# Selecting CTs and Panel Meter



## DML2112 (Nov 12, 2021)

Looking for advice on how to choose CTs and a panel meter for the following application:
3 phase, 480VAC, 5 HP motor, rated 8 amps full load.
Operator needs to be able to monitor amperage when a change in process conditions causes motor to draw high current.
After doing an initial search, I am not finding much in the way of straightforward info. I have found wiring schematics on how to properly set up the system. But not so much on how to choose appropriate ct's and meter for this specific application.
Any advice or recommendations would be highly appreciated!


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## just the cowboy (Sep 4, 2013)

Welcome
Here you go.
https://cdn.automationdirect.com/static/specs/acuampct.pdf for your CT.



https://www.automationdirect.com/adc/shopping/catalog/process_control_-a-_measurement/graphical_panel_meters/adm100-ln?gclid=CjwKCAiAnO2MBhApEiwA8q0HYTsQ7dUtAY0P2bCnp0RRRz2K0C3emnigN-ZQKcMA85SsUK3Or70NPRoC2K0QAvD_BwE


panel meter.
Your CT is selected by a current to current ratio. Secondary is usually 5 amp. Split core allows you to install it without removing wires.
Most meters will take this current and scale it in the proper ratio.

Cowboy


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## joe-nwt (Mar 28, 2019)

For best results, select a CT that puts your expected current somewhere in the middle of the CT range.


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## just the cowboy (Sep 4, 2013)

joe-nwt said:


> For best results, select a CT that puts your expected current somewhere in the middle of the CT range.


I'm from the old school with analog meters, 20% so you don't bend needles


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## splatz (May 23, 2015)

If I was going to use a digital meter I'd want a single meter that displays all three phases. This one can do more than just current and they make a three wire CT to go with it. 

Simpson A201 

Amik Digital Power Meter (simpsonelectric.com)


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## SWDweller (Dec 9, 2020)

I always try to be in the mid range of CT's. Remember the bottom 10% and above 90% can generate errors.
Would be handy to add an alarm to the display, a set point you can control with a light.


https://www.automationdirect.com/adc/shopping/catalog/process_control_-a-_measurement/power_monitoring/socomec_power_meters/4825u010


Expecting the operator to "watch a screen" for problems is not something I have any luck with.
Make sure you use shorting blocks and fuse protection for your wiring. Shorting blocks will save a ton of problems, as you can reverse the polarity of the CT's with out removal.


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## ScooterMcGavin (Jan 24, 2011)

If you don’t need to know power or energy you could just use a current transducer that feeds a 4-20ma signal to a loop display or the DCS/PLC if you have one. Assuming the motor is balanced you only need to monitor one phase to determine if the process is overloading the motor. You also get to avoid all the potential issues that using traditional CT’s can have.

NK technology current transducers


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## micromind (Aug 11, 2007)

If you're looking for something simple that doesn't need external power and a basic analog meter is ok, it'd be pretty easy to find a CT that's 50:5. Since the load is 8 amps, and the CT is 50 amps, you'll need to loop one of the motor wires through the CT 4 times. This will 'trick' the CT into putting out roughly 3 amps with 8 amps on the motor. 

Next, get a basic analog meter that has a 5 amp input (very common) and a 0-150 amp scale. This will read percentage of full load. 

When the meter reads 100, the motor current will be pretty close to 8 amps. If it reads 150, the motor current will be close to 12 amps. 

If you'd like, use the same CT but loop the motor wire 7 times and use a 0-200 amp CT. 

These are rough currents, if you need the reading to be closer, loop one of the CT secondary wires through the core. You can go through all the calculations and polarity (yes, AC does indeed have polarity) and very likely get it wrong or you can just pick one, loop it through and compare the meter reading with the actual value and experiment until you get it right. If the reading is worse, loop it through in the opposite direction. If it's not enough, loop it through twice. 

One thing to remember about CTs and meters, never have the secondary an open circuit while there's current through the primary (the hole in the middle). The secondary (small wires) always must be either shorted or be connected to a meter. Shorting them is fine, the CT doesn't care but if the secondary is open and there's current through the hole, the CT will be destroyed quickly.


