PURPOSE: To always balance an upper load with a lower load to prevent a slip by connecting one by one out of the two field windings of respective motors for the upper and lower rolls in series with each other and by causing electric current to flow from each field-control thyristor apparatus which is allotted to each field winding.

CONSTITUTION: The first field winding 15 of an upper roll motor 3 and the second winding 16 of a lower roll motor 4 are connected in series and a field current If1 outputted from an upper roll field-control exciter 24 is caused to flow through each winding. Similarly, the second field winding 14 for upper roll and the first field winding 17 for lower roll are connected in series and a field current If2 outputted from a lower roll field-control exciter 25 is caused to flow through each winding. A field current Ifn (unshown) for the upper roll motor and field current Ifd not (shown) for the lower roll motor are Ifu=If1+ If2=Ifd, and the upper roll motor 3 and lower roll motor 4 are equal in magnetic flux and torque. Thus, loads are balanced and a slip is prevented.