3 phase motor rotation direction

hello I work with motors every day i used to replace tow phases when I want to change the 3 phase motor rotation direction my question is why motor change its rotation when I replace any two phases?

Edited By David Clark 1 on 08/03/2012 10:11:33

Think about the three sine waves 120 degrees out of phase. If for example you look at the positive peaks you will see that each peak occures 120 degrees later. (Assuming you drew them with each phase lagging the previous one.) If you now cut your diagram into three strips then swap any two you will see that the peaks now occur 120 degrees earlier.

Les.

ABCABC forwards ACBACB reverse i.e. swap B & C to reverse.

Billy.

I think the clearest way to see it is to imagine three lights equally spaced around a circle, 1 at 12 o'clock, 2 at 4 o'clock and 3 at 8 o'clock. If you power them 1,2,3 the light appears to travel clockwise round the circle; if you power them 1,3,2 it appears to travel anticlockwise. The 3 rotor windings on a 3-phase motor behave in a similar way.

David

I find this worrying, somebody who is let loose on motors every day does not know why swapping the connections, changes direction of the rotor? Do we have yet another case for Health and Safety??

Edited By KWIL on 01/03/2012 12:05:47

Kwil,

I know what you mean but by the same token I work with women every day but I've never understood them!

Bob

As a car mechanjic about 'stoichiometric ratios'. these days there's a tendency just to teach people what they need to know, not why.

Credit to Amr for wantiong to know why and having the courage to ask.

Neil

Apparently in a recent interview Steven Hawking admitted that he didn't understand women - what hope is there for the likes of us?

But seriously as a chartered electricl engineer some of the questions and answers about electrical topics that I frequently see on this forum absolutely TERRIFY me.

I for one would be far hrappier if all electrical questions in here were banned by the moderators and the very sound advice "If you are in any doubt then consult a qualified and registered electrician" was placed after every electrical question

You can't hear it, you can't see it, you can't smell it and by the time you feel it then its usually too late.

I was trying to resist but just can't. I was brought up by an electrical engineer (FIEE), did degree in EE myself and been in the profession all my life and survived to 61 without anything more than a minor jolt despite both professional and amateur dabbling.

But I'm appalled by this attitude that no-one should touch electricity unless they are "qualified and registered" especially since there is so much evidence that the training given is just so deficient these days - just learn how to do the wiring and never mind why it works. Some of the stories you hear about solar installations and the reasons given for why things don't work just make my hair curl. And I've had "professionals" install RCDs who obviously just don't have a clue about how they work. When I was 20 or so everyone I knew buying houses would do their own minor wiring jobs and I don't think there was much evidence of major fires being started or large numbers of people electrocuting themselves. Since then we have had the elfin safety brigade in cahoots with the restrictive trades cartels trying to make sure that everyone is dead scared of doing it themselves.

Yes, electricity is dangerous, but so is machining - so should people not take up ME unless they have served an apprenticeship? Actually I'm much more nervous when I have a chunk of metal flying round in a chuck or a fly cutter on the mill than I am when doing an experiment with high voltages.

Rant over, now I expect the flame war to start!

Well said John Haine. I worked with electicity all my life until retirement, but because I was in electronics I never got the necessary papers to qualify me for electrical installation. Initially I repaired domestic televisions. Did you know that the old shadowmask CRTs had a lethal 25Kvolt supply.

I rewired my first house in 1969 (and it didn't catch fire). Mid '70s I moved to CCTV. I lost count of the number of installations where I had to sort out the mains electric supplies that had supposedly been installed by qualified electricians. At my present address I wired the garage in its present workshop configuration just before all the new regulations came in.

I am now in the position where I have a brand new consumer unit for the house, but will have to get someone with considerably less experience than me to install it ( or check my installation) because he has the correct certificate and I don't.

Yes there have been alarming posts on electrical matters however the same is true of chemical, fire and mechanical risks on this forum. Pointing out the risks and explaining what is sensible seems a kinder route than "get an electrician/fireman/chemist" , it should then be up to the individual to decide what to do after getting the info they seek in a HOBBY context.

