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How to check the Windings of a 3-Phase AC motor with an OhmmeterAuthor
Giorgos Lazaridis
January 4, 2012

PAGE 2 of 2 - Checking the coils of a motor connected in Star or Delta


Checking the coils of a motor in permanent Star connection
Let's first see the leads of the motor in permanent Star connection:







From the schematic diagram you can see that if the first probe of the ohmmeter is connected to one lead (for example 1) and the other probe is connected to another (for example 2 or 3), the ohmmeter must show the total resistance of two coils connected in series. The ohmmeter must show double the resistance of each coil (since all coils have the same resistance).

So, here is what you have to measure: With the first probe connected on lead 1, you connect the other probe to leads 2, 3 and ground. If the motor has no problem, then you must find exactly the same resistance between 1-2 and 1-3, and infinite resistance between 1 and ground.

Then, you repeat the same process, but with the first probe connected on lead 2. Then you connected the second probe on lead 3 and on ground. The resistance between 2 and 3 must be exactly the same like before, and the resistance between 2 and ground must be infinite.

You don't need to make any other measurement to verify that the motor is ok. As you see, with this method, you don't need to have physical access in the motor's electrical connection. You can measure the coils from the protection relay in the electrical cabinet instead. This way, you can verify both the motor AND the power cables:


 
 



Of course, it is VERY IMPORTANT to ensure that there is absolutely no power in the circuit!!!


In short
  • You must find the same resistance between the three leads that power is applied to the motor.
  • There must be absolutely no connection (infinite resistance) between the leads and the ground
  • The measured resistance must be double the resistance of the coils.



  • Checking the coils of a motor in permanent Delta connection
    Let's first see the leads of the motor in permanent Delta connection:







    From the schematic diagram it is obvious that there are three points that you can measure with the multimeter without disconnecting the ohmmeter, the three nodes 1,2 and 3. Like before, you must find exactly the same resistance between 1-2, 1-3 and 2-3, as well as between 1-ground and 2-ground.

    For educational reasons, let's calculate the resistance that must be measured. We assume that all 3 coils have the same resistance RL. Suppose now that we measure between nodes 1 and 2. It is obvious that the coils between the nodes 2-3 and 3-1 are connected in series. The total resistance of these coils is 2RL. Moreover, the coil between the nodes 1-2 is connected in parallel with the previous 2 coils. So, the total resistance which will be measured with the ohmmeter is the result of the RL parallel to 2RL:


    RTOTAL = ( RL x 2RL ) / ( RL + 2RL ) = 2RL2 / 3RL => RTOTAL = 2/3 RL


    From the above formula we see that if the coil resistance is 20 Ohms, the resistance that the ohmmeter will measure will be 2/3 x 20, or 0.66 x 20 = 13.3 Ohms


    Once again, it is obvious that the coils can be measured either directly into the motor's electrical box, or on the motor protection in the electrical cabinet. Just make sure that there is absolutely no power in the cabinet.



    In short
  • You must find the same resistance between the three leads that power is applied to the motor.
  • There must be absolutely no connection (infinite resistance) between the leads and the ground
  • The measured resistance must be 2/3 the resistance of the coils.







  • Continue reading. Click here to go back to the first page <<<.



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  • At 20 October 2014, 18:05:29 user sahil wrote:   [reply @ sahil]
    • undoubtly good article


  • At 12 October 2014, 1:54:44 user Anil kumar wrote:   [reply @ Anil kumar]
    • This article is very good


  • At 4 October 2014, 13:49:11 user dinesh kumar thakur wrote:   [reply @ dinesh kumar thakur]
    • good content


  • At 17 September 2014, 1:07:57 user bulelani wrote:   [reply @ bulelani]
    • I was doing my trade test, motor test task. I found out my windis are not da same while doing continuity test with my meter. Now I had to balance the windings, I had 21.3 on U and 10 on V but infinity (OL) on W windimg. I could not calculate my percentage difference becouse onfinity is too big it has mo value. My qiestion what was I surpose to do?? I knw da formula to calculate percentage difference its=

      Percentage diffence = Resistance high-Resistance low all/ divoded by resistance high tymes 100%. Whts infinity value?? Or my amswer


  • At 13 September 2014, 15:46:07 user Manfred Frost wrote:   [reply @ Manfred Frost]
    • Thanks. Concise and easy to understand, although not what I was looking for.


