When Curtis-Toledo Inc., St. Louis, an Original Equipment Manufacturer (OEM) of industrial air compressors, was looking for an innovative way to make its rotary-screw compressors more energy efficient and operator-friendly, they turned to ABB Inc., Low-Voltage Drives, New Berlin, Wis. In addition to supplying drives to control the speed and torque of the AF-series compressors, the ABB project team designed software to control the system pressure, air delivery, oil temperature and duty cycle directly from the drive.
Payback on the system depends on the air demand; as the drive runs the motor at lower speeds, it uses less energy. Energy savings up to and possibly exceeding 50% are possible, which could result in an ROI of 12 months to 18 months on the drive. Other benefits include increased efficiency in the plant, as operators need to make fewer manual adjustments; easy and accurate evaluation of systems operations; and improved working conditions due to reduced noise and heat levels.
Curtis-Toledo started offering its AF-series compressors with the ACS800 drive and software in late 2004. At this time, ABB has not plans to add the software package to any drives other than the ACS800.
Drive and software add up to visible savings
Curtis-Toledo markets five 300-hp screw compressors that can provide the necessary air for operating pneumatic tools and machines found in various applications and industries (Figure 1). While many facilities operate these compressors in standard fixed-speed mode, energy costs are prompting some to retrofit theirs with variable-speed drives (VSDs) to control air-flow supplies in direct response and proportion to real-time operating conditions. By matching the compressor output flow and pressure to the instantaneous demand of the customer’s process, the compressor operates at its best efficiency.
Virtually all compressor manufacturers use either a standard programmable-logic controller (PLC) or a custom-designed, OEM board-level controller; in both cases, the machine’s control logic is handled via a separate device. ABB’s ACS800 drive not only controls the motor, but the entire compressor, allowing Curtis-Toledo to eliminate separate controllers and the expenses associated with them (Figure 2). Doing away with PLC or board-level controllers creates a more reliable system.
The standard ACS800 drive includes a set of application function blocks not usually available for compressor drives, says Steve Boren, application engineer for ABB who customized the software for Curtis-Toledo’s machines. These functions include, but are not limited to, and/or, multiply, divide, add, subtract, ramp, counters and timers. These function blocks give the drive its PLC functionality. Because the drive firmware was customized, it can efficiently control a complex screw compressor.
“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,” Boren says. “Typically, air compressors operate at fixed speeds and, when operating at partial loads, they are expending unneeded energy. With a drive built into the compressor as an integral part of the machine, compressors operate at greater efficiencies – including matching air volume to demand; providing energy savings as the variable speed slows the motor down when air demand decreases, and audible noise reduction because, at lower speeds, compressors generate significantly less audible levels than at full speeds.”
The software regulates the drive and changes its speed to supply the exact air demand; the compressor does not load and unload, wasting energy. Air is compressed once -- and only in the volume needed to meet the demands of the system.
“Having a VSD integrated into our compressors is precipitated by industry demand,” says Jerry Elsen, national sales manager for Curtis-Toledo. “VSDs are coming into play where energy costs are higher, such as in California or the Northeast. And demand is getting stronger.”
The ABB drive software continuously calculates the energy savings, allowing the user to see the dollar savings.
“This whole application is about saving energy, so having a touch screen 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 Elsen.
Control at the touch of a finger
The user interfaces with the drive using an integrated touch screen. The screen features a custom menu, through which the operator can adjust operating pressure, external pressure and oil temperature. Monitoring signals are provided in either a digital read-out format or analog meters, and provide information such as amps on the motor, input voltage of the drive, produced torque and energy consumed. The color touch screen is connected to the drive via a standard Modbus connection.
John Fahey, ABB sales representative, who worked with Boren on the customized software, says feedback indicates that an operator can be trained to use the touch-screen menus in a few minutes. “This means that the customer is able to avoid setup errors and eliminate time-consuming factory calls,” he says.
Boren says it’s not necessary to have drive technology knowledge to use the menus. “An operator can set up the machine parameters for operation, monitor all running compressor system signals, access preventive maintenance timers and diagnostic screens, and even view a fault history log, which has a built in troubleshooting manual, all at the touch of a finger and all without any understanding of VSD technology. The touch screen is set up to give the user a complete snapshot of the compressor operation and set-up, while making it very simple for the operator to navigate the screens,” he says.
The drive software indicates when to service all of the components that need to be changed periodically, including oil, separator elements, oil filter, drive enclosure filter and air filter. Adjustable maintenance timers count down until they reach zero; an alarm is then displayed on the touch screen indicating which maintenance function needs attention. Once the issue has been addressed, the operator resets the timer, which then starts counting down again.
At the request of Curtis-Toledo, the control system also provides automatic lead/lag control for two machines: When one machine can’t keep up with demand, it starts the secondary machine automatically. Conversely, the secondary machine is taken off line when not needed, eliminating the need for any external device.
The ACS800 drive has a fault history log to help the operator troubleshoot problems. If the machine faults, an alarm is activated and a window pops up on the touch screen. The display tells the operator what the fault is and how to correct it.
Warnings and faults displayed in the fault/warning history include:
• E-stop: a safety circuit required by OSHA. In case of a plant emergency, a push button on the machine immediately shuts down the drive;
• Remote start/stop: a hard-wired push button that can start or stop the machine from a remote location. This is a convenient when an operator cannot get to the touch screen;
• Input air block detection: This safety measure shuts down the machine if the airflow is blocked; and
• Motor temperature protection: This safety circuit will shut down the machine in case of extreme overheating.
The ACS800 drive has a feature called Direct Torque Control (DTC), which 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, which can minimize down time due to random faults.
A unique feature on the Curtis-Toledo installation is the addition of a secondary drive in the machine to cool the oil. “We found through testing that if the speed varied on the main compressor drive, and the cooler ran at a constant speed, the oil would overcool and moisture would build up, resulting in a machine breakdown,” says Boren. “What we’ve done is put another ABB drive (the ACS550 series) on the fan motor. We take the discharge oil temperature and run a PID loop on temperature, meaning that the speed of the cooler fan varies so that the oil temperature is maintained.”
Better temperature control can result in longer oil life since the oil won’t breakdown if its temperature is regulated and controlled within safe limits. Controlling the temperature also removes moisture from the oil, thereby eliminating a major cause of mechanical breakdowns.
The fan drive is also integrated into the software so the operator can use the touch screen to monitor its current, torque, voltage, power, temperature feedback and temperature set point.
John Wilmes is district manager for ABB Low Voltage Products. He works closely with manufacturers to provide automation products and solutions to electrical distributors and end users. E-mail him at [email protected].
