The application of using induction motors/generators in submerged cryogenic applications (i.e., LNG, LPG, ethylene, etc.) presents certain design, manufacturing, and testing challenges and advantages when compared to traditional “air motors” (i.e., induction motors operating in ambient temperature air). Since the operating fluid passes through the rotating machine, significant design and testing philosophies have been developed to address the influence of the cryogenic fluids.
The unique application of moving cryogenic fluids such as LNG effectively removes the “normal” concern of heat generation in the motor itself during normal operation. Due to this effect, traditional standards which apply to temperature rise and other related temperature influenced concerns are not relevant. Historical testing has shown that the stator coils of a submerged cryogenic machine typically experience an 8°C – 12°C temperature rise depending on the fluid temperature and motor/generator size during full load operation. For example, an induction motor/generator operating in LNG (fluid temperature typically at -161°C) will result in a coils temperature of approximately -151°C during full load continuous duty operation. Since these temperatures do not approach typical air motor coil temperatures and the standards that govern them, typical motor winding temperature testing is neither relevant nor performed.
Each motor/generator design is project specific and based on the requirements of a given application, and each design is verified by complete performance testing prior to shipment. However, individual dynamic performance testing of the motor/generator itself is not possible due to the operating environment in which the machine is operated (i.e. they must be submerged in a cryogenic fluid of appropriately low temperature). Since the hydraulic portion of each unit is integral with the motor/generator (i.e., single shaft for motor/generator and pump/expander) separation of the hydraulic and drive section is not physically possible. Also, since installing a motor dynamometer is not practical to operate in the fluid, traditional motor performance testing is not performed. During performance testing, Cryodynamics® does measure motor/generator inputs and hydraulic outputs allowing motor performance to be determined.
Manufacturing of motors/generators for cryogenic pumps/expanders employs proprietary insulation systems, best-in-class assembly techniques, and the highest quality materials to ensure that each unit lives up to the standards for which Cryodynamics is known. From our certified welders, experienced winders, double VPI process, to our automated electrical test equipment, each and every motor/generator provides the highest levels of performance and reliability required for the application. Cryodynamics motors/generators are tested to applicable NEMA MG-1 test standards including DC High Potential Testing, Insulation Resistance Testing (Megger), Surge Testing, as well as Polarization Index (PI). These tests provide a proven method of testing the turn to turn insulation, phase to phase insulation, ground wall insulation, as well as the proper connection of the coils. Each step in the manufacturing process is tracked for quality assurance and retained for future reference.
One of the additional benefits apart from lack of heat generation (and therefore the need to remove the heat) is that due to the cold operating temperatures, the continuous load rating (for motors and generators) is significantly higher than an equivalent sized air motor. This allows the motor package to be much more compact than equivalent sized air motors or generators.
Overall, there are many challenges which Cryodynamics has addressed in operating an induction machine in cryogenic fluids with regard to design, manufacturing and testing. However, the unique nature of having the cold fluid removing heat from the system offers benefits in these areas as well. Cryodynamics has a track record in producing pumps and expanders which offer tremendous ruggedness and reliability in a most challenging environment. As with any technically driven company, we are continuously striving to offer the best combination of design and manufacturing techniques to our customers in the handling of cryogenic liquefied gas fluids.