Subject: Re: Shaft Voltage in Rotating Machines
From: g.mathijssen@pi.net (gaston)
Date: Thu, 14 Nov 1996 15:54:29 GMT
>..
>>
>>In article <52r443$pg5@purple.iap.net.au>, pd05831@gold.iap.net.au
>>says...
>>>
>>>Can anybody assist with a problem I have in a gold mine in
>>western
>>>Australia.
>>>
>>>We have a 36 foot Sag Mill with a gearbox driven to a 11kw or
>>3.8
>>>Megawatt Electric Motor.
>>>
>>>The gearbox has recently failed due to shaft voltage being
>>conducted
>>>through the internal bearings.
>>>
>>>A check was done recently by the sparkys and they found 6 volts
>>>travelling through the g/box.
>>>
>>>Can anybody explain what is shaft voltage in rotating machines,
>>how
>>>it is caused and how to eliminate it.
>>>
>>
>>
>>
Answer:
marshall.geoff.gr@bhp.com.au (Geoff Marshall) wrote:
Many years experience working with large drives
>and generators shows that the ideal situation is to have
> only one point earthed on the whole drive train. This
>can easily be done by a carbon brush (insulated holder )
> on the end of the shaft or some form of wire brush
>running against some exposed part of the shaft. It is
> not a good idea to earth through a bearing housing.
>Earthing should be through a low value, high wattage
>resistor as this will provide an element of current
>limiting in the event of a fault and also give a
>monitoring point, where regular checks can be carried
>out to check the integrity of bearing insulation. All
>other bearings and services to them, e.g. cooling water,
>lube oil, instrumentation must be insulated---pipes to
>have insulated flanges, instruments through insulating
>bushes, etc.
> An alternative to insulating all of the other
>bearings in the train is to provide an insulated coupling
> on the motor. I have experience of these on
>1 MW 3000 r.p.m. drives, upsizing should not be too much
> of a problem.
> Food for thought:-- it is not unknown for shaft
>voltages to be mysteriously generated in machines with
>an East-West orientation.
> Regards,
> Geoff.Marshall
> Senior Engineer, Engineering Services
> BHP New Zealand Steel.
> (Formerly U.K.Electricity Supply Industry)
Since I couldn't find the original article I reply on the article of Geoff
Marshall.
I'm a product engineer of multistage turbo-compressor and pumps in the power
ranges from 500 KW to approx 12MW. The mentioned problem is very familiair to
us. We have solved the problem on a different method as sketch by Geoff
Marshall. We have had one case (in approx 30 years) were a good earth point lead
to a solution of the problem and that was in a generator- steamturbine train the
measured voltage difference were in this also much higher.
We have measured voltage differences over the electricmotor shafts in the
values from 2 V to 5 V. Which lead in some cases to bearing failure and
sometimes also in gear failure. It's very interesting to hear that the
"sparky's" in your case measured 6 V which gives me the feeling that your facing
the same problems as we had.
How can it be dealt with? -
The answer:
Make sure that your non drive end (NDE) bearing of the electric motor is well
insulated. If so you are avoiding a closed circuit and the potential difference
can not harm the train.
When you buy an electric motor at a good supplier you receive one with a totally
electrically insulated bearing at non drive end (NDE). Insulating a bearing is
a tricky thing which is done by insulating the bracket and all things that go
through (vibration probes, temp measurements etc.). As an engineer you will
automatically search for a way of measuring this. If the other bearing (NDE) is
not isolated you have a problem. That's why electric motor suppliers have also
insulated the drive end (DE) bearing as wel. The trick with this is that it's
short circuited with a wire (I come later to this).
If you suspect to have a problem with a voltage difference over the shaft here's
what you do:
Check the insulation of the NDE bearing:
- Remove coupling to isolated shaft from rest of train.
Has your DE bearing a wire attached
YES: Remove this wire and put an ohm-meter in between if you measure
infinite resistance the insulation is okay.
NO: If you are nevertheless sure that the DE bearing is also insulated put a
ohm meter between shaft and electric motor earth point.
If you are not sure the DE bearing is insulated then measurements become more
difficult and a stepwise approach is difficult to give.
Why put isolation on the driver coupling?
A lot of our contractors require insulating of the drive coupling the one who
are willing to listen we can convince that it is not necessary for the others we
can't convince there are two possibilities or they don't understand it
themselves and are doing what their customer requires or they have understood
the problems very well (see point 1below)
I can think of two reasons why to put insulation
1) In the above mentioned check you can easily check the insulation of the NDE
coupling without removing the coupling.
