Transcript
Introduction
I’m Dylan. I work in software here, and I’m gonna be talking a little bit about my white paper, three-phase systems voltage rotation. We’re gonna start out with a scenario.
The Scenario: When Motors Run Backwards
Common situation you’ve probably seen: a crew finishes restoration or a new install, they energize the service, and everything looks fine. Lights work, loads are good, everyone goes home. Then a day or two later, a motor starts running backwards or conveyor belt starts going the other way, pump might start working in reverse, and now that’s a problem.
Something important here is that nothing is broken electrically. If a motor starts running backwards, that’s not gonna damage the motor, usually. However, the problems emerge when you get to what the motor actually powers. A conveyor running backwards is really not something you want, and that’s where damage comes up. This is why it’s really important to understand where does this come from, what’s the problem, and then how do you diagnose this? We’re gonna go into a little bit about how you do that in PQ Canvas.
What Is Phase Rotation?
Quick fundamentals: it’s a three-phase system with three sinusoidal voltages. You can call them VA, VB, and VC, equal in size and offset from each other by 120 degrees. Now, the normal ordering is ABC, and we call that forward or positive. The opposite of that is negative or reverse, and that’s ACB.
Something interesting is at a glance, these look the same. Assuming you have very little unbalance, then a reverse and a forward rotation is going to look identical. That’s really important because rotation is not unbalance.
Rotation Is Not Unbalance
This is something important to nail down because it trips up a lot of people. You can have a perfectly balanced source, three phases, equal magnitude, exactly 120 apart, and still have the rotation wrong. The opposite is true. You can have a badly unbalanced source that has perfectly correct rotation, but they come from different causes. They look different in the data, and they get fixed in different ways.
I’m flagging this now because when we get to the numbers, you’ll see that the two cases are really easy to tell apart once you know what to look for, and PQ Canvas makes that easy.
Why Wrong Rotation Is Bad
When things start running backwards, that’s where a lot of problems start to show up. It’s hard to know how to predict how much damage that’s going to do because it really depends on the context of the installation. Depending on the load, the motor can also end up either overloaded or stalled, and that can damage the motor.
The easy version of this is that the installer who finds this at startup, they swap any two or three leads and walks away, and you’re done. The hard part about this is rotation gets reversed during a restoration and nobody notices. Because again, the single-phase loads don’t care. It just sits there until something starts acting up, and that could be hours or days. And that delay, that’s what can make it really expensive.
Variable Frequency Drives
Variable frequency drives deserve a footnote because they behave differently. A lot of common AC drives rectify the incoming AC into a DC bus and then synthesize their own output waveform. So for those, the line side rotation doesn’t set the motor direction, the drive does through its own output sequence, its parameters, or swapping two leads at the drive output.
The caveat is that some drives still watch the incoming phase sequence as a protection or configuration check, and they’ll throw a fault on it. So the rule is here, the drive’s manual governs the field diagnosis, not the general motor.
Parallel Transformer Banks
Now the higher stake cases. Parallel transformer banks are unforgiving. If you close a parallel switch onto two banks whose rotation doesn’t match, you get very large circulating currents, sometimes large enough to damage the banks before protection clears it. These mismatches tend to show up during restoration when conductors are being re-identified and reconnected under time pressure. You might not have labels, or you do have labels, but they’re not right. So it’s easy for a technician to potentially misconfigure.
Generators and Transfer Switches
Generators and transfer switches are similar. In an open transition transfer, the two sources are never connected, so a reversed generator rotation doesn’t fault the transfer itself. Instead, your downstream three-phase loads just run backwards after the switch completes.
But in a closed transition or in-phase transfer where the sources can briefly be paralleled, it’s much worse. Corresponding poles aren’t the same phase anymore, and closing the tie can put line-to-line voltage across the switching path and creates fault level current. So depending on the system, wrong rotation ranges from it’s annoying to catastrophic. It’s again really important to be able to identify this.
Symmetrical Components and Measurement
All of those features are the same physics. The positive sequence voltage, the balance ABC component, is what creates a field rotating in the intended direction and does the useful work. Negative sequence content in a normally running machine is a counter-rotating component. It makes heat and torque pulsation instead of useful torque.
