Patent GB-628386-A (GB)
Title: Improvements in and relating to electric motor control systems
IPC: B60L50/11
Filed: 1945-05-11
Abstract: 628,386. Transmitting mechanical power. BRITISH THOMSON-HOUSTON CO., Ltd. May 11, 1945, No. 11827. Convention date, May 12, 1944. [Class 38 (iv)] [Also in Group XXXVII] A system for controlling the parallel and series connection of D.C. motors to a common variable voltage generator 11 comprises a voltage responsive relay V for automatically transferring the connection of the motors from series to parallel circuit relation when a predetermined higher operating voltage is exceeded in the generator circuit and from parallel to series when the voltage in the generator circuit falls to a predetermined lower level. In the example a pair of D.C. motors 15, 16 are supplied by generator 11 which is driven by an internal combustion engine 10 in a vehicle traction system. The engine 10 provided with both a manual and governor control of the throttle and is normally run at a fixed throttle opening with the engine fully loaded by the generator 11, so that the output characteristic curves of the generator for various throttle openings have the form shown in Fig. 2. The voltage responsive relay V has its actuating winding 30 connected across the generator armature in series with a resistance 31 which is short-circuited by a contact 14 when the coil 41 of a contactor GF is deenergized. Closure of a battery switch 33 energizes a coil 34 and closes contactor S<SP>1</SP> and, over normally closed contactor 37, energizes coil 36 of contactor S<SP>2</SP>, thereby connecting the series wound driving motors 15, 16 in series with the generator supply. Actuation of contactor S<SP>2</SP> also opens a pair of normally closed back contacts 38 to prevent actuation of a paralleling contactor 22, and substantially simultaneously closes contacts 40, thereby closing a battery fed circuit over line l and normally closed contacts 42 to energize coil 41. This causes contactor 14 to short-circuit a resistance 13 in the shunt field circuit of the generator, which is then fully energized to supply accelerating current to the motors. The generator voltage builds up as shown in the "Full Throttle " curve of Fig. 2 until, for example, voltage V 1  is attained, whereupon transfer relay V becomes operative and opens contacts 42 so that coil-41 is de-energized and resistance 13 is inserted in series with generator shunt field 12. The field flux accordingly begins to collapse before any of the transfer switches have operated. Operation of relay V closes contacts 43 and, through contactor 44 completes the battery fed circuit through coil 45 of paralleling contactor P<SP>1</SP>, at the same time opening contactor 37 to de-energize the series contactor S<SP>2</SP> with the result that back contacts 38 of the latter are closed and the coil 39 of paralleling contactor 39 is energized. The motors 15, 16 are now in parallel, and in the meantime the resistance 31 in series with the transfer relay has been short-circuited by contactor 14 to obviate the possibility of the relay being de-energized in the event of the generator voltage falling to an unwanted low value during the transfer. The closure of contactor P<SP>2</SP>, however, closes a normally open contactor 48 and completes a circuit from the battery through the coil 41 of generator field relay GF, so that the field resistance is cut out, and the generator operation is stabilized in the region of point 50, Fig. 2, with the voltage approximately half as large as at the forward transfer and the current twice as great. If the engine output is reduced suddenly by the operator, or the motors encounter a sudden increase in load, as in climbing a hill, so that the generator voltage is reduced to that denoted by V 2 , re-transfer to series running takes place.