Electricity + Control November 2016

DRIVES, MOTORS + SWITCHGEAR

Electric Induction Motors: Power Management System

Henry du Preez, Consultant

The author responds to the quote by Dan Jones which suggests that: ‘Competition pushes for innovation ahead of regulations in the electric motor industry’.

manufactured in set standard ratings, for example − 37, 45, 55 and 75 kW. So if the load requires a 63 kW peak, a 75 kW motor has to be installed. On average this means that medium rated motors operate as low as 50% of rating. The regulation IEC 60034-30-1 [1] shows the efficiency minimum requirement to be met to comply with categories IE1 to IE4. As can be seen in Figure 1 the improvement in efficiency for mo- tors above 37 kW is small. Coverage of IEC 60034-30-1 [1] Generally speaking three-phase inductionmotors for the intermediate range 37 kW to 500 kW are relatively efficient when operated at load close to full rated load. If we wish to save power the manufacturers would have to improve efficiency at a load lower than the motor rating − bearing in mind that many motors do not operate at a constant load peak ef- ficiency and the Power Factor (PF) should be maintained throughout the operating range. In terms of numbers of running motors (installed stock), small motors are the most common: 2 billion out of an estimated global total of 2,23 billion are rated at less than 0,75 kW. The relatively few large motors account for a considerable share of overall motor electricity consumption. It is estimated that medium size motors consume almost three-quarters of the global electricity demand of all motors (Wikström, 2009). Studies carried out show the approximate world-wide power consumption admittedly 10 years ago, but I do not believe the trend has changed radically. Top-down analysis provides several preliminary results: • The estimated total global electricity use of all electric motors in 2006 was between 6 900 TWh and 7 200 TWh • Electric motors account for between 44% and 46% of total global

Dan Jones, acting chief engineer for St. Petersburg, Florida-based Revolution Mo- tor Industries (RMI), claims that: “Government regulations meant to boost electric motor efficiency and curb carbon emissions may have started the race for achieving ever-increasing output with lower energy intakes. But lately it’s the industry itself, propelled by competition that has developed engines that can not only meet − but also exceed - the toughest European and U.S. standards”. ****** A re we barking up the wrong tree? Should we not be looking at the systems where motors are used? I believe that this is where the greatest savings are possible – by using a system’s approach to optimising the motor management. Yes, it is important to improve efficiency but in general, electric motors are efficient at rated power and less efficient at low loads. It is therefore important to use motors more efficiently: This can be done by optimising the energy consumption under varying loads. On average motors do not operate at, or consistently at, rate motor loads. There are a number of reasons for this; these include the application of compressors, pumps and mechanical presses and others, where the load could drop considerably − even as low as no load for a fair period of the operation. Medium range motors are

Figure 1: Efficiency curves for 4 pole induction motors as per IEC specification IEC 60034-30-1 [1].

Electricity+Control November ‘16

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