A wrong rotation install in those terms is a system where essentially all of your voltage shows up as negative sequence, and the positive sequence drops to nearly zero. It’s the exact mirror image of a healthy system, and that flip is something we can read straight off the data.
Diagnostic Signature
What does each case look like with our symmetrical components? Well, balance positive sequence voltage is basically equal to your RMS voltage, and negative sequence is tiny. Ordinary voltage unbalance tends to push this negative sequence up, but only by a few percent, and that’s really important. For reference, ANSI C84.1 recommends keeping unbalance to 3% at a revenue meter under no load. So single digit percent, keep that in mind.
Wrong rotation is a totally different beast. Positive sequences collapse to zero and negative sequences climbs towards the full phase voltage. Since PQ Canvass reports unbalance as a ratio of the negative to the positive, the ratio goes to the roof. It’s not single digits, but hundreds or thousands of percent, and that’s nonsensical. But it’s also really easy to notice. If you see that going into huge, huge, huge numbers, you know you have something wrong.
Catching It in PQ Canvass
I’m gonna switch over to PQ Canvass in just a second here, and we are going to do a walkthrough, an example from the paper. PQ Canvass does try to make this super easy.
This is the same recording we used in the paper. It’s an example of a recording with some really clean data, and we’re gonna see why that’s useful. Just as an example, if you go to the waveforms, take a look at a periodic capture here. You’ll see that current is extremely low. It’s basically noise. And we have a really nice sine wave here for our voltage. So this is a really clean system, and then we’re gonna look at how we can actually diagnose the problem.
Using the Channel Swap Tool
First go, we’ll pick number six. These are periodic captures, so these weren’t triggered by any faults. This is just basically a snapshot through the recording, just regular data. PQ Canvass has this tool down here in the bottom left corner that lets us swap voltage channels two and three with each other, and we’re gonna see why that’s really useful.
Let’s go here. This is probably gonna look a little familiar. I just gave an example. This is our vector graph, and as we can see, it’s V1, V2, V3, A, B, C. It’s a forward setup. Great. Then if we look here, this looks good. Again, current is basically nonexistent.
I’m gonna flip this. And now we have V1, V3, V2. So again, it looks fine. At no point in the waveform does anything look wrong. It’s just this reversal. However, we’re gonna go over to the meter. We’re gonna go down here and 20,000. 20,000%. And this is your cue. This is how you find out when something is terribly wrong with your rotations and you have it reversed.
Why the Leads Don’t Tell You
Now again, keep in mind, if you do have this set up wrong, this is what you will see. You’ll just see V1, V2, V3. You’ll think it’s set up right because again, the leads don’t care. If you set it up such that it’s V1, V2, V3, but in reality it’s V1, V3, V2, you’re not going to notice. That’s what this tool, single button, lets you sanity check this. And that is how you do it.
Of course, it goes in the opposite direction too. If you happen to be looking at a waveform and this is just what you’re seeing, you can again use the same thing to flip it around. And if it goes to something totally normal and you’re like, “Oh, we’ve done something wrong.”
The Detection Workflow
What you just saw, that was the whole detection workflow, and it really is just that simple. You just open the vector diagram, you check the angular order, open the meter view. Again, you flip it around and you can see it.
However, before you declare field wiring at fault, confirm the recorder’s own channel mapping. Sometimes a unit was hooked up the wrong way rather than the service itself. In a clean recording with known mappings like this one, the two cases are unmistakable. But do check the mapping first.
Wrap Up
Phase rotation is a foundational property of any three-phase system. It’s separate from voltage unbalance. Again, very important. It has a clean quantitative signature that you can read right off of symmetrical components. PQ Canvass exposes this in a very easy-to-see, convenient way. The vector diagram is for your angular order. Symmetrical components shows you the magnitudes, and that’s basically all you need.
If you want to do some further reading, White Paper 371 covers symmetrical components in more detail, while White Paper 398 is a kind of a companion paper on voltage balance. And that’s it for me. If you guys have any questions, you can always shoot us an email. We’re happy to answer more. White papers are also available on our website, so do check that out. Thank you all so much for coming.