2) Designers from such heavy equipment are primarily mechanical engineers
(including myself) which have (I hate to admit) not much sense about what
happens within a electric motor. The attitude of: "put electrically
insulation in the coupling because I hear very strange stories and I don't
understand a thing what's happening there but with the insulation at least it
won't reach the rest of the train" is to a certain height valid and easily made.
Primarily I would advise you to check the insulation of the NDEbearing first
because if that's okay no closed electrically circuit can be found and your
problems are over.
Hope my answer is clear to you if not E-mail me at
g.mathijssen@pi.net
Best Regards,
Gaston J.F.Mathijssen
Subject: Re: Engineering Contests
From: James
Date: Thu, 14 Nov 1996 08:03:24 -0700
While the egg drop may scenerio may be over done, there are many
contests that require engineering and innovative ideas.
My university once had a contest that gave students a "grab bag" of
items. Inside was a description of the problem to be solved. You had
no proir knowlegde of the contest until you were handed the bag and
given one hour to turn in your entry. In my case, we had to create a
mechanism that could propel an object about the size of a pingpong ball
the furthest. The weight and size of the projectile could vary within
certain limits.
The bag contents included: a paint stir stick, rubber bands, tacks,
nails, a bottle cap, cloths pins, etc. Out of the 10 or so groups that
entered 8 made catapults and 2 made rubber band canons. While the
catapults performed wonderful (8-10ft), the canons blasted 25-30 ft.
You always hear about engineers designing things that cannot be built or
are not practical. Well, in one hour, students proved they could
design, build and operate solution to the problem. Now thats a
challenging engineering contest. I think someday when I am in the
market for entry level engineers, I will hand out a "grab bad" instead
of collecting resumes from my applicants. **grin**
Subject: Inclusive workplace language (was Re: Drafting Software Survey)
From: rongraham1@aol.com
Date: 14 Nov 1996 17:37:32 GMT
In article <328B352F.446B@wildcat.selma.etn.com>, Mike Ross
writes:
>I THOUGHT YOU ALL WERE GOING TO TALK ABOUT
>DRAFTING OR SOMETHING.
What, when people can pull opinions out of their backsides instead?
You must be kidding. :-) No need to shout, by the way.
>Gary Lynn Kerr wrote:
>> To whom ever wrote that the term draftsMEN was not correct. GET A
LIFE!
>> When the word men/man is used in a word it is generic and means
mankind.
The statement about the meaning of the term "man" is correct.
Nevertheless, if "getting a life" means to have one like Mr. Kerr's,
I think I'll pass, thank you very much. My life is meant to serve
others, not to be self-serving.
The fact of the matter is that this usage is not "generic" any more.
And it's becoming less so all the time. Furthermore, the only people
who have a gripe about using more "inclusive" language are simply
too lazy to try. "Drafters" and "designers" are terms that are both
inclusive and simpler to use than "draftsmen." So why not use them?
>> And a female boss has a problem with it she has too much time on her
>> hands. Also that same feMALE boss probably would not liked being
>> referred to as feMALE/woMEN. As I said before GET A LIFE!
And if Mr. Kerr can't keep up with a changing workforce, that makes me
wonder what the heck else he can't keep up with.
Dr. Ron Graham
Project Engineer for Robotics, GreyPilgrim LLC, Washington DC
founder of sci.engr and keeper of the sci.engr.* FAQs
EMMA Robotic Manipulator online -- http://www.greypilgrim.com/
Subject: Re: Ground
From: altavoz
Date: Thu, 14 Nov 1996 11:30:29 -0800
bikerbabe in black leather wrote:
>
> In article <5665u7$3pu@Nntp1.mcs.net>, Tommy E. wrote:
> >
> >>From: DC, 75277.3335@CompuServe.COM
> >
> >>>What completes the grounding circuit in an electrical system?
> >>Let's say a refrigerator shorts and the current goes to ground (thus
> >>protecting anyone touching the refrigerator).
> >>The current goes through a wire down to metal pipe down to the earth
> >>itself, but how does it come back from the soil in the back or front
> >>yard to complete the circuit to the refrigerator in the house?
> >>Thanks for help and info.
>
> To clear up a bit of a misnomer, ground has several meanings. In
> general it's used to indicate something that is at zero volts with
> respect to a source. The earth ground is used mainly for protection
> from lightening or similarly induced voltages, and is used to ensure
> that all wires into a house are at the same ground voltage with
> respect to each other. This includes phone lines and cable.
