GB1395996A - Dc motor circuits - Google Patents

Abstract

1395996 Braking D.C. motors ASSOCIATED ELECTRICAL INDUSTRIES Ltd 14 Sept 1973 [15 Sept 1972] 42889/72 Heading H2J A D.C. motor circuit comprises a braking arrangement wherein the armature 12 is connected in a first series circuit with the field winding 11, in a second series circuit with a load resistance 5 and in a third series circuit with the D.C. supply terminals, and circuit means are provided and are arranged to control the series circuits cyclically so that in a first part of a cycle energy is generated by rotation of the armature 12 in the first circuit, in a second part aportion of this energy is dissipated in the resistance 5 and in a third part a portion of this energy is returned to the D.C. terminals. In a preferred embodiment the first circuit comprises thyristors 2, 3, field coil 11, resistor 13 and armature 12, the polarities of the thyristors being such as to pass currents generated by movement of armature 12. In the first part of the cycle both thyristors are fired and energy is stored in the winding 11 and the polarity of the right hand plate of the commutating capacitor 4 rises to that of the positive D.C. rail. Thyristor 1 is then fired causing a reverse bias to be applied across thyristor 2 turning it off thereby terminating the first part of the cycle and the polarity of capacitor 4 commences to reverse. The field winding 11 is included in a freewheel circuit comprising diode 10, resistor 13 and saturable reactor. The effect of this reactor is to delay the build-up of field current in the freewheel circuit after termination of the first period and to cause the right hand plate of capacitor 4 to reach a lower negative potential owing to field e.m.f. acting through thyristor 3. As the current builds up when the reactor saturates the thyristor 3 is reverse-biased and extinguished. The second circuit now in operation comprises the thyristor 1, resistance 5, diode 9, resistance 13 and armature 12, and rheostatic braking takes place. The second part of the cycle is terminated by firing thyristor 2 to extinguish thyristor 1 and regenerative braking via diode 9, resistor 13 then occurs. At low speeds, the generated armature current will be insufficient to maintain diode 9 forward-biased against the free-wheeling current and this latter will then be identical with the armature current. In a modification, Fig. 2 (not shown), resistor 13 is omitted and the anode of diode 10 connected to the negative D.C. supply line.