Variable Frequency Drives

Variable frequency drive energy saving on air compressors


Courtesy of Courtesy of Variable Frequency Drives

When we are looking for an innovative way to make its rotary-screw compressors more energy efficient and operator-friendly, they turned to Gozuk, Low-Voltage VFDs. In addition to supplying VFDs to control the speed and torque of the AF-series compressors, the Gozuk project team designed software to control the system pressure, air delivery, oil temperature and duty cycle directly from the VFD.

VFD Payback on the system depends on the air demand; as the VFD 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 VFD. 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.

We started offering its compressors with the EDS1000 VFD and software in late 2012. At this time, Gozuk has not plans to add the software package to any VFDs other than the EDS1000.

VFD and software add up to visible savings
Our markets five 300-hp screw compressors that can provide the necessary air for operating pneumatic tools and machines found in various applications and industries. While many facilities operate these compressors in standard fixed-speed mode, energy costs are prompting some to retrofit theirs with VFDs 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. Gozuk's EDS1000 VFD not only controls the motor, but the entire compressor, allowing us to eliminate separate controllers and the expenses associated with them. Doing away with PLC or board-level controllers creates a more reliable system.

The standard EDS1000 VFD includes a set of application function blocks not usually available for compressor VFDs, says Steve Boren, application engineer for Gozuk who customized the software for our machines. These functions include, but are not limited to, and/or, multiply, divide, add, subtract, ramp, counters and timers. These function blocks give the VFD its PLC functionality. Because the VFD firmware was customized, it can efficiently control a complex screw compressor.

'We've enhanced the capability of the VFD 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 VFD 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 VFD 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 Variable frequency drive integrated into our compressors is precipitated by industry demand,' says Jerry Elsen, national sales manager for us. 'Variable frequency drives are coming into play where energy costs are higher, such as in California or the Northeast. And demand is getting stronger.'

The Gozuk VFD 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 VFD 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 VFD, produced torque and energy consumed. The color touch screen is connected to the VFD via a standard Modbus connection.
John Fahey, Gozuk 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 VFD 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 Variable frequency drive 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 VFD software indicates when to service all of the components that need to be changed periodically, including oil, separator elements, oil filter, VFD 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 us, 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 EDS1000 VFD 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 VFD;
  • 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 EDS1000 VFD 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 VFD 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 installation is the addition of a secondary VFD in the machine to cool the oil. 'We found through testing that if the speed varied on the main compressor VFD, 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 Gozuk VFD 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 VFD 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.

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