The Big 5
Welcome back loyal blog readers! This week, we’d like to introduce you to the Big 5. Sadly, we’re not talking about the Big 5 Personality Traits, although we’re going to get to those. If you have a lot of extra time on your hands – as many of us do right now – you can google that and take a very, very extensive personality test. No pressure though, it’s seriously very long. And, you already read this blog, so your personality is probably great. But to each their own!
Anyways, this big 5 is much cooler.
The Big 5 Critical Motor Tests. Are you as excited as we are?
To make things even more interested, we’re going to attempt to connect each of the Big 5 Motor Tests to the Big 5 Personality Traits. Wish us luck!
Let’s get to it.
Kelvin Resistance Testing - Agreeableness
If this test was one of the Big 5 personality traits, it would be agreeableness. There’s truly no test as agreeable as this one. If you’re looking for a helpful test with little effort, this is your guy! Plus, the results are never questioned, and the fixes are usually just a crimp away from solution.
Low Voltage or Kelvin Resistance testing – when performed on motors specifically - is looking for imbalance conditions. Usually, we don’t know what the resistance of a motor should be – since it’s not written on the nameplate – but we do know that it should be balanced.
So, we use a constant current source and measure the voltage drop across the circuit – at a point of contact – for phase to phase. Then, you’ll get a measurement from each phase to phase test and you can do a few different calculations from there – like calculate individual phase resistance or imbalance. You can either take the deviation from average and divide it by the average or take the maximum minus the minimum measurement and divide that by the average. Either way, you’re going to be looking for a percentage value.
While there’s no established standard for the balance, we recommend having a balance of less than 3%. Ideally, less than 1% would be your best bet. But keep in mind, motors with concentric windings can have inherently higher imbalances due to their design.
We should also mention that if you’re comparing values to a standard, you’ll need to temperature correct the value to actually have a meaningful comparison. Typically, 25°C is the common value that temperature is corrected to.
Nominal DC Testing - Neuroticism
Moving right along. The DC test – or insulation resistance test – is one you’re probably already familiar with. If for some strange reason, you’ve just stumbled upon the Megger blog, you should know that Megger actually comes from the words megohm-tester, which is just another way of saying insulation tester.
Now that we’ve got the history covered, let’s talk more about the test.
This test is typically performed at or around the nameplate voltage, therefore, it’s not intended to be stressful to the insulation. It’s simply an evaluation of the groundwall insulation resistance value, which can then be compared to standards or prior readings. Technically, we aren’t truly measuring the value, we’re calculating it.
How is it calculated? We’re glad you asked! This test begins with a charging phase – where we increase the voltage to our test voltage. Then, we hold the voltage for one minute, allowing the current to decay, while the insulation polarizes. At the end of one minute, we take the measured voltage, divided by the measured leakage current, to get our calculated resistance value. Make sense?
If this test was one of the Big 5 personality traits, it would be Neuroticism.
But not in a bad way. This test is just so fickle about the details of current shape. Polarization index wiggles could indicate moisture or contamination, for example. If your motor is moody, this test will tell you all about it.
High Voltage DC Testing - Conscientiousness
Okay, next up – high voltage DC testing, not to be confused with the nominal DC test, alright? Except we are basically using the exact same test. We’re just slowly increasing the voltage up to the level of stress that the motor would experience in its normal life - like when the motor is turned on and off, for example.
For this test, you’re going to be looking at least at a voltage of twice the line voltage – plus 1,000 volts. For quality testing, you may be pushing this voltage even higher.
In the realm of high voltage testing, you do have options though. You could run a step voltage test, walking the voltage current up slowly and measuring the resultant leakage current at each of the “steps”. If this response is linear, that’s a great sign – letting you know that the insulation resistance is staying constant throughout the test, despite increases in voltage. Now, if you see a departure from linearity (like an arc), that’s not a great sign for your motor. It is a pretty great indication that your groundwall insulation system is starting to degrade. Sorry, but at least you have time to start looking for a replacement! You’re welcome.
If High Voltage DC Testing was a Big 5 personality trait, it would totally be conscientiousness. This test loves its set voltage levels and timetables. Preparation is key for this test, as it likes to pay close attention to every detail of voltage and current, along the way.
Surge Testing - Extraversion
Almost done. Everyone still with me?
The surge test is another high voltage test, but it looks at the copper-to-copper insulation, rather than groundwall insulation. By the way, copper-to-copper insulation is anywhere between individual turns in a coil or two adjacent coils in a motor, so we can be referring to either turn-to-turn or phase-to-phase insulation. Don’t let that confuse you!
So how does the surge test work? Pretty sure, we’ve gone into this on the blog before, but let’s just do it again for good measure. You’ll start by charging a capacitor with a certain amount of energy, which you’re then going to release into the winding. The energy produces a fast-rising impulse that will flow back and forth between the winding and the capacitor producing a decaying sinusoidal wave. After the wave dissipates, we pulse it again at a slightly higher voltages until test voltage is achieved. This produces a non-linear voltage drop across the turns of the winding. If the insulation has be compromised this difference in voltage will cause a change in the inductance of the circuit. The change in inductance results in shifting of the ringing wave pattern. It is this change in wave pattern that signals us to a problem with our turn to turn insulation.
Surge testing would be extraversion, since no other tests loves the spotlight and center of attention, quite like the surge test. Always finding faults that no other test can, the surge test really lets the power get to its head.
Dynamic Testing - Openness
Finally, dynamic testing. You made it. So, dynamic testing is performed while the motor is running, which is great because you want a motor to be running, right? If the motor is running, you’re making money, essentially.
Dynamic testing will evaluate the power quality and the motor and load operational quality, which can potentially be a source of heat – a motor’s mortal enemy. We even wrote a whole blog about heat last summer. It’s a big deal. If you weren’t yet a fan of the blog back then, we’ll remind you that excessive heat can lead to insulation degradation and accelerate mechanical failure. So, that’s definitely something you’ll want to avoid.
Dynamic testing will look at a lot of different parameters, while evaluating the motor’s environment, as well as the condition of the motor’s mechanical components.
Last, but not least, if dynamic testing was a personality trait, it would be openness! Dynamic testing is basically an entire motor laboratory in a suitcase. It is open to tackling new concepts in parameter evaluation and analysis – essentially the next great frontier of reliability.
So, there you have it – the big 5 motor tests. While low voltage testing alone is great, it’s not giving you the complete story. Without a surge test, you’ll never know what’s going on with your copper-to-copper insulation, which is scary. Plus, high voltage testing allows you to safely stress a motor with voltages that it’s guaranteed to experience in its lifetime. Together, the big 5 give you a complete picture of your motor’s health. Want to learn more? Check out our Blind Spots of Testing Motors with Low Voltage Testing webinar.