SPEED CONTROL DEVICE FOR DC MACHINE

PURPOSE:To perform speed control stably, by a method wherein field current is reduced when input AC voltage of a thyristor rectifier drops beyond allowable voltage drop value determined by alpha limit value of the thyristor rectifier. CONSTITUTION:A thyristor rectifier 2 is controlled corresponding to deviation between speed signal 3a and speed reference signal 5a. On the other hand, a thyristor rectifier 12 is controlled corresponding to deviation between field current 8a and field current set value. When discrimination is that AC input voltage becomes alpha limit value of the thyristor rectifier 2 in view of AC input voltage 14a and DC output voltage 16a, an operator 16 closes a contact 17. An operator 19 closes a contact 21 when main circuit current is less than rated current. If a relay 22 is excited, field current set value is reduced from field current reference and decreased at prescribed rate to the field current reference.