St. Louis-based Curtis-Toledo, Inc is a 150-year-old OEM of industrial air compressors (5-300 hp) used to power equipment and operations in a variety of applications and industries. With energy savings as a top priority, Curtis-Toledo wanted to upgrade its screw compressors with adjustable-speed drives to provide and control airflow supplies in direct response and proportion to real-time operating (load) conditions.
To satisfy Curtis-Toledo's needs for energy efficiency and ease of use, ABB created a unique control package -- using the ABB ACS800 drive built into the compressor, coupled with customized software that controls the system pressure, air delivery, oil temperature, and duty cycle directly from the drive.
ABB's approach eliminated separate logic controllers and uses the ACS800 drive to perform the basic function of controlling the speed of the motor and, at the same time, uses its processing power and extensive I/O to act like a traditional programmable logic controller.
"We've enhanced the capability of the drive without the cost and inconvenience of add-ons such as PLCs, which require external mounting, wiring, and added space," says Steve Boren, the ABB application engineer who customized the software for Curtis-Toledo's machines.
With a drive built into the compressor as an integral part of the machine, the compressors operate at greater efficiencies, including matching air volume to demand and providing energy savings, as the variable speed slows the motor down when air demand decreases. Through the software, the drive regulates and changes speed to match the exact air demands from the plant or process.
The control system (drive and software) also provides lead/lag control automatically with two machines: when one machine can't keep up with demand, it brings on the secondary machine automatically; and the secondary machine is taken offline when not needed, eliminating the need for any external device.
Energy Savings Easy To Calculate
The ABB drive software was designed to include a formula that continuously calculates the energy savings, so the end user can see the dollar savings on demand by viewing a touchscreen.
"This whole application is about saving energy, so having a touchscreen that displays the calculated energy savings as the machine is running is the biggest benefit, followed by its ability to monitor the working conditions of the compressor," says Jerry Elson, national sales manager for Curtis-Toledo.
End users can expect annual energy cost savings with ROI in 12-18 months on the drive and savings up to and possibly exceeding 50%, depending on the air demands of the facility.
Touchscreen Provides User-Friendly Functionality
The drive is easy to use, with an integrated touchscreen that interfaces with it. At the touch of a finger and without any prior knowledge of variable-speed drive technology, an operator can set up the machine parameters for operation, monitor all running compressor system signals, access preventive maintenance timers and diagnostic screens, and view a fault history log that has a built-in troubleshooting manual.
Direct Torque Control Provides Precise Motor Control
The Direct Torque Control (DTC) technology feature provides fast, accurate control of both motor speed and torque without pulse-encoder feedback from the motor shaft. This enables the drive to calculate the torque and flux of the motor 40,000 times per second and makes the motor controllers tripless. Having a tripless drive operating the Curtis-Toledo compressor helps minimize downtime due to the random faults that can plague competitive drives.
DTC optimizes motor flux control, yielding up to a 10% increase in efficiency at 25% load for added energy savings. DTC also maintains the service factor of the motor, because when DTC controls the motor, less heat is generated due to reduced slip. Currently, the Curtis Toledo/ABB design is the only one in the market that maintains the 1.15 Service Factor without needing to oversize the motor.
Customized Solution Benefits The End User
"Having a variable speed drive (VSD) integrated into our compressors is precipitated by industry demand," Elson says. "VSDs are coming into play where energy costs are higher, such as in California or the Northeast. And demand is getting stronger. With the ABB drive and software, we feel that Curtis-Toledo is ahead of the curve."