>
>
>
> >
> >The problem, as you might imagine, is when for some reason, say defective
> >wiring, you have a high resistance ground connection. In this case, the
> >protective overcurrent device will not trip, leaving the circuit hot and
> >the frame of the device at an energized, possibly dangerous potential.
> >If you suspect that you have a high resistance ground, your installation
> >should be immediately checked by a qualified electrician.
>
> This is also one of the reasons for ground-fault interrupted circuits.
> These can either be at the outlet or an entire circuit, and are
> designed to sense when there is a voltage with respect to ground that
> may not be high enough to trip a regular breaker. altavoz: Any normal load will cause a volt diff to appear from nuet to
bare(gnd). It's not the volt diff ( nuet to bare) that fires a GFI ,
its a diff in currents between nuet and hot . I used to think it was
bare current that fired the GFI but was corrected . Its a matter of where
you measure it cause the diff nuet to hot is equal to the bare current .
Anytime a circuit
> is to be used in a bathroom, basement, kitchen near a sink, garage
> or similar outbuilding, or outdoor circuits, a GFI should be used.
>
> --
> Anmar Mirza # Chief of Tranquility #How many of our #I'm a cheap
> EMT-D # Base, Lawrence Co.,IN #Bretheren die for#date, but an
> N9ISY (tech) # Somewhere out on the #every glass of V8#expensive
> EOL DoD#1147 # Mirza Ranch.C'mon over#Juice?. TBTW #10 #pet.
--
______End of text from altavoz___________
Subject: Re: Fighting Complexity
From: altavoz
Date: Thu, 14 Nov 1996 11:43:37 -0800
> You know, this is funny, because I am attempting to start a competition
> here at Carleton University in which competitors (engineering students)
> are required to perform relatively simple tasks, like getting a ping
> pong ball out of a cylinder, using only the tools provided (duct tape,
> tubing, etc.). There are some extremely simple solutions that most
> students would overlook, and that's the point of the competition. It is
> my experience as a student and as a teaching assistant that students
> believe all problems are difficult. Professors never ask questions with
> simple common sense answers (in the students view). The reason for
> this, of course, is to challenge the students, weed out the bad ones,
> and reward the good ones for finding the solution. But something is
> missed. On the job, the best engineer is the one who can come up with
> the simplest, cheapest solution. In school, the best engineer is the
> one who can solve the most difficult problem. I'm hoping this
> competition will make some things click in the students mind. I
> definitely agree that this is a problem, even at the academic level.
>
> --
> ("`-''-/").___..--''"`-._
> `6_ 6 ) `-. ( ).`-.__.`) Chad English
> (_Y_.)' ._ ) `._ `. ``-..-' cenglish@mae.carleton.ca
> _..`--'_..-_/ /--'_.' ,' http://www.mae.carleton.ca/~cenglish
> (il).-'' (li).' ((!.-'
altavoz: The problem is GOVT SCHOOLS ! If all schools were private
there'd be competetion to satisfy industry , not some group of regents.
The solution is not to try the simple solution 1st , but to know thru
experience if the solution is or is not simple ( Einstein "everything
s/b made as simple as possible but not simpler"). Experienced Eng's dont
need to try 20 different solutions then say "it must be complicated" ..
so we'll now try complicated solutions .
Then the other fight i see constantly is " it worked 23 times
before , so therefore it will work again " NOT !
If we could challenge tests to get a Eng degree , then many would study
on their own . The Govt schools wont allow that.
ITS THE DAMN GOVT SCHOOLS ! PRIVATIZE .
______End of text from altavoz___________
Subject: Re: Pipe Secti~Z or S?
From: "hobdbcgb"
Date: 11 Nov 96 19:04:15 GMT
Some further background on the S & Z comments.
mwcraig wrote in article
>
> I would refer you to the latest edition of the American Institue of Steel
> Construction (AISC) "Manual of Steel Construction" which contains both
the
> "Load and Resistance Factor Design Specification for Structural Steel
> Buildings" and the "Code of Standard Practice for Steel Buildings and
> Bridges" as an appropriate standard for the use of terminology on plastic
and
> elastic section moduli.
This was one of the books in my library that I had in mind when commenting
on the Z vs S "standard" comment. While I do use parts of it quite
regularly, I do not rely on it as a definitive standard. It has its place
when the base parameters of the AISC standard are used, such as using coped
beams which limit moment transfer, steels as listed, etc.
