the actuator does not continue to compress the carbon pile after the field windings have been reversed in relation to the direction of rotation of the armature. The actuator 110 comprises means for compensating for changes in length of the carbon pile due to variations of temperature. An hydraulic chamber 131 is separated from an air chamber 132 by a piston 135 attached to diaphragm 137, and fluid under pressure directed through inlet port 134 will move the piston 135 to the right, compressing return spring 140 and permitting ring 170 to act as a seal when pressed between shoulders 165 and 171. Fluid is thereby locked between seals 158, 170, so that force exerted on piston 135 is transmitted directly to the carbon pile through piston 111. When the accelerator pedal is released, a finger portion 175 of pilot plunger 162 engages adjustable stop 180, so unseating the ring 170 and equalizing the pressure in chambers 131 and 163. The communication of the fluid between the rings 158, 170, with the chamber 131 accommodates the position of piston 111 to compensate for changes in length of the carbon pile. If the length of the regulator increases, the ring 170 is unseated to permit escape of fluid into the chamber 131 to allow movement of the piston 111 relative to tubular piston 150 toward the plunger 162. If the regulator decreases in length, the spring 112 moves the piston 111 to the right when the actuator pressure is relieved. Thus, the space between the rings 158, 170 is supplied with fluid from the chamber 131 when the ring 170 is unseated.