How to Evaluate Motor & Drive Energy Efficiency

By Evan Kaiser, Director, Warehouse and Logistics and Erik Lange, Senior Product Specialist, Rockwell Automation

Selecting a variable-frequency drive (VFD) and motor is a critical opportunity to deliver energy efficiency and cost savings to an industrial process. The cumulative energy consumed over the life of an electric motor often is misunderstood and underestimated.

In fact, over their lifespan, a VFD and motor’s energy consumption costs can be as much as 10x or more than the price paid for the system.

Users can get meaningful energy cost savings by performing comparative evaluations and implementing optimized VFD and motor combinations.

Energy Savings Opportunities

Selecting a VFD for improved energy efficiency can be a complex undertaking. Fortunately, with the passage of the EU directive on IEC 61800-9-2 compliance, tools are available to start the process.

IEC 61800-9-2 is the calculated efficiency of a VFD. It provides a basic evaluation of a VFD manufacturer's design choices relative to energy efficiency. This is a static, mathematical calculation. It doesn't consider application-specific dynamic conditions.

IEC 61800-9-2 requires a compliant manufacturer to make public their VFD data. Ease of access to the data is variable depending on the manufacturer, but most provide information on their website.

Comparative analysis of the IEC 61800-9-2 energy efficiency information provides the foundation for making an informed selection for VFD energy efficiency.

Fortunately, IEC 61800-9-2 provides a process for energy efficiency testing (IES). Manufacturers test the VFD and motor combination on a dynamometer through the IES-specified speed and torque combinations, as show in Figure 1. Energy efficiency is measured using an energy analyzer and high-accuracy current measurement (±0.1%). The test environment, including the dynamometer and test instrumentation, should be ISO 17025 certified.

The IES system-efficiency test evaluates the VFD motor control algorithms. These algorithms build on hardware design choices to deliver energy efficiency. To fully understand the system efficiency of the VFD and motor, IES test results deliver a compelling data set to guide technology choices.

Motor selection would be needed before performing the IES testing. The efficiency performance between the three more common choices — induction, permanent magnet (PM) and synchronous reluctance — is important. However, motor selection for the appropriate application is outside the scope of this discussion.

In addition to the fundamental hardware and control algorithms design, additional energy efficiency functionalities are available to improve efficiency further. Economizer mode is an excellent example for applications that operate at lower speeds and torques (see Figure 2). If the application doesn't experience sudden load changes, efficiency can be improved dramatically in low speed and torque applications.

Comparing VFD energy efficiency from IEC 61800-9-2 data, IES testing and applicable energy-efficiency functionality structures the data users must make a selection.

When Does a Difference In Energy Efficiency Matter?

A key question is, at what value does a difference in energy efficiency make economic sense?

Figure 3 illustrates an estimated VFD life of seven years, as an example, with the VFD operating below rating, or on and off, representing an overall 60% duty cycle. This is 36,792 hours of rated operation. A VFD and motor combination with a combined system efficiency of 90%, providing 100kW to an application consumes approximately 111kW/h or 4.088MkWh over the expected life of the VFD motor and drive combination.

A 1% difference (91% efficiency) will consume 4.043MkWh. A difference of 41,000 kWh. At $0.1/kWh the savings value is $4,100.

For applications that run closer to 100% duty cycle and power rating, the savings increase significantly.

Ten years at 90% duty cycle, 90% efficiency: 8.760MkWh. At 91% efficiency: 8.664MkWh. The difference is 96,263kWh or $9,626 in savings.

This straightforward approach supports a direct comparison. It does exclude application-specific dynamic conditions and power-related factors such as power factor. But these exclusions apply equally to all VFDs being compared.

Motor Selection Impact on Energy Efficiency

Energy efficiency and motor type are related. IEC 61800-9-2 efficiency and IES testing provide insight into cost vs. value when choosing a motor.

Induction motors deliver the lowest energy-efficiency performance. Permanent magnet motors, in their various forms are regarded as having the highest energy-efficiency performers. Synchronous reluctance motors are typically less expensive than PM motors but deliver marginally less efficiency.

The question is not whether a VFD can control a motor. The question to ask is, "Which VFD and motor combination delivers performance and efficiency?"

The VFD motor-control algorithm must efficiently rotate the motor. It's possible for a VFD to operate the motor effectively from an application viewpoint, but when tested is found to be inefficient. Without IES testing, the energy efficiency performance will be unknown.