Individual Resistance Measurement

Discussion in 'General Electronics Chat' started by viju, Mar 6, 2013.

  1. viju

    Thread Starter Senior Member

    Sep 28, 2008
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    Can I seek the members help to measure the individual resistance of the circuit shown here? Each resistance joint is accessible for measurement.The circuit shown here measures the combination of 4 resistances in series in two arms and parallel combination of both.Is there any method that could measure R1 ~ R8 individually?
     
  2. MrChips

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    Oct 2, 2009
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    You have to break the connection in only one place in one of the two chains
    in order to measure any of the resistances in that chain.

    Repeat in order to do the same in the other chain.
     
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    theoretically you could measure the total reistance across each resistor in circuit and the solve 8 equations, but I don´t think anyone would want to do it like that.
     
    Last edited: Mar 6, 2013
  4. MrChips

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    Actually, remove the current source and voltmeter.
    Break the connection at any one resistor.
    Then you can measure the resistance across any one of the resistors.

    Or you can leave the current source in circuit, break the connection in one chain and measure the voltage across each of the four resistors in the chain still connected.
     
  5. viju

    Thread Starter Senior Member

    Sep 28, 2008
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    Resistance cannot be broken as they are fused together.Actually the model depicted is an eight coil pmdc motor armature.
     
  6. MrChips

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    Thanks for telling us now. Next time please provide all the info so that we can provide you with proper answers.

    So your question is, how do you determine a short or open circuit in an eight coil motor armature wired as shown?

    My guess is, you cannot.

    If the resistances are correct, the equivalent resistance is 2Ω.
    Anything lower than 2Ω means there is a short.
    An open circuit in one arm would give 4Ω.
     
    Last edited: Mar 6, 2013
  7. viju

    Thread Starter Senior Member

    Sep 28, 2008
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    Finding open or short is not my intention.Measuring as per the attachment will not identify turn to turn short in the winding due to tolerence level.Is there any other method to measure the individual coil resistance.

    I have already tried with a system in which I apply two current sources ( one fixed and one variable).I get good results but the process is cumbersome and requires huge electronics.

    I am seeking and exploring alternate measuring methods.
     
  8. The Electrician

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    Oct 9, 2007
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    What is your intention? Are you trying to detect turn-to-turn shorts?

    Is this a production problem where you must make these measurements quickly, and on a large number of units in a day?

    Are the eight coils connected in a ring as shown on the armature?

    Are the coils connected to a commutator?

    Can you post a picture of the armature?

    The more details you give, the easier it is for people to suggest help.
     
  9. viju

    Thread Starter Senior Member

    Sep 28, 2008
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    Yes for your questions and thanks for guessing. I could have elaborated in detail.In the production system I measure the resistance as per the attachment in first thread.Likewise I measure in clockwise direction for 4 combinations(1-5, 2-6, 3-7, 4-8).Each combination will have the same set of series - parallel combination ( measured 180 deg opposite).Which will not detect turn to turn short due to tolerence level.Measuring adjacent commutator bars will also does not yield result.I too use impulse testing to detect turn to turn short.But I cannot hit the root coil which is short.

    As already said I have tried with the above said method of using two current sources and get results.

    I just came up with an idea of shorting all the commutator bars except two and measure the equivalent resistance.Continuing with rest of the combinations could locate the exact coil which is short.But it would give indication that the coil which is short but not exact value.

    With the above combinations I tried the equations which all finally ended up with indeterminate.

    Can somebody help me to identify the individual resistance value
     
  10. The Electrician

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    Are all the nodes between coils available at the commutator?

    Does this mean that if you connect to the terminals of 1 coil, what you have then is one coil in parallel with 7 other coils which are in series. The series combination of the 7 other coils is connected across the single.

    So, you can work your way around the commutator, with each step of the way giving you access to one coil, with the 7 remaining coils in series, across the one coil?
     
  11. viju

    Thread Starter Senior Member

    Sep 28, 2008
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    The method described by you is measuring adjacent commutator bar. As mentioned I have already tested this and the result fall short of my expectation.
     
