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


Close the Loop on Power!

Volts, amps, watts, kilowatts, and power. I don’t know about you, but until today, nothing has earned my ire and frustration quite as much as trying to clearly understand the difference between these electrical terms once and for all: volts, amps, watts, kilowatts, and power. They’re all to do with electricity, but what exactly do they each mean, and how do they all fit together? Today is the day for closing this loop!

A practical example involving all of these terms makes it easy to grasp them. Let’s say we have a motor with a dual rating of 230V/460V. Will running a motor at its higher voltage rating save money by using less amperage?

It won’t because we pay for power in watts or kilowatts. Volts are units measuring electrical potential, while amps are a unit of measurement for electrical current. Power is the combined value of amps (electrical current) and volts (electrical potential), and it turns out be the same for each “rated value” (meaning the 230V or 460V voltage values rated to the motor on its nameplate). Power, which is (amps * volts) is measured in watts or kilowatts.

So, for example, if we’ve got:

*14 amps run at 230V, (14 * 230) that’s 3,220 watts, or 3.2 kw of power

*7 amps run at 460V (7*460) is also 3,220 watts, or 3.2 kw of power

What’s interesting to note is that the higher voltage rated value of 460V is double that of the lower value of 230V. It takes half the amps (i.e., running electrical current) to get the same power (measured in watts or kilowatts) with 460V (of electrical potential) to attain 3.2 kw of power.

Running current at the higher rating can only save money on installation costs because smaller diameter wires can be run at the higher rating than are required for the lower rating.

This explanation, which I was lucky enough to stumble on in web research, finally had each term making clear sense, while showing how it all fits together in practical terms.

This is the source article via El Paso Electric: http://c03.apogee.net/contentplayer/?coursetype=md&utilityid=elpaso&id=12592


Thermodynamics in Cavitation

I’m not sure I ever really understood the underlying dynamics involved with hydraulic or liquid cavitation until now. I only knew that it involves pressure and equipment damage potential. Thanks to the common sense writing in Mike Volk’s publication, Pump Characteristics and Applications, it finally makes sense. Let me start by saying that there is no way to grasp this topic without learning something totally new to a lot of us.

Cavitation involves vapor pressure creating bubbles that collapse in implosion. I kept seeing images of bubbles headed toward pump impeller eyes in various books with the caption: “cavitation”. That was a start in the right direction in grasping this. It turns out that actually getting it means learning a little about thermodynamics. Those bubbles are boiling water! Now, wait a minute. How can that be? We are talking about ambient temperature water that is boiling, not high temperature water over a stovetop.

If you’re like me, you might have only known about raising temperature as a means to get liquid boiling. This is where it gets interesting. There is another way to get water or another liquid boiling starting with a lower or even ambient or cold temperature, and that is to lower pressure below vapor pressure. I was reading this in awe because I’ve gone through life without this fascinating and useful information. It turns out that pressure and heat are correlated. Raising or lowering either of these, pressure or temperature, has a direct impact on when cavitation happens.

I’ll use a few examples from Mike Volk’s book in my own words on this. So, if we’re talking about 14.7 psia, which is baseline sea level atmospheric pressure, water boils at 212 degrees F. That sounds familiar so far. Now, let’s go climb a mountain where the psia is lower than sea level atmospheric pressure and boil water via raising temperature at that higher level elevation. Water will actually boil at several degrees lower than 212 F, so that means the temperature level is relative to pressure level.

Now, let’s take the psia in the other direction. Let’s say we have 100 psia with a 300 degree temperature. That liquid will not be boiling at 300 degrees with that higher psia. Incredibly, it will just remain in a liquid state. You need to raise the temperature higher to get boiling in that case, and this is how pressure cookers work. Drop the pressure to 67 psia on it and it will boil. At 60 degrees F, vapor pressure is 0.2563 psia, so if pressure is dropped below that, cavitation results. That boiling water with vapor bubbles collapsing and imploding throughout equipment in a significant pressure drop situation even at ambient temperature causes cavitation!

There are predictable causes for water pressure dropping below vapor pressure in equipment such as a pump. Those causes for pressure loss are a topic for another time. And, I’ve been talking about water here. This topic applies to any liquid. If it’s liquid other than water, specific gravity needs to be factored in and taken into account. Thanks for reading!

Why flow coefficient in valve selection matters

Happy New Year and welcome to my first official learning blog post! Couple my current interest in water strewardship with my inherent love for writing, and it should come as no surprise that I’m ringing in the New Year with a blog of my own. I’ve benefitted so much from the blogs of others. Here is my small part and piece to add to it all from the perspective of a relatively new learner.

In the world of pumps, pipes, and valves that I’m in these days, there is a question of how to size a valve for a process system allowing for optimal performance. Undersizing valves can mean restriction upstream and back pressure buildup which can lead to flashing or cavitation. Oversizing via a too large flow/valve coefficient can lead to the opposite problem: a drastic pressure drop and velocity speed up. Here again, there is the real chance of encountering flashing or cavitation. Trim parts inside a valve can start eroding, causing a “bathtub stopper effect” wherein the closure element of a valve gets sucked into its seat. 

What are flashing and cavitation? These are pressure problems at opposite ends of each other, both with real potential for damaging effects to anything in their paths. When we’re thinking about the concept of pressure, it has to due with a strong or weak flow of water.

  • With high pressure, i.e. low/weak water flow, we want to boost water flow up to meet demand.
  • It is the opposite with strong/high water flow. We need to do the opposite and decrease it.
  • Too fast of a velocity or speed boost of the water flow results in a large pressure drop. A large pressure drop can lead to flashing, whereby water “flashes” into a partially vaporized state with the potential for equipment damage.
  • Slow the speed, i.e. velocity, down too far and we get a large pressure increase. This is where cavitation comes in. Now we have too much pressure, so we’ve got water static pressure dropping below vapor pressure instead. The vapor evaporation collapses on itself and that’s cavitation. Think of caved in or deformed pipes, valves, or equipment and you’ve got the idea here.

This, in part, all has to do with the valve “restriction”. What is restriction? Drumroll, please…

You say you’ve gotta valve in the path of a pipe, do you? Drumroll square roots, because they are all over this and so many other of these doggone mechanical engineering problems. There is an equation called the “Square-Root Law”, whereby flow = restriction * square root of pressure drop. I need to figure out how to get these symbols in my posts properly. Until then, there are calculators and charts that punch these out in no time.

Back to where we started here: valve coefficients. There is a sizing formula for this, known as Cv. Cv = flow * square root of (specific gravity/pressure drop).  Don’t get hung up or caught up in this. And please don’t let anyone try to say that you *just can’t* possibly understand or grasp this due to it being formulaic. Everyone uses calculators. Get your hands on one and you’re as good to go as anyone else making this proclamation. They’re not doing anything else we wouldn’t.

We need to get this right to size valves properly to optimize performance and avoid expensive wear and tear. Flow characteristics come into this, where there are inherent flow and installed flow characteristics. WTH, you might ask. I know I did. Inherent flow does not take into account the effects of piping, while installed flow does. There is more to this, but I am putting a close to this post for now. Thanks for reading. Please feel free to add or comment. Where is the rest of the topic here? I’m not done yet! If there is interest, please stay tuned for more.

All my best wishes for health, happiness, and exciting new learning and development in the New Year!

In gratitude,

Jennifer Zadka
