What Are the Different Types of Adjustable Speed Industrial Motor Drives

International Electrotechnical Commission (IEC) standard 61800 recognizes two types of adjustable speed electrical power drive systems (PDS) for industrial applications. 61800-1 applies to direct current (DC) PDS, and 61800-2 applies to alternating current (AC) PDS. The term PDS applies to the entire system of drive plus motor.

Other sections of 61800 discuss test methods, safety requirements related to thermal and energy conditions, functional safety, electrical and environmental requirements for encoders, electrical interfaces, and performance measurements. The newest part, IEC 61800-9, covers ecodesign for motor systems, including energy efficiency determination and classification.

While IEC 61800 defines adjustable speed AC and DC PDS, there are also general definitions for variable speed drives (VSDs) and variable frequency drives (VFDs) in industrial applications. IEC 61800 applies to mains-powered PDS connected to up to 1.5 kV_AC 50 Hz or 60 Hz. It also applies to DC input voltages for battery-powered systems like industrial autonomous mobile robots (AMRs) that use adjustable speed drives. Traction and electric vehicle drives are excluded from IEC 61800.

This article briefly presents the common definitions of VSDs and VFDs and looks at why VFDs are widely used. It then reviews the efficiency classes defined in IEC 61800-9 for AC drives and, considers exemplary mains-powered VFDs from Delta Electronics, Siemens, Schneider Electric, Omron Automation, and closes by looking at the use of VFDs in AMRs and other battery-powered systems using an example system from MEAN WELL.

The standard definition of a VFD is a drive that uses changes in frequency to control motor speed, making them useful with AC motors. At the same time, a VSD varies the voltage to control the motor, making it useful for both AC and DC motors.

But it’s not quite that simple. Both types of drives can be used to control the speed of motors. As a result, sometimes, the term VSD is applied to VFDs. VFDs can be used with brushless DC motors (BLDCs); strictly speaking, they are not limited to AC motors. VFDs are suitable for use with a variety of motors like:

• Induction (IM), or asynchronous AC motors, are widely used in industrial applications since they are self-starting, reliable, and economical.
• Permanent magnet synchronous motors (PMSM) are highly efficient AC motors and can enable precise control of torque and speed in high-performance applications that demand high energy efficiency.
• BLDCs are also used in applications that require high efficiency and precise control and typically have long operating lives.
• Servo motors can be AC or DC and support rapid, high-precision responses. VFDs with specialized control algorithms can be used with servo motors in robots, computer numerically controlled (CNC) machines, and similar applications.
• Synchronous AC motors (SMs) are suited for applications that require constant speed and precise synchronization. While VFDs can control the speed of SMs, other (lower cost) drive options can support constant speed operation.

There’s a variety of control algorithms used with VFDs that increase their versatility. For example, there are four primary types of VFD control algorithms just for induction motors: volts-per-Hertz (V/f), V/f with encoder, open-loop vector, and closed-loop vector. All use pulse-width modulation and provide different levels of control over speed and torque.

The importance of VFDs in a wide range of industrial applications is evidenced by the development of IEC 61800-9, which is focused on the efficiency and ecodesign of VFDs and related motor drive systems.

BDM, CDM, and PDS

There are two sections of IEC 61800-9 related to VFDs. Part 1 delineates the methodology for determining an application's energy efficiency index or reference. Part 2 details methods for evaluating efficiency based on a series of classifications.

While the efficiency of VFDs, called basic drive modules (BDMs) in IEC 61800-9, is important, it’s not the primary focus of the standard. The standard is more broadly based and considers complete drive modules (CDMs) that consist of a frequency inverter (the VFD), a feeding section, and input and output auxiliaries (like filters and chokes) and on the power drive system (PDS) that consists of the CDM plus the motor (Figure 1).

Figure 1: IEC 61800-9 efficiency classes apply to the CDM (black section) and PDS (red section) in VFD systems. (Image source: Schneider Electric)

CDM efficiency classes

CDM international efficiency (IE) classes are defined from IE0 to IE2. They are determined by comparing the total loss of the CDM with the performance of a reference CDM (RCDM). IE classes for CDMs are defined relative to the 90, 100 operating point using 90% motor stator frequency and 100% torque current to avoid overmodulation and ensure comparability of the performance measurements of drives from different makers.

The performance of the RCDM is defined as IE1. A CDM with greater than 25% lower losses than the RCDM is classified as IE2, and a CDM with greater than 25% higher losses than the RCDM is classified as IE0. The RCDM also enables the comparison of the energy consumption with an average technology CDM at eight pre-defined operating points (0, 25), (0, 50), (0, 100), (50, 25), (50, 50), (50, 100), (90, 50) and (90, 100) (Figure 2).

Figure 2: IEC 61800-9 CDM operating points and efficiency classes. (Image source: Siemens)

PDS efficiency classes

PDS international efficiency system (IES) classes are like the CDM IE classes and are defined as IES0 to IES2. They are based on a reference PDS (RPDS) and reflect the efficiency of the complete drive module plus the motor.

