Jump to content

Talk:Variable-frequency drive/GA1

Page contents not supported in other languages.
From Wikipedia, the free encyclopedia

GA Review

[edit]
GA toolbox
Reviewing

Article (edit | visual edit | history) · Article talk (edit | history) · Watch

Reviewer: Pyrotec (talk · contribs) 14:03, 17 May 2012 (UTC)[reply]

I will review. Pyrotec (talk) 14:03, 17 May 2012 (UTC)[reply]

Initial comments

[edit]

I've had a quick read of the article and now I'm going to work my way through the article section by section, but leaving the WP:Lead until the end of the article.

On the basis of this quick reading, the article appears to have sufficient material to be considered for GA-status; but further improvements will be needed to obtain GA status this time round. For example: the current Lead is non-compliant with the requirements of WP:Lead since it is only provides an introduction for the article and makes no effort to summarise the main points. (I shall be covering this later in detail). There are also whole paragraphs, such as the first paragraph of Energy savings subsection of Benefits, that are unreferenced. The grammar/prose in some sentences is poor.

The article appears to have sufficient material to be considered for GA-status'
ExcellentCblambert (talk) 04:33, 20 May 2012 (UTC)[reply]

The requirements for GA can be found in WP:WIAGA, which is what I will be assessing the article against.

  • System description and operation -
  • There are two inconsistent types of referencing: ref 4 is "Jaeschke, pp. 210-211" and there is (an earlier) ref 2 a&b "Jaeschke, Ralph L. (1978). Controlling Power Transmission Systems. Cleveland, OH: Penton/IPC. pp. 210–215." I would argue that ref 4 could be combined with ref 2.
  • Ref 5 is "NEMA Standards Publication (2007). Application Guide for AC Adjustable Speed Drive Systems. Rosslyn, VA USA: National Electrical Manufacturers Association (now The Association of Electrical Equipment and Medical Imaging Manufacturers). p. 4. Retrieved Mar. 27, 2008" and this is followed by ref 6, i.e. "NEMA, p. 13".
  • Harvard-type citations should be made consistent. Using Jaeschke as an exmaple: "Jaeschke, Ralph L. (1978). Controlling Power Transmission Systems. Cleveland, OH: Penton/IPC." is quoted as a reference. Jaeschke, pp. 210-220 and Jaescke, pp. 210-210 are used a citations (possibly a poor example, as arguably the same reference could be used for all three occurrences).Pyrotec (talk) 18:13, 20 May 2012 (UTC)[reply]
My understanding is that first instance of reference is provided with full details with subsequent citations being in form 'last name (year/date), p. xxx).Cblambert (talk) 19:20, 20 May 2012 (UTC)[reply]
There is this Wikipedia:Citing sources, and if you wish to see a good example of a GA using that style try Leoš Janáček. Arguable this is Scientific article (well, engineering) so there is also Wikipedia:Scientific citation guidelines. Pyrotec (talk) 20:09, 20 May 2012 (UTC)[reply]
I have cleaned up citations to restore them to my current citation forma understanding in light of recent re-org of sections, which is my preference for now. Can go to Harvard format if need be. I have hesitated to make changes to old citations' pages because those old sources are in most cases not easy to access.Cblambert (talk) 01:47, 21 May 2012 (UTC)[reply]
    • Motor -
  • A good, clear and well referenced paragraph. My only concern is that citation for the claims is NEMA Standard MG-1, which appears to be domestic USA standard. In Europe, where I am, we would be using EN, ISO or IEC standards. comment struck out. Pyrotec (talk) 20:09, 20 May 2012 (UTC)[reply]
This is why it says '. . . accordance to such requirements as Part 31 of NEMA Standard . . .' but I can see about European standard equivalent.Cblambert (talk) 17:46, 20 May 2012 (UTC)[reply]
Internet search for NEMA MG-1 Part 31 equivalent states in Eaton Application Note 'The Reflective Wave Phenomena' that ". . . 380V AC motors are typically European manufactured and have 1000V AC insulation. Therefore, the theoretical worst case voltage doubling effect is still within the capability of the insulation. Ergo, likely no equivalent European standard exists for NEMA MG-1 Part 31.Cblambert (talk) 18:48, 20 May 2012 (UTC)[reply]
Is the general statement "Elevated voltage stresses imposed on induction motors that are supplied by VFDs require that such motors be designed for definite-purpose inverter-fed duty " correct, i.e. can I use my existing (European) AC motor and just add a VFD drive, if I need a new motor and a drive I'll strike out this comment. Pyrotec (talk) 19:04, 20 May 2012 (UTC)[reply]
Not sure what you mean by 'strike out' but, yes, European general purpose low voltage motor can be used when adding a VFD drive.Cblambert (talk) 19:31, 20 May 2012 (UTC)[reply]
    • Controller -