  • At 10 September 2014, 5:04:58 user Kevin Brierley wrote:   [reply @ Kevin Brierley]
    • A very clear laid out explanation, been a electrician mate i have no understanding of motors, we had a fault the other day on a motor, & i wondered how they would test it, Well now i do, now i even no about STAR DELTA.

      Nice one


  • At 9 September 2014, 6:42:07 user SS SHAH wrote:   [reply @ SS SHAH]
    • How do we know standard resistance/inductance value of 3-phase induction value for different capacity and speed of individual motor ?


  • At 8 September 2014, 19:20:55 user monu wrote:   [reply @ monu]
    • How do we know the terminal a1a2 b1b2 c1c2 if there are not marked A B C 1 2 3 ? I hope you understand my question that there is no identification of A1A2 B1B2 C1C2 SO HOW DO WE UNDERSTAND THAT A1A2 B1B2 C1C2


  • At 29 August 2014, 16:29:43 user Wellawattage Prabath Madushan Peris wrote:   [reply @ Wellawattage Prabath Madushan Peris]
    • you have explained this very well.you have done a big task.thank u.


  • At 11 August 2014, 7:14:05 user hashim m rusambi wrote:   [reply @ hashim m rusambi]
    • how can instol the winding motor


  • At 28 July 2014, 6:42:37 user DEEPAK wrote:   [reply @ DEEPAK]
    • How do we know the terminal a1a2 b1b2 c1c2 if there are not marked A B C 1 2 3 ? I hope you understand my question that there is no identification of A1A2 B1B2 C1C2 SO HOW DO WE UNDERSTAND THAT A1A2 B1B2 C1C2


  • At 13 July 2014, 18:11:37 user ramprakash wrote:   [reply @ ramprakash]
    • I want a motor winding checking list by ohm and any checking method because I was a face a last time problem I am checked the motor proper by continuity all ok so start the motor and its blast and I am face manny problem because I done the maximum work 5kw to 200 kw.


  • At 12 July 2014, 15:20:33 user subrata mukherjee wrote:   [reply @ subrata mukherjee]
    • in a single phase oil circulating motor,i test in my section continuously 4 hours.there were no probelem.rating of the motor is 7.5Amp,230V,but when it connect in motorcoach (inCOMPLETE OIL CIRCUIT) sometimes it appears taking high current,motor was not running,abnormal sound etc.what is the reason? please tell me.


  • At 1 July 2014, 23:48:04 user coltmanneil wrote:   [reply @ coltmanneil]
    • I have a twin speed 3 phase motor high/low speed ,at some point terminal block has been taken away.How can i find the high and low speed windings.??Thanks


  • At 22 June 2014, 5:52:13 user ADAMSON wrote:   [reply @ ADAMSON]
    • A 3 phase motor for a washer with the following details:
      type A12H4d-23
      Y = 190v 3phase 50hz speed:1455 RPM Curent:7.5A P2:1.5Kw
      IP:20 I.C:F part number 438970561

      The machine is indicating Error motor too hot after running for 4minutes.The winding resistance is 1.4ohms across the phases in star(uniform values).When running,one the current results obtained are::
      L1=2.5Amps L2=1.6amps L3=2.5Amps while phase to phase voltage=62.15volts and phase to neutral= 135volts(in low speed).The motor is supplied from a motor control unit which takes in single phase(220v) and delivers three phase to the motor.

      Please help.


