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First trial of IE4 synchronous reluctance drive delivers 6% energy saving

10 December, 2012

The world’s first commercial IE4 synchronous reluctance motor-and-drive package has been installed on a pump operated by a UK water utility, and is achieving a 6% energy saving compared to the previous induction motor and variable-speed drive package. ABB, which developed the new technology, says that when replacing less efficient drives, it can deliver savings of up to 15%, with paybacks measured in months in some installations.

In the trial installation, at South Staffs Water’s Somerford pumping station, the new system, called SynRM, has replaced a 20-year-old, 115kW IE2 induction motor used to control a vertical-shaft-driven borehole pump which abstracts 2.5 million litres of water every day. The old motor was controlled by an ABB ACS800 drive which was saving energy using its flux optimisation function.

The utility wanted to assess whether the synchronous reluctance technology would deliver ABB’s claimed benefits, which include high efficiency and reliability, coupled with low heat losses, noise levels and maintenance costs.

The existing drive was replaced by an ABB ACS850 drive which incorporates a more powerful processor needed to control the synchronous reluctance motor. The drive also provides the latest version of ABB’s DTC (direct torque control) system, avoiding the need for encoders and speed feedback devices.

The biggest potential attraction for South Staffs Water was the new system’s predicted energy saving. As the utility’s supply director, Keith Marshall, explains: “While we are one of the most efficient companies in the industry, our electricity bill is more than £9m per year – and rising, through increases in wholesale energy prices. Pumping water accounts for some 90% of this spend as we have one of the highest pumping heads of any UK water utility because of the deep boreholes and hilly terrain in our area.”

After a few months’ use of the new installation, South Staffs Water calculates that the energy being consumed by the pump had dropped by about 6%

“A 6% reduction on one pump in a system that was already very efficient is massive news for us,” says Marshall. “Including the UK government’s Enhanced Capital Allowances (ECAs), we estimate the return on investment to be between five to six years. Given that the existing drive and motor were already fairly efficient and therefore squeezing more savings was always going to be difficult, then this is a very acceptable return.”

The SynRM rotors, which ABB unveiled at the 2011 Hannover Fair, have no conducting short-circuit cages (as found in conventional induction motors), no permanent magnets, and no field excitation windings. Instead, they operate on the magnetic reluctance principle. A computer-optimised laminated rotor structure eliminates rotor cage losses, thus increasing efficiency and reducing the motor size. Achieving standard power and torque levels at a low temperature-rise class improves the lifetime of the motor bearings and insulation, or can be used to extend the greasing intervals.

At South Staffs Water, the new motor has been operating at a much lower frame temperature than the previous induction motor. “In the summer, lots of sites run hot and using SynRM across more sites means we can reduce the need for forced ventilation dramatically,” Marshall comments. “As the rotor effectively has no losses, this lowers the bearing temperature. This means we can either choose to extend the period between greasing, or increase the bearing life expectancy.”

The utility has also found that noise levels have dropped from 78dBA for the old motors to 72.3dBA for the new machine, running at 1,450 rpm. “While Somerford is remote, the use of SynRM at booster stations close to residential areas is fantastic news for our neighbours and employees,” Marshall remarks.

“What is significant is that we chose to trial the SynRM on an application that was already using leading-edge drive technology – although it was controlling a 20-year-old high-efficiency induction motor,” explains Glen Hickman, technical director at Coventry-based Sentridge Control, which designed and commissioned the installation.

“We could have reduced the size of the motor, but for the purpose of a like-for-like comparison, we opted to retain the same frame size,” he adds. “The additional 6% energy saving is incredible given that the application was already deemed to be extremely energy-efficient. If we choose an older motor-and-drive combination elsewhere on the plant, we believe the energy savings could easily reach 10-15%. We are now looking at applications that are using really old drive technology or where a motor-drive combination is deemed suitable.”

As a result of the trial, South Staffs Water has brought forward its investment programme to capitalise on the benefits of the synchronous reluctance technology. “This is such a sizable leap-forward in technology over traditional induction motors that we are now considering other applications across our sites,” says Marshall. “Suddenly other applications that might have had low priority become easy targets within our investment programme.

The SynRM motor is designed specifically to work with variable-torque loads controlled by variable-speed drives. “Traditional induction motors are designed for fixed-speed operation, which accounts for some 90% of the market,” explains Steve Ruddell, global marketing manager for ABB Motors and Generators. “For the first time ever, we have a motor technology that has been developed, from the outset, to work specifically with the variable-speed demands of pumps and fans.

“In the future, I believe that every variable-torque pump and fan application that needs a variable-speed should use the SynRM package,” he adds. “Most pump and fan applications are over-dimensioned by some 20-40%. At low loads, the efficiency gap between synchronous reluctance and induction motor technologies is at its greatest. There is simply no other combination out there that can achieve this.”

ABB claims that the new package delivers IE4 performance, even though there is no IE4 standard yet, and the latest draft standard for IE classes (IEC 60034-34-1 1.0) covers motors which are “rated for line operation”. The SynRM is not rated for line operation, so cannot have an IE mark on its rating plate. But Ari Tammi, ABB’s product manager for special VSD motors, argues that “if you take a line-operated IE4 motor and use it together with a drive, you get certain package efficiency for the whole motor-drive package. When you take an ABB IE4 SynRM package the package efficiency is at the same level.”

The SynRM motor is similar in size to an IE2 induction motor, unlike IE4 induction motors which are usually bigger than IE2 induction machines. No modifications are needed to convert an installation from an IE2 motor to an IE4 SynRM machine. ABB claims that the new motors have better part-load efficiencies than induction motors and that their bearings will run cooler, leading to improved reliability.

Although there is a premium of 10-15% to pay for the SynRM motor-and-drive package compared to conventional drive packages, ABB is forecasting a payback time of 1-1.5 years for new installations, compared to IE2 packages in applications with high running hours, and 2-3 years for those with lower running hours.

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