Ohm’s Law: Resistance, Conductance, and Application

“A professor who preached such heresies was unworthy to teach science.” This was the phrase used to describe Georg Ohm back in the day. Ohm was a German professor written off for his “web of naked fancies” by the German Minister of Education in 1825. Technology has come along a bit since then, but humans? Well, not quite as much. If you’re gonna hit someone up with an insult, try a little harder. Or at least have credibility beyond status quo bias as a basis for defense.

Now that the components of power and how they fit together have been covered, ohms as units of measurement for electrical resistance can be explained with a practical application example. Let’s say we have a handheld ohmmeter for motor insulation resistance (IR) testing, or what we in the pump and motor world call megger testing. We perform a field test to determine electrical current resistance in stator windings, which have an electromagnetic insulation varnish applied in manufacturing, in order to resist damage from the motor potentially overheating. The ohmmeter returns a value of 75 megohms of electrical resistance, or high current resistance. Is that good news or bad? Let’s have at this topic right now.

  • Ohm’s Law tells us there is voltage across two points, and that current in amperage is directly proportional to it. It’s expressed this way: I = V/R, where:
  • I is amps as current through the conductor (that conductor, or you can think of it as a passageway, is the motor stator windings in my example) = V, which is voltage across our conductor / R, meaning resistance of current in amps (how much resistance to current flow in amps are encountered across the stator diameter).
  • The freer the current flowing across the stator windings, the faster those windings can overheat and wear down their magnetic function in relation to the stator.
  • All kinds of other variables can also factor into insulation breakdown: excess moisture or humidity, corrosive vapors, hot, cold, vibration or mechanical damage.

We need that magnetic function to work in order to create rotating magnetic flux across poles. It’s not going to work if it’s worn down. That’s because flux creates a magnetic field in the air gap between the stator and the rotor. That, in turn, induces a voltage which produces current through the rotor bars. The rotating flux plus the current create the force for the torque needed to start the motor. When I read about and saw the insulation for myself, it was really confusing. Insulation for what? The insulation is to create a barrier to the free flow of electric charges. If windings sufficiently overheat, the insulation wears down, allowing for resistance to drop and the magnetic flux to lessen. That electric energy is transformed to the mechanical, or kinetic energy powering the motor shaft drive. That’s how this all fits together. There is more to this topic, but this is enough for now.

Now, back to the original question.

  • If we’re testing motor windings with a handheld ohmmeter that returns 75 megohms of resistance, is that a good thing?
  • It is, because the higher the resistance, the better shape that motor is in, where it will perform to its name plate rating while powering something that might have a critical requirement for power such as a pump.
  • If that ohmmeter returns a resistance of 10 megohms or lower, that means the DC current running the megger test is returning higher than acceptable conductivity.
  • Conductivity is the inverse of resistivity, measured in siemens.
  • It’s not a good thing and likely an indication that it’s time for a motor repair or more commonly, a motor replacement.
  • And yep, that test is typically performed in direct current. AC megohm testing could be done, but typically isn’t because the higher voltage has more potential to wear stator winding insulation down, causing current leakage.
  • As we know from thermodynamics, stray current manifests as excess heat. It kind of seems like once the insulation starts breaking down, the excess heat hastens the integrity degradation process.

If you’re taking the whole “this guy was European, not American” bit to mean that the ohm could be a metric system (SI) derived unit, you’re right about it. Knowing that, it follows that kilohm and megohm units are just multiples from the base ohm units.  In case you’re ever corrected for pronouncing kilohm as kiloohm and megohm as megaohm, from what I’m seeing now, those longer prefixes are right. But they’ve been shortened by convention. Who needs three syllable words instead of two for just one extra letter?

Now, I know what you must be thinking. Says who? Buck the trend and take the plunge! Use these three syllable variants to rivet and captivate your audience. Insist that these legitimate finer points receive their proper due instead of resigning – and then settling on a lifetime of obedience to a staid status quo. Just do it. And watch out for another type of resistivity: any flying tomatoes or pies coming at you. Don’t say I didn’t warn you.

*The image shown atop this post is of Ohm’s notebook, where he wrote out this practical and useful electrical relationship named after him that we take for granted today.

*Reference sources:

Click to access Megger-Guide-to-Insulation-Testing.pdf


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