In 1983 a retrofit project was completed at Dallas/Fort Worth Airport in which a 7800 hp (5819 kW) centrifugal chiller drive was replaced with a 5000 hp (3730 kW) variable-frequency, variable-speed, synchronous electric motor. The operating control for the reconfigured unit consists of an industrial-type digital controller. This paper defines the control system inputs and outputs and the basic logic that governs the machine's operation. Since certain elements of the applied control strategies are unique to this machine and are not commonplace in the industry, it is recommended that they NEVER be used without exhausting testing and concurrence of the manufacturer of the machine to which they are to be applied. The test results of this application have been excellent and were well documented in ASHRAE Research Project RP-409, 'Analysis of the Energy Use and Control Characteristics of a Large Variable- Speed Drive Chiller' by J.E. Braun, J.W. Mitchell, S.A. Klein, and W.A. Bechman.

KEYWORDS: chillers, variable speed motors, controls, modernising, chilled water supply, computer controls, programmable controllers, cooling towers.