Not surprisingly, considerable effort has been applied to developing motors with higher efficiencies, and governments are using financial incentives to encourage the adoption of such motors in an attempt to reduce national carbon dioxide emissions.

Improvement

While a small per centage improvement in motor efficiency can make a significant overall difference to energy consumption, there are other factors that should also be considered if the motor-driven plant or equipment is to perform efficiently as a complete system.

For applications where the motor load is not constant – such as pumps and fans – the use of variable-speed drives (frequency inverters) can
be far more energy-efficient than running the motor at full speed and using a throttling device or damper to reduce flow.

Indeed, reducing the motor speed by 20percent can cut energy consumption by half.

Other areas where energy can be wasted range from inefficient gearboxes and worn belt drives, to misaligned shafts and accumulated dirt that raises the motor’s operating temperature. However, this present article focuses on the main opportunities for saving energy, namely motors and drives.

For three-phase, two- and four-pole,
squirrel-cage, totally enclosed, fan-cooled (TEFC) motors in the range 1.1 to 90kW, European energy efficiency ratings identify motors as
EFF1, EFF2 or EFF3 standard (EFF1 being the most efficient). This CEMEP (European Committee of Manufacturers of Electrical Machines and Power Electronics) scheme works well, but it has its limitations.

Firstly, recent developments have enabled motors with even greater efficiency than EFF1 to be produced; WEG, for example, offers motors with a premium efficiency rating designated EFF1+.

Specifically, WEG’s premium-efficiency W21 motors have higher
quality and thinner steel stator laminations, more copper in the windings, better quality insulation, reduced fan losses and closer machining tolerances.

They therefore save energy by reducing the motor losses, including stator and rotor resistance, friction (in the bearings and brushes), windage (in fans or auxiliary machines) and load losses.

Baldor similarly claims that its SSE motors exceed EFF1 levels, though the most notable feature of this range is that these are all-stainless-steel, washdown-duty ac motors.

Excellent for use in food and dairy processing, as well as pharmaceutical and brewing applications, the motors can cope with caustic solutions and high-pressure jets. Premium electrical materials are used throughout the SSE range to ensure very high operating efficiencies. In addition, the stator windings employ special inverter-spike-resistant (ISR) magnet wire to maximise reliability and thermal performance, which is especially advantageous when the motors are used with variable frequency inverter drives.

Beyond CEMEP

Although the scope of the CEMEP scheme is restricted to the motors identified above, this problem has been mitigated to some extent in the UK by the adoption as a benchmark of the Water Industry Mechanical and Electrical Specification (WIMES3.03). This lays down minimum full-load efficiencies for two- and four-pole motors in the range 110-400kW and six- and eight-pole motors in the range 5.5-315kW, as well as minimum requirements for power factors and three-quarter load efficiency values.

For motors outside the scope of the CEMEP and WIMES specifications, however, specifiers have little option but to work from the motor manufacturers’ published efficiency data.

For example, at the 2007 Hannover Fair in Germany, Siemens Automation and Drives (A&D) exhibited a new series of high-performance torque motors for applications requiring high torques or low speeds.

These HT-direct series motors are based on permanent-magnet technology and are characterised by compact dimensions, ease of installation and maintenance, reduced noise, and up to three per cent higher efficiency compared with geared motors.

High torques

The use of direct drives can be advantageous where high torques and low speeds are involved. Typical examples are presses and rollers in paper machines, cutters, edgers, winders and small rolling installations in the steel industry, as well as pumps and fans, plastic extruders, sugar centrifuges and gear testing benches.

Direct drives for high torques can be readily implemented with permanent-magnet synchronous motors. In contrast to induction machines – whose reactive power requirement grows as the number of poles increases – a high-pole design for permanent-magnet synchronous machines can be created relatively simply.

The high-pole permanent-magnet synchronous machine is characterised by short winding heads and thin stator yokes, resulting in a compact, space-saving construction.

Siemens says that the HT-direct motors, which
are designed for use with frequency converters, can handle speeds of up to 800rpm and deliver torques of up to 42kNm, corresponding to a power output of 2100kW.

Speed control

Gaining a few percentage points of additional efficiency by installing a better motor will certainly bring benefits, but often it is possible to gain even more by installing a variable speed drive (VSD).

Pumps and fans are the classic examples of applications where, traditionally, motors have been operated at full speed, with
flow control achieved by throttling.

