Transcript
Introduction to PMI’s Iron Core CTs
Today, we’re talking about PMI’s iron core CTs. We call them TLARs, and these are iron core transformers used for picking up current for our reporters. These are in comparison with the flexible regalsikls of these. And here today, we’re going to be talking about the iron core version.
These are most useful for low currents. If you’re measuring five-amp metering CT-CT secondaries in particular, you only got an amp or two of current, the iron core CTs are inherently more accurate at these low currents. So we encourage you to use the TLAR CTs whenever possible if you’re measuring very low currents under an amp or two. That’s where these things shine.
But they’re also useful for measuring safe ram circuits, where you’re just measuring the output of a breaker, or even in small residential or light commercial situations where the entire building current is only under a couple hundred amps, because you can choose the current range between 20 amps and 200 amps on these TLARs.
How Current Transformers Work
Current transformers are a safe way to measure current, small or large, that an AC load is drawing. This is accomplished through the main components of a CT that we outline in figure one here. The primary winding is typically the conductor that you are measuring the load on, and this winding will go through the core of the CT versus wrapping around the core of the CT like the secondary winding does.
The core itself can either be a solid or split type, depending on the clamp you have. For instance, all of our clamps are a split core type to allow them to be field installable, whereas a solid core CT is simply installed during installation of a conductor.
The burden resistor that we see here in the middle, that goes between the leads of the secondary winding, this is used to induce a load on the secondary winding of the current transformer, and this helps prevent any kind of high voltage conditions that might occur if the secondary winding were to experience an open circuit. This ensures a safe operating session, so to speak, for the end user.
CTs are generally expressed as their amperage outputs. For instance, 200 to 5 would mean that for every 200 amps on the primary, there’s a 5 amps output on the secondary. Figure two shows how we calculate the secondary winding output, where the current around the secondary is equal to the number of turns on the primary divided by the number of turns on the secondary times the current on the primary. Then for our case, where we only have one primary winding, we can use the simplified formula shown in figure three, where the secondary current is equal to the primary current divided by the number of turns on the secondary.
TLAR vs. TLARS Size and Features
One of the big things between the two types of TLARs we offer, the TLAR and the TLAR-S, is their size. The TLAR is a larger cable. It is our red and black offering. We have a picture of it in figure four, and the larger size on it allows us to be able to fit a direct burial aluminum cable through the jaws of this clamp.
Then in figure five, we can see the TLAR-S. It’s our blue and gray offering. It’s a smaller, more compact version able to fit in tighter spaces, and it also has a locking jaw on it to help prevent any kind of accidental releases.
Figure seven shows a set of the TLAR-S installed in a very tight clearance cabinet to monitor the draw of a three-phase motor. In a small space like this, you might be looking at using a set of nine-inch flex CTs to fit in this, since they’re a little bit able to contort to the small space. So this motor, under full load, only draws 32 amps, so with that low amperage, these TLARs shine a lot better and are a lot more accurate in that scenario than the flex CTs would be.
Tips for Using TLARs in the Field
Keep the Jaws Clean
One of the big things is to make sure that the jaws of the TLARs are fully clean when you’re going to use them. Any kind of dirt or debris that would be on the jaw face could result in the core not being fully closed, and this would result in a buzzing sound being introduced that you can audibly hear.
This gap here, where the two metal faces meet is really important to make that a solid contact. They need to be completely in contact. A little bit of grit or dirt in there will prevent the clamp from closing completely, and that prevents the magnetic circuit from being completed in the clamp. And so it will not function effectively. Just a tiny little bit of dirt or grit in there will stop that. These two metal faces need to be joined completely and perfectly, and can surface together. Same thing on the blue TLAR, or any iron core clamp for that matter. These faces need to be clean and touching.
Keep Clamp Bodies Away from High Current Conductors
Another item for the TLARs would be you want to try and keep the clamp bodies themselves away from any kind of high current conductors or bus bars. The clamps can become magnetically coupled to these, and they can cause inaccurate readings to be sent to your recorder and kind of screw up your recording that you’re trying to achieve.