... to be continued. Pyrotec (talk) 17:27, 20 May 2012 (UTC)[reply]

  • checkY Pyrotec (talk) 08:14, 22 May 2012 (UTC) - Citation 10, "Eisenbrown, Robert E. (May 18, 2008)" at the moment is coming up as a dead link - claimed to "Dead since 2012-04-28". Possibly, just removing the web link would (still) leave a valid citation; but a valid web link would be better.[reply]
Link removed. Cannot find link in Internet.Cblambert (talk) 22:37, 20 May 2012 (UTC)[reply]
  • checkY Pyrotec (talk) 08:14, 22 May 2012 (UTC) - "V/Hz drive control" is starting to appear in the text without any prior definition. I might assume that its shorthand for variable frequency drive(s), the title of the article being variable-frequency drive, which has an existing defined abbreviation of VFD (sometimes VF drive), also AFD and VSD. However, it is used as engineering unit: Volts per HZ.[reply]
In V/Hz control a VFD's voltage magnitude output is proportional to frequency or speed. V/Hz is also referred to as scalar control because there is only one controllable variable, say frequency or Hz with V being derived from actual Hz. V/Hz is suitable for low performance load application, which is most loads, i.e., centrifugal pumps, fans, compressors, etc. Trend however is for vector control or direct torque control due to trend to standarize on DSP-based microprocessors.Cblambert (talk) 22:37, 20 May 2012 (UTC)[reply]
Thanks for the explanation. Its first occurrence seems to be here: "In variable-torque applications suited for V/Hz drive control, AC motor characteristics require that ....". A minor change such as "In variable-torque applications suited for scalar control drives (V/Hz drive control), AC motor characteristics require that ..." would satisfy me. You don't have to do it this way, its merely one suggested approach. Pyrotec (talk) 10:40, 21 May 2012 (UTC)[reply]
  • checkY Pyrotec (talk) 08:32, 22 May 2012 (UTC) -I'm having trouble understanding: "In variable-torque applications suited for V/Hz drive control, AC motor characteristics require that the magnitude of the inverter voltage output to the motor be adjusted to match the required load torque with inverter frequency being derived from this voltage magnitude on a linear V/Hz relationship.".[reply]
At link as http://www3.sea.siemens.com/step/flash/STEPACDrives/index.htm, (Basics of AC Drives), go to part about 'AC Motors', slide 7 or 10 entitled "Motor Performance Under Load". You will see number of motor and load torque curves:
* One is a standard NEMA B type motor torque curve
* Two are variable torque load torque curves
A motor is designed to operate in the part of the motor torque curve to the right of the breakdown (i.e. highest) point of the curve, as one of the variable torque curve shows. (see in same 'AC Motors' part slides 4 and 5 for detailed treatment of NEMA B )
In order for this to happen, VFD inverter output voltage magnitude (and hence speed) must be adjusted to match the intersection of the two above torque curves.
See also in same 'Basics of AC Drives', part about 'AC Motors and Drives' slide 1 of 5 entitled 'Volts Per Hertz Ratio'.Cblambert (talk) 22:37, 20 May 2012 (UTC)[reply]
Thanks. It's the prose that I'm having problems with. Could it be written as (for instance): "In variable-torque applications suited for V/Hz drive control, the AC motor characteristics require that the magnitude of the inverter's output voltage supplied to the motor be adjusted to match the required load torque, such that there is linear (V/Hz) relationship between the inverter's frequency and output voltage."? (this is British English, I'm not too good at American English)? Pyrotec (talk) 10:40, 21 May 2012 (UTC)[reply]
    • Operator interface -
OK.
    • Drive operation -
  • checkY Pyrotec (talk) 11:46, 23 May 2012 (UTC) - Labelling between the chart and the first paragraphs is not consistent. The paragraph describes I & III as motoring and II & IV as Generation, but in the chart its Driving and braking. Possibly a minor difference in terminology, but it does stand out.[reply]
Labelling changed accordingly.Cblambert (talk) 22:12, 22 May 2012 (UTC)[reply]
  • Benefits -
    • Energy savings -
  •  Not done Pyrotec (talk) 11:27, 25 May 2012 (UTC) - The first paragraph makes a number of claims, but there are no references to support them.[reply]
  • checkY Pyrotec (talk) 15:45, 20 May 2012 (UTC) - Technical terms such as torque are used without explanation, they should be wikilinked on their first occurrence (I wikilinked horsepower).[reply]
  • checkY Pyrotec (talk) 15:45, 20 May 2012 (UTC) - The first paragraph appears to be confused. It starts talking about "variable speed drive"s, then it goes onto variable torque (twice), there are discussions of the relationships of torque, speed and horsepower, then it talks about speed and horsepower, and the relationships are named (the affinity laws). The second paragraph is mostly about variable torque, so perhaps the first was as well? The paragraph needs to be rewritten to improve readability.[reply]
  •  Not done Pyrotec (talk) 11:27, 25 May 2012 (UTC) - References 6 and 8 are 44 and 132 pages long. The citations should therefore give the page number or numbers where "evidence" for the claimed statements can be found.[reply]
I have done editing accordingly. References can easily be provided.Cblambert (talk) 04:33, 20 May 2012 (UTC)[reply]
The reference numbers have changed due as a result of editing, ref 6 now appears to be ref 32 and ref 8 now 34. Pyrotec (talk) 11:27, 25 May 2012 (UTC)[reply]

