LH Marthinusen (LHM), a division of ACTOM (Pty) Ltd, is a leading repairer of transformers and large rotating machines and manufacturer of specialised transformers, recently completed a complex repair operation on a hybrid integrated AC motor unit used to drive a large pump in a South African pump station.

The 1150kW unit, one among several identical electromechanical units deployed at the pump station, was brought into LHM’s main repair facility in Denver, Johannesburg, after it had failed. The repairs were done by a special project team managed by Boris Breganski, LHM’s Operations Manager, Power Generation. “The hybrid unit, which was manufactured in 1970, consists of a variable speed AC commutated motor and an induction regulator,” explained Boris.

“This design of variable speed AC motor was innovative in its time and it is still today a highly efficient and energy-saving unit for its purpose. This is the main reason why the pump station continues to use these motors, preferring to have them repaired when they fail through long use rather than replace them with more recently developed alternative systems,” he commented.

“The design features that make it unusual are, firstly, the fact that it incorporates a commutator, which is more customarily linked to a DC rather than an AC motor. Secondly, the rotor closely resembles an armature, which is also usually associated with DC motor technology.”

On examination both the rotor and stator were found to require rewinding.

In addition to having to familiarise themselves with the unique features of the unit, the repair team had to start from scratch as there were no original design drawings available to work from, necessitating a reverse engineering process in order to duplicate the original windings.

“We manufactured new coils with up-to-date improved insulation characteristics. The winding configuration for the rotor was unusually complex and required very systematic documentation of the original windings to ensure that the replacement windings were done correctly,” Boris remarked.

The project, which was completed in August 2016, took seven months. “It was the most complex AC motor repair project LHM has performed in recent years,” said Boris.