I would agree with John that many "professionals" get it wrong. There is the classic normalisation of red/blue/yellow/ black to black /brown/ grey/ blue or just all black with number sleves. That progress is retrograde-it has already killed electricians in the UK. If you have ever seen a blown armoured cable sliced up down a clay hole or worked under bad lighting you are left thinking why we abandoned bright colours- including red- for the darkest possible combinations. That's my rant for today.

Billy.

PS well done AMR! at least you asked the question to find out WHY.

Edited By Billy Mills on 02/03/2012 00:34:37

Hi, far better for someone to be allowed to asks questions about electrical situations, or any other for that matter, than for them to be band on this forum. People will do things that they are not qualified to do regardless of whether they are allowed to ask questions, so it is much better if they can ask and get an answer that will advise them not to do something that may well be fatal to them selves or others.

Yes the phrase "If in doubt consult a qualified electrician" should be put forward, but I think most people who are likely to want to do any electrical work will have a good level of understanding of the nature of electricity. The majority of people that I've been acquainted with who don't have much understanding, will not even replace a plug on a lead.

Simple electrical work isn't "rocket science" and if the basic rules like switching off and removing plugs from the supply before working on equipment, replacing covers before switching back on, and using the correct fuses as recommended by the manufacturers, and not attempting to do something that you are not sure of, ect., the dangers and risks are kept to a minimum.

Regards Nick.

HMMM Just as I expected.

Maybe if like me you had been involved in electrical accident investigation work then you would have different views. As a junior engineer with the local board I well remember my first 'big one'. This was a housefire caused by faulty wiringwhich had been done by 'grandad' - he knew exactky what he was doing - 'he was in REME during the war and was a TV repairman after demob'

Result - house fire where his daughter and her 5 year old twin daughters DIED

When you have stood in the garden of a house like that and witnessed the burned bedding and furniture, The charred teddy bears and the melted toys - the same toys that yiour OWN six year old plays with - then you might take a slightly different view

Then go into the charred and blackened house and childrens bedroom and witness the smoke blackened remains of the bunk beds and see the actual CLEAN UNBLACKENED outline on the floor under the bed where the twins were 'hiding' in terror

NOW TRY NOT THROWING YOUR GUTS UP

The photograpf of those two little girs is still so vivid to me and often comes back to me in nightmares over 25 years later

Nobody can say that these things don't happen because they do - thankfully they are rare but they happen. Maybe the worst that happens is that you get a 'jolt' - perhaps you have a pacemaker?Perhaps ypur wife has one? Or a visitor has one?

Apart from killing yourselves (which is fine by me) you stand the chance of killing OTHERS and also stand the chance losing your HOME - for which you WILL NOT be insured.

The fire may also spread to neigboring houses / adjoining property, vehicles, caravans, boats or whatever else might be close enough - again you will NOT be insured I can assure you that in my career that this is NOT the only incident I have investigated and am now glad to be working in another area of the electricity supply industry although I still get called upon to be a professional witness from time to time which when it is a domestic incident is always harrowing.

BS7671 and part P regulations are there for a reason and no, its not some cartel / conspiracy / government plot - they are there to protect YOU and I and are here to stay like it or not

NICEIC registered electricians have to have £M2 third party insurance liability - have you?

There is nione so blind as he who will not see

Edited By Kinlet Hall 4936 on 02/03/2012 10:23:03

Hi All,

I feel I have to add my thoughts here as some others have. I have worked in a variety of industrial situations where I have been/am responsible for new electrical installations/replacement installations of old equipment. We always use qualified electricians for the on-site work, and often consult with competent individuals (consultants) for design phases. I have experienced on occasions situations where our trained electricians are unfamiliar with some of the types of technology often mentioned in this forum e.g VFD's or servo drives etc. Obviously they are first class at installing new equipment, installing containment (cblke protection systems), specifying suitable cable sizes for power capacities, cable runs, installation method etc etc etc. They are very capable of installing new distribution boards etc or terminating new supplies. They do come stuck where it comes to some electronic controls, and this is where specifically trained electricians come into play. I take on board the opinions of "Kinlet Hall", however if an individual is familiar with the nature of the work and carries out the design/installation according to the 17th edition IET Wiring regs there can be little room for error. Surely the thankfully rare cases of fire are generally caused by poorly installed circuits, poorly terminated cables (with a high resistance/low continuity) or a badly selected cable with too low a current carrying capacity.