... to be continued. Pyrotec (talk) 12:49, 19 May 2012 (UTC)[reply]

    • Control performance -
  • checkY Pyrotec (talk) 21:51, 20 May 2012 (UTC) - This whole subsection could do with a clean up. It is not clear what this subsection is about. Unless someone can explain what this stuff is here, I would be strongly tempted to fail the article on WP:WIAGA clause 3(a).[reply]
Most important benefit, for great majority of AC drives, is energy savings but AC drives inherently benefit in other ways including especially first sentence: 'AC drives are used to bring about process and quality improvements in industrial and commercial applications' acceleration, flow, monitoring, pressure, speed, temperature, tension and torque.'
  • The subsection, unfortunately, looks much like a bullet-point list without bullet points. It consists of one single-sentence paragraph (1st paragraph), two two-sentence paragraphs (2nd & 4th) and one three-sentence paragraph (3rd). The first paragraph is badly written, but it seems to be about the benefits of AC drives. The second is unreferenced and appears to have a technical term (Across-the-line single-speed starters) that is not wikilinked or otherwise explained; it also appears to be comparing variable speed drives against AC motors that don't have them. The third is also unreferenced and appears to be comparing VFD's against what I assume is variable voltage drives (is reduced-voltage starting a drive?). The fourth paragraph gives a table comparing DC drives against what I assume is different types of AC drives, only one of which (AC V/Hz Control) appears to be a VFD. Since the Lead makes no mention of DC motors why are they here. Four-quadrant operation and hazardous area operation are technical terms that should be wikilinked or otherwise explained.
1st sentence re-written and sentence about 'reduced-voltage starting' has been deleted because largely redundant.Cblambert (talk) 04:33, 20 May 2012 (UTC)[reply]
Article would benefit from having a 'History' section to put things in perspective.Cblambert (talk) 04:33, 20 May 2012 (UTC)[reply]
'System description and operation' section moved to beginning of the article to ease in understanding of 'Benefits' and 'VFD Types and Ratings' sections.Cblambert (talk) 04:33, 20 May 2012 (UTC)[reply]
In 4th paragraph DC drive is compared to three AC drive/VFD control options.
  • checkY Pyrotec (talk) 21:51, 20 May 2012 (UTC) - Having looked at the table: AC V/HZ has the poorest speed regulation, it can't use a feedback device and it has the worst performance at zero/low speed and low speed constant torque operation. Its sole advantage is multiple motor operation. Am I right?[reply]
With all due respect, you are more or less wrong. In terms of performance:
* Speed regulation for DC drive and AC closed-loop (i.e., feedback) vector control drive is 100 time better than AC open-loop vector control drive
* Low end of speed range is infinity better for DC drive and closed-loop vector control AC drive than for the two open-loop AC drive options; in motion control/robotic application standstill is only game in town.Cblambert (talk) 04:33, 20 May 2012 (UTC)[reply]
* It should really point out that V/Hz control is unsuitable for high-performance applications which can become unstable especially as low speed end of the range.Cblambert (talk) 04:33, 20 May 2012 (UTC)[reply]
In summary, beyond obvious benefit of soft starting, other control performance factor are crucial in many industrial applications.Cblambert (talk) 04:33, 20 May 2012 (UTC)[reply]
I will think further on the section and advise.Cblambert (talk) 04:33, 20 May 2012 (UTC)[reply]
It's my job to asses the nomination against WP:WIAGA. Considering clause 1 (a) "the prose is clear and concise, respects copyright laws, and the spelling and grammar are correct", it not clear what the drives labelled "AC V/Hz Control", "AC Open-Loop Vector", "AC Closed-Loop Flux Vector" are. The introduction to the table states; "The following table compares AC and DC drives according to certain key parameters", so one of the few things that is clear is that there is one DC and three AC drives in the table. I searched for "Flux vector" and "loop vector" and they only occur once in the article, and that is in this table. Its' not even clear that "AC V/Hz Control" is a VFD, I sort of assumed that it was. Let's have an unambiguous declaration in the table was to which AC drives are VFDs and which are not: as I said above "AC V/Hz Control" looked like it was a VFD simply because I recognised Hz as measure of frequency. Pyrotec (talk) 15:45, 20 May 2012 (UTC)[reply]
Drive type DC VFD VFD VFD
Criteria Brush Type DC AC V/Hz Control AC Open-Loop Vector AC Closed-Loop Flux Vector
Typical speed regulation (%) 0.01 1 0.5 0.01
Typical speed range at constant torque 0-100 10-100 3-100 0-100
Min. speed at 100% torque (% of base) Standstill 8% 2% Standstill
Multiple-motor operation recommended No Yes No No
Fault protection (Fused only or inherent to drive) Fused only Inherent Inherent Inherent
Maintenance (Brushes) Low Low Low
Feedback device Tachometer or encoder N/A N/A Encoder
Done. Per article's 1st paragraph, last sentence,
Variable-frequency drives are also known as adjustable-frequency drives (AFD), variable-speed drives (VSD), AC drives, microdrives or inverter drives.,
it should be clear to you that the terms 'VFD' and 'AC drives' are used interchangeably.Cblambert (talk) 18:07, 20 May 2012 (UTC)[reply]


