Now that the components of power and how they fit together are 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 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 ohms 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 circumference 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.
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 creates the force that generates the torque needed to start the motor.
“A professor who preached such heresies was unworthy to teach science.” Georg 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 haven’t 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.
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 source: https://en.wikipedia.org/wiki/Ohm%27s_law