New life for old compound DC motors in industrial applications?

DC motors have been used in the past as the workhorse of industrial systems until the introduction of the AC induction motors, about twenty years ago. Since then, the DC motors in the industrial plants have been replaced with more robust, less complex AC ones and the speed control was achieved by digital system controlled PWM inverters. But there was a time when the power electronics, digital control or even the analogical control was far to be introduced and the speed regulation, if even possible, was deputed to electromechanical devices. These equipments have been designed and manufactured in such a robust way that some of them are still working and used for the production, i.e. in metal industries. A particular configuration, the compound DC motor was widely used, for example for lifting and traction. The aim of this paper is to present how an actual supply chain for old DC compound motors can be upgraded with static DC converter achieving less losses and better speed response without the modification of the mechanical assembly. The paper begins with the description of the existing supply configuration, hence describe an analytical method to identify the motor parameters and behaviour and than presents the new supply configuration and an original calculation of the motor voltage suitable to achieve the required speed in a wide range of torque load without the need of a speed sensor.