System for driving DC motor

A system for driving a DC motor having an armature and a field winding and driven by using a three-phase anti-parallel thyristor bridge circuit having a three-phase alternating voltage applied thereto, said system comprising:

• a first means for controlling an armature current I.sub.a flowing through said armature of said DC motor;
• and
• a second means for controlling a field current I.sub.f flowing through said field winding, said second controlling means including a first detecting circuit for detecting said armature voltage E.sub.a, a second detecting circuit for detecting said field current I.sub.f, a field setting circuit for setting the maximum level of said field current in response to said three-phase alternating voltage, said maximum level being constant when said three-phase alternating voltage is higher than a predetermined value and said field current being proportional to said three-phase alternating voltage when said three-phase alternating voltage is lower than said predetermined value, a field control circuit for integrating the sum of the output signals generated by said first and second detecting circuits and an output signal generated from said field setting circuit and field controller means for controlling said field current in response to the integrated signal such that, when said three-phase alternating voltage decreases to a value lower than said predetermined value, said field current I.sub.f is decreased so that said armature voltage E.sub.a is decreased, and when said three-phase alternating voltage is higher than said predetermined value, said field current I.sub.f is held constant wherein said armature voltage E.sub.a is constant.