Energy-efficient motors: new standard supports market

Paul Boughton

International standard IEC 60034-30:2008 defines new energy efficiency classes for electric motors. Jon Severn looks at the standard's scope and limitations to find out whether it will really help equipment suppliers and end users.

Of all the electricity generated worldwide, it is estimated that between 30 and 40 per cent is consumed by industrial electric motors. Given the global concern about diminishing resources, and recent high prices for energy, it is no wonder that there is increasing interest in the energy-efficiency of electric motors. Of course the electric motor is only one element in a motor-driven system that offers the potential for savings, but it has been estimated that optimising motor-driven systems could deliver overall savings of between 30 and 60 per cent.

A commonly used approach to improving system efficiency involves installing an inverter drive to give variable-speed control of the motor. A Motor Workshop organised by the International Energy Agency (IEA) in July 2006 concluded that energy-efficient electric motors in combination with frequency converters could reduce global electricity consumption by about sevenpercent, with approximately one-quarter to one-third of those savings being attributable to the improved efficiency of the motor.

Around one-third of the energy-saving potential for industrial electric motors is accounted for by motors rated in the range 0.75 to 4 kW, with another third accounted for by 4 to 30kW motors. The remaining energy-saving potential is almost all accounted for by motors rated from 30 to 500kW. Already there are numerous energy-efficiency standards for AC induction motors, such as NEMA, EPACT, CSA, CEMEP, COPANT, AS/NZS, JIS and GB, but the IEC (International Electrotechnical Commission) has focused on single-speed, three-phase, 50Hz and 60Hz AC cage induction motors in the range 0.75 to 375kW in its recently published standard IEC 60034-30:2008 Rotating electrical machines - Part 30: Efficiency classes of single-speed, three-phase, cage-induction motors (IE-code).

The standard's scope is intended to cover the vast majority of industrial motors and therefore includes motors, geared motors and brake motors rated up to 1000V, with two, four or six poles and capable of operating direct online, for continuous duty or operation on an 80 per cent duty cycle or higher. Specifically excluded from the scope are motors manufactured solely for converter operation (these are covered by Technical Specification IEC TS 60034-25:2007 Rotating electrical machines - Part 25: Guidance for the design and performance of ac motors specifically designed for converter supply) and motors that are integrated within machinery such that they cannot be tested in isolation - such as in some pumps, fans and compressors.

IEC 60034-30:2008 defines three IE (International Efficiency) classes for single-speed, three-phase, cage induction motors as follows:

IE1 = Standard efficiency (with efficiency levels similar to the existing European CEMEP Eff2 class).

IE2 = High efficiency (with efficiency levels similar to the existing CEMEP Eff1 class and identical to USA Epact levels for 60Hz motors).

IE3 = Premium efficiency (a new efficiency class in Europe and identical to the USA NEMA Premium class for 60Hz motors).

In addition, the standard provides for an IE4 class to be defined in the future.

The efficiency levels defined in IEC 60034-30 are based on the test methods whcih arte specified in IEC 60034-2-1:2007, which give different results from those obtained using the test methods in the previous standard IEC 60034-2:1996. To aid transparency in the market, IEC 60034-30 states that both the efficiency class and efficiency value must be shown on the motor rating plate and in the product documentation.

European EuP Directive

In Europe the Directive 2005/32/EC on the Eco-Design Requirements for Energy-Using Products (known as the EuP Directive) will, for the first time, mandate energy-efficiency classes for industrial electric motors (subject to approval by the European Parliament and adoption by the European Commission). In contrast, the CEMEP Eff efficiency classifications scheme has only been voluntary.

Under the EuP Directive, the IE system will be implemented in phases, gradually replacing the Eff scheme. From June 2011 the intention is to require all motors placed on the market that fall within the scope of IEC 60034-30 to be at least IE2. From January 2015, motors rated from 7.5 to 375 kW will have to be IE3 or be IE2 and installed with a variable-speed drive. The same requirement will be extended to motors in the range 0.75 to 7.5 kW from 2017.

Depending on the duty cycle, energy-efficient motors can easily deliver a lower total cost of ownership than less efficient alternatives despite the purchase price being as much as several times higher, so this development in Europe sounds like good news for end users as well as the environment. Nevertheless, there are some commentators that have accused the European Commission of being slow to act in comparison to other legislatures, and there are others that feel the measures do not go far enough.

Meanwhile, motor manufacturers are already starting to offer motors with the IE class marked on the rating plate. WEG, for example, has recently introduced the W22 range that it says exceeds the new efficiency standards. In addition, the motors are quieter, smoother and more reliable - though these characteristics can generally be expected on high-efficiency motors that have necessarily been designed with tighter manufacturing tolerances. One of the more notable features on the W22 motors is the 'aerodynamic' frame that increases airflow and reduces the motor's operating temperature.

SEW Eurodrive is another manufacturer that is well prepared for the new efficiency standard, having launched the all-new, modular DR series of AC induction motors in early 2008 (Fig.1).

