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  1. #1
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    Question 3 phase resistance calculation


    problem: called to site to find mains 3 phase supply fault, that is red phase 199v yellow 199v blue 230v. resulting in 1 phase not registering voltage. obviously supply fault and i'm still waiting for utd utils to dig the road up.
    ac and fridge cond unit comps red hot and internally tripped or tripped on thermistor.
    with systems isolated the only test i can carry out on scroll compressors is a winding resistance check. this will tell me if the motor will try to run once power is restored or if the motor has been damaged and i need to try to get a replacement before power supply fault is rectified. i know its not conclusive but it's all i've got.
    ok during the day the compressor manufacturers tech helpline is open and i can use that to get the resistance readings. compare that to the readings i get using an ohm meter and deduce whether or not the windings are damaged.
    i can't do that in the middle of the night.
    So the point of this post is to find out how i can calculate the winding resistance of a compressor, knowing the voltage, full load current and output watts of the compressor



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    Re: 3 phase resistance calculation

    Personally, I wouldn't bother trying to do the calculations.

    With an Ohm meeter, check the resistances:
    L1 - L2
    L2 - L3
    L3 - L1
    All 3 values should be well within 5% of each other.

    With a Megger, Check all terminals to earth (Cu-pipe by compressor is the best).
    All should be open circuit.

    NOTE: THESE TESTS SHOULD BE CARRIED OUT WITH THE COMPRESSOR LEADS REMOVED!

    If it passes the above tests then the likelihood is that the compressor will start.
    Mind you, your customer should try and get the electricity supplier to pay for any failures in the near future, even if it starts tonight, there is no way of knowing what longterm damage has been done.

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    Re: 3 phase resistance calculation

    hi viking sad to see you have nothing better to do on a friday night
    i know how to check the resistances, BUT if the comp has overheated to the extent that it has tripped how would i know what the original readings should be without the technical details.
    call me anal if you like but i asked 4 good engineers the same question and noboddy knew the formula.
    if nothing else it's useful to know

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    Re: 3 phase resistance calculation

    Ok,
    The thing being that it's almost unheard of that all 3 windings would be damaged, therefore you would get the imbalance between them.

    But if you are sad enough to spend a Friday night with a calculator....

    Ohms=Volts squared/Watt
    (more info HERE )

    Just remember that this doesn't include for power factors and such.


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    Re: 3 phase resistance calculation

    Quote Originally Posted by bill1983 View Post
    call me anal if you like but i asked 4 good engineers the same question and noboddy knew the formula.
    if nothing else it's useful to know
    There is no such a formula. To many different factors are in game to make any meaningful formula. You need to find technical details of compressor. All manufacturers have such data for their compressors. If it is Copeland, Dorin, Bitzer, Tecumseh, Maneurop, I could try to find that resistance for you if you provide model no.
    Now, when I am officialy citizen off EU, I am looking for decent job! For any job offer please check my profile!

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    Re: 3 phase resistance calculation

    Hello Bill,

    no idea to answer your question.

    We need a 'motor rewind mans' opinion.

    Although, i've tested many a compressor in the small hours of the morning & they all have similar resistances (generally only a few ohms a winding). Thats small compressors (under 10 H.P.) & big 'uns' up to 100 H.P. (i'm only a comercial boy).

    Billy Ray

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    Re: 3 phase resistance calculation

    Quote Originally Posted by The Viking View Post

    Ohms=Volts squared/Watt
    (more info HERE )

    Just remember that this doesn't include for power factors and such.

    Compressor motor are not only resistive load, it is also (mostly) inductive load, and that makes that formula incorrect for that use.
    Now, when I am officialy citizen off EU, I am looking for decent job! For any job offer please check my profile!

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    Re: 3 phase resistance calculation

    Quote Originally Posted by nike123 View Post
    Compressor motor are not only resistive load, it is also (mostly) inductive load, and that makes that formula incorrect for that use.

    Hence the note at the end of my post...

    As you said, there isn't a calculation that will give the right answer accurately..(at least not to my knowledge)

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    Re: 3 phase resistance calculation

    Bill,

    it appears there is no formulae to calculate.

    It makes me feel a little bit better, as i've had similar service calls in the past & i've always jumped in he van & tol the customer 'sort the power out'

    '& i'll be back'

    Billy Ray

    'there's a lot of milk tanks in Norfolk with iffy power supplys'

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    Re: 3 phase resistance calculation

    Also if the motor is hot the resistances will change, can't remember which way though. Why not make a note of resistances when you do maintenace visits for different motors, then you will be able to look them up in future.

