Best practices for energy-efficient machines

Machine control designs are changing to integrate best practices in energy management. These steps include measurement and monitoring of energy use and control and optimization to minimize energy consumption.

05/29/2015


Figure 1: AC current transducers are often used to monitor and measure machine and equipment power use. Courtesy: AutomationDirect.com.Energy efficiency should be designed into a machine, by measuring and monitoring energy consumption, and optimal states of machines, production lines, and entire facilities can be determined and implemented through control systems and other changes. 

Measuring and monitoring

Electrical power usage is always specified on the nameplate, but this number is typically a worst-case usage estimate, and the actual energy demand will vary depending on the application of the equipment. For example, a cycling machine will consume power at a variable rate throughout the course of its programmed process.

The best way to measure and monitor this dynamic demand is to use a power meter. With the ability to measure the actual power consumed at any point in time, the power usage for each operating mode of the machine can be determined.

Power is measured during all parts of the programmed process to better determine when energy consumption is the highest and to identify possible areas for improvement.

By measuring the power, the amount of energy consumed at each production state can be determined using proper software and analysis. Using this power measurement data, the energy cost of a machine idling during production stops can be determined and reduced if found to be excessive.

Monitoring power can also point to process and scheduling problems that cause unnecessary energy usage. For example, heating up an oven for 30 minutes to reach a stable operating temperature and then only running it for an hour of production is not as efficient as planning for a longer production run.

It may be possible to have machines with particularly high-power consumption run at off-peak hours when electricity rates are typically lower. If a manufacturing facility is charged for peak demand by its electric utility, it often makes sense to take this into account when scheduling machine, line, and facility operations. Power measurement information can be used to reduce energy usage and expenses.

Power monitoring details

A variety of equipment is available to provide power monitoring. Voltmeters and handheld clamp-on ammeters can be used to measure a machine's main power and auxiliary equipment energy use. These manual measurements require the opening of a control enclosure, measuring the power, and then recording the collected data.

This data must then be translated to approximate power usage by multiplying the voltage by the current. Power usage data won't be exact unless the power factor is at unity, a rare condition for most industrial equipment and machinery. While this is an acceptable way to get an approximately established baseline energy consumption reading, other methods should be considered for accurate, long-term data collection and analysis.

For accurate real-time, energy-consumption power measurement, a power meter is typically connected just downstream of the machine's main disconnect. Modern power meters are panel-mount devices capable of measuring true power usage in real time, transmitting this measured data to higher level controls, and monitoring systems via a digital data link such as the Ethernet.

Measuring actual ac and dc running current can also provide valuable information. This is especially important when measuring distorted waveforms found on variable frequency drive (VFD) or silicon-controlled rectifier (SCR) outputs, or on linear loads in electrically noisy environments where a true RMS value is required.

True RMS transducers and current transformers, when connected to a VFD output, can indicate how the motor and attached load are operating (See Figure 1). True RMS transducers can also measure actual current in phase-angle fired, burst-fired, or time-proportioned SCRs typically used in heating applications. 

Design for energy efficiency

There are many ways machine design affects efficiency. Something as simple as supplying excess voltage to the machine can waste energy.

For example, many machines need 480 V ac to power motors, but others don't. If no machine equipment requires this high voltage, a step-down transformer will be needed to reduce a 480 V ac supply to a typical control voltage level of 120 V ac. This transformer is not 100% efficient, resulting in losses of up to 5%.

Figure 2: Reducing the speed of a centrifugal pump instead of throttling its full-speed output can provide significant energy savings. Courtesy: AutomationDirect.com.Most industrial facilities will have a variety of voltages available, and machines with equipment requiring 480 Vaac power should be designed to accept power at this voltage. Machines not requiring 480 V ac will be more efficient with a 208Y/120 V ac supply. In this design, 208 V ac will be available for relatively higher power equipment, and 120 V ac will be available to power control circuits, eliminating the need for a transformer.

Machine designers should also consider the efficiency of synchronous motors such as steppers and servos. These motors have similar efficiencies, typically more than 85%. However, servos can be 50% more efficient in some applications. If a motor spends much of its time at zero speed, a servo motor will use little if any power at this level, while a stepper motor will use up to 50% of its rated power at zero speed.