While a useful standard, I would hold that the AISC Standard is much too
limited and specific to be considered anything but a narrow structural
steel standard. Unfortunatelty, it addresses only steel, and only a few
types at that, and is often referred to as a "cookbook" in its assumptions
of compliance and limit when the included equatuons are to be applied, and
in that most equations are is used to find a limit, not find a stress.
> I would estimate that more tons of steel structure
> are designed and built to this standard than to > most others combined.
True, but only tons of A36 and a few other types of steel, and only using
bolts and rivets over 1/2 " or larger, but not too many tons of stainless,
aluminum, concrete, copper, or HSHA steels.
I would also estimate that many more non-structural fasteners are used
than the fasteners required in this structural standard.
>
> Just as the elastic section modulus assumes a linearly elastic material
when
> applied to actual materials, the plastic section modulus assumes ideal
> plasticity to infinite strains when applied to the bending problem.
However,
> both are purely theoretical geometric properties of a given section
because
> they can be found by purely geometrical considerations.
>
Fine for the steels of this standard which have curves suitable for the
included equations, but for anything else, first, there is the problem of
reversing plastic deformation--do you use the initial area or the reflexed
area?
Second, referring to the stress strain curves for structural steels and
comparing them to HSHA, glass, or aluminums of various types, the
plasticity is extremely varied, including the aforementioned reversing.
(And this does not address the ongoing argument of using true section
stress/strain vs orginal section stress/strain curves.)
The geometry for most materials straining in the plastic range depends on
the stress.-- almost circularly so in the definitions
> Recall that all allowable or ultimate strength estimations are just
that...
> estimates or reference states.
Which is one of the main arguments for designing to yield-- the actual test
specimens from which yield strength is found has much less question of
definition, preworking, and less scatter than ultimate tests.
> Use of allowable stress formulae cannot take
> into consideration stress concentrations and residual stresses nor can
all
> sections exibit ideal placticity to infinite levels of strain. Just as
many
> sections cannot reach strain levels high enough to produce their full
plastic
> strength, many sections cannot even reach their initial yield strength
before
> local or overall buckling limit states are reached.
>
I have worked extensively in vibration and fatigue applications since '73,
and we almost always rely on allowable stress (with at least eight stress
limit modifiers, including stress concentraions) Allowable stress is
defined by the engineer, and not by a standard, nearly all which are mute
on the subject of fatigue limits.(with good reason)
This is a major problem in parts of the AISC standard in that it defines
allowable stress as though it is a static value, (which I agree it is when
most of the load is steel and concrete and where the sizing criterion of
the code is deflection-selected to minimize human discomfort and to
interface with brittle material) In many fields, however, engineers allow
statistical applications of stress, complex applications, and multiaxial
loading more complex than that covered in many of the equations of AISC.
The AISC user also receives ongoing project information in the form of
load, making a limit concept useful.
Fatigue project changes are usually changes in material and cycle, making
allowable stress equations more useful.
Having done structural steel and its interface for many years, I would not
use yield strength as a criterion for column strength. It is a measure of
different limits. Compression failure locally, however, is a function of
yield strength.
> Another thing to note is that the use of the term safety factor is
rapidly
> becoming out of date. The lastest probabilistic studies and the LRFD
> Specification mentioned above use overload factors
I agree the term safety factor is not appropriate to the AISC standard, as
I had advised that body several years ago. Overlaod factor is more
appropriate to a standard which requires compliance at or above a certain
level.
Safety factor is, however, appropriate to many other areas of
engineering where the engineer must decide, without benefit of rigorous
requirements, what margin of error or risk they are willing to accept
rather than to what level they are going, and where they must report to a
more experienced engineer who may
accept the work but choose a higher margin of protection without re-doing
the project or including the cost and time of commissioning a statistical
study.. .
Thanks for the comment,
...
Subject: Titles in the Workplace (was Re: Drafting Software Survey)
From: "S. Yoder"
Date: 15 Nov 1996 03:46:39 GMT
Tom_Austin wrote in article
<328B9F7F.31DF@optilink.dsccc.com>...
> >
> > I personally never gave a flip. Yet I don't like "draftswoman", but I
do
> > take it as a sign of politeness when a guy goes out of his way to
> > say "drafter". "Draftsperson" doesn't cut it either. Makes me think of
> > an old Far Side cartoon ;)
>
>
> Maybe it's a generational/geographical thing. I'm 35, and in all my
> time in the workplaces, and all the places I've worked, Drafter and
> Designer are the standard terms, even by the Old Guard guys, who are
> traditionally sexist in every other way.
