Speed controlling device for rolling mills

Claim 1

A speed controlling device for a rolling mill, comprising: a speed detector for detecting the speed of an electric motor for rolls of said rolling mill; a rolling pressure detector for detecting a rolling pressure applied to said rolls and for detecting introduction of a work to be rolled between said rolls to output an introduction signal; a speed controlling circuit responsive to an error of an output signal of said rotational speed detector from a prescribed reference speed signal and to the introduction signal for changing a gain and a time constant of a speed control system of said electric motor; a thyristor device for controlling an electric current flowing through said electric motor; a current detector for detecting an electric current flowing through said electric motor; a current controlling circuit responsive to an error of the output signal of said current detector from a current instruction to said electric motor for outputting a firing phase angle signal to said thyristor device to cancel the error; a work presence detector for detecting if the work to be rolled is present or not at a time directly before the work is introduced between said rolls; a circuit responsive to an output of said work presence detector and also to an output of said rolling pressure detector for outputting signals respresentative of rolling pressure before and after introduction of the work to be rolled; and means responsive to output signals of said work presence detector representative of presence or absence of the work to be rolled before and after introduction of the work to be rolled and also to an output signal of said rolling pressure detector for changing the gain and the time constant of said speed controlling circuit to reduce the amount of drop of the speed of said rolls upon introduction of the work to be rolled and to reduce the speed restoration time.