  • At 7 June 2014, 0:56:48 user jithendra wrote:   [reply @ jithendra]
    • thank u @ zithu


  • At 31 May 2014, 13:01:04 user chris wrote:   [reply @ chris]
    • Ohmmeter can not detect a partially shorted windings specially when you're dealing with a larger motors. The best equipment to use is the microhmeter. And also you can use LCR to measure the inductances of each winding. May I request you to give a procedure in measuring the inductances using the LCR.

      Thank you


  • At 18 May 2014, 19:36:26 user Deepak kumar purohit wrote:   [reply @ Deepak kumar purohit]
    • Very easily you explained it, very nice


  • At 29 April 2014, 2:02:52 user Hereimjay_81@yahoo.co.in wrote:   [reply @ Hereimjay_81@yahoo.co.in]
    • Thanks. ....


  • At 3 April 2014, 13:12:54 user vijay sagar pyda wrote:   [reply @ vijay sagar pyda]
    • it's useful for Je


  • At 31 March 2014, 16:02:12 user Bill Bastenbeck wrote:   [reply @ Bill Bastenbeck]
    • Great info- very informative. Thanks!


  • At 23 January 2014, 21:14:27 user Mike wrote:   [reply @ Mike]
    • Great Site!


  • At 7 January 2014, 18:14:52 user Sankarappan wrote:   [reply @ Sankarappan]
    • Relly, very useful to my knowlage.


  • At 31 December 2013, 21:08:43 user Skip wrote:   [reply @ Skip]
    • Sir,

      Simply put your explanation of star and delta connections is probably the best one I have come across!
      Keep up the good work, you have a rare quality you must use to its max !


  • At 30 December 2013, 8:37:09 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Asokarasan Once i had a motor from a coil repair and the guy had this mistake as you mention it. It stalls and makes noise.


  • At 26 December 2013, 16:23:37 user Asokarasan wrote:   [reply @ Asokarasan]
    • if A-A1,B-B1,C-C1 is coil sequence in delta connection ,if we change A1-A, B-B1,C-C1 what will happen motor


  • At 4 December 2013, 8:01:57 user krishnamoorthy wrote:   [reply @ krishnamoorthy]
    • really interested in your lecture on resistance measurement. is highly knowledge to me.


  • At 31 October 2013, 10:03:41 user Mohamed Rafi wrote:   [reply @ Mohamed Rafi]
    • I really loved your lecture on resistance measurement. is highly facilitating.


  • At 25 October 2013, 2:00:11 user selladurai wrote:   [reply @ selladurai]
    • i'm like this circuit connection . Any more impede send mail


  • At 14 October 2013, 6:05:52 user LAKSHMI NARAYANA REDDY Y wrote:   [reply @ LAKSHMI NARAYANA REDDY Y]
    • Dear Sir,
      I would like to know the connection (star star/star) two speed three phase squirel cage motor also the connection (star/-star) that i saw those connection in ASIA type motors and supporting your answering by drawings(line voltage is 380vac)thankyou


  • At 14 September 2013, 2:40:05 user ch zeeshan wrote:   [reply @ ch zeeshan]
    • i like him


  • At 13 September 2013, 21:38:50 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Daramola Dele It depends on the power and type of the motor. It may have less than 1 ohm resistance or much higher.


  • At 13 September 2013, 18:38:49 user Daramola Dele wrote:   [reply @ Daramola Dele]
    • I really loved your lecture on resistance measurement. is highly facilitating.
      however, what is the minimum acceptable resistance and insulation resistance for a motor?


  • At 24 July 2013, 7:15:50 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Shawn Hall Always talking about Y connection, between common and any terminal is the lowest resistance, between two terminals is the highest.


  • At 17 July 2013, 5:22:42 user Joseph Wilfred Unekwu wrote:   [reply @ Joseph Wilfred Unekwu]
    • Thanks alot you just open my a new chapter to my knowledge.


  • At 12 July 2013, 15:02:01 user Shawn Hall wrote:   [reply @ Shawn Hall]
    • When testing the windings od a single phase motor,
      Technician #1 says that the common and the run terminals show the lowest resistance.
      Technition #2 says that the pair of terminals with the highest resistance will indicate that the other terminal is the common.
      Who is correct?
      A. Technician #1 only.
      B. Technitian #2 only.
      C. Both technicions #1 and #2.
      D. Neither technicions #1 or #2.

