628,326. Control of D.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd., and HARVEY, A. F. July 17, 1946, No. 21430. [Class 38(iii)] A control system for supplying a varying voltage to a load, such as traction motors, of the type employing a plurality of dynamo-electric machines coupled together mechanically, with one or more of the machines interposed between the line and the load, the dynamo-electric machines including a reducer-booster set, comprises a pilot motor-operated field rheostat for varying the excitation of the exciter supplying the field current of the booster, the pilot mo- .tor having two independent sets of field coils, one energized through a master controller and the other being energized in proportion to the load current. When a master controller 1 is moved to an " Off " position a contactor 27 is closed to energize another contactor 24. With a change-over switch 9 in the position shown a circuit is closed from a higher voltage supply line 31 through starting resistors 38, 39, and 40, two coils of an overload relay 12, and the armatures of machines 46 and 54 in series to the negative line 58. The field windings of the machines 46 and 54 are connected in series across the lines. The relays 24 and 27 are provided with holding circuits. Contactors are closed in sequence to short-circuit the sections of the starting resistance which is finally removed by closure of a relay 21. With the master controller stillin. the "Off" position, the armature of. a pilot motor 91 is energized, segments 92 and 98 of a rheostat 18 being respectively. afforded negative and positive polarity. A field winding 103 of an 'exciter 104 for a booster 110 is energized. The booster is connectable, in series with load- -motors 114, across the armature of the machine 54, and at starting it balances the voltage across the said machine and the load voltage is zero. Segments 127-132 of a field rheostat follow-up panel 133 are negatively energized, the arm 135 of the panel being mechanically coupled to an arm 134 of the field rheostat and driven in common by the motor 91. The arm 135 carries contacts 139 and 140 which are separated by a resistor 141. Adjacent to segments 127-132 are disposed segments 142-146, each connected to a previous segment through a resistor 147. In the "Off" position of the master controller all the segments of the follow-up panel are energized negatively, and a field winding 150 of the motor 91 is energized to drive the arms 134 and 135 to an "Off" position. A field winding 151 of the motor 91 is connected. across a series field of one of the motors 114 and thereby carries a current proportional to the load current. A segment 170 of the controller 1 is positively energized through a resistor 167, which is variable by means of a controller. 161, from a segment 95. When the master controller is moved to " Position 1 " a relay 15, which connects the load motors to the booster, is closed, provided that the relay 21 is closed to short-circuit resistors 38-40. A relay 180 provides means to ensure that the voltage across the main contacts of the relay 15 is substantially zero at closure thereof. The segments 127 and 142 are now positively energized, being connected to the segment 170, and the field winding 150 is energized through the resistor 167 to drive the motor to move the arms 134 and 135 in a clockwise direction. Resistance is introduced in the exciter field winding circuit and the field excitation of the booster 110 is reduced to increase the load voltage. The field winding 151 is energized in accordance with the accelerating load current and tends to balance the field winding 150 and stop the motor 91. When the arm 135 contacts the segment 142 the resistor 147 is introduced into the circuit of the field winding 150 to check the forward movement of the field rheostat. Overtravel of the arm 135 results in a negative potential being applied to the contact thereof and the motor 91 returns the arm to the previous segment. In a modification the resistor 147 may be connected to a contact and mounted on the arm 135 oppositely to the resistor 141,. the segments 142-146 being disconnected. Further movement of the master controller positively energizes further contacts of the follow-up control panel, the field rheostat 18 being moved to reduce the negative booster voltage to zero, and thereafter increasing it positively. The change-over switch may be moved to connect with the lower voltage supply line. The armatures of the machines 46 and 54, and the field windings thereof, are connected in parallel. A resistor 97 is inserted in series with the field winding of the machine 46. Subsequent operation is similar to that.described. The motor 91 may be provided with restraining means, for exarnple, an eddy-current brake.