Can we beat the heat?
Heat wave. Two words we are never thrilled to hear. Unless, of course, you thrive on triple digit temperatures, or maybe, you own an air conditioning company. For the rest of us, we struggle through.
While we are swimming, hiding in the AC, and eating ice cream to stay cool, what’s happening to the resistance of insulating materials all around us? We know it's exactly what you’ve all been wondering.
Unfortunately, as temperatures rise, the resistance of insulation decreases noticeably. Luckily, all hope is not lost. The time-resistance and step-voltage methods are essentially independent of temperature, so we can get relative values no matter what.
Another option is temperature correction. If you want to make accurate comparisons between readings, we suggest correcting to a base temperature, such as 20°C. Let’s talk about some general guidelines for temperature correction.
In general, for every 10°C increase in temperature, halve the resistance; or, for every 10°C decrease double the resistance. We should probably note, these rules also apply to the opposite temperature issues – the brutal, freezing cold winter temperatures. Nobody really likes those either.
For example, a two-megohm resistance at 20°C reduces to about ½ megohm at 40°C. Not so fast, though. Different types of insulating materials will respond differently to the temperature change. Great news though – we have a table for that! Motors, transformers and cables with all respond differently in elevated temperatures. In Table 1, below, the temperature correction factors are listed, and you can easily multiply your measured resistance by the listed factor to obtain the true measurement. Keep in mind, you must measure the temperature yourself, before you use the table.
Table 1. Temperature Correction Factors.
Let’s go through a quick example before we let you go on your own. You are using a class A motor. You measured the temperature in the windings to be 95°F and you got a reading of 1.9 megohms. If you look at the table, you will see your correction factor is 3.24. Excellent, it’s math time.
There you have it. Your corrected reading at a temperature of 95°F is 6.159 megohms. Simple, right?
Many engineers also recommend building your own custom correction factor table – specific to your piece of equipment. Just record two resistance values at two different temperatures and then graph resistance (on a logarithmic scale) vs. temperature (linear). This plot will be a straight line and can then be extrapolated (extended) to any temperature, so that correction factors can be read directly from the graph.
You’re probably wondering – is this really something I need to do? Um, yes! We are going to show you why. Consider the example below (from our Guide to Diagnostic Insulation Testing Above 1 KV). Jim (our made-up test engineer) has been testing his motor and keeping a detailed log for quite some time now. Great work, Jim.
Now, imagine if Jim had not corrected for temperature. We are going to show you two plots that he could have made. Which one is better?
Unless you were trying to make a great zig-zag design, the top graph is relatively useless. Whereas, the bottom graph (with temperature correction) gives us a valuable insight into the motor’s health and the deterioration of the insulation.
Now that we’ve covered heat, let’s get into humidity. If you’ve ever lived in a humid place, you know how miserable it can be. So, you may expect that the insulation is experiencing the same pain and suffering that you are. However, if your equipment is regularly running above the dew-point temperature, the test readings will likely not be compromised by elevated humidity levels. By the way, the dew point is the temperature at which the moisture in the air condenses as water droplets. If your equipment is idle, but stored at a temperature above the dew point, you are still in the clear. This assumes that the insulation surfaces are free of contaminants with absorbing properties, though, such as some lints, acids, or salts. The presence of these hydroscopic materials can alter your test readings and should be removed before performing an insulation test.
For electrical test equipment, we are mostly concerned about moisture collecting on the exposed surfaces, which would affect the overall resistance of the insulation. Unfortunately, dew is sneaky and it can form in the nooks and crannies of insulation before it is ever even seen on the surface of the equipment. Ugh! We recommend recording a measurement of the dew point, so you have an idea of the potential conditions lurking below the surface that maybe affecting your test results.
Which leads us to our next point – your maintenance records. With regards to humidity, there are a few things you should be writing down each time you make a measurement. First, it’s a great idea to make a note of whether the surrounding air was dry or humid when the test was made. If you are having trouble determining this, you could always just check the weather on your phone or computer. Next, you should be recording whether the temperature is above or below the ambient. What’s the ambient? It’s the temperature of everything surrounding the device under test. Got it? Finally, write down the ambient wet and dry bulb temperatures, so that you can obtain a dew-point and percent relative or absolute humidity later.
It’s not too complicated. When it comes to electrical testing, you can definitely beat the heat. With a little extra math and a few more notes, your equipment’s predictive maintenance is ready to go!