      I think the answer is D?


  • At 15 June 2013, 6:32:38 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Robert Chadwick 3-phase AC induction motor Star connection? Then it is dead.


  • At 13 June 2013, 22:56:29 user Robert Chadwick wrote:   [reply @ Robert Chadwick]
    • I have a 3 phase motor, and I am able to the 3 leads that go into the motor I get no resistance between 2 of the leads, and 33 OHMS from either one of those to the third lead. what does that mean


  • At 3 June 2013, 17:43:16 user Eswaran wrote:   [reply @ Eswaran]
    • @samantha @samantha. if we change the connection RPM will not change. this 2 connections related for voltage & current only. if the connection is star current will low sameas voltage should be costant 415v. delta current will high but motor will start at littlebit low voltage also. RPM related only NO OF POLES and Effeciency of motor


  • At 3 June 2013, 13:50:17 user danilo de vera wrote:   [reply @ danilo de vera]
    • I had a 3 phase ac motor i want to to connect to power supply and the control was starting by star then delta (6 leads) my problem is the si lead came from motor has no marking for the 1,2,3,4,5,6 i cant determine the 1,2,3 and the 4,5,6.please help me


  • At 11 May 2013, 12:03:24 user Tim Rigsby wrote:   [reply @ Tim Rigsby]
    • Will a fifty hz 415 .37kw work on 480 60hz.....?????,I


  • At 2 May 2013, 5:06:31 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @kazi sahriar U1-V1-W1 goes to RST. U2-V2-W2 shorted all together for star connection.


  • At 29 April 2013, 7:41:01 user kazi sahriar wrote:   [reply @ kazi sahriar]
    • dear sir,
      my problem is, I have a 3 phase motor which r 6 probe those r U1 V1 W1 & U2 V2 W3.so,how i can connect those terminal.Plzz tell me sir.


  • At 23 April 2013, 6:22:16 user Shelvindra Bali wrote:   [reply @ Shelvindra Bali]
    • I want know if there is any type of book or software showing me all the connection daigrams of the motor. For example like 2star 2delta 3star 3delta 4star 4delta for different rpm and poles


  • At 16 April 2013, 16:38:23 user Idriss wrote:   [reply @ Idriss]
    • That is a very clear explanation. I have read so many topics on this, but this is the first time I actually get it.


  • At 3 April 2013, 8:57:29 user RADHAKRISHNAN wrote:   [reply @ RADHAKRISHNAN]
    • sir,

      IN the Motor Winding end wire in 3 phase A1A2, B1B2, C1C2 connected thro RYB the motor will run But if any changes in the terminal connection

      A2A1 / B2B1 / C2C1 if the motor winding will get Abnormal heat.

      Further, pleas give the diagram for the How to identify the A1, A2 B1, B2,C1,C2 by using TEST LAMP METHOD


  • At 22 March 2013, 6:53:42 user Geofry wrote:   [reply @ Geofry]
    • Thanks for the info above. Its help me a lot.


  • At 16 March 2013, 12:15:37 user tanveer shah wrote:   [reply @ tanveer shah]
    • sir many regards

      kindly send me any post notification as to be more informative and conscious about my related field.
      thanx a lot


  • At 15 March 2013, 15:01:50 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Vasilis Caravitis This is the worst case scenario... Star-Delta switches though have schematics how to connect them. You have to consult this schematic. There are 6 connectors, RST and UVW. You connect the first row of motor connectors to R-S-T and the second row to U-V-W


  • At 11 March 2013, 4:34:37 user Vasilis Caravitis wrote:   [reply @ Vasilis Caravitis]
    • Hi, My problem is that I need to replace the Star-Delta change-over switch. The existing 5-position (broken) switch allows also reverse rotation, but I only need one direction of rotation. The rplacement is intended for single direction operation and has a different terminal pattern. How can I determine which cable goes where?