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    Thumbs up Re: 3 phase resistance calculation

    Quote Originally Posted by bill1983 View Post
    problem: called to site to find mains 3 phase supply fault, that is red phase 199v yellow 199v blue 230v. resulting in 1 phase not registering voltage. obviously supply fault and i'm still waiting for utd utils to dig the road up.
    ac and fridge cond unit comps red hot and internally tripped or tripped on thermistor.
    with systems isolated the only test i can carry out on scroll compressors is a winding resistance check. this will tell me if the motor will try to run once power is restored or if the motor has been damaged and i need to try to get a replacement before power supply fault is rectified. i know its not conclusive but it's all i've got.
    ok during the day the compressor manufacturers tech helpline is open and i can use that to get the resistance readings. compare that to the readings i get using an ohm meter and deduce whether or not the windings are damaged.
    i can't do that in the middle of the night.
    So the point of this post is to find out how i can calculate the winding resistance of a compressor, knowing the voltage, full load current and output watts of the compressor
    Bill
    If you follow the link below and click on the "Download our field Compressor Guide" Link at the bottom of the page.
    Mentions a few more tests that have not been discussed.
    You will find some handy testing tips within.
    I carry a printed off version of this with me to remind me when I forget. As you say the brain don't function to well, when it knows it should be sleeping.
    Grizzly

    http://www.thermagroup.com/compressors.php

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    Re: 3 phase resistance calculation

    Quote Originally Posted by nike123 View Post
    Compressor motor are not only resistive load, it is also (mostly) inductive load, and that makes that formula incorrect for that use.
    Exactly Nike 123, you can't calculate it, it all depends what wires and how many the manufacturer uses to make his stators.
    If you measure 3 equal windings, then you can decide almost for sure that the motor will be fine, whatever readings there may be.
    The thermal protection is fitted for this purpose to cut the power as soon the windings becomes too hot, still on a safe temperature.

    I disagree anyhow that the power utility company should pay for this because a 3 phase motor must be protected against a phase loss.

    Resistance will decrease when windings becomes hot Monkey Spanners. Reason why an outside light when it's very cold is prone to fail, resistance is then much lower whereas the current will increase and burn the filament.

    Bill 1983, I don't see the clue in your sentence hi viking sad to see you have nothing better to do on a friday night
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: 3 phase resistance calculation

    Quote Originally Posted by Peter_1 View Post
    Resistance will decrease when windings becomes hot Monkey Spanners. Reason why an outside light when it's very cold is prone to fail, resistance is then much lower whereas the current will increase and burn the filament.
    You should read this sentence carefuly once more Peter!
    Last edited by Peter_1; 26-01-2008 at 01:48 PM.
    Now, when I am officialy citizen off EU, I am looking for decent job! For any job offer please check my profile!

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    Re: 3 phase resistance calculation

    You should read this sentence carefuly once more Peter!
    Oeps, resistance will increase when wires becomes hot of course. The rest is correct.
    Last edited by Peter_1; 28-01-2008 at 12:59 AM.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: 3 phase resistance calculation

    Quote Originally Posted by Peter_1 View Post
    hi viking sad to see you have nothing better to do on a friday night
    You'll have to include me in that too peter,i didn't go out last night and flicked some pages on here while watching the telly.

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    Re: 3 phase resistance calculation

    Many years ago while working as a technical service engineer for an a/c manufacturer I was called to a shop with nine 10hp splits.
    The local sub station had a problem & one phase had failed.
    The a/c units had continued to run for a period of time on 2 phase & all nine compressor motors had burnt out.
    I had to attend a site meeting with the shop manager & a representative from the shops HQ, the mechanical services consultant,the mechanical services a/c maintenance contractor, the electrical maintenance contractor & engineers from the power supply company.
    Alloction of blame for the cost of replacing the compressors was the agenda.
    The power supply company advised that the contract did not garantee the correct power supply & therefore no claim could be made against them for the loss of one phase. They advised that it was the responsibilty of the end user to ensure that adiquate phase loss protection is installed to protect electrical equipment.

    I had to advise that the manufacturer of split systems do not include phase loss protection as standard.

    So the enduser had to cover the cost & consider if they would upgrade the complete chain of shops to include some phase loss protection.