Matching the combined motor and CDM to the specific application requirements provides greater potential for overall efficiency optimization. That efficiency optimization is reflected in a higher IES classification. Like the RCDM, the RPDS enables the comparison of energy consumption with an average technology PDS at eight pre-defined operating points.

The operating points are based on a percentage of torque and a percentage of speed, and the IES value is calculated based on 100% torque and 100% speed, which is the (100, 100) operating point.

Instead of using the 25% changes of the IE classes, IES classes are based on 20% changes. A PDS with an efficiency class IES2 has greater than 20% lower losses, and a class IES0 PDS has greater than 20% higher losses than the RPDS performance defined as IES1 (Figure 3).

Figure 3: IEC 61800-9 PDS operating points and efficiency classes. (Image source: Schneider Electric)

VFD examples

VFD makers don’t always report efficiency based on 61800-9. That’s because the simplest efficiency measurement using IEC 61800-9 is for the CDM, which consists of the VFD (frequency inverter) plus numerous additional components, including the feeding section and input and output auxiliary devices. The use of specific additional components is beyond the control of VFD makers, and 61800-9 doesn’t apply directly to VFDs.

Some VFD makers have adapted the 61800-9 methodology. When IE2 compliance is claimed, the data is reported in various formats, including charts, tables, and Excel files.

For example, Siemens uses the IEC 61800-9 methodology with its SINAMICS V20 drives and reports them as efficiency class IE2 (Figure 4). These drives are offered in nine frame sizes, ranging from 0.16 to 40 horsepower (hp). These drives have been optimized for basic drive systems in manufacturing and process applications like pumps, fans, compressors, and conveyors. Numerous optional components include input filters, input and output reactors, braking resistors, and so on.

Figure 4: Efficiency class IE2 7.5 kW CDM that has 36.1% lower losses compared with the reference converter (90% / 100%). The percentages show the losses in relation to the rated power of the basic drive without optional components. (Image source: Siemens)

Delta Electronics has also adapted the 61800-9 methodology and reports IE2 efficiency for its 1.7, 3.0, 4.2, 6.6, 9.9, and 12.2 kVA MS300 Series compact drives. The data is detailed in a tabular format rather than as a chart. The MS300 series includes drives from 0.2 to 22 kW (Figure 5). These drives feature several built-in features, including a programmable logic controller (PLC) function for programming, MODBUS communication, a communication slot that can support additional protocols, and a USB port for uploading and downloading data.

Figure 5: Delta Electronics’ MS300 series includes 0.2 to 22 kW drives. (Image source: Delta Electronics)

Omron reports its “variable speed drives with three phases input,” like the MX2 Series VFDs, meet the requirements of IE2 efficiency. The company provides the test data as an Excel file. MX2 drives are available with ratings from 0.1 to 2.2 kW for 200 V single-phase input, 0.1 to 15.0 kW for 200 V three-phase input, and 0.4 to 15.0 kW for 400 V three-phase input. These drives are designed for IM and PM motors and support smooth control down to zero speed with 200% starting torque at 0.5 Hz.

While other VFD makers focus on sections 1 and 2 of IEC 61800-9, Schneider Electric takes a more holistic approach and describes how to integrate its drives with the appropriate motor to meet the ecodesign directive and section 3 of IEC 61800-9 that delineates a quantitative ecodesign approach by means of eco balancing including product category regulations and related environmental declarations.

The company’s Altivar Machine ATV320 drive family includes IP20 and IP6x rated VFDs from 0.18 to 15 kW (0.25 to 20 hp) for three-phase synchronous, asynchronous, PM and BLDC motors in open loop control and includes functions like:

• Low-speed torque and speed accuracy and high dynamic performance using flux vector control without a sensor
• Support for high-frequency motors
• Integrated functions for compliance with functional safety standards

What about AMRs?

AMRs use VFDs, but a different type of VFD. The VFD Series of industrial BLDC motor drives from MEAN WELL are a good example. They comply with the relevant sections of IEC 61800, such as 61800-5-1 safety requirements and 61800-3 electromagnetic compatibility (EMC) requirements. However, these VFDs are not packaged drives, so the efficiency categories of 61800-9 don’t apply.

The VFD Series includes eight models with DC and AC input versions ranging from 150 to 750 W. The model VFD-350P-48 operates with an input of 48 V_DC for battery-power applications like AMRs and can supply up to 350 W and 20 A of output current.

This 350 W BLCD driver is packaged on a 4"x 2" circuit card, and the fanless design can support 200% peak loads for 5 seconds (Figure 6). All the models in the VFD Series include only the power drive section and require an external control card. MEAN WELL also offers an optional control card.

Figure 6: Block diagram of a VFD drive power section (left) and the power section ready for installation in an AMR (right). (Image source: MEAN WELL)

Conclusion

Various adjustable speed drive designs are available for industrial applications, including machine controls and AMRs. They can support both AC and DC motors and have varying levels of compliance with sections of IEC 61800. In addition, since the performance of individual VFDs is not a focus of IEC 61800-9, there are several different approaches to reporting performance relative to those efficiency standards. Some VFD makers focus on sections 1 and 2 and report VFD efficiency levels like IE2. In contrast, others focus on section 3, which is related to overall ecodesign considerations, including product category regulations and related environmental declarations.