SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS
ELECTRICAL POWER COORDINATING COMMITTEE
ROTATING MACHINES WORKING GROUP
Minutes of Meeting held on 15th February 2006 at Zest Electric Motors
Present
Alan Meyer
Andre Maritz
C.D. Pitis
Chris Tute
Du Toit Grobler
Flip Janse van Vuuren
Gary Daines
George Jeans
Graham Paton (Zest)
Grant Muller
Henk de Swardt
Henry du Preez
I.D. (Dave) Braude
Iain Grayer
James Cowling
Johan van Niekerk (Zest)
Kahesh Dhuness (University of Johannesburg)
Mario Kuisis
Meyer Naude
Nic Grobler
Peter Heim
Richard Anderson (Zest)
Robert Holm (University of Johannesburg)
Thys Botha
Uresh Naidoo
Walter Schwellnus (Rand Water)
Apologies
Alan Mitchell
At Greyling
Derek Wood
Dries Wolmarans
Johan Nell
Mark McNally
Mark Spencer
Marten Scholtz
Mohamed Ahmed
Robert Melaia
Tony Phillips
Willie Cronje
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Opening
Mario Kuisis opened the meeting at 14:15 and welcomed everyone present. As Mark
Spencer was out of the country on business he had tendered his apologies with the
request that Mario Kuisis open the meeting in his absence. He had also requested that
his appreciation be expressed for the excellent support he had received from the
members of the RMWG during his term of office as Chairman.
Minutes of the Previous Meeting
The minutes of the previous meeting were accepted.
Matters Arising
1. Election of Chairman
No further nominations for the position of Chairman of the RMWG had been
received or were put forward at the meeting. Mario Kuisis confirmed
acceptance of his nomination previously proposed and accepted and was
thereby elected into office. He then proceeded to chair the meeting.
2. SANS
Comment on SANS is covered under Section Feedback.
Presentations
Two presentations were delivered and both elicited useful discussion.
1. Line Effects of PWM Variable Speed Drives
This presentation was delivered by Johan van Niekerk of Zest Electric Motors
and Drives. The presentation covered harmonics and the effect of long cables.
Johan reviewed the origin of the harmonics produced by VSD’s and the various
options commonly available and used for dealing with them. The cost vs
performance trade-off of these options was examined and Zest’s approach to
the problem was discussed.
The problems caused by long cables which can result in nuisance tripping on
earth protection systems was highlighted. This was explained to be due to
leakage currents which become excessive when high capacitance of the cable,
due to greater length, is combined with high frequency components from the
drive. This was normally countered with compensation techniques in the drive
itself and reactors on the drive output.
The problems associated with switching transients possessing high rates of rise
and high peak values due to reflections was then explained. The way in which
increasing cable length amplifies the peak magnitudes was also discussed.
Several examples of actual measured values were used to demonstrate the
extent of the stresses imposed and how they varied according to cable length.
The WEG solution to these problems was presented with illustrations of how
the transients are effectively mitigated with respect to rise time, peak value and
repetition by incorporating chokes at the drive output. Johan also presented
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figures illustrating how resistance of the WEG motor winding to these
transients is increased through use of an insulation system with higher levels of
withstand to both voltage level and rise time.
In discussion it was noted that alternative solutions, such as capacitors
connected at the motor terminals, can also deal effectively with many of these
transient voltage problems caused by rapid rise switching surges and reflections
at the end of long cables.
2. Broken Rotor Bars and Shaft Currents
Thys Botha delivered a presentation in which he showed numerous examples of
rotors with various forms of defective squirrel cage windings. These were in
the form of photographic case studies from his experiences at LHM. He also
discussed the puzzling symptoms exhibited by some of the faults and the nature
of the investigative work required to identify the causes.
The presentation included much useful discussion on why many of the failures
occurred and suggestions for repair methods and design improvement
strategies. Tightness in the depth of the slot was agreed to be probably the
single most important factor.
Thys said that in his experience vibration analysis is a reliable tool for
identifying the presence of broken rotor bars, but that the standard approach to
vibration analysis is insufficient since the frequency range is above that
normally looked at. It is also necessary to have load on the machine and more
than just one broken bar is required before there is sufficient effect on vibration
for it to be detected.