    If you look at manufacturers of splits & vrf you wont find any specific phase loss protection component in wiring diagrams or specification. But you will find that they will all stop & therefore save the compressor when the phase which powers up the circuit boards, safety circuits & transformers is lost.
    But if the circuit boards, transformer & safety circuits are all powered from one phase then when any one of the other two phases fail for any reason the outdoor unit does not see this & the compressor continues with one or two phase & burns out.
    So the unit will protect itself by default if the phase which goes down is also the phase which the unit needs for the controls.
    You will find some a/c units use one phase for the transformer & another for the circuit board & another for the safety circuit. So if any phase fails then the unit will stop & save compressor damage.
    But nobody seems to consider this as a design benifit ,,, until that is they have to deal with an expensive failure due to the loss of a power supply phase.

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    Re: 3 phase resistance calculation

    If compressor have thermal protection in windings, it should protect motor, because with 2 phase, he will overheat.
    If compressor have thermal over current relay together with contactor, it should react and protect compressor, because other two phases now draws higher current.
    If compressor have electronic (hall type) over current relay together with contactor, it should react immediately at phase drop and protect compressor.

    It is often case that one phase fail (acting of fuse etc...), but it is rare case that compressor burns because that phase loss, unless someone has fiddling with protective devices.
    Most likely, if for some reasons (work on power transformer or distribution boxes), someone change phase order (rotation direction) scroll compressors is going to rotate in reverse direction than you could get damaged (blocked) compressor. That is is why most (reputable) manufacturers have phase rotation and presence relay when they have scrolls.
    Last edited by nike123; 26-01-2008 at 06:10 PM.
    Now, when I am officialy citizen off EU, I am looking for decent job! For any job offer please check my profile!

  18. #18
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    Re: 3 phase resistance calculation

    Quote Originally Posted by Thermatech View Post
    They advised that it was the responsibilty of the end user to ensure that adiquate phase loss protection is installed to protect electrical equipment.
    This is very good advice

    I had to advise that the manufacturer of split systems do not include phase loss protection as standard.
    This is incorrect.
    A lot of manufacturers include phase rotation monitors as standard on 3 phase equipment

    If you look at manufacturers of splits & vrf you wont find any specific phase loss protection component in wiring diagrams or specification. But you will find that they will all stop & therefore save the compressor when the phase which powers up the circuit boards, safety circuits & transformers is lost.
    See the attached pdf from an old R407C service manual
    Attached Files Attached Files
    I'm back on the Pale

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    Smile Re: 3 phase resistance calculation

    thanks for all your interest guys the advice you give is as usual interesting and helpful but i'm thinking that if the motor manufacturers have the technical info then somewhere up the line someone must have calculated the requirements of said motor.
    is the resistance purely a byproduct of those calculations or is it an integral part of those calcs.
    i know we can all make the phone call when required, but in that situation when you dont know the readings and its weekend or the wee hours is there a test on the compressor that would give you an idea whether or not the motor would turn on restart.
    thanks grizzly for the link to the booklet/pdf thermacon will also send you as many copies as you require through the post, i did exactly this 2 years ago for all our engineers, how many of them use it i don't know

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    Re: 3 phase resistance calculation

    Frank

    You are completely right lots of modern 3 ph splits do include some phase rotation protection but often it is to ensure scroll compressors rotate in the correct direction.

    My little story pre dates 407c equipment by quite a few years though & lots of engineers looking on this site still working on basic R22 kit.

    The point I tried to make was that nobody tends to think about this issue until its to late & some one has to pay.

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    Re: 3 phase resistance calculation

    Quote Originally Posted by Thermatech View Post
    The point I tried to make was that nobody tends to think about this issue until its to late & some one has to pay.
    Thermatech
    It's not just the reverse phase protection though. If the phase imbalance goes over 10% the electronics shuts down the equipment under fault conditions to prevent damage.
    I agree that not all manufacturers include this but most of the 'better' ones do.
    I've worked on lots of old 22 units where this is so. Chillers especially (OK I know that's not splits)
    I'm back on the Pale

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    Re: 3 phase resistance calculation

    Frank

    Ive been to lots of VRF systems with burnt out compressors & inverter damage which was caused by faulty 3ph isolator.
    Good 3ph from distribution board to external isolator but only 2ph from isolator to outdoor unit.
    The units have reverse phase protection which stops the comp running when you turn on the 3 ph power to the unit. But that did not protect the comp when it was running & one ph was cut off.
    After all any one can open up an RS catalogue & order a phase loss protection relay as an extra protection for the power supply but no one seems to even consider it until after the problem has happened & then they find out that the a/c system did not have that level of protection.

