This page contains a Flash digital edition of a book.
Motors and Drives


Fig. 1. With so many different motor-driven applications, ABB has an AC drive to match virtually every need.


Warning: harmonics can ruin your power supply


Sanna-Kaisa Ehanto looks at the problems caused by harmonic distortion. H


armonic distortion is a form of pollution in the electricity supply that can make equipment behave erratically. The water industry, with its large installed base of AC drives and generators is particularly affected. Harmonics


can be difficult to identify and users often attribute the symptoms to other causes, frequently applying mitigating action that fails to solve the problem. Harmonics can be effectively dealt with if properly identified and there is no substitute for a skilled on-site survey. Harmonic distortion is


a type of pollution to the electricity supply that can be hard to detect. Water companies, in particular, operate many loads that are both susceptible to harmonics and which generate harmonics. A harmonic frequency


Fig. 2. Sanna-Kaisa Ehanto, Market Manager/Water and wastewater, ABB Oy, Low voltage AC drives, Finland


56 www.engineerlive.com


is a multiple of the network frequency, for instance a 250 Hz waveform on a 50 Hz network is the 5th harmonic. A harmonic waveform represents energy that cannot be used


by the connected equipment and this can make the equipment behave erratically - motors can overheat or become noisy, cable insulation can break down and meters can give false readings, for instance. Harmonics are produced by non-linear loads, such as


computer power supplies, fluorescent lights and welding supplies, as well as both AC and DC drives. Water companies tend to have large numbers of AC drives across many sites. Many of these sites are in remote locations with weak supplies and these are particularly prone to harmonics. Also the back-up generators used at many sites can be an additional source of, or be susceptible to, harmonics. In addition, many booster stations in built-up areas operate large pump installations that have the potential to distort the network.


Identifying the cause


Harmonics caused by AC drives can be managed and suppressed to a level where they no longer cause a problem, or can be eliminated altogether. However, problems caused by harmonics are frequently


not recognized as such. As the user is not aware that the problems are caused by harmonics, inappropriate action is often taken, for instance additional cooling and higher rated transformers, cables and capacitors. Although such action may limit the problem it does not address the fundamental cause.


Reducing harmonics Most low voltage AC drives use diode rectifiers which create high levels of harmonics. These have to be mitigated after


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68