A line-start permanent magnet motor with gentle starting behavior

The line-start permanent magnet motor combines a permanent magnet rotor for best motor efficiency and an induction motor squirrel-cage rotor to permit starting on a conventional AC power source. Three new concepts have been introduced to improve starting performance by eliminating objectionable torque pulsations during the starting interval. A new rotor concept, called the divided magnet rotor, has been devised to provide the best compromise between the magnetic needs of a permanent magnet rotor and the magnetic needs of an induction motor rotor. Here, the rotor slots superficially resemble that of a double-cage induction motor rotor, but the top portion of the slot contains a block of permanent magnet material openly exposed to the airgap. The bottom portion of the slot is devoted to the squirrel-cage. There are more rotor slots than poles, and the magnet polarities are arranged to form the desired number of poles for the intended running speed. The second new concept is the separate starting armature. The stator has two armatures, a running armature having the same number of poles as the magnets, and a starting armature having at least one fewer pole-pairs than the running armature. The starting armature accelerates the rotor by induction motor action, and the power source is switched over to the running armature when synchronous speed of the running armature is attained. The third new concept is an automatic synchronizer that switches the power source when the conditions of optimal speed and optimal electrical phase difference on the running armature are met.