Patent GB-331834-A (GB)
Title: Improvements in or relating to speed control devices for step-by-step electric motors
IPC: H02K7/065
Filed: 1929-04-08
Abstract: 331,834. Ollerenshaw, R. E. April 8, 1929. Reciprocating and oscillating machines. - A stepby-step motor, shown in Fig. 3, comprises a Z armature 57 adapted to be oscillated by a field winding 50 against the action of a spring. An end-plate on the armature carries a spring-pressed pawl 60 cooperating with a ratchet-wheel on the shaft 58, over-running being prevented by a spring-pressed catch 65 engaging a notch in a plate 63 also secured to the shaft 58. On the return movement of the armature a pin 66 releases the catch 65. The speed of the motor is controlled by a switch device comprising one or more thermo-electric switches operable automatically to control the intermittent energization of the motor circuit and provided with adjustable means to vary at will the periodicity of this intermittent energization. In the form shown in Figs. 1 and 4 the device consists of a series of bimetallic strips 18 surrounded by heating coils having varying time characteristics and adapted to short-circuit the motor windings after a predetermined interval, the particular thermal switch in circuit being selected by a rotary contact arm 40 engaging one or other of the studs 32 - - 38. The strips 18 are mounted on a common boss 22 supported by an insulating base 17 and connected by a lead 24 to a terminal on the base 39 of the stud switch. The strips each have a contact point 47 adapted to engage, when deflected, with a ring 48 mounted on the base 17 and connected by a lead 49 to a second terminal on the base 39. One end of each heating coil is connected to the fixed end of each bimetallic strip, while the other end is connected to a contact stud 32. The contact arm 40 which is manually adjusted by a knob 43 provided with a pointer 44 is provided with a ball contact 45 slidable over a ring 46. Each thermal element may be provided with a lagging of different thickness between the element and the heating coil to vary the time taken to heat the element enough to close the switch, or heating coils of different heating values may be used. In a modification, a single thermal switch is employed and the current flowing through its heating coil varied by a shunt resistance. In a further modification, the thermal element is fitted with an armature co-operating with an electromagnet having a variable number of turns in series with the heating coil. To compensate for variations in the initial temperature the fixed contact may be replaced by a contact on a second thermal strip. In the arrangement shown in Fig. 5 the heating coil 20 for the strip 18 is shunted by a variable resistance 68 and the strip 18 has a pair of contacts working between contacts carried by a pair of bimetallic strips. Initially current passes by way of strips 70, 18, heating coil 20, and resistance 68, and field winding 50. As the armature moves over, contacts 72, 73 engage and current passes by way of these contacts to the heating coil and field winding. On the strip 18 contacting with the strip 71 it short-circuits both heating coil and field winding and the device returns to its initial position. In a modification, a second strip is joined to the free end of the strip 18, both strips having heating coils, one of which is shunted by a variable resistance. According to the Provisional Specification a single beating device may be used with a series of strips, or the strip may be heated by the current passing through it or a plurality of strips may be placed individually in circuit. Further, it is stated that the device is applicable for controlling traffic-control signals. Specification 325,296, [Class 38 (v), Electric switches &c.], is referred to.