I know and work with many electrical engineers and electricians, who work in very different circumstances - domestic, industrial, local utilities, automation etc, and I have to say there is no one size fits all electrician, and as a result you may find that your NICEIC electrician may not have an awareness about things such as motors, drives etc.

It has also been mentioned about levels of Public Liability insurance cover - there are normally various levels of cover depending upon the level of risk and the loss that can be incurred - I was in the belief that NICEIC required up to £2M PL cover. Of course if a contractor were working in a large factory where losses can be higher then PL may need to be more. The potential losses under other works may be a lot less.

Just my own thoughts.

Neil

Is not the answer that we all rush out and spend several hundreds of pounds on the Course (short) and then we can have our paper to avoid having to be inspected?

Edited By KWIL on 02/03/2012 16:56:24

I quite agree,

It is not only yourself or others who are killed in a fire caused by faulty wiring installed by someone who thinks they know better, but also the following generation who may inherit, buy or rent the property. I stripped out my daughters kitchen in a newly aquired property only to find a most horrendous DIY wiring bodge which was too extensive to describehere but it could have been lethal.

Further, I suffered a devastating workshop fire which destroyedeverything within and almost set fire to my nearby bungalow. I was lucky in that I lost a lifetimes collection of tools, models and machinersrty as well as two Triumph Stags, but I am alive to tell the tale. It was caused by a fault in the wiring by a previous owner (who considered himself competent in the work because he had only had a couple of light jolts in his career).

Regards

Terry

Reading the posts above, I can agree with the arguments of both sides (although it would be interesting to know what kind of error in an installation can lead to a fire after working ok for months or years).

However, the mention of "qualified electrician" brought up in memory a post from a colleague in another forum; I hope he will not be offended when I quote here his message verbatim. Read and enjoy!

>>

Just had an interesting discussion with a Paraguayan electrician. I was commenting to him that I was re-wiring a house, and had been dismayed to find in many parts circuits consisting of a 4 square milimetre live cable, and a 2.5 sqmm neutral. His reply (translated) : "But that is normal, because after coming out of the aparatus the electricity has done work, so it is weaker and only need a smaller cable." Hope you enjoy this one as much as I did; and think that this is what he teaches to his apprentice... << "qualified" has not the same meaning everywhere...! Greetings, Hansrudolf

Edited By Mark Web Tech on 15/03/2012 10:59:21

Well I for one think it is good that someone can ask the question why when all he has been trained to do is the task required, not why the task has the effects it may have.

I also think that much simple electrical jobs, like simple building or plumbing plumbing jobs, can be done safely by those who are sensible and able by virtue of experience and capability, as opposed only by someone who has the necessary qualifications.

I remember on a ship having an engineer come aboard who had a degree and had written a thesis on boilers. All went well during his tour of the engine room until he came upon the boilers and asked what they were. He could write all about the theory of boilers, but knew nothing of how to run and maintain them - or even recognise them!

I also remember sailing on ships in the 60's and 70's, with both DC and AC mains, with no electrician on board. One of the 'mechanical' engineers sorted out any and all electrical problems. DC ships were interesting as the main switchboard was a 'live front' board, protected by just a handrail across the front, meaning all the electrical contacts etc were totally exposed. Despite the natural movement of the ships in heavy weather I never heard of an accident occuring, of someone touching the live electrical parts - we were all too aware and careful. AC ships were a revalation after DC ships, the main switchboard was a 'dead' board, all live contacts safely hidden behind panels. But AC ships also used to generate 380 or 440 volts; us 'untrained', in the modern view, engineers had to deal with problems when they arose with 380/440 volts too as well as the 240v ones.

