Equipment and technology improvements can be sound investments.
by Dave DePasquale, Siemens Industry, Inc.

High-speed motors with active magnetic bearing systems bring operational benefits—such as wide speed range, multiple start-stops, fast ramp-up, unmanned or remote operations and monitoring, and lower the environmental footprint.

Answering the CCU Challenge

The solution was a combination of technology, an in-depth understanding of the products and situation and expertise to deliver one of the first solutions of its kind to the refinery. 

Each CCU would be supported by two 4,500-horsepower, 4.5-kV, 78- to 105-hertz (4,700- to 6,300-rpm) totally enclosed, water-to-air cooled, high-speed motors with complete active magnetic bearing (AMB) systems. These were the first of their kind installed in the U.S. and used in a refinery.
These high-voltage motors featured a unique rotor design. The rotor was made from a special, single-piece forging. The rotor slots were then machined. Once this process was complete, a proprietary fusion process shaped and bonded the copper bars into the slot. The result was a solid rotor that could withstand the centrifugal forces associated with high-speed operation. The overall process led to reproducible vibration behavior under all conditions and throughout the life cycle of the product. 

Ensuring Continued Operation

“One of our key requirements was reliability, as an unscheduled shutdown of our CCUs is unacceptable, and has a significant impact on our production levels and operating costs,” says the refinery engineer. “We had a high confidence level in the motors and drives to help us deliver uninterrupted service of our compressor train for 5 years, but these products had to operate in tough conditions as well.”

The AMB system brought several other operational benefits, including operating at high speeds, a wide speed range (API 617 and API 546), multiple start-stops, fast ramp-up, unmanned or remote operations and monitoring, reduced life-cycle costs, controllable rotor dynamics, no lubrication system and an oil-free string (including compressor) that reduced the environmental footprint.

The motors levitated the shaft and permitted motion without friction or wear, unlike traditional oil-lubricated or grease-lubricated motor bearings. The AMB consisted of an electromagnet assembly, a set of power amplifiers that supplied current to the electromagnets, a controller and gap sensor with associated electronics to provide the feedback required to control the position of the rotor within the gap. Each AMB was equipped with a backup bearing for emergency coast-down in the event of a power failure to the AMB system.

While electronic sensors of the AMB were located on the shaft at the bearing positions, the backup bearings were provided next to each of the two magnetic bearings, just in case the AMB closed-loop control was to unexpectedly fail. These backup bearings had an air gap of approximately 0.5 millimeters, while the air gap of the magnetic bearings and the sensors is about 2 millimeters. Further, the backup bearings were dry-lubricated, and the sleeve bearing shells had a coating of a special material and were split. This would make for easy replacement, if necessary, without disassembling the complete stator/rotor of the motor.

The corresponding sleeves of the backup bearings on the shaft featured a galvanic anti-adhesion coating, which helped prevent too much material from being transferred from the sleeve bearing shells to the sleeves when the backup bearings must be used.
Installing Drives for Reliability 

To further increase reliability and efficiency, the refinery opted to connect the CCU’s new high-speed motors to two medium-voltage drives. Variable frequency drives (VFDs) provide the motors with the ability to accommodate the changing demands of the CCUs, thereby providing additional energy savings.

The motor/drive combination did not require any complex gear unit, which facilitated a more compact drive train system and eliminated the costs associated with gear unit inspection and service.

Commissioned in spring 2010, the VFDs’ low-voltage cells’ topology could be scaled precisely for a wide range of voltage and output power. With the ability to bypass any one cell during operation, the VFDs could maintain the full output voltage necessary for the process to continue uninterrupted. This cell-based configuration also provided the refinery with easy access to drive components for scheduled maintenance. This reduced system repair time to minutes. 

The cell bypass ensures automatic bypass of a failed power cell in less than 500 milliseconds. Instead of shutting down the entire drive, a process-tolerant protection system (ProToPS) provides a hierarchical system of warnings. This control strategy allows time to evaluate the situation and respond appropriately.

“We had severe thunderstorms, resulting in a power failure at the refinery. Due to the power loss ride-through capability of both the motors and the drives, all continued to operate at full-rated power,” says the refinery team leader.

An integral transformer with phase-shifted secondaries provided 24-pulse or better input harmonic cancellation with a power factor above 0.95 under any operating conditions. This eliminated the need for input harmonic filters or power factor compensation. It completely removed any common-mode voltages from being imposed on the motor. The VFDs supplied an output voltage that was so close to a perfect sine-wave shape that the CCU motors could be operated without any additional stress or overheating that might result from excessive dV/dt or harmonic distortion.