Choosing where to use variable speed drives

London - New European energy efficiency legislation for electric motors and their use with electronic variable speed drives is now in force, in order to enhance the EU climate protection efforts, and to deliver significant savings in electricity consumption by 2011.

Electric motors consumed around 70% of the energy used in industry in 2005, corresponding to 427Mt of CO2 emissions in Europe, according to the EU Commission. For the industrial plant manager, another interesting figure is that 90% of the lifetime cost of a pump, fan or compressor is the energy it consumes, and these three are the major energy consuming equipments in industry. The greatest savings are made when speed control is used to continuously adapt operation to the demand.

The EU Directive 2005/32/EC means that by 2011 all new motors will have to meet the new ‘IE2’ efficiency rating, which is similar to the current ‘Eff1’. By 2015, motors from 7.5kW to 375kW will have to meet an even higher efficiency rating, ‘IE3’, or alternately, meet IE2 and be driven by an electronic Variable Speed Drive (VSD). This limit motor power limit will be lowered to 0.75kW by 2017. “

In most cases, the additional initial capital outlay of buying higher efficiency motors and variable speed drives should be recouped relatively quickly through reduced energy bills” comments John Murphy, VP Strategic Planning for Control Techniques: “We at CT can advise customers on the most cost-efficient route to Directive 2005/32/EC compliance”.

However, even the apparent EU endorsement of the benefits of the technology does not mean that all installations will derive a significant benefit from applying VSD drives: Emotron of Sweden points out that an EU survey showed 50% of installations are not suited to variable speed control, and a further 15% are already controlled efficiently. They estimate the remaining 35% of installations could benefit on average by a 40% saving in energy consumption: for pump applications the typical estimated payback for a VSD is between 12 and 24 months.

Dust blowers

At the Civil and Marine Middlesbrough plant, situated on the coast adjacent to the Corus steelworks, blast furnace slag, otherwise a waste product from iron production, is ground into a fine powder called Ground Granulated Blastfurnace Slag (GGBS). This is used to enhance the properties of concrete, replacing between 20 and 80% of the normal Portland cement. Excessive wear and tear on dampers in particular prompted an examination of alternative means of control of the air flow that extracts the highly abrasive fine powder from the ball mill and Sepol separator.

“As neither fan operated at full speed, we also saw an opportunity for energy-savings”, explains electrical engineer and assistant works manager Rob Thwaites. “We brought in Control Techniques who calculated likely savings and predicted that the drive for the Sepol separator would have a payback of just seven months, whilst the ball mill would pay for itself in nine. The calculations have proved to be very accurate and we have made precisely the savings that Control Techniques predicted”.

The Sepol separator is driven by a 200 kW motor running at 71 to 75% of full speed. With the original damper, its energy consumption was 146 kWh. Now, under inverter control using a CT Unidrive SPM drive, this has been reduced to 61 kWh, giving a saving of 85 kWh. Total savings on the two installations resulted in a £700 per week reduction in the power bill.

“A further benefit we hadn’t anticipated is the reduction in noise,” comments Thwaite, “which makes the plant much more comfortable for operators. We also anticipate that fan motors and bearings will last longer and require less maintenance.”

Further Control Techniques Unidrive SP AC drives have been installed on the pre-grinders, which has resulted in an overall increase in throughput from 50 to 70 tonnes of GGBS per hour, enhancing the energy efficiency of the plant by reducing the energy consumed per ton of output produced.

Control Techniques recommends that any higher power installation operating a pump or fan, where the output is throttled using a valve or damper, should be reviewed closely, to see how the energy consumption could be reduced, using VSD control.

Compressor control

Glaston Compressor Services of Skelmersdale provide comprehensive maintenance and total site management services, specialising in intelligent control & energy management for compressed air systems. A recent installation on a 90kW rotary compressor used a Vacon NXS variable speed drive controller to replace the existing single-speed unit, in conjunction with a compressor management system developed by Glaston. This arrangement ensures that the output of the compressor always matches the instantaneous demand for compressed air. The project was undertaken for Albion Automotive, one of the UK’s prominent automotive tier 1 suppliers, following the results of a free detailed energy survey and audit by Glaston, which forecast £7000pa in electricity savings.

Michael Douglas, managing director at Glaston, confirmed “We’ve been using Vacon VS drives for some time to upgrade fixed-speed compressor systems, and we’ve found them to be robust, reliable and easy to work with”. In addition to the anticipated energy savings resulting from continuously matching the operating speed of the compressor to the instantaneous load, it also proved possible to make further savings by reducing the nominal operating pressure of the installation. With the much more precise and responsive control achieved by the Vacon drive system, the nominal delivery pressure could be reduced from 7.5bar to just 6.05 bar, with total confidence that the actual pressure in the installation will never fall below the required 6 bar minimum. As a result of this pressure reduction, and a number of other minor energy saving adaptations, the total cost savings achieved in practice after the installation increased to £14,000pa.

Shaft power monitoring

International Specialty Products and Chemicals (ISP) in the US serves the pharmaceutical, beverage and personal care industries. Reliability engineer Ken Myers needed to prevent regular pump failures and production interruptions, where three pumps were costing over $90,000pa in maintenance and downtime: “Our goal was to protect our magnetic drive pumps from dry run conditions”.

Having tried a number of monitoring technologies which were inefficient at low load conditions, ISP installed Emotron M20 shaft power monitoring systems. The M20 uses the drive motor as a sensor, feeding a unique shaft power calculation technique that immediately detects pump load changes due to dry running or other abnormal process conditions, across the whole motor load range. This offers highly reliable monitoring with direct correlation to the pump curve.

Myers summarises the results: “After the monitors were installed, the failure costs for the next years dropped to zero. The functionality of the Emotron M20 is built into the new Emotron soft-starters and variable speed drives installed on other ISP applications, such as positive displacement pumps, transfer elevators and product blenders”.