SYNC or ASYNC – How to Start a Synchronous Motor Driving a Reciprocating Compressor

Slow speed reciprocating compressors are typically rigidly coupled to single bearing synchronous motors. The compressor load during starting is seldom above 30% of the motor rated torque. In addition, synchronous motors have a relatively low starting current and a relatively high torque-peramp ratio. Consequently, most applications allow for an across-the-line start. There are however occasions where an adjustable speed drive is used for starting. In these instances, the user has two options to consider, start as an induction motor (asynchronously) or start as a synchronous motor with the main rotor field energized from zero speed. This paper will discuss the merits of each of these options for both new installations as well as retrofits. It will consider the views of the motor designer, ASD designer, and end user as well as provide guidance on how to evaluate each option.