Patent GB-783655-A (GB)
Title: Improvements in or relating to control apparatus for direct-current electric motors
Inventor: WALZ ALFRED
IPC: H01H9/54
Filed: 1955-08-31
Abstract: 783,655. Control of motors; controllers; interrupters; sparking preventing in switches. FRAKO KONDENSATORENUND APPARATEBAU GES. Aug. 31, 1955, No. 25074/55. Classes 38 (3) and 38 (5). An electric motor 2 driving a truck is supplied from battery 1 through a vibratory switch 3 the ratio between the open and closed periods of which is adjustable. In order to provide smooth starting operation without excessive starting current and sparking, a capacitor 4 is connected, in series with a choke 6, across the motor, whilst an inductor 5 may be connected in series with the motor. The choke 6, which prevents excessive charging current of the capacitor, may be replaced by a choke connected in series with the switch 3 and carrying the full motor current. A rectifier connected in parallel with the choke 6 may be poled so as not to pass the charging current but to offer low impedance to the discharge of the capacitor. Suppression of sparking when the contacts 3 open may also be effected by shunting the contents by a capacitor connected in series with a parallel arrangement of a rectifier and choke. A function of the inductor 5 is to reduce the current when the motor is stationary and when the period of contact-closure is at a minimum; furthermore, the inductor reduces the rate of discharge of the capacitor 4 and so prevents arcing at the contacts of switch 3, complete discharge of the capacitor being possible only when the switch has its shortest contact-making period. The value of the capacitor 4 is such that complete charging takes place, even at the lowest speeds, whilst the inductance of the motor circuit is such that a damping effect is produced and reversal of current when the vibratory contacts open takes place, if at all, only when the switch is set for a short contactmaking time. A suitable construction of the switch 3 comprises a shaft 55 driven by a constant-speed pilot motor 57 and slidably mounting a cylindrical cam body 47 the cross-section of which is variable along its length and which co-operates with a cam follower in the form of a rotatably mounted ball 45. A pivoted switch lever 40 carries contacts 43 which co-operate with contacts 37 and can thus establish a connection between one pair of spring laminations 32 and another, the switch lever being urged to the contact-breaking position by springs 39. The spring laminations 32 carrying the contacts 37 press resiliently against an insulating bar 34, and the resilience is aided by insulated springs 36, When a control lever 53 is rotated, the cam body 47 is shifted axially on the driving shaft 55, so adjusting the relative closing and opening times of the switch-contacts. The cam body may be in the form of a generally cylindrical tube the outer curved surface of which is tapered in an axial direction, and so formed that the contacts close slowly but are broken rapidly, Figs. 7-11 (not shown). In another arrangement, the cylindrical cam is formed with a step in its outer curved surface, the step winding helically around the cam, Fig. 16 (not shown). In order to reduce the transfer of material from one contact to another, the current - direction through the contacts is changed each time the working motor is disconnected by appropriate movement of the lever 53, Fig. 19 (not shown). In another arrangement, the contact-polarity is changed over each time the motor is reversed. When controlling a low-power motor, the switching frequency may be reduced by reducing the speed of the auxiliary motor operating the switch.