The problems associated with shaft currents were also highlighted and the
requirements for effective elimination of the problem discussed. His
experience showed that in some instances voltage values much lower than
generally accepted limits had resulted in severe damage to bearings. Examples
in the form of photographic evidence were shown in the presentation.
Some of the difficulties in problem identification were illustrated, especially
with regard to misleading information and the typical situations faced in real
life with a multiplicity of defects usually acting together. Some of the
approaches to vibration analysis that have yielded good correlation with the
actual defects subsequently found were also discussed.
Copies of the presentations were requested and the authors agreed to make them
available. It was noted that large file sizes create a problem for emailing. Henk de
Swardt suggested that the RMWG set up a web facility where members can access
such material. The chairman undertook to investigate this option.
Research and Development
Alan Meyer reported that the University of the Witwatersrand is conducting research
into linear wave generators. No representatives from other universities were present to
add to this.
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Section Feedback
1. Machine Insulation Assessment
Nothing to report
2. IEC / Cigré
Nothing to Report
3. SANS (PREVIOUSLY SABS)
The chairman reported having made contact with Marten Scholz to enquire
about having the Rotating Machines Working Group represented on the SANS
committee dealing with rotating machines (StanSA TC 61: ROTATING
MACHINERY). Marten Scholtz reported that this would indeed happen as part
of SANS’ plans to get associations rather than individual companies making up
the committees. However no time scale was provided.
Alan Meyer and others who had attended the latest (and quite recent) meeting
of TC 61 commented that this was at odds with discussions at the meeting
when SANS indicated a reversal of the plan to do away with individual
membership. There may also be complications since the SAIEE is already
represented on TC 61. Since there was no SANS representative present to
clarify this issue, it was decided that it would be investigated and cleared up for
the next RMWG meeting.
4. Motor Specification Guide
At Greyling had submitted a request via the chairman for a report on progress
with the Medium Voltage Motor Specification guide.
Chris Tute reported that, based on his recollection from the last SANS meeting,
the document had been circulated as a committee draft for comment. No
responses had been received and SANS were going ahead with circulating the
document to industry for comment. However, he was not altogether certain and
undertook to investigate and report back on the status. (Chris has since
confirmed this situation).
5. RMWG Membership
The Chairman suggested that Lindo Hauptfleisch (of Anglo Platinum) could
make a useful contribution to the RMWG through his knowledge and wealth of
experience as an end user of electrical machines of all kinds. It was agreed that
he be invited to join the group
6. Electrical Machines Forum
James Cowling reported that the EMF (Electrical Machines Forum) had held a
special meeting to decide on the future of the association. The only options
tabled were to wind up the EMF or to amalgamate with the Technical
Committee of the EEAIA (Electrical Engineering and Allied Industries
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Association). In discussions it became clear that there was much to be gained
in the amalgamation so the decision to go ahead was taken and confirmed.
The EMF will therefore re-establish it’s program of two monthly meetings, but
now combined with the EEAIA Technical Committee meetings. The first
combined meeting had been held on 7th February at the Johannesburg Country
Club (the future venue for these meetings). This well attended meeting began
with a presentation delivered by Mr. Justus Mavhungu – Energy Expert from
the Department of Public Enterprises.
General
EMF and RMWG
At Greyling had submitted a suggestion via the Chairman that there may be merit in
combining the activities of the EMF and RMWG. After some discussion it was agreed
that this no longer made sense since the EMF was now attached to the EEAIA and also
the RMWG was purely focused on technical matters.
Next Meeting
The next meeting will be held at Megawatt Park on 13th July 2006 at 12:00. The
earlier starting time was chosen at the request of several country members who have to
travel long distances after the meeting. Details of the venue will be circulated at a later
date. The date of the last meeting for the year was also set: Thursday 16th November.
The venue still to be found.
Presentations:
Jack Mc Murdo will be invited to deliver a presentation on his field experience
with testing for broken rotor bars.
Closure
The Chairman thanked the members for their contributions and Zest Electric Motors
for hosting the meeting, for providing refreshments and arranging the tour of Zest’s
facilities which followed the meeting. He then declared the meeting closed.