    So I have for many years suspected that the the reverse phase protection is a self test which the outdoor unit only carries out when the 3ph power is turned on & the unit has to decide if the phase is good for forward rotation.

    On your Daikin sky Air units have you ever seen the protection fault when you have power drop on one phase or only when you commission the new system & you then have to switch arround the incomming cables for correct rotation of the comp ?

    Perhaps this question warrants a new thread which asks service engineers of the world which units they have found to save the comp when 1 of 3 ph power is cut while comp is running ?

    This is the type of issue which any sales person will tel you the units have every protection you care to imagine but you can be sure the reality is completely different & service manuals are not always completely clear.

  23. #23
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    Re: 3 phase resistance calculation

    Quote Originally Posted by Thermatech View Post
    Frank


    The point I tried to make was that nobody tends to think about this issue until its to late & some one has to pay.
    I do, always a overcurrent relay on a 3 phase motor.
    Besides this, many hermetic compressors have a clixon in the starpoint of the windings.
    Most semi hermetics have an additional Kriwan protection and together with the overcurrent, you almost have a perfect motor protection.
    If you have a phase loss on a 2 phase motor, then it simply will stop to run.

    The electronic devices in splits and VRV/VRF's not only control phase rotation but also phase loss, it's incoporated in teh same enclosure.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: 3 phase resistance calculation

    Peter 1

    never mind ,,,, forget I mentioned it

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    Re: 3 phase resistance calculation

    i spoke to a guy from RS today and he confirmed what many of you have said, that there is o formula to calculate the resistance. he did tell me that the resistance is a result of the motor design and that the motor is designed to rotate at a certain speed which is determined by the work the motor has to do. i don't understand it fully yet, but i will report back on this forum, hopefully before the end of the week, with a better explanation.
    thanks again for your interest

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    Re: 3 phase resistance calculation

    i am going to try and answer the origanal question.

    you are wanting to do a quick calc to check a motor winding. on the side of any motor there is a data plate ( as i am sure you know). the info you need is motor KW, V, A and cos.

    formula W (kw/1000) = 1.732 * VL * IL * cos

    The motor KW rating is the culmative total of all three balanced windings. so motor kw/3 is approx the power rating of 1 winding. ANS/Vp = Ip. then do Vp/Ip = winding Z ohms.

    in reality its a bit more complicated than that, but in the middle of the night on a wet and windy roof, it should give you a good idea.

    hope that helps

    sparky matt

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    Thumbs up Re: 3 phase resistance calculation

    try this,


    resistance=power/root3 voltage

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    Re: 3 phase resistance calculation

    Quote Originally Posted by praveen008 View Post
    try this,


    resistance=power/root3 voltage
    Hi and welcome ,you should make it a habbit to look at the date of posts!!!
    THE DEFINITION OF A SMART PERSON IS ONE WHOM LEARNS FROM HIS MISTAKES!!!

  29. #29
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    Re: 3 phase resistance calculation

    Quote Originally Posted by bill1983;91170...
    but i will report back on this forum, hopefully before the end of the week, with a better explanation...
    Still waiting since more than 2.5 years now
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: 3 phase resistance calculation

    apologies Peter and to all who contributed. Thanks Praveen for reminding us of this thread. The problem at the time became academic, as the power company upgraded the mains, the comps all ran which i already suspected they would as there was no winding resistance imbalance. I'll get some workshop time in the next 2 weeks and try all the options and formulas you have provided on a variety of compressors to see which works. and this time i won't leave it for 2 years before i return to the thread.

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    Re: 3 phase resistance calculation

    Peter did you deliberately post your latest at a ridiculous time just to prove you have nothing better to do, or had you just returned from a night out on the town? 3.52 am shocking.

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    Re: 3 phase resistance calculation

    Strange, if there's one who needs his minimum 7 hour sleep , then it's me. It was posted on a regular hour. Strange.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: 3 phase resistance calculation

    hi all dear
    i have question that if any comp. has motor with part winding. it mean motor has 6 wires for check amp. so my question is how much difference in each wire of amp.
    i have no exact idea i hope my senior can teach me.
    GOOD LUCK FOR ALL

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    Re: 3 phase resistance calculation

    My claculate guess is 3 wires should take the same current, the other 3 another current.
    Assuming a 40:60 ratio the I'd imagine a similar ratio for the current.
    If the ratio was 50:50 then they'd all be the same.

    Cheers,

    Andy
    Health and safety first..........unless I'm in a hurry.

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