Line-Start Permanent-Magnet Machines using a Canned Rotor

Various line-start permanent-magnet machines have been around for many years: these offer the advantages of line-starting as an induction machine and running synchronously as a permanent-magnet machine, both from a fixed frequency supply. The major disadvantage is the uncertainty of successful synchronisation particularly with loads combining high inertia with high load torque. Virtually all conventional line-start pm machines use a cage-type induction winding embedded into the rotor along with the magnets. This paper describes an unusual variant of this: comprising a very simple rotor construction of surface-mounted rotor magnets contained within an annular conducting rotor can which acts as the induction winding, but can also provide environmental containment if the application requires it. The purpose of this paper is to describe the canned motor concept in detail so that its potential for other motor/drive applications can be assessed. The paper also develops and describes an analytical dynamic electromechanical model using 'layer theory'. Experimental results demonstrating successful operation of a 2.5 kW prototype machine are presented together with the simulated results using the dynamic model. The influence of certain key design features on the synchronisation process is also included.