How to check the Windings of a 3-Phase AC motor with an Ohmmeter

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How to check the Windings of a 3-Phase AC motor with an Ohmmeter

Author
Giorgos Lazaridis
January 4, 2012

PAGE 1 of 2 - How to check the Windings of a 3-Phase AC motor

The rotor of a squirrel-cage motor

There are several different AC motor types, each one with different operating and mechanical characteristics. The most common type though is the -so called- squirrel cage rotor. It is called squirrel-cage, because it's rotor looks like the exercising wheel found in squirrel or hamster cages.

A typical 3-phase squirrel-cage motor has six connection leads in the electrical connection box for the three coils. If someone works with AC 3-phase motors, then it is important to know how to connect these motors in Star and Delta connection, and how to detect an electrical problem. There are basically 4 problems that the motor windings can suffer from:

Broken coil (infinite coil resistance)

Short-circuited coil (less than normal or zero coil resistance)

Leaking coil to ground (current leaking from one coil to ground/neutral)

Two or more coils short-circuited with each other (current leaking from one coil to another coil)

All of the above problems can be detected with a simple Ohm-meter. First of all, you need to understand how the coils are connected with the six motor leads that exist in the electrical connection box. Here is a photo from a typical electrical connection box:

As you can see, there are indeed six leads arranged in two rows. Since each coil has 2 endings, it is easy to understand that these six leads can be separated in three pairs, and each pair is connected to one coil. It sounds logical to separate these 3 pairs in a vertical pattern, but that's not the way it goes. Instead, the pairs are in cross-pattern like this:

You may now wonder, why the coils are connected in cross-pattern and not vertically... The answer is simple: To easily bridge a permanent connection. A permanent connection is when the motor is connected in either Star or Delta, and this connection is not supposed to changed during the operation of the motor. This is usually done if the motor is small (smaller than 3.5 KWatts) or if the motor is driven by an inverter or some kind of electronic driver. A permanent connection is accomplished with 2 or 3 metallic bridges.

Suppose for example that the motor is connected in Star. In a star connection, each one of the 3 phases (R-S-T) is connected at one end of each coil. The other ends of the coils are connected together in a common point. A star connection can be easily accomplished simply by bridging one of the two horizontal rows in the connection box of the motor. The phases are then connected on the leads of the other horizontal row:


This is why we call this type of connection "Star" (or Y)	A star connection is accomplished simply by bridging one horizontal row in the connection box	This is a photo from the connection box of a motor connected in Star

Suppose now that the motor is connected in Delta. In Delta connection, the end of each coil is connected with the start of another coil. The 3 coils are then connected in a circle, creating thus 3 nodes. The 3 phases are then applied on these nodes. A Delta connection is easily accomplished by bridging the 3 columns in the connection box vertically:


This is why we call this type of connection "Delta" (from the Greek letter Δέλτα)	A Delta connection is easily accomplished by bridging the 3 columns in the connection box vertically	This is a photo from the connection box of a motor connected in Delta

There is one last thing to know: All 3 coils must have the same resistance. That is of-course easy to understand why, so there is no need to explain any further.

Checking the coils of an unconnected motor

Being a motor unconnected, means that there are no Star or Delta bridges on its leads. This is the most straight-forward case to understand. All you have to do is try to find the coil pairs in the electrical connection box. Let's give numbers to the 6 leads:

Suppose that you start from lead 1 and you want to find it's pair. You connect the first probe of the ohm-meter to lead 1, and then you connect the other probe of the ohm-meter to leads A, B, C, 2, 3 and ground (motor chassis).If the motor has no problem, then you must find infinite resistance between all leads and ground, except from one lead. This one cannot be lead A though, because -as we said- coils are connected in cross pattern.

Then, you repeat the same process but this time you connect the first lead of the ohm-meter to lead 2, and the second lead to leads A, B, C, 3 and ground. Notice that you do not need again between 2 and 1 since you've measured already before. Again, you must find infinite resistance between all leads and ground, except from one lead. This lead cannot be the one opposite to 2 (which is the lead B) and of course cannot be the lead that pairs with lead 1 (found from the previous measurement).

Finally, you repeat the same process with the first probe on lead 3, and the second probe on leads A, B, C and ground. Now, you know exactly which of these 3 leads pairs with lead 3. If for example you found that lead B pairs with 1 and lead C pairs with 2, then obviously lead A pairs with 3.

Additionally, you can compare the resistances between the pairs. If they are all equal, then you have a very well manufactured motor. A tolerance of 5% (and some times 10%) is generally accepted. If the difference is bigger, this could mean that some wires of the coil are short-circuited and the overall coil length is shorter. This is the most difficult problem to identify in such a motor. Usually, if the motor has short-circuited windings, it won't take long until this specific coil it is totally destroyed.

