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Multimeter, testing and reliable readings.

Discussion in 'Electrics UK' started by dishman, 15 Sep 2021.

  1. JohnW2

    JohnW2

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    Ah, yes, I'd totally forgotten about the ('electrical') 'resistance zeroers' (of which, if I recall correctly, and rather uniquely, the AVO-8 had three - one for each resistance range) - and was only thinking iof the ('mechanical') needle-zeroing adjuster (which most analogue meters have/had).

    Now having remembered, the 'electrical' adjusters are/were clearly essential since the method of measuring resistance is so relatively 'crude' that it has to be adjusted for changing battery voltage.

    I still am not convinced that (given the non-linearity), measurements would necessarily remain accurate if one had to adjust one of those appreciably to compensate for a 'low voltage' battery - so would have to experiment to see if that is the case. The adjuster is presumably either altering an 'in series' resistance or altering shunting of the meter movement - and, without thinking too deeply, my first intuitive thought is that the latter might not distort resistance readings but the former might well do so.

    Kind Regards, John
     
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  3. JohnW2

    JohnW2

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    I think what you are talking about is essentially the concept behind the 'Kelvin Bridge', in which case I agree.

    If one is effectively looking at the voltages across the resistance under test and a standard resistor in series with it, then then the ratio of those voltages (hence ratio of resistances) will be the same regardless of what current is flowing through them - and it is that current that will be influenced by the 'quality of the connections' (and conductor lengths etc.).

    However, I don't think that is what Bernard illustrated.

    Kind Regards, John
     
  4. dishman

    dishman

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    Thanks for all the replies thus far. A small update with images!

    Just to remind everyone:

    I am just using this as way of testing/using my new multimeter. I thought it was a good little project to see if I could identify where the fault may lie without just using visual clues etc.

    Motor background: It suddenly lost power in my mower. I immediately assumed the carbon brushes may be worn, and they were. Down to the nub. When I swapped them out however, I noticed excessive sparking when the motor ran. It seems to me there was a short of some kind.

    In the meantime I had a spare working motor which works fine, so this became a "can I identify the issue and potentially repair" project.

    I will try to refine my attempt to problem solve as per @conny 's advice.

    For now I just went around the commutator testing the resistance between each adjacent bar. Without noting anything down, I concluded that they all seemed within a similar range of readings. Usually 0.3 to 0.5ohms. I pressed down hard with the tips to try and get a good reading. If I got anything wild I tried again until it gave a seemingly stable reading.

    But, everything seem ok based on this test.

    Visually the commutator looks fine. It may just need a clean. However there are a few areas that I have noted where the issue may lie.

    motor-9616-2.JPG motor-9616.JPG

    It looks like some of the insulation has popped out here. This is also where there is more blackening underneath. It is possible that a short only presents itself when the higher mains voltage is applied? It arcs between those contacts?

    One of the bars here also seems slightly raised, and when I say slightly I mean maybe a micro millimetre. I can just about feel it with my finger when running around the commutator.

    Further around the commutator are more areas with less insulation. But there is no blackening/scorching in those areas, so I don't think there is an issue here.

    motor-9617.JPG

    Thoughts? Worth even trying to repair? Or any further suggestions for testing with the limited tools I have.
     
    Last edited: 16 Sep 2021
  5. Harry Bloomfield

    Harry Bloomfield

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    This might prove helpful, if you want to progress your knowledge of motor testing..

     
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  7. conny

    conny

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    @dishman In the middle picture it seems the comm has 'thrown' a small amount of solder from the joint. There also appears to be excessive carbon contamination in the area and there is carbon between the segs, (commutator bars).
    Depending on how keen you are to further your knowledge/experience there are a few things you can experiment with. The first thing is to clean away the carbon deposits. If you have access to a high pressure air line you can blow MOST of it away. This may improve the readings a bit. In an industrial setting we used electrical solvent cleaners but for big ones we would often upend them into a 45 gallon drum of boiling water which had had a box of ordinary washing powder, (Tide or Omo were favourites), tipped in and left to bubble away over night. It was then thoroughly rinsed off with cold water and put in a baking oven for about 12 hours to dry out. You could do similar with a pan of hot soapy water for a couple of hours then rinse it thoroughly under the tap and dry it off in the kitchen oven, (temp no more than 135°C). You need to ensure it is completely dry by testing to earth with a megger before continuing. Minimum earth leakage is 1MΩ but the higher the figure the better. Access to a lathe/turner to lightly skim the comm would be a huge bonus but if you can rig up an electric drill to behave like a mini lathe you can polish the comm with fine emery held as a loop around the comm. (BE VERY CAREFUL IT DOESN'T SNAG! If it snags it can be a nasty injury. If doubtful of your capabilities it's best not to use this method). Last resort is to hold the armature in a strong bench vice with padding around the body and the comm overhanging. Loop a stirp of smooth emery around the comm and polish with a sawing motion, turning frequently until the comm is bright copper. Avoid flat spots by turning often. You won't get those deep indents out by this method but you will certainly reduce their impact on the brushes. The final stage is to undercut the micas' between the segs with the hacksaw blade mentioned in my previous post. You don't needd to cut deep, just enough to show clean mica below the blade. Don't forget to also clean the small slivers of mica which will stick to the sides of the segs. Clean the full length of the slot including between the joints. Brush/blow/vacuum the dust away when you have finished and before testing.
     
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  8. Keithmac

    Keithmac

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    These are my weapons of choice, both read zero ohms when test leads are shorted, both accurate and repeatable.

    As the saying goes "you get what you pay for", I use both daily at work (I've had the bottom one probably 11 years now and still my favourite).

    I bought the top one for it's Bluetooth pairing and recording/ graphing features (via my Phone).

    20210819_145706.jpg

    20210820_093857.jpg
     
  9. Gasguru

    Gasguru

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    Most multimeters are inaccurate measuring sub ohm resistances but the only reason those snapon readings are zero is due to the ability to dial out the lead resistance by using the REL (ative) button.... a feature pretty standard on multimeters at a certain price level.
     
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  10. Keithmac

    Keithmac

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    I've found them remarkably accurate in the sub 1 ohm region measuring stator windings etc, obviously there will be better than this for sub 1 ohm but for my uses these are more than adequate.
     
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