Applying permanent magnet motors in an ex environment

CATEGORY: LARGE MOTORS
Petroleum and Chemical Industry Technical Conference (PCIC), 2013, Digital Object Identifier: 10.1109/PCICon.2013.6666034, Page(s): 1 - 7
Applying permanent magnet motors in an ex environment
McElveen, R. ; Martin, B. ; Massey, E. ; Stelzner, D.
Abstract
Ceretain characteristics of permanent magnet (PM) motors create new challenges in certifying for oil and gas environments. Currently, certifications of permanent magnet motors are handled on a case by case basis. Authors identify areas in the standards that require additional clarification or assumptions when applying PM motors.
Many of the differences between PM motors and traditional induction motors revolve around the working voltage of a PM motor. Often the working voltage is not the input voltage, but rather the voltage generated by the machine as the rotor spins at a given RPM. This generated voltage can reach or exceed the nameplate voltage of the machine. The generated voltage increases linearly with speed. Accordingly, the operational speed of the motor plays a large part in determining whether or not the motor meets a certain Ex definition.
Motor manufacturer must be engaged early in the certification process. As discussed for a cageless PM motor, it may be desirable to design the stator windings for 90% of the nameplate voltage to avoid any heating problems when caused by being forced to lower the drive input voltage to 90% of rated voltage.
There are differences between certifying a cageless PM motor and a line start PM motor. Many of the standards contain specific requirements for motors with cage windings in the rotor. All of these requirements are also applicable to line start PM machines. The standards typically do not specifically address cageless designs. Standard revisions are underway to better address this type of PM motor. Until standards are revised, the users and motor manufacturers must work closely with certifying agencies to properly interpret the intent of each protection concept.
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