Patent GB-872715-A (GB)
Title: Improvements in or relating to electric circuit arrangements including an electric motor
IPC: H02P7/298
Filed: 1958-01-24
Abstract: 872,715. Control of motors. PHILIPS ELECTRICAL INDUSTRIES Ltd. Jan. 24, 1958 [Jan. 26, 1957], No. 2440/58. Class 38 (3). An electric motor having an armature 1 and a shunt field winding 2 is operated at a stable speed, despite variations in the load torque, by connecting the emitter-collector path of a transistor 7 in series with the field winding, the transistor being controlled in accordance with the armature current so that, with increasing armature current, the field of the motor is reduced. The base electrode of the transistor is connected to a constant-potential point 11 in battery 10 and a control resistor 9 is included in both the emitter and armature circuits whereby the transistor is controlled in accordance with the difference between a constant voltage and one varying with the armature current. The value at which the speed is stabilized is dependent upon the value of resistor 8 which is much higher than resistor 9. To prevent excessive weakening of the motor field due to the heavy armature current obtained during the starting operation, a timedelay capacitor 12 prevents cut-off of the transistor until the capacitor has been fully charged. In another arrangement for providing satisfactory starting, a second transistor has its emitter-collector path connected in series with the motor armature so that the armature current is limited to a value at which reduction of the field does not prevent starting of the motor, Fig. 2 (not shown). Alternatively, a resistor having a high positive temperature coefficient may be connected in parallel with the resistor 9, Fig. 3 (not shown). In a further embodiment the motor has a series field winding 2 which is shunted by the transistor 7, the base electrode of which is coupled to control resistor 9 through voltage divider 17, 18. Constant speed is maintained by making the collector current of the transistor a linear function of the armature current. An additional transistor amplifier may be provided having its collector electrode connected to the base electrode of the transistor 7. The collector circuit of the additional transistor includes a load resistor through which the base electrode of the transistor 7 is connected to the negative terminal of the battery 10 through the armature of the motor, Fig. 5 (not shown). Two transistors of opposite conductivity types may have corresponding electrodes connected in parallel, the motor being supplied from either a D.C. or an A.C. source. The emitter electrodes are connected to a tapping in a voltage divider across the source, Fig. 6 (not shown). For controlling an A.C. series-wound motor, the voltage divider is replaced by an auto-transformer and the resistor 9 by an inductor. The reduction of field current is effected by two parallel-connected transistors of opposite conductivity types, and these are controlled by two further transistors, Fig. 7 (not shown). The motor may have a permanent magnet field which is opposed by the field produced by a control winding connected in the collector circuit of the transistor. Satisfactory starting operation is afforded by connecting a capacitor between the emitter and base electrodes. With increasing armature current, the voltage-drop across a control resistor increases until the transistor becomes conductive and the motor field is reduced by current passing through the control winding, Fig. 8 (not shown).