# Kostov 9 armature resistance question



## Tesseract (Sep 27, 2008)

727jjmc said:


> ...
> The other day, I tested the armature circuit resistance with a DC ohmeter, and the resistance was very high (1.5 Ohms).....


You can't measure armature resistance with a regular "DC ohm-meter"; you need to force a known current through the armature and measure the voltage drop instead. Then just divide the measured voltage drop by the applied current to get the resistance. E.g., if you force 10A through the armature and measure 0.5V of drop then the resistance is 50 milliohms.


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## 727jjmc (Aug 15, 2013)

Interesting. I did not know that. Thanks for the post!


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## major (Apr 4, 2008)

Tess is correct inasmuch as a common Ohmmeter or multimeter is useless in measuring the armature resistance of the type of DC motors like the Kostov. However, although he doesn't specify it, I think his method would include the brushes in the armature circuit being measured. If so, then the resulting value would not be representative of the actual armature resistance. The resistance of the brush and contact to the commutator is a nonlinear (V per A) quantity. Calculations of the motor circuit will most often use a voltage drop to characterize the brush not a resistance. Typically look-up tables or charts are used for various currents, RPM, temperature, humidity, age, percent seating, altitude and phase of the moon.

All kidding aside, the proper way to measure armature resistance on the 4 pole Kostov is to lift the brushes off the comm and probe with a Wheatstone bridge at 90º on the comm. With a good calibrated instrument this is very accurate. I've done it and have seen it done thousands of times as it is how we used to determine armature temperature for loaded rating testing. Precise change in resistance of copper indicates change in temperature.

Probably more than you needed to know and the Tess method may give you a good enough number to use for whatever you're doing, however the brushes will be a big part of that number.


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## Tesseract (Sep 27, 2008)

Aye, crusty ol' maj is correct that my method does include the brush resistance, and that the brush resistance is likely the dominant contributor to armature circuit resistance.

However, measuring the resistance of just the armature is a bit more involved and probably only warranted if one suspects one or more loops are open, in which case a regular multimeter will work fine for such go/no-go testing.

And since poor seating of the brushes can cause a huge increase in total armature circuit resistance*, and since that itself is important to know, I think my approach is not without merit, even if it is easier... 



* - which includes the resistance of the armature loops in contact with the brushes, plus that of the brushes themselves, and their pigtails, bolts, etc.


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## 727jjmc (Aug 15, 2013)

Wow. I'm speechless. I had no idea this was so involved. Thank you both for sharing your expert technical advice. It all makes sense now. I can now sleep at night and finish my conversion. Much appreciated!


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