Cooling Solutions for the First Norwegian Series of Permanent Magnet Machines

Abstract

SmartMotor AS has an ambition of developing a new series of permanent magnet (PM) machines. The aim is to offer one range of machines for marine applications such as propulsion, anchor and winches, and a separate range for industrial applications. In this regard, there is a need to identify possibilities for improvement of the cooling system. A study of competitors cooling solutions revealed that cooling methods of the International Cooling (IC) code IC71W, IC81W, IC01 and IC411 occurred most frequently in marine applications. The machine, of which the design of the new range will be based, is a newly build 1,4 MW unit, commissioned by Wärtsilä. This machine was tested to verify the performance characteristics of both machine and cooling system, which consists of combined air- and water-cooling. A surrounding water jacket primarily cools the stator and housing, representing 88 % of the total losses. The remaining 12 % loss from rotor, magnets and support is air-cooled by external fans. Losses were estimated to be 44,1 kW and tests verified these estimations by recording actual losses of 43,3 kW.The potential of replacing the external fans with a shaft-driven centrifugal fan was mapped out for five machines, representing the range project. A model of each machine was developed using COMSOL Multiphysics 4.3b. These models were used to obtain a system resistance curve, which was compared to the performance of centrifugal fans in the market. A single shaft-driven fan proved to be an impossible solution. Long active parts of 900-1200 mm required higher static pressure than the shaft speeds of 100-600 rpm could supply. Calculations and simulations stated an average deviation from required to obtainable pressure of 800% for the five machines. Two parallel shaft-driven fans proved to be a theoretical option for machines with speed of 400-600 rpm.Theoretical heat transfer to the cooling air was calculated for comparative purposes. This indicated overcooling of close to 200 % for the simplified design of the Wärtsilä machine and between 20-120 % overcooling for the estimated design of the range project. The magnets, which can handle temperatures of 80 ℃, were registered with an average temperature of 43,2 ℃ for a test with 75 % load at full speed. Simulations reported 41,5 ℃, a deviation of 3,9 %. These results indicated potential overcooling.