I am going to a customer site to measure machine power, while it is in an
idle state. It has a 3 phase & neutral supply(50Hz).
My plan is to use a clamp meter to measure current,
is it simply a case of P=IV for power or is there a need to multiply by
sqrt(2). Also, do I need to measure the 3 phases and add the reults together
to get the total power consumption.
Thanks.
From the question- you really don't know what you are doing so I suggest
that you get someone who does know even if it costs a bit. Your customer
might be far happier if someone competent did the measurements.
If you want real power measurements a clamp on ammeter won't do. You need to
a) have a wattmeter or some form of phase measurement
b) know what you are doing.
--
Don Kelly snipped-for-privacy@shawcross.ca
remove the X to answer
Firstly,
I can't remember saying I did have a clue what I was doing.
Secondly,
I may not be a competent electrician, but I am a qualified one so, at least
in the UK, people will just have to put up with cowboys like me doing
electrical work and giving superstars like yourselves a bad name.
Regards.
Well in the US, to be a 'qualified electrician' you would have to know that
for a *balanced* three phase supply the total real power flow is = sqrt(3) *
V* I * pf. Where pf is the power factor. Power factor can range from zero
to unity based on two general things, the phase relationship between voltage
and current, and the harmonic content of the current. Phase displacement
happens with inductive loads (motors most common) or capacitive loads
(rare). Harmonic content can be high if supplying non-linear loads
(electronic power supplies most common).
So using a clamp on ammeter will give you one of these terms and you can
easily get the voltage term with a simple voltmeter. Finding the power
factor means you need a different type of instrument altogether. The most
common instrument for this is a watt-meter. It combines all three terms
(voltage, current, power factor) in one unit, eliminating a lot of the work
in measuring power.
Now, if the load is *not* evenly balanced (quite common in office
environments), then you have more trouble. You can measure the current in
each phase individually along with its harmonic content and phase
displacement. Sum each of the three measurements (without the sqrt(3)
multiplier). But you also have to consider that if you are measuring
voltage line-to-line, there is a 30 degree displacement from line-to-neutral
that can skew your phase displacement measurements (instrument transformers
used in permanent metering can complicate this even more).
Now, if you think the others are giving you a rough time about 'not knowing
what you're doing', it's probably because nothing in your original post
suggested you had any idea about any of these factors. If you did know this
stuff, then you wouldn't have suggested just using P=VI and you would at
*least* have known that its the sqrt(3) that gets tossed around in
three-phase calculations, not sqrt(2).
daestrom
Wow, just 4 days ago Sue/Palindrome said:
“The system in the UK is, admittedly, wierd.”
“The actual person doing the wiring needs no qualifications at all,
specific or unspecific. He could have been at school the previous week.”
You dismissed Don’s answer as “drivel”.
You apparently didn’t take my suggestion and look up “power factor” in
Wikipedia. If you did, you would have found a significant portion of
what daestrom posted, and perhaps could have asked an intelligent question.
Somehow "superstars" like me have been given a bad name. "Superstars"
like me include most licensed electricians in the US and about everyone
that follows this newsgroup.
Your behavior is not conducive to getting responses.
--
bud--
Unfortunately, I am not familiar with the legal position surrounding the UK
system.
All I know is I got a job as an apprentice electrician, had a combination of
4 years mixed work/study,
sat an achievement measurement test (AM2), followed by a further 6 months
work then was given a card
by the JIB stating I was an electrician. Not sure if there is any other
route and to be honest I don't particularly care.
It was! I was looking for a few pointers. The fact of the matter is, after a
couple of years electrical work,
I done a BSc in software development then a masters in networking. I work in
machine integration, getting
hardware and software together. It has been nearly fifteen years since I had
to do any basic electrical theory
and when my boss asked me to do this, I thought I would ask for suggestions
before commiting. I wasn't
looking to be told how clueless I was.
Stick to electrcal work, claivoyance clearly isn't your thing. I did in fact
look at Wiki as suggested.
Then I looked out my old college notes and found the information I was
looking for. I am still unsure
where you got the idea I hadn't looked up the reference you gave me, albeit
after you told me.
After asking a fairly basic question, I didn't get a conducive response so
what difference does my behaviour make?
I don't doubt the knowledge each person in this newsgroup has in this area,
but please remember that out of your particular
area of expertise, you may also ask questions that seem ridiculous.
If you don't know don't mess. Would you measure your gas supply?
Trained industrial electricians are hard to come by, people change a plug
and think they know it all
Unqualified and incompetant what a combination
Obviously you have met house bashers and not industrial trained sparks
Yes you are within your ability to ask questions but not within your ability
to understand the most basic fundamentals of electricity ie Ohms law ( yes
it is spelt correctly)
Might i suggest you get you copy of the Yellow Pages and look up a good
industrial sparky and sub contract the job out at least this way you wont
end up killing yourself or anyone else.
Gavin
Perhaps they ought to change their job title to something more fitting, a
word of caution though
GOD is already taken.
I am very qualified, but I don't dispute the incompetant part
I have never worked on a house, only industrial projects and most of the
people I worked with
were struggling alcoholics who could hardly string a sentence together. This
in no way reflects on
you personally, or the people you work with. I am not in a position to
comment on that. I am,
however in a position to comment on my personal experience and stand by what
I have said.
Speed = Distance x Time. I understand perfectly
I don't need the yellow pages, all I need to do is go to the local boozer
and I will find dozens there.
Power factor improvement & three phase theory (Module No 2160070) June 1994.
Much of it was covered again in the HN unit Electrical principles(Module No
2400040)
which I passed with merit in June 1995 as part of an HNC
electrical/electronic engineering.
Then why did you ask the stupid questions that you did? Did you remember
anything about the course except the it's name?
My original answer was not to put you down. You have done that to yourself.
Bye
--
Don Kelly snipped-for-privacy@shawcross.ca
remove the X to answer
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