  • At 5 March 2013, 20:13:07 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Decker auto electric This motor must be connected in star probably (i may be mistaken though). Connect the 3 poles on one side all together, and then apply RST to the other 3 opposite poles


  • At 4 March 2013, 22:04:59 user Decker auto electric wrote:   [reply @ Decker auto electric]
    • I got this electric motor that i can not
      Figure out how to connect the internal
      Wires coming out of the motor..
      Could anybody help me on it???

      24 slot. 2 pole lapwinding 2 coils per group
      6 groups. 295 rounds per coil. Pitch 1 to 11

      440/220 volts 3 ph 3200 rpm
      There are only 6 leads coming out of motor
      But just can't get the motor running
      Thank a lot


  • At 16 February 2013, 16:33:43 user kevin wrote:   [reply @ kevin]
    • Hi does anyone have any ideas about wiring a six wire motor to use on a rotary phase converter because ive tried a few different ways but im getting nowhere, any help is appreciated.


  • At 6 January 2013, 21:00:17 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @samantha none, both will give you the same rpm. Delta will give you (if motor permits) more torque.


  • At 6 January 2013, 19:10:14 user samantha wrote:   [reply @ samantha]
    • pls tel me which connection is giving more RPM? (star or delta)


  • At 2 January 2013, 8:35:49 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Abbas musa there is no formula. It depends on the wire thickness and length.


  • At 2 January 2013, 2:28:13 user geoff wrote:   [reply @ geoff]
    • Well presented explanation. This is a good site for electrical mechanics.


  • At 1 January 2013, 15:42:05 user Abbas musa wrote:   [reply @ Abbas musa]
    • helo... Am 4rm Nigeria.please, what is the formula used for calculating the winding resistance of a.c asynchronous motor using the volt-ampere method?


  • At 20 December 2012, 2:16:37 user John Romano wrote:   [reply @ John Romano]
    • Six wires are not a problem. I have a motor that I looked at today that had 9 wires, and two ways to set up. The first is high voltage and the other is low voltage. What is the correct way to check the motor for proper connection?


  • At 13 December 2012, 15:46:52 user KHALID wrote:   [reply @ KHALID]
    • GOOD EXPLANATION, I LEARN TOO MUCH FROM THIS TOPIC.


  • At 12 December 2012, 20:36:44 user Thamir Kareem wrote:   [reply @ Thamir Kareem]
    • Dear Sir,
      I would like to know the connection (star star/star) two speed three phase squirel cage motor also the connection (star/-star) that i saw those connection in ASIA type motors and supporting your answering by drawings(line voltage is 380vac)thankyou


  • At 23 November 2012, 11:43:56 user Aamir mehmood wrote:   [reply @ Aamir mehmood]
    • I like very much improve knoldge my self
      thanks


  • At 12 November 2012, 16:54:39 user mahroof.o wrote:   [reply @ mahroof.o]
    • it'S EASY TO UNDERSTAND............


  • At 3 November 2012, 19:04:58 user jonas hamata wrote:   [reply @ jonas hamata]
    • This is good and clear


  • At 31 October 2012, 7:27:30 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @atish kumar read 'Checking the coils of an unconnected motor"


  • At 28 October 2012, 11:29:59 user atish kumar wrote:   [reply @ atish kumar]
    • I have the same doubt that..
      How would you go about testing the start & end of each motor winding prior to powering up the motor, either by the use of a multimeter or a battery and a volt meter. I understand that one could power up the motor and check the current being pulled by each phase, if there is a serios unbalance on a single winding, then turn that winding around, BUT I would like clarity on how to sort out the windings start and end before power up.


  • At 24 September 2012, 14:30:09 user ganesh wrote:   [reply @ ganesh]
    • may be ur ohm meter problem.


