The present invention provides a permanent magnet rotor arrangement that is particularly suitable for low-speed large-diameter electrical generators. The arrangement includes a rotor 2 having a radially outer rim 4. A circumferential array of magnet carriers 12 is affixed to the outer rim 4 of the rotor and have a radially outer surface. An inverted U-shaped pole piece retainer 18 made of non-magnetic material such as stainless steel is affixed to each magnet carrier 12 and is formed with an axially extending channel. At least one pole piece 16 made of a magnetic material such as steel is located adjacent to the radially outer surface of each magnet carrier 12 and in the channel formed in its associated pole piece retainer 18.