Patent GB-643180-A (GB)
Title: Improvements in or relating to d.c. electric motor control systems
IPC: B60L3/10
Filed: 1947-12-15
Abstract: 643,180. Control of D.C. motors; electromagnetic switches. GENERAL ELECTRIC CO., Ltd., and TURRALL, J. C. Dec. 15, 1947, No. 33005 [Classes 38(iii) and 38(v)] An electric motor control system comprises a plurality of direct-current motors arranged to drive independently two or more axles or the like, a relay having an armature and two pairs of windings disposed on separate limbs of a magnetic core, the magnetic circuit being so arranged that a magnetic flux produced in either limb can assist but not oppose flux produced in the other limb carrying a pair of windings, each of said windings being connected to the motors so that, when the ratio of the speeds is normal, the magnetic flux produced by one winding of each pair opposes and substantially balances the flux produced by the other winding of the pair, but when the ratio of the motor speeds is abnormal, said magnetic fluxes do not balance in one or both legs and so cause actuation of the relay. Four series motors which individually drive the wheels of a vehicle have their armatures 1A, 2A, and 3A, 4A, connected in pairs, the two pairs being connected either in series or parallel. An electromagnetic relay 12, Fig. 2, is associated with the motors to provide an indication or to effect a control in the event of wheel slip. Legs 14 and 15 of the relay each carry two like windings, 21, 22, and 23, 24, which are respectively connected across the motor armatures. Each pair of windings is connected so that, when there is no wheel-slip, the currents in the windings are in opposition and produce substantially no resultant flux in the leg. When the motors are connected in series and one of the motors slips, to produce, for example, an excess excitation of one of the windings on the leg 14, a resultant flux is produced in that leg which causes movement of a spring-biassed armature 16 to close contacts 18 and 19, the resultant flux in the leg 15 remaining at zero under these conditions. The likelihood of more than one motor slipping at the same time is small since the effect of one motor slipping is to reduce the total current and to reduce the tendency for the other motor to slip. Similar operation is provided when the pairs of motors are connected in parallel. If one motor in each of the parallel-circuits slips there will be resultant fluxes in both legs 14 and 15 and the total flux, whether the resultant fluxes are in the same or opposite directions, will be such as to lift the armature 16. When the system comprises two series motors which may be connected in series or parallel, Fig. 3 (not shown), a relay, similar to that shown in Fig. 2, is arranged to have two windings on one leg connected across the armatures of the motors, the other relay-windings being connected across the series field windings. The relay is again not operated under normal conditions and, when the motors are connected in series, the current-responsive relaywindings balance each other. If one motor slips, however, the voltage-responsive windings effect the required operation. When the motors are connected in parallel and one of them slips, the difference in voltages across the armatures may not be sufficient to operate the relay. The current of the slipping motor will fall, however, and the relay will be operated by its current-responsive windings. In a modification of the system shown in Fig. 1 an additional relay may have its contacts connected in parallel with those of the previously-mentioned relay. One of the windings of this additional relay is connected across one or both of the field windings 1F and 2F, an opposing winding being likewise connected across one or both of windings 3F and 4F. In the system of Fig. 4 (not shown), six series motors are arranged in pairs to drive three axles. These motors may be connected all in series, in two groups of three, or three groups of two. It is only necessary to detect slip between three of the motors driving the three axles, and this may be done by means of a relay of the type described. Two relay-windings, one on each leg, are connected across the armatures of two of the motors, the other two windings are connected in series or parallel across the armature of the third motor. The windings are arranged so that when there is no slip, the two windings on each leg are balanced. With the motors connected in any of the three combinations stated, the relay will operate when the wheels of any one axle or any pair of axles slip. When six motors drive wheels individually, two similar relays may be associated with each group of three motors, the contacts of the relays being connected in parallel.