Fan- or centrifugal-pump applications with dynamic demand should be evaluated as candidates for a VFD installation (See Figure 2). In an application where a pump typically operates at a low-flow rate, controlling motor speed with a VFD will result in much lower energy costs when compared to running the pump at full speed and throttling flow with a control valve. A review of pump affinity laws shows that a 50% reduction of motor speed results in a 75% reduction in power use. If less work is needed, then less work should be done, particularly in situations such as these where the amount of energy used decreases much more rapidly than the amount of work performed.

Limiting the pressure of a pneumatic system can also help improve energy efficiency. Although full pressure may be needed, as a cylinder extends and performs work in an application, the retract stroke may not need as much force. Retracting at a lower air pressure or with spring force can save significant energy, particularly if the cylinder operates frequently. 

Turn data into actionable information

Monitoring power or true RMS current on a machine can help with understanding machine metrics such as overall equipment effectiveness (OEE). This monitoring helps identify and eliminate wasteful practices, reducing unnecessary usage. A first step is collecting real-time data to create a baseline of current power consumption.

Analysis of historical machine-power usage data provides useful statistics when compared over time with baseline machine-idle power and running power. Without this historical data it is difficult for plant engineers or management to know where to start or to understand where energy-efficiency techniques will have the greatest impact.

In a single-machine application, a human-machine interface (HMI) with basic historical data acquisition capabilities can be used in conjunction with a panel-mount power meter to collect and display both real-time and historical energy usage data. As energy usage monitoring expands to additional machines in a production line, data logging in an historian is a good option to make data available for further analysis.

Historians are specialized databases, generally PC-based and optimized for storage of large amounts of data. With the data stored, it can be used to create custom reports or HMI trend screens. This information can also be used by external data analysis platforms.

With the proper software and networking, this data can also be pushed to operators, engineers, and management in a variety of ways. Management may see the energy-efficiency data on a report analyzing machine efficiency. Manufacturing engineers can have access to this data on their tablet or smartphone for quick analysis and action, and the operator can view the data on a local display to ensure safe operation. 

Shut it off or turn it down

Figure 3: Conveyors should be closely monitored and controlled by automation systems to make sure they aren’t running when not needed, or running faster than required. Courtesy: AutomationDirect.com.With proper power measurement and monitoring in place, efficient control of machines becomes an adaptable and adjustable capability. Using power measurement data, machines and equipment can be shut down when not needed, or turned down where possible.

Actions as simple as turning off the lights in an assembly cell, turning off a machine tool, or turning off a vision inspection station can save energy. This is particularly true when there are multiple machines used in large assembly lines or manufacturing facilities.

Automatically shutting off idle machines and related equipment is another way to improve efficiency. Idle machines often still use power, so turning this equipment off can save more energy.

In some motor applications, the best strategy is to turn the motor off. If a conveyor has something to move, run it. If it is running empty for an extended period, turn it off. Run to meet demand, then stop, and don't run at a speed in excess of demand (See Figure 3).

This is also an option for applications where the motor must run at full motor speed, such as with grinders and mixers. These processes should be carefully controlled and monitored to ensure they turn off when complete. (See Table.)

With proper monitoring of machines and equipment, energy usage can be optimized. As a bonus, many of the techniques used to cut energy consumption also will extend equipment life and reduce required maintenance.

Table: Basic techniques to reduce energy use

Power down idle machines and equipment

Turn off lights

Turn off conveyors

Adjust speed to match demand

Install VFDs.

- Cindy Green is an industrial engineer at AutomationDirect; edited by Eric R. Eissler, editor-in-chief, Oil & Gas Engineering, eeissler@cfemedia.com.

Key concepts

  • Look for equipment that runs idle often to power it down when not in use.
  • Adjust speeds to match production demands.
  • Use historians to determine peak energy usage.

Consider this

Actions as simple as turning off the lights in an assembly cell, turning off a machine tool, or turning off a vision inspection station can save energy.

ONLINE extra

More about the author

Cindy Green, an AutomationDirect industrial engineer, provides assistance to a variety of product groups. She holds an MS in Manufacturing Systems Engineering from the University of Pittsburgh and a BS in Industrial Engineering from the University of Tennessee and has more than 10 years of experience in manufacturing.

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