  • VFD types and ratings -
    • Generic topologies -
  • This subsection as written is not really acceptable as it stands. Much of it appears to be technobabble pretending to be explanation.
Technobabble!!?? Pretending!!?? I don't agree.
By all means disagree. The pass fail criteria is WP:WIAGA, so if I fail it, it will be against clause 1(a). I fail very few articles at WP:GAN (less than 10% of what I review), but I do fail articles. Pyrotec (talk) 16:05, 20 May 2012 (UTC)[reply]
Figure 1 from link at http://www05.abb.com/global/scot/scot239.nsf/veritydisplay/fec1a7b62d273351c12571b60056a0fd/$file/voltstress.pdf, "Principles of Operation for Variable Frequency Drives", provides good visualization relating AC drive topology elements with corresponding waveforms and functions. I will see about adding such treatment at beginning of 'System description and operation' section.Cblambert (talk) 19:04, 20 May 2012 (UTC)[reply]
  • checkY Pyrotec (talk) 16:05, 20 May 2012 (UTC) - The concept of Topology is not explained, but to me it seems to be how the AC is converted to DC and back to AC with an inverter (or not in one case). This should be stated explicitly. Note: the two figures for the VSI and CSI drives appear to show this diagrammatically. Very little is explained in English, not even DC is defined as Direct current (see the disambiguation page DC for instance). There is also one occurrence of D.C. in the Six-step inverter drives discussion. Is that the same as DC?[reply]
The concept of Topology has been explained.Cblambert (talk) 04:33, 20 May 2012 (UTC)[reply]
Article would benefit from having a 'History' section to put things in perspective.Cblambert (talk) 04:33, 20 May 2012 (UTC)[reply]
  • checkY Pyrotec (talk) 16:05, 20 May 2012 (UTC) - There are numerous technical terms such as: Diodes,capacitor and SCR-bridge, that are not wikilinked or even named. Darlington Pair is one exception that is wikilinked, but IGBT inverter, quasi-sinusoidal, stiff volatage and stiff quasi-sinusoidal appear without explanation nearby.[reply]
Wikilinks added where possible.Cblambert (talk) 04:33, 20 May 2012 (UTC)[reply]
Need to explain terms 'quasi' noted.
checkY Pyrotec (talk) 10:43, 21 May 2012 (UTC) - "stiff" - does that mean the same as stiff in English, i.e. (1) not easily bent, (2) not relaxed/friendly, (3) strong, (4) full of, and if so which one? Pyrotec (talk) 16:05, 20 May 2012 (UTC)[reply]