However, installing a VSD and controlling the speed of the motor can deliver far greater energy savings than would be achieved by exchanging the motor for a more efficient model.

Inverter drives are often viewed as a 'box of electronics', with little to differentiate between models available from one supplier and the next. In fact various aspects of the basic drive have been the subject of developments in recent years, as illustrated by the MVW-01 range of medium-voltage (MV) drives from WEG, which are claimed to achieve a drive efficiency of 99percent through the use of high-voltage (6.5kV) IGBTs (insulated gate bipolar transistors) that reduce motor harmonic currents to extremely low levels.

Reduced components

WEG says that, unlike many MV drives on the market that have three- or five-layer control, its MVW-01 drives have just two layers. This reduces the number of components in the drive, leading to commensurate improvements in efficiency and reliability.

Engineers sometimes avoid using medium-voltage drives because of high purchase costs and added complexity compared with low-voltage drives.

WEG counters this by
saying that the equivalent low-voltage drives have to operate
at higher currents, resulting
in more expensive cabling – due to EMC shielding being necessary – and higher installation costs.

Nevertheless, for smaller applications where there is no need for a medium-voltage drive, suppliers have focused their attention on improving various drive characteristics.

Rockwell Automation, for example, has introduced the Allen Bradley Powerflex 700L liquid-cooled drive with the aim of reducing space requirements.

A proprietary liquid-cooled heatsink is said to increase power density significantly, resulting in a drive that is up to 65percent smaller than similarly power-rated air-cooled drives.

The simplified coolant system makes it easier for users to integrate the coolant supply with existing systems, such as recycled plant water; alternatively, a
water-to-air heat exchanger loop can be used to dissipate heat from the drive into the air.

Aside from applications where space is a major issue, the energy-efficient liquid-cooled drives are also useful whereair-cooled drives might result in unacceptably high costs for climate control, or in dusty environments where an air-cooled drive is simply unsuitable.

Additional functions

As mentioned above, pumps and fans are one type of application that can benefit from drives, but there are others where modern controls enable standard induction motors to deliver a level of performance that is similar to that available from servo motors.

Hitachi’s SJ700 sensorless vector drive, for example, incorporates a new patented power switching technology and can be used for multi-point positioning applications requiring responsiveness and accuracy.

If even finer control is needed, Hitachi offers an optional feedback card for closed-loop vector control.

Other features of the SJ700 include intelligent energy saving, regenerative braking under emergency stop conditions, integral EMI/RFI (C3) filters across the whole range of 0.75-400kW, a customisable display panel, and active frequency matching for restarting and for catching flying loads.

Communications

Another area where drives manufacturers have been concentrating their research and development resources is communications.

The HitachiSJ700 has a range of communications options, while Baldor recently introduced the Motiflexe100 three-phase ac drives that benefit from Ethernet Powerlink and TCP/IP connectivity.

Compatibility with the Ethernet Powerlink protocol provides a high degree of flexibility, with each drive incorporating an Ethernet hub that enables systems to be built using simple daisy-chain connections.

The high-speed and deterministic Ethernet Powerlink network cuts cabling substantially and can significantly reduce the cost of building large multi-axis systems. For example, a single Baldor Ethernet Powerlink machine controller can manage systems with up to 16 interpolated axes.

Each Motiflex drive can operate independently or as part of a shared dc bus system. When operating in a shared dc bus system, power regenerated back into any drive during deceleration may be utilised by the other axes, thereby saving energy.

As each drive has a local capacitor bank, an external braking resistor is often not required because the total capacitance of the system may be sufficient to store the energy without reaching the over-voltage limit.

Baldor says that, in contrast to traditional shared dc bus systems, Motiflex drive systems do not require a separate power supply unit.

Instead, the ac-dc converter stage in each drive
is capable of supplying power not only to itself, but also to a drive or combination of drives of the same total rating.

For many multi-axis applications, this often means that the highest-rated drive is able to power the rest of the system.

Enhanced functionality

Clearly there is considerably more to state-of-the-art drives than there was five years ago, offering designers of new equipment and those retrofitting older equipment the opportunity to reduce energy consumption and enhance functionality through additional control and communications features.

In addition, motors should be selected with care due to their relatively high energy consumption.

In most cases the best advice is to look at the application as a whole to ensure that the optimum combination of motor, drive and other transmission elements are working together as efficiently as possible.