Energy Management

Make your machines more energy-efficient: 5 basics

How to monitor, measure, and control the power consumption of industrial machines and production lines.

By Cindy Green, AutomationDirect

Machine builders, OEMs, and system integrators are designing machines with energy-management best practices in mind. Atop the best-practices list is monitoring and measuring power use and then programming operation of machines, equipment, and production lines to reduce energy consumption.

Power consumption measuring and monitoring helps optimize facilities, production lines, and machine control – and knowing just where and when power is being used will help you identify where improvements are needed. Here are five keys to getting greater efficiency from your assets.

1. Commit to keeping track of your power use

The best way to determine power use is to monitor power consumption in real time, as it provides a means to measure actual, dynamic demand. This is important because machines use varying amounts of power depending on their operating mode or where the equipment is in its sequence.

Relying on the nameplate ratings for electrical equipment such as motors and drives will reveal only worst-case values; these aren’t a reliable measure of power consumption because actual energy use varies depending on the equipment and how it’s used. Machines typically consume power at a variable rate, and power supplies and servo drives and VFDs often use much less power than their nameplate ratings indicate.

Power consumption measurements help determine where energy use is the highest and point to areas for possible improvement. When a machine is stopped and waiting for product or downstream equipment, the reduced power use at idle can be recorded. The power usage also is recorded continuously during the machine’s cycle and at a variety of production rates.

When you have a comprehensive picture of your power use during production, you can better decide where and when to change a machine’s production state. If the idle state is using too much power, the machine can be shut down. Data analysis also may indicate that the machine can be switched to idle state while waiting for product instead of waiting in a ready-to-run state, with conveyors in motion and waiting for product, for example.

Power monitoring can highlight scheduling issues that are causing inefficient energy use. If an oven is heated and waiting for product for 30 minutes before cycling, then preheat can likely start at a later time. In a similar example, running an oven for only an hour’s worth of production when it takes 30 minutes to heat up is inefficient.

Monitoring and understanding power data can make it possible to run machines and processes that consume large amounts of power at off-peak hours to take advantage of lower electricity rates. This information can also be used to reduce demand charges, which many utilities assess based on peak power consumption.

2. Perfect your power monitoring

Power can be monitored manually with a handheld clamp-on ammeter and a voltmeter. However, these measurements require manual labor to access the equipment, measure the power, and record the values every time the data is collected. For the most basic power monitoring applications, manually measured power can be combined with an AC or DC current-operated switch. The switch becomes an input to the programmable logic controller (PLC) or other controller, and the assumption is that the measured power is being consumed when the switch is activated.

However, the required labor continues as the data is input to a spreadsheet, with the voltage and current converted to approximate power use. This power use data doesn’t include power factor, so the values will be accurate only if the power factor is at or near unity – obtained when current and voltage are in phase, a condition unlikely to be seen in industrial facilities. Although this is a good way to establish a power use baseline, there are other, more-accurate approaches to continued power monitoring.

A power meter can be connected to the load side of the main disconnect in the control enclosure of a machine or an item of equipment such as an air compressor. Modern panel-mount power meters can measure real-time, true power use. This measurement data can then be sent to the PLC or another controller and from there to a human-machine interface (HMI). Power meters typically have multiple communication options built in, allowing information to be transmitted via 4-20mA analog signals or via a single digital data link such as an Ethernet protocol.

Many applications require true RMS measurements of AC and DC running current to obtain useful power consumption values (Figure 1). This is necessary in electrically noisy environments and with measurement of VFD or SCR outputs (which can be distorted waveforms), for example.

When connected to a VFD, true RMS transducers and current transformers provide accurate measurements. SCRs, often used in heating applications, can also be measured using true RMS transducers to determine actual current in time-proportioned, phase-angle-fired or burst-fired heating applications.

3. Know how much power you really need

Unfortunately, inefficiencies are sometimes designed into machines and equipment, and a designer’s lack of experience or poor attention to detail frequently is the culprit. Some of these issues can be corrected by modifying existing machines and equipment, but it’s much more effective to design for energy efficiency up front.