It is not to say that all supposedly qualified electricians are squeeky clean either. I remember early in married life going into our new bungalow. 2 wall light points were available in the lounge. I duely fitted twin light fittings to each point, added 25w bulbs, and found the lights dim. Fitted 40w bulbs and dimmer still. Took one bulb out of one fitting and the other bulb went brighter! At the same time the reason clicked and I was next minute up in the loft rewiring the light fittings supply wiring correctly. I also checked what fuse served what on the distribution board, as other than indicating 'lights' or 'ring main' nothing was marked. 2 ring mains were shown. Left a table lamp lit in the lounge and pulled one ring main fuse. Lamp still lit. Replaced fuse and pulled the other - lamp still lit. Pulled both fuses and lamp went out. Reason, both circuits had been wired as one in the distribution board. So much for a qualified electrician; an non-qualified electrician (me) had to sort out the qualified guys mistakes.

We should also not get too up ourselves that we do everything best in this country either. In France for example, flat twin and earth cable is not allowed. Why, because the earth cable has to be sized the same as the live and neutral cables. Makes sense no? Ring mains are not allowed, it has to be radial, and the fuse breakers or MCB's when switched off isolate both the live AND the neutral, instead of just the live as here in the UK. MUCH more sensible.

We who pursue this model engineering hobby are generally very sensible and careful people. We have to be to operate the machinery safely. We generally think about what we are doing and the likely consequences of our actions. To ban us from doing anything electrical regardless, and disregarding also our knowledge and capability and force us to consult a 'trained electrician' who may be a genius or maybe a complete idiot is in my opinion stupid, but typical of this increasingly nanny state. Soon we will all be walking about in bubblewrap in case we fall over.

So well done amr refat for asking the question, better to ask than not to ask I say whatever comment you might attract in doing so, as it says elsewhere, a question isn't stupid if you don't know the answer, or something like that. Sorry if I digressed on your thread a bit!

Chris

ps. I also think it stupid that I am qualified to sail in any ship in the British Merchant Navy as Chief Engineer, and have operated and maintained steam boilers operating at 62 bar and 540 deg.C superheat, that I am not allowed to service and commission my own oil fired boiler and cooker because I don't have the right bit of paper to satify nanny, but that is another story!

Edited By ChrisH on 03/03/2012 00:40:50

Edited By ChrisH on 03/03/2012 00:45:14

While discrete examples of what can go wrong are, of course appaling, I really can't see that they say very much without context. Such as what proportion of unqualified installations are a problem?

As an extreme example imagine that 1 million unqualified people undertook house wiring activities and one of those activities resulted in such a tragedy. Would that be sufficient reason to ban unqualified electrical work?

1 in 100,00? ... 1 in 10,000? ......

Moreover, the context needs to include also the number of such problems resulting from qualified installations for comparison. (I presume there are some).

Firstly to Terry - I'm very sorry to hear of your devastating fire and am glad that your loss was only replaceable items. You seem to be the only person so far who has understood what I am saying.

I am not going to go into further details about specific incidents as I feel they will serve absolutely no purpose and that whatever 'professionals' , qualified' people, 'experts' - call them what you like - say, you all obviously know far better. Far preferable to keep your heads stuck firmly up your own bums and carry on regardless. After all the fire brigade, ambulance service and hospitals are there for you if you are fortunate enough to survive when it all goes wrong. Sadly in my experience it is innocent third parties who suffer the most.

You won't take any notice of my experience, you wont take any notice of Terry's experience as you all know FAR MORE and can quote apocryphal stories about 'quqlified' people getting things wrong that you havehad to put right!

I would be absolutely FASCINATED to hear details of what was actually wrong with your installation ChrisH - you seem to be implying some sort of high impedance supply from your description but I cannot fathom out exactly what you are saying - but then i'm only a chartered electrical engineer wit almost 40 years EXPERIENCE in the industry and obviously know ABSOLUTELY NOTHING.

There is none so blind as he who WILL not see.