WIP - July - Global Media Publishing Ltd. - UK.COM

ALTERNATORSAlternators - what to look forEssentially, you want the greatest performance from an alternator for the least cost andwithout creating difficulties, explains Steve Wells of Mecc Alte UK Ltd.Selecting an alternator for a givenapplication is something often overlooked,writes Steve Wells.Even some very well-known hirecompanies will generally oversize agenerator to ensure that there are noproblems encountered at site. Probablythis is quite right as there are major costsinvolved in shipping generators around thecountry.For fixed generator applications theload, present and future is usually knownat the start, and such loads are becomingincreasingly more complex. It is still quitelikely that a specification for a generatingset will have 80% written around theengine and systems, but little left for thealternator. Alternators are important withregards to the load, to economy and toreliability and should command morethought.Different alternator manufacturers havediffering priorities, whether group-led orsimple design philosophies. Generallyspeaking, you want the most performancefrom the alternator for the least cost andwithout creating difficulties.The higher the efficiency, the less fuel isburned for a given power output. Here,there are differences betweenmanufacturers that are quite marked - upto 5% difference in efficiencies in somecomparisons.Although not directly proportional tofuel saving, it is in fact a fuel-saving usingthe higher efficiency machines. This is anod to the 'greens', and also for thoseseeking optimum power from the gensetto give them an edge in a tender, or touse the slightly lower power engineoption.Again, for optimum sizing and winning oftenders, the transient ability of thealternator could result in a smallermachine. Some projects will need to sizethe gen-set based upon the transient andnot the continuous load. In these casesLV alternator from Mecc Alte UK Ltd.the steady state condition means it is avery lightly loaded genset with all theconsequential problems encountered withengines and then ultimately its reliability.The alternator does not give a jot aboutlight loads, although if it has an goodoverload ability - 'current forcing' thenyou can have a smaller machine to thesame job. Systems exist now with theMecc Alte 'MAUX', the CGT's 'PMG/EBS' andthe Leroy Somer's 'AREP/PMG', but not allof theses are standard fixtures so bewareof the cost penalties.Some loads will create currentharmonics and affect the waveformproduced from the alternator and alsoadd to the heating effects in thealternator windings. Some loads arevariable peed motor drives, UPS systemsand furnaces. Here it is important tounderstand the level of harmonicsproduced by the load and the amount ofvoltage distortion the load will allow.Factors to consider when sizing themachine will include:- The AVR should have a dedicatedpower supply which is either separate tothe main power windings or derived froma differing vector. These will include theMecc Alte 'MAUX', the CGT's 'PMG/EBS' andthe Leroy Somer's 'AREP/PMG'.- Ideally, the regulator shouldsense on all three phases to get an overallpicture of what is happening to themachine - because it will still be sensingthe distorted output waveform from thestator.- Oversize for either heatingand/or to reduce output voltagedistortions.- The oversizing has the effect ofreducing the sub-transient reactance (x"d)of the machine. The lower the value thebetter the machine will cope with thedistorted loads. Some manufacturers havemachines with a low x"d across most ofthe range which means less oversizing -smaller machine, more compact, less cost- win tenders.Not all manufacturers have the samephilosophy - to provide all of this in acompact, reliable and proven package ata competitive price. So look around atwhat is on offer from the most popularmanufacturers. Do not follow like sheepto the 'perceived' best - because they maynot be the best at all.WIPInternet linkwww.meccalte.co.uk14JULY 2009 • WORLDWIDE I NDEPENDENT P OWER