Depending on the machine and equipment, it may be necessary to specify a 480 VAC main power feed, usually to power motors. However, other machines and equipment may not require this high of a voltage. If none of the components, motors, or drives requires 480 VAC, then a stepdown transformer is commonly used to supply common control voltages in the 240 to 120 VAC range. A transformer is not 100% efficient, with heat losses up to 5%.

With careful design, only machines and equipment needing 480 VAC will require that voltage feed. If it’s not needed, a 208Y/120 VAC supply will be more efficient. With this supply, higher-power equipment can still receive the 208 VAC 3-phase voltage – and 120 VAC control voltage will be available without the requirement of a stepdown transformer on the machine.

Motors also affect the efficiency of machines. Most 1 hp and larger electric motors today are well past the 85% efficiency specified in U.S. Department of Energy’s motor efficiency standards, and efficiency continues to improve. The efficiency of synchronous motors such as servos and steppers should also be considered.

For example, in some applications, servos can be 50% more efficient than steppers. If much of a motor’s time is spent in an idle, zero-speed state, a stepper will use up to 100% of its torque in this state, depending on how it’s set up. A servo will use only what is necessary to hold its position.

Centrifugal pumps and fans experiencing frequent load change should be supplied with variable-frequency drives (Figure 2). Instead of controlling flow with a throttling valve or damper, the pump or fan should be run at a speed to match the load. Pump affinity laws show that reducing pump motor speed by 50% can reduce power consumption by 75%, with similar savings found along the speed-versus-power curve.

Compressed air systems are a big power user, and reducing air pressure to only what’s needed can increase efficiency. Even though full pressure may be needed while the cylinder is extending and performing work, the retract stroke often will work well with much less force. Using a spring or lowering the retract pressure can save air consumption (and thus energy) on cylinders that operate frequently.

4. Make the most of your power data

Collecting real-time current and power data over time is a first step to creating a baseline for power consumption. Measurement of true RMS current of a machine or item of equipment is also helpful in determining overall equipment effectiveness (OEE). Through monitoring, wasteful practices can be identified and eliminated to reduce energy use.

Real-time power data, collected in a historical data base, can be compared with that for running and idle power. This historical data is a good place for management and plant engineers to start in identifying the low-hanging fruit where energy-efficiency efforts will have the largest impact.

An HMI connected to a panel-mount power meter through the PLC or controller is a good place to start collecting real-time and historical power usage data. An HMI works well in single-machine or production-line applications and can connect to several controllers to receive additional data. The HMI also enables production of custom reports, power-use trend graphs on the screen, and integration with external data analysis programs. As the size of the production line increases, data logging to an upstream SCADA system can support power-monitoring and analysis efforts.

Collecting power use data also supports smarter decision-making by management, engineers, and operators. While operators monitor data locally to ensure efficient operation, engineers can use the same data, often viewable on their smartphone, to make production decisions. Managers can view the power efficiency data to track progress toward long-term equipment efficiency goals.

5. Slow it down or turn it off

Power measuring and monitoring provides the information needed to make good decisions regarding machine and equipment operation. Data may prompt you to make production changes such as slowing equipment down or shutting off a machine or equipment when it’s not needed. Simply turning on a vision system, a machine, or an assembly line promotes efficiency, especially if there are multiple or large production lines involved.

Many machines, production lines, and pieces of equipment operate only one shift per day, yet the power is often left on overnight. Turning off idle production machines can save significant energy. A similar strategy should be used for motors. Instead of letting motors run for extended periods in idle mode, they should be shut off. It is simple to monitor and control this energy-saving concept. Run when needed and not faster than necessary, and stop and power down whenever possible (Figure 3).

Optimizing energy use by monitoring machine power consumption not only cuts electric bills, but also it extends equipment life and often reduces maintenance requirements. It’s best to require machine and equipment suppliers to build energy efficiency into their designs, but it’s also possible to retrofit older plants to improve energy efficiency.