  12. The Electrician

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    What I want to know is whether or not my understanding of the connections:

    "Does this mean that if you connect to the terminals of 1 coil, what you have then is one coil in parallel with 7 other coils which are in series. The series combination of the 7 other coils is connected across the single."

    is perfectly accurate?
     
  13. MrChips

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    Oct 2, 2009
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    I say this is perfectly accurate, assuming that there are no interwinding shorts.
     
  14. The Electrician

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    Oct 9, 2007
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    A technique I have used to detect shorted turns in a transformer should work for you as well.

    I use an LCR meter to measure the inductance and Q of the winding. If there is a shorted turn, the inductance and Q will be greatly reduced. To show how this works, here is a picture of a small power transformer with a single turn of magnet wire threaded around the center leg of the transformer. To produce a short I simply pressed the ends of the wire together with my fingers--I didn't solder the ends together. This is not a very good short, but I can still detect it. I measured the Q on the low voltage winding of the transformer.

    [​IMG]

    I thought I would show how the amount of the decrease in Q depends on the measuring frequency. Here is an image showing the measured Q versus frequency taken on an impedance analyzer. The upper curve shows the Q without the short, and the lower curve shows the Q with the ends of the single turn pinched together with my fingers. It appears that the maximum change in Q occurs around 1 kHz. The Q decreased from 4.77 to a little less than 1 when measured at 1 kHz. The inductance also decreased from about 22 mH to about 6 mH.

    [​IMG]

    I think that if you are connecting to the commutator bars of the shorted winding, the fact that there are 7 other "good" windings in series/parallel may not prevent the low Q of the shorted winding from dominating your measurement. It is certainly worth a try.

    You wouldn't need to use an expensive impedance analyzer to make the measurement. A small LCR meter like this:

    http://www.ebay.com/itm/Handheld-LC...874?pt=LH_DefaultDomain_0&hash=item3f22e2781a

    would do the job. You might even have one around your lab. The best measuring frequency for you might not be 1 kHz. These meters typically can measure at 100 Hz, 120 Hz, 1kHz and sometimes 10 kHz and 100 kHz. You can try the various frequencies and determine what works best for you.

    Please report back to the forum if this works for you, and tell us how well (if at all!) the method was able to detect a shorted turn on your armature.
     
    Last edited: Mar 8, 2013
  15. viju

    Thread Starter Senior Member

    Sep 28, 2008
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    Excellent and detailed reply. I have LCR meter with me and will give it a try.If it works I can build of my own LCR meter to implement in the production system.
     
  16. The Electrician

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  17. The Electrician

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    Another question occurs to me. Are these armatures sufficiently expensive that you repair them, or do you just throw them away if they're bad?

    Do you need to know which particular coil is shorted, or is it enough to simply know that at least one coil on the armature is shorted?

    What are the dimensions of your armatures? Are they relatively small?
     
  18. viju

    Thread Starter Senior Member

    Sep 28, 2008
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    I was away from work due to illness last week.

    Armatures are miniature in size and are not expensive.These motors go into automobiles which is as important as the engine.

    Growler can detect the coil to coil short.

    My intention is to measure the individual coil resistances so that inter turn short can easily be identified.

    Winding impulse tester would do this job and I have them in plenty.

    Also I am able to measure individual resistances of 5 ,7,8,10,12 and 24 slots armatures with the set up made with two current sources( one fixed and one variable). Current sources will be connected in anti parallel and and by adjusting the variable current source the current in the arm beteen two current sources would become "0".This elimiates the series - parallel combination of other resistances. The voltage drop across the two current sources are the individual resistances.

    But I want to explore other possibilities with less electronics.

    I am grateful to the members who have given me ideas and suggestions.

    Let's explore.

    Thanks Mr.Electrician
     
  19. The Electrician

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    Even if you can measure the individual coil resistances accurately, is that sufficient to detect an inter-turn short if the short is only a single turn short?

    What is the tolerance on the DC resistance of an individual coil? How many turns of what gauge wire comprise a single coil in your example armature?
     
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