To illustrate the modularity, up to three different sized brakes can be mounted on one motor size, and there is a choice of three types of encoder that can be mounted behind the fan, rather than in the conventional position on the B end, which results in a shorter overall length for the motors.

The Standard Efficiency (IE1) four-pole ac motors are designated the DRS models and are available in sizes 71 and larger, with power ratings from 0.37 to 200 kW. High Efficiency (IE2) four-pole ac motors are designated the DRE models and are available in sizes 80 and larger, with power ratings from 0.75 to 200 kW. Premium Efficiency (IE3) four-pole ac motors are designated the DRP models and are available in sizes 90 and larger, with power ratings from 0.75 to 160 kW. All three motor types feature die-cast aluminium or copper rotor cages. Compared with an IE1 standard motor, the IE2 energy-efficient motor of the same power is one stage larger; the IE3 motor with the same power is two stages larger.

Variable-speed drives

Inverter drives are often promoted as a good way to improve the overall efficiency of a system, as a motor driven at a lower speed will consume less energy than a motor driven at full speed with a physical restriction (such as a throttle or baffle) being used to control the work done.

However, drives also exhibit losses - typically in the range three to five per cent, depending on the drive design, size and loading - and there is currently a lack of test standards and efficiency classifications for drives. This is more than just an inconvenience, as a small difference in energy efficiency between two otherwise similar drives can result in substantial differences in the lifetime running costs of the associated motor. And specifiers need to be aware that an increase in inverter efficiency often comes at the expense of motor efficiency and, consequently, overall system efficiency.

Furthermore, a motor that has a low duty cycle or is only used occasionally may not even benefit from an inverter drive in the same way as one operating continuously or on a high duty cycle.

For equipment designers or maintenance engineers considering upgrading equipment to take advantage of a more energy-efficient motor or a variable-speed drive, the procedure may not be as simple as might be thought. For a start, a higher-efficiency motor is likely to have different physical dimensions, typically being longer and sometimes also larger in diameter. Electrically, a high-efficiency motor is likely to draw a lower full load current, yet the inrush current at start-up could be higher. These two factors mean that the motor protection measures may need to be upgraded at the same time as the motor itself.

Elsewhere in the system, the higher operating speed of the high-efficiency motor may need further adjustments to be made. While speed control is straightforward using a variable-speed drive, constant-speed applications will need to be considered carefully, as a small increase in the speed of a conveyor, pump or fan, for example, could have serious consequences.

In some applications there may be times when no operational output is required. Typically a variable-speed drive might, for example, keep a pump running at minimal speed and producing no flow, or a conveyor may continue to run without any load. Mitsubishi has therefore created an intelligent threshold control feature for its D700 inverters that enables the motor to be stopped, then returned to speed using a fast algorithm that avoids any demand dwell (Fig.2).

For drive systems in which loads must be decelerated, some drives are capable of being used with regenerative braking, which means much of the energy is recovered instead of being lost to the environment as heat from a braking resistor or friction brake.

Beyond the motor and drive, there are various ways in which 'intelligence' can be used to deliver large savings in energy. For example, a sensor can be used to detect when a conveyor is empty, thereby enabling the conveyor to be switched off. Many of the current generation of drives have built-in processing capability, which means they can be connected directly to inputs and outputs without any need for a separate PLC (programmable logic controller). In dynamic applications, fine-tuning of the speed profile can yield benefits, as employing the maximum acceleration available may be unnecessary and wasteful of energy.

Dynamic applications, in particular, can benefit from exchanging ac induction motors for permanent-magnet motors that are inherently more efficient; the amount of energy needed for a typical positioning application can be as much as 30 per cent lower.

In addition, a synchronous servo motor is likely to draw a lower current, so it might be feasible to reduce the size of the inverter as well.

Siemens has recently taken the interesting step of launching a single family of motors, known as the 1PH8 series, that encompasses both induction and synchronous versions (Fig.3). Customers can choose from forced ventilation or water cooling, and power outputs ranging from 2.8 to 1340kW.

The publication of an international standard for motor efficiencies should help to make the selection of energy-efficient motors simpler, but designers and specifiers need to do far more than pick a motor with an IE1, IE2 or IE3 rating.

First, it has to be remembered that IEC 60034-30 only covers limited types and styles of motor, and it may be that switching to a motor of a different type will bring greater benefits. And motors that are seldom used might be best left as they are. It could also be that installing an inverter drive with an existing motor could bring greater benefits than swapping the motor for a more efficient model of the same rating.

As we have seen above, it is seldom a simple matter to upgrade to a more efficient motor, as there may be mechanical and electrical changes that need to be made to accommodate the new motor.

Nevertheless, the potential to reduce the running costs over the lifetime of a motor can, in many cases, easily outweigh the additional investment required to purchase a higher-efficiency motor.

With the end user being the beneficiary rather than the equipment manufacturer, it is therefore the end user that should be considering specifying high-efficiency motors - and manufactures should emphasise the potential lifetime savings when supplying equipment that is fitted with energy-efficient motors.

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