The way you might get in trouble is, for example, if you have a bus bar and the clamp is pressed against that bus bar and that has a lot of current flowing through it, say hundreds of amps, that will couple into the magnetic circuit here and cause your readings to be a bit off. So ideally, the body of this clamp, especially the magnetic portion, is not directly against a metal that’s also some sort of conductor that has hundreds of amps flowing through it. A circular conductor is not nearly as bad, but if it’s a bus bar where the whole body of the clamp is against it, that’s the worst case.
Current Flow Direction Arrow
On our TLARs, we have an arrow on the side of them to indicate the direction of the current flow, so these should always be pointed toward your load. We have them highlighted here in figure eight, but in practice, they don’t have the red outline around them. They’re molded into the plastic, so they can be not so obvious sometimes.
This controls the relative polarity of the waveforms for voltage and current for each phase. If the clamp is backwards, or the arrow is not pointing towards the load, you’ll have negative powers instead of positive power as recorded by the recorder. So it’s important that they’re all pointing towards the load, or at least all pointed in the same direction so that you have consistent polarity with respect to the voltage across all three phases.
It can happen that you’re out in the field and you accidentally install one backwards and you don’t find it out till after the fact. In your header report in PQ Canvas, you are able to change your scale factor. If you put in a negative one scale factor for that channel, that will flip it around so it’ll be the correct orientation when you’re looking at the recording.
Increasing Resolution with Multiple Wraps
When you’re trying to read a very low current, for your metering CTs, the five amps are gonna be your max output. But generally, it’s probably gonna be around one amp, one to two amp nominal. One way that you can increase the resolution of this is to wrap the conductor around the jaw of the TLAR multiple times, as we can see here in figure 10.
For instance, this wire in figure 10 is wrapped around the jaw four times. That’ll give us four times the amount of current going through that CT, and we’ll measure four times higher. So we can do a .25 scale factor on our header report so that we can drop that back down to where we’re looking for.
That’s a great way of boosting the resolution, especially if you have just like a tenth of an amp, or under one amp nominal current. If you have a metering CT or something like that where it’s only a fraction of an amp in normal conditions, wrapping multiple turns like this can help increase your resolution. Then as Charlie said, you would undo that multiplier in the software so that you are back to the correct number of amps.
Setting the Current Range in ProVision
Our TLARs, both the TLAR and TLAR-S’s, have two current ranges that we utilize. It’s 20 amp and 200 amp, and it’s fairly simple to set these up. If you’re in your initialization screen in ProVision, under your current range dropdown, we have a 20 amp or the 100, 200 amp setting for Flex or TLAR. But the 20 amp is specifically for TLARs only. The Flex CTs are not designed to go down to a current range that low.
The default if you’re using TLARs is the higher 200 amp range, just so you don’t accidentally over-range and have clipping. But if you know that you’re measuring five amp metering CT secondaries, go ahead and set this down to the 20 amp range. That gives you some over-range capability, too. If there’s a fault downstream of the meter, that five amps may shoot up to 10 or 20 amps and you’ll still catch that accurately. But if you’re measuring small currents, pick that 20 amp range.
Physical Differences Between the TLAR and TLARS
The blue TLAR-S is a bit smaller. It’ll fit into tighter spaces than the red ones. But the red jaw is a little bit larger diameter, and that allows them to clamp around some direct burial cables that the blue TLAR-S will not clamp around.
The other difference mechanically is the red TLAR just opens and closes. The blue TLAR-S actually has a locking mechanism. To open, you push this mechanism forward and that unlocks it and then you can open it. And when it locks, it goes back. So if you don’t have that positive lock, that means that there was grit or some sort of interference in these jaws. That also keeps it from accidentally opening.
One pitfall of using this style is that it may look good in your cabinet, but then when you go to close the cover, the cover may push against that jaw, or indirectly the jaw gets opened while you’re buttoning things up. And if it opens slightly, again, that’s gonna cause some inaccuracies. That’s not really possible with the blue TLAR that has a locking mechanism. Once that’s locked, that jaw is not gonna come open. That’s another advantage of the blue TLAR versus the red. But if you have situations where you have a large conductor that you want to clamp around, the red TLAR is larger.
Closing
That wraps up the white paper. If you have any questions about using these clamps, feel free to type them here in the questions box. Or if you have questions later, give us a call anytime at 1-800-296-4120 or send an email to support@powermonitors.com. Thanks for attending, everyone. Everyone have a great day.