...stopping for now. To be continued. Pyrotec (talk) 19:01, 19 May 2012 (UTC)[reply]

"Stiff" relates to electrical distribution system's (AC line power's) ability to not sag; in case of VFD, stiff relates to voltage in DC link capacitor's, or current in DC link inductor's, ability to not decay or sag when acted on by the inverter's motor load. Hence, why the two main inverter topologies are called voltage-source or current-source inverters. Has to do with circuit theory's Thévenin's theorem whereby ideal voltage and current sources behave as if there are 'infinite'(i.e., zero impedance) sources.Cblambert (talk) 17:12, 20 May 2012 (UTC)[reply]
The prefix term 'quasi-sinusoidal' implies 'close to sinusoidal'. A perfect inverter output is pure sine wave, which costs due to VFD rectification function's non-linearities. Older VFDs therefore used six-step, i.e., quasi-sinusoidal, inverter topologies, which in addition to the pure 'fundamental' sine wave component produced an infinite number of higher harmonic components. Some various small low performance VFDs even operate with square wave output, which can also be considered quasi-sinusoidal but is even higher in harmonic content.Cblambert (talk) 16:00, 21 May 2012 (UTC)[reply]
  • checkY Pyrotec (talk) 11:46, 23 May 2012 (UTC) - It was not obvious why six Generic topologies were dumped in this subsection without discussion; and for them to reappear in tables 1 and 2 without discussion. However, further discussion does appear later in the "Controller" subsection of System description and operation. It would be helpful to refer to this.[reply]
'System description and operation' section moved to beginning of the article to easy enhance understanding of 'Benefits' and 'VFD Types and Ratings' sections.Cblambert (talk) 04:33, 20 May 2012 (UTC)[reply]
Re tables 1 thru 4, this is admitedly a work in progess needing accompanying discussion.Cblambert (talk) 04:33, 20 May 2012 (UTC)[reply]
    • Control platforms & Load torque and power characteristics -
  • checkY Pyrotec (talk) 09:18, 22 May 2012 (UTC) - Again, I can't see the point of these subsections, as they are here without explanation.[reply]
  • The first of these consists of a sentence with three bullet points and two see also's. "Control platforms" seems to only appear once again, as a single-sentence paragraph in "Controller" subsection of System description and operation.
Will think on this but this is intended as a summary of AC drive types.Cblambert (talk) 04:33, 20 May 2012 (UTC)[reply]
Will think on this but this is intended as a summary of AC drive types.Cblambert (talk) 04:33, 20 May 2012 (UTC)[reply]

...stopping for now. To be continued. Pyrotec (talk) 21:31, 19 May 2012 (UTC)[reply]