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## paulengr (Oct 8, 2017)

DML2112 said:


> Looking for advice on how to choose CTs and a panel meter for the following application:
> 3 phase, 480VAC, 5 HP motor, rated 8 amps full load.
> Operator needs to be able to monitor amperage when a change in process conditions causes motor to draw high current.
> After doing an initial search, I am not finding much in the way of straightforward info. I have found wiring schematics on how to properly set up the system. But not so much on how to choose appropriate ct's and meter for this specific application.
> Any advice or recommendations would be highly appreciated!


Europe uses 1 A as a standard. US uses 5 A. Process equipment uses 4-20 mA. There are sort of 3 types of CTs. The powered type are 4 wire or often these days 2 wire and output 4-20 mA. Big thing to remember is that they CANNOT work above their rating so if you select say 0-10 A and it’s pulling 20 A, all you will see is 20 mA (10 A+).

The others are either metering or protection grade. A typical metering grade spec is under 1% error and accurate in its range (say 10:5 so 5 A is the output with 10 A input current). Generally performance is bad at high current (above rated). Protection grade generally has higher errors…say 3% accuracy grade, but can easily handle at least up to 20 A before it saturates.

With this in mind you might find a meter with say a 10 A display and a 0-5 A input. If the motor FLA is 8 A you can easily see the normal operating range. If you want good readings up to stall then you’d want to see 50 or 100 A ranges. Also low ratio CTs like 10:5 are rare and more expensive so a 50:5 or 100:5 might be the better choice.

OR since it’s a motor look at say a microprocessor based digital relay like say the Benshaw SPD-60S. So this is a microprocessor based overload relay that direct mounts on most of the smaller IEC contactors and has a very nice external display that reads Amos, trip status, etc. it direct mounts so you don’t need to figure out CT ratios or worry about meter ratings. Downside is this is generally an overload meant for larger (250+ HP) motors so the price tag is a bit high. It does a lot though including recording some historical data so can be useful troubleshooting. It’s not SEL grade but it’s not SEL price. You see the same overload labeled the “RX” from Toshiba and Motortronics too. The actual manufacturer is LS (part of LG electronics) but it is heavily brand labeled. Street price is around $250-300. Only offering this as an option since everything is “done” for you and you can program it with limits, overloads, etc., to trigger an alarm without involving extra hardware.

My employer reps this under most of those names but we’d usually go to Benshaw first due to better inventory.


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## derslo (11 mo ago)

Assuming you have a power estimation project coming up, odds are you have reduced your quest for a power meter to a couple of decisions. Whether it's a multi-circuit application or a high-accuracy metering in a modern setting, the subsequent stage in project readiness is choosing the right current transformer to amplify the exhibition of your power meter. While going through the determination cycle, it very well may be useful to answer a couple, fundamental application inquiries to arrive at a choice and consider a few boundaries including current transformer yield, guide size, amperage reach, and exactness. Assuming you want assistance choosing, connect with your power meter producer so they can assist with directing you to the CT that best meets your undertaking's estimation goals and spending plan.


snaptube vidmate


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## paulengr (Oct 8, 2017)

Last thing!!

For non-powered CTs I know the temptation is to buy 10:5. You will have a hard time finding low ratios. Might be easier to just find a 0-10 A meter. This low if a ratio is hard to manufacture. Generally around 3:1 is sort of a limit. You could just use a 10:1 A but there is another trick. Buy a 20:5 and run the wire through twice. That makes it a 10:5.

Also before you go too far down this road, many drives and soft starters can be programmed to output power or current. And they are a little more (google Shark 100) but power is MUCH better. A motor draws both flux and torque currents. So at zero load a 4 pole motor draws 10-25% current, and because it’s almost pure reactive power the current doesn’t ramp smoothly with load. Power is directly related to how much work the motor does so it will be nearly zero at no load (just fan and bearings) and ramps evenly with the load. Plus it’s easier to see when a motor is “giving 110%”.


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