Safety aspects of permanent magnet motors in paper machine applications

The worldwide pulp & paper industry has significantly increased its use of 3-phase AC permanent magnet motors in recent years. Many precision applications for squirrel cage induction motors can now use permanent magnet motors with improved project costs and application performance. The different characteristics between the newer permanent magnet motor compared to the common squirrel cage induction motor necessitate a discussion of the safety implications when implementing this change in motor technology. The primary reason for use of a permanent magnet motor is to eliminate the negatives associated with a gearbox. This reduces the mechanical drive between the AC drive motor and paper machine section to one coupling. Because of their built-in field, permanent magnet motors generate voltage onto their stator terminals whenever the rotor is turned. Maintenance work such as roll inspection and cleaning, or accidental events which turn the rotor, can cause this condition. This condition also exists in the event of an operator initiated coast stop or during any other coasting stop such as a power failure or upstream trip. The generated voltage can be hazardous to those servicing the motor and its associated electric drive. This paper presents configurations and procedures for achieving optimal safety when using permanent magnet motors. At the conclusion of this paper, optimal safety configurations are offered.