    • Available power ratings & Drives by machines & detailed topologies -
  • OK.
  • Application considerations -
    • AC line harmonics -
  • checkY Pyrotec (talk) 11:46, 23 May 2012 (UTC) - I suspect that "utility" (used three times in different paragraphs) is a USA term for the mains electricity supplier, we would probably call them power supply company, supplier, utility company. Its more obvious the second and third times what the term is being used for (substations and grid are the "clues" that clarify - I wikilinked both).[reply]
Changed accordingly to electrical power company in 1st instance and 'power company' in subsequent mention.Cblambert (talk) 22:12, 22 May 2012 (UTC)[reply]
    • AC line harmonics & Long lead effects -
  • The second subsection starts off with "The output voltage of a PWM VFD consists of a train of pulses switched at what is called the carrier frequency." but carrier frequency is first discussed in the first subsection. I suggest that these two subsections be reviewed.
I have combined 1st two sentences of 'Long lead effects' in order to de-emphasize compard to 1st sentence in 'AC line harmonics'. AC line harmonics phenomena have to do with upsteam side of VFD whereas long lead effect phenomena have to do with downsteam side of VFD. Line harmonics are of relatively low frequencies compared to long lead effects. A big problem that PWM solved compared to older six-step inverter is PWM's ability to better approximate sine wave output to the motor. PWM thus allowed harmonic-free inverter output for frequencies below the carrier frequency. Which leave PWM drive with the two disctint problems:
Lower frequency harmonics on the AC line side of VFD
High frequency long lead effects on the motor side of the VFD.Cblambert (talk) 22:12, 22 May 2012 (UTC)[reply]
Changed accordingly.Cblambert (talk) 22:12, 22 May 2012 (UTC)[reply]
I made it into a footnote using the {{efn}} - three edits - this is not mandatory for GA, so you can undo it if you have objections. Pyrotec (talk) 12:05, 23 May 2012 (UTC)[reply]
    • Motor bearing currents -
  • I'm not sure what those three citations are doing at the start of the subsection. The first paragraph is unreferenced as is the last one, so do they belong there?
I have repositioned two the three citations at the end of the paragraph. The remaining 'general' citation is still there because it is an old citation and I would have to dig back to see if it warrants keeping.Cblambert (talk) 22:12, 22 May 2012 (UTC)[reply]
That 3rd citation is in fact relevant to 1st sentence and has been positioned as such.Cblambert (talk) 16:37, 23 May 2012 (UTC)[reply]
    • Dynamic braking -
  • The "slip" in "Torque generated by the drive causes the induction motor to run at synchronous speed less the slip" needs an explanation.
In first instance, I have noted 'slip' as being described in induction motor.Cblambert (talk) 22:12, 22 May 2012 (UTC)[reply]
I had added a citation accordingly.Cblambert (talk) 22:12, 22 May 2012 (UTC)[reply]
    • Regenerative drives -

...stopping for now. Pyrotec (talk) 09:33, 22 May 2012 (UTC)[reply]

((tick|15}} Pyrotec (talk) 11:10, 25 May 2012 (UTC) - The function of the Lead is to both introduce the topic and to summarise the main points of the article. The current lead at 1.5 paragraphs is rather short. It does introduce the article but (in my opinion) is not all that good at summarising main points of the article.[reply]
I have no strong opinion on what is important (this is not my field), however, energy saving seems to be important. Different topologies, applications considerations could be important.
Possibly the current lead needs to be twice or three times as long (this is not a requirement just a hint at what I'm looking for), but only three (at most) four paragraphs long.

Pyrotec (talk) 12:24, 23 May 2012 (UTC)[reply]

Any comment on my kick at lead cat?Cblambert (talk) 16:50, 24 May 2012 (UTC)[reply]
I'm happy. Pyrotec (talk) 11:10, 25 May 2012 (UTC)[reply]

Overall summary

[edit]

GA review – see WP:WIAGA for criteria

  1. Is it reasonably well written?
    A. Prose quality:
    B. MoS compliance for lead, layout, words to watch, fiction, and lists:
    Yes, various edits during the course of this review have resulted in considerable improvements.
  2. Is it factually accurate and verifiable?
    A. References to sources:
    B. Citation of reliable sources where necessary:
    Please note that there are two long citations 44 and 132 pages long. The relevant page number(s) have not been provided for these two reference publications
    C. No original research:
  3. Is it broad in its coverage?
    A. Major aspects:
    B. Focused:
  4. Is it neutral?
    Fair representation without bias:
  5. Is it stable?
    No edit wars, etc:

I'm awarding this article GA status. Congratulations on producing what is now fine article. Pyrotec (talk) 11:37, 25 May 2012 (UTC)[reply]

  2. Does it contain images to illustrate the topic?
    A. Images are copyright tagged, and non-free images have fair use rationales:
    Well illustrated.
    B. Images are provided where possible and appropriate, with suitable captions:
    Well illustrated.
  3. Overall:
    Pass or Fail:
    In the light of the corrective actions that have been undertaken, I'm happy to award GA status.

I'm awarding this article GA status. Congratulations on producing, what is now, a fine article. Pyrotec (talk) 11:37, 25 May 2012 (UTC)[reply]

Retrieved from "https://en.wikipedia.org/w/index.php?title=Talk:Variable-frequency_drive/GA1&oldid=960744300"