In short

You must find the same resistance between 3 pairs of leads ONLY

These pairs must be in cross-pattern as explained before

There must be absolutely no connection (infinite resistance) between all other combinations

There must be absolutely no connection (infinite resistance) between the leads and the ground

Continue reading. Click here to go to the next page >>>.

Comments

At 15 March 2016, 5:53:51 user Ashraf Mortada wrote: [reply @ Ashraf Mortada]

What is the allowed tolerance for winding resistance as per standard?

At 9 March 2016, 17:18:43 user Naseem wrote: [reply @ Naseem]

I m very thankfull to u bcoz it is very clear concept. Today my all doubts are clear abt motor connection.

At 6 March 2016, 12:38:34 user moses mathe wrote: [reply @ moses mathe]

it help me a lot

At 21 February 2016, 23:59:37 user narendra wrote: [reply @ narendra]

Excellent article and photos

At 11 February 2016, 2:55:18 user rizuan wrote: [reply @ rizuan]

thanks bro

At 28 January 2016, 19:27:07 user innocent olo kubacha wrote: [reply @ innocent olo kubacha]

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At 30 December 2015, 5:16:05 user Shahin wrote: [reply @ Shahin]

Very good

At 29 December 2015, 19:15:38 user santhosh kumar wrote: [reply @ santhosh kumar]

Thank u very much.

At 28 November 2015, 2:41:10 user Thein wrote: [reply @ Thein]

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At 24 November 2015, 13:07:22 user Vins James wrote: [reply @ Vins James]

It was written so well, in clear and simple language , without leading to any confusion. Really good.

At 3 November 2015, 13:00:53 user rajatoseer wrote: [reply @ rajatoseer]

its really such a good thing which i did not know before. Thanks

At 2 November 2015, 6:25:09 user M.Ebin Nisanth wrote: [reply @ M.Ebin Nisanth]

motor details

At 29 September 2015, 11:59:08 user parthiban wrote: [reply @ parthiban]

5 HP MOTOR resistance value how much

At 19 September 2015, 13:53:32 user Shakeel ahmed wrote: [reply @ Shakeel ahmed]

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At 19 August 2015, 12:44:29 user Joan Myers wrote: [reply @ Joan Myers]

At 16 August 2015, 18:11:32 user Interesting wrote: [reply @ Interesting]

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At 13 June 2015, 2:10:49 user Kiran Kumar wrote: [reply @ Kiran Kumar]

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At 21 May 2015, 9:16:44 user k.prakashkesavan wrote: [reply @ k.prakashkesavan]

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At 20 May 2015, 14:09:45 user SAMUEL VARGAS wrote: [reply @ SAMUEL VARGAS]

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At 9 May 2015, 21:23:32 user Anthony wrote: [reply @ Anthony]

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At 14 April 2015, 11:05:46 user yogender sharma wrote: [reply @ yogender sharma]

easy way to known the connection of induction motor .

At 10 April 2015, 1:33:03 user Subhranil Mallick wrote: [reply @ Subhranil Mallick]

Thank you, its very easy to get the basic knowledge.

At 16 March 2015, 15:31:09 user Isuru Jayaranga wrote: [reply @ Isuru Jayaranga]

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At 10 March 2015, 13:53:35 user husnain wrote: [reply @ husnain]

At 8 February 2015, 18:24:00 user johnny magtulus wrote: [reply @ johnny magtulus]

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At 8 February 2015, 17:29:28 user vishwanath badiger wrote: [reply @ vishwanath badiger]

beautiful illustreted

At 17 January 2015, 13:57:19 user Ganesh wrote: [reply @ Ganesh]

If you give how to identify starting end and finishing end in a coil it is more useful. Just try to give that information

At 8 January 2015, 22:14:26 user danny wrote: [reply @ danny]

nice informative scripts

At 4 January 2015, 16:31:14 user jackking wrote: [reply @ jackking]

Guud....

At 28 November 2014, 19:48:12 user Guest wrote: [reply @ Guest]

Brief and thorough. Just a great piece of technical writing!

At 18 November 2014, 14:35:54 user Michael L. Rappaport wrote: [reply @ Michael L. Rappaport]

At 11 November 2014, 15:45:58 user SAHIL wrote: [reply @ SAHIL]

USFULL .. LIKE IT

At 10 November 2014, 17:07:16 user SUNIL GHORPADE wrote: [reply @ SUNIL GHORPADE]

You cleared all my doubts. Thanks

At 10 November 2014, 16:27:30 user kishore wrote: [reply @ kishore]

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At 23 October 2014, 1:09:22 user Michael A. White wrote: [reply @ Michael A. White]

Do you have same information for 12 lead 3 phase motor? thank you

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]

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At 17 September 2014, 1:07:57 user bulelani wrote: [reply @ bulelani]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

At 23 November 2012, 11:43:56 user Aamir mehmood wrote: [reply @ Aamir mehmood]

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]

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]

At 5 January 2012, 23:39:19 user George wrote: [reply @ George]

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]