Concatenated Induction Motors for Rolling Mill Drive

The paper describes a six-speed concatenated induction motor set for driving the finishing rolls of a 12-stand continuous mill. Analysis of the conditions in this mill showed that the use of a low-pressure turbine would lead to the waste of a large amount of exhaust steam and that the alternating-current turbine plant which was in operation had ample reserve capacity for driving the mill. The large number of speeds required suggested the use of a direct-current motor, but as this required a motor-generator set it was found to be more expensive than the concatenated motor. An induction motor operating with a speed regulating set was also considered, but its disadvantages lay in the fact that a commutating motor or synchronous converter would be required, the performance of which at the frequency employed was questionable. The concatenated set installed consists of a double-wound main motor and single-wound secondary motor, both stators mounted on a common bed-plate and both rotors on a common shaft. The larger motor has both a 14- and 16-pole stator winding and the secondary motor has its winding arranged for four- and eight-pole connections. The control system is described in detail; its operation is very simple, so that no regular attendant is needed. The motor has six definite speeds and two additional resistance speeds, and while the control is somewhat complicated it has been made so nearly fool-proof as to make a special attendant unnecessary.