>
> I'm in California. Maybe that's it.
Eh, could be. I'm in the "bible-belt" and graduated from a high school
that, only 6 years ago, still strongly discouraged females from taking
the drafting/machine shop class. But then that makes me younger than
you by about 10 years.
I'll cast my vote for attitude, though.
Enjoy,
Stef
Subject: Re: Attn.: English students
From: jmh@supernet.ab.ca (J.M. Holloway)
Date: Fri, 15 Nov 1996 04:34:48 GMT
HW Raedt wrote:
>Hi English Students !
>
>
>I am studying Metallurgy & Material Sciences in Aachen, Germany, and I
>am going to graduate next spring.
>
>I am looking for an oppurtunity to do a PhD at an
>English University.
>
>If you happen to know of a vacancy in Engineering, especially in Metal
>Forming / Rolling / Die Forging etc. and / or Finite Element Analysis of
>such processes, please let me know.
>
>I you know any department researching in the above mentioned field,
>please contact me, too.
>
>Reward: If your help leads to an employment at your college /
>university, I offer a crate of German beer of your choice.
>
>
>Thanking you in advance
>
>J. Raedt
Try pointing your web browser to "http://www.ucalgary.ca/" and
"http://www.ualberta.ca/" for information at the University of Calgary
and the University of Alberta respectively. Both located in Alberta,
Canada. Both pages provide direct links to their respective faculties
of engineering.
J.M. Holloway
jmh@supernet.ab.ca
Subject: I-DEAS Master Modeler Implementation
From: TechV@infoave.net (Tech Visions)
Date: 15 Nov 1996 04:33:24 GMT
Tech Visions announces various hi-level CAD/CAM support positions.
Join a long established and highly successful company, with wide
ranging contracts in design automation. Join a team with an exciting
new approach to design software implementation engineering. Help
develop new innovative approaches in design producibility and
feasibility for end-users. Work with talented individuals and top
industry managers in a state-of-the-art design environment and with the
latest software tools and hardware. Be a part of what has been called
by company officers as Òone of the most exciting and ambitious CAD/CAM
efforts in modern times.Ó
High salaries will be offered for exceptional candidates as Sr.
Technical Leaders. The company is public, development in this division
is well funded and long-term. These are not contracts and not
short-term positions. Benefits, incentives, bonuses, and full,
extended relocation may be offered.
Requirements:
FEA (GNC a plus);
I-Deas experience, Master series and solids a plus;
Software development experience;
good communication skills and experience in one or more: beta-test of
CAD software, customer service, CAD training, defining enhancements, or
application support.
To apply in confidence please send your resume and / or contact Tech
Visions.
Subject: Re: Titles in the Workplace (was Re: Drafting Software Survey)
From: C
Date: Thu, 14 Nov 1996 08:05:07 -0800
S. Yoder wrote:
>
> Tom_Austin wrote in article
> <328B9F7F.31DF@optilink.dsccc.com>...
> > >
> > > I personally never gave a flip. Yet I don't like "draftswoman", but I
> do
> > > take it as a sign of politeness when a guy goes out of his way to
> > > say "drafter". "Draftsperson" doesn't cut it either. Makes me think of
> > > an old Far Side cartoon ;)
> >
> >
> > Maybe it's a generational/geographical thing. I'm 35, and in all my
> > time in the workplaces, and all the places I've worked, Drafter and
> > Designer are the standard terms, even by the Old Guard guys, who are
> > traditionally sexist in every other way.
> >
> > I'm in California. Maybe that's it.
> Eh, could be. I'm in the "bible-belt" and graduated from a high school
> that, only 6 years ago, still strongly discouraged females from taking
> the drafting/machine shop class. But then that makes me younger than
> you by about 10 years.
>
> I'll cast my vote for attitude, though.
> Enjoy,
> Stef
Just call me a CADD Operator
Subject: Re: Engineering Contests
From: "M.E. Moore"
Date: Fri, 15 Nov 1996 07:27:28 -0700
James wrote:
>I think someday when I am in the
> market for entry level engineers, I will hand out a "grab bad" instead
> of collecting resumes from my applicants. **grin**
Dittos. I've spent my career (12 years now) in experimental fluid
mechanics. This invovles a lot of work in and around wind tunnels,
gages, piping, etc. etc. The computer jocks and pencil pushers were
abysmal in the required work, IMHO. However, they would probably roll
their eyes and sigh if I had to do any FORTRAN coding, especially now.
;-)
--
M.E. Moore
Los Alamos, NM