  • At 20 September 2012, 3:03:18 user kaac.perera wrote:   [reply @ kaac.perera]
    • i like this web site


  • At 24 August 2012, 7:16:52 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @marcus usually 3-phase motors with 3 wires are internally Y or delta connected. It must be on the plate.


  • At 24 August 2012, 5:23:58 user marcus wrote:   [reply @ marcus]
    • and if your three phase motor only has three wires not six - what way do you test it then?


  • At 21 August 2012, 21:29:29 user T.Sethuram kumar wrote:   [reply @ T.Sethuram kumar]
    • Today i got different problem what actually happen one motor got tripped continually so i checked the motor amps its high 12A so i checked the name plate there full load current 12A so that i removed the load and checked after that also same current 12A how it's i confused because with load 12A without load also 12 A so that i checked the winding resistance that is ok 3phase equally, after that i checked bearing that also good winding also good then how this it's happen anybody knows kindly reply me


  • At 18 July 2012, 8:19:57 user Francisco Gavina wrote:   [reply @ Francisco Gavina]
    • (Portugal) Great tutorial. I am 51 years old, I am self taught. I'm unemployed. I like learning things of quality. I like to diversify knowledge. It will not let me grow old. Thank you. Gavina.


  • At 21 May 2012, 11:30:55 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Ian Ellis there are 6 connectors on a 3-phase squirrel cage motor. You can test them with multimeter, ohm-meter or battery by following the instruction if the first page of this theory, at the bottom (Checking the coils of an unconnected motor)


  • At 20 May 2012, 5:29:49 user Ian Ellis wrote:   [reply @ Ian Ellis]
    • How would you go about testing the start & end of each motor winding prior to powering up the motor, either by the use of a multimeter or a battery and a volt meter. I understand that one could power up the motor and check the current being pulled by each phase, if there is a serios unbalance on a single winding, then turn that winding around, BUT I would like clarity on how to sort out the windings start and end before power up.


  • At 6 January 2012, 19:43:50 user Kammenos wrote:   [reply @ Kammenos]
    • @George excellent! I am working on a simple coil checker tool (that is why i uploaded this theory after all). I will check this link and see if i can add it as a function. ;) thanks.


  • At 5 January 2012, 23:42:46 user George wrote:   [reply @ George]
    • one other thing.

      NEVER WORK ON AC system by yourself, ALWAYS have another person there as the SAFETY person. To switch off the power, and/or save your butt/ life if anything goes seriously wrong


  • At 5 January 2012, 23:39:19 user George wrote:   [reply @ George]
    • @Paul - there are two methods for checking phase of windings mentioned on cr4.globalspec.com/thread/71498

      One uses a push button DC voltage and a galvanometer .

      The other uses live AC voltage on 1 winding against the other 2 windings connected in series to see if there is a net 0V AC or double the test winding Voltage, BUT beware YOU ARE WORKING WITH LIVE AC POWER. So make sure that you have tested for earth faults. [A multimeter is not really the greatest here a megger would be better] AND DO NOT TOUCH ANYTHING WHEN LIVE - use INSULATED PROBES


  • At 5 January 2012, 16:17:57 user Kammenos wrote:   [reply @ Kammenos]
    • @Paul :D :D i'll tell you a little secret. The motor that is the photos, is a repaired motor that had a burned coil. The (not so good) technician that repaired the coils connected one of them the opposite direction. I tried hard to find a way to measure that problem, but had no luck with simple instruments. You connect it in the supply and if it makes noise and does not rotate, or rotate with no torque, then change connection...


  • At 5 January 2012, 16:03:36 user Paul wrote:   [reply @ Paul]
    • Just wonderin...

      Some time ago I also had to figure out how to connect a 1.5KW three phase motor but on this motor there was not such a nice board for connecting the wires. It basically had 6 loose wires dangling from it.

      A big problem would be if one of the phases is connected backwards relative to the other 2.

      How do you check for that?
      In the end I just (sort of) guessed, but I took some extra precautions to prevent destroying a perfectly good motor.
      I guessed right but didn't feel happy about it.





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