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Measure the current of 3 phase induction motor

Zee613

Jan 26, 2017
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I am working on a project to the measure the load current of 3 phase induction motor through CT's, can someone guide me through this project e.g
- what type of CT i have to choose (ACS 758?)
-should i use the three CT's for three phase separately
The ultimate goal of this project:
-to the measure the Current and display it on pc/hmi/scada at about 100-200 meter distance (not sure, and also not sure whether i should use Arduino for it or Plc)
- to store the current data for specific time (2,3 days)
 

Harald Kapp

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Nov 17, 2011
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- what type of CT i have to choose (ACS 758?)
There are many possibilities. This one is just one. The sensor you use needs to match the expected current to be measured.

-should i use the three CT's for three phase separately
Definitely.

-to the measure the Current and display it on pc/hmi/scada at about 100-200 meter distance (not sure, and also not sure whether i should use Arduino for it or Plc)
At a distance of 200 m an anlog signal runs the risk of being severly distorted, especially a voltage signal as from the ACS 758. I recommend you either use a current transfomer (currents are not that easily distorted) or you put the A/D conversion next to the sensor and transmit the measured data as a digital data stream using a serial protocol, e.g. UART via RS232 or RS484. Covering 200 m does not allow you to use SPI or similar, even RS232 or RS485 will be challenged at this distance. Even Ethernet is not designed for >100m.
You may consider using optical transmission using optical fiber. This is especially robust and insensitive to distortions from teh environment (noise etc.)

The type of microcontroller is of no great importance as long as the processing power is enough for your purpose. Use whatever you are familiar with.

- to store the current data for specific time (2,3 days)
The duration alone doesn't tell us anything about the amount of data you want to store. What's the interval (every second, every minute, ...)? What's the amount of data per sample (3 phases, number of bits per measurement, with or without timestamp)?
How do you envision to store the data? Non-volatile in a FLASH or EEPROM? or volatile in a RAM, possibly backed-up by a battery to 'survive' power outages?
 

Zee613

Jan 26, 2017
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@ Harald Kapp
Really sorry someone misguided me about the distance it's not more than 30 meter. The really problem is to take the data form PLC/Arduino (after A/D conversion) and display it at 30 meter distance as well as store it to 1-3 days in PC.
-We are using DSL for internet so i think Ethernet and WiFi options are available.
- I don't know at this time that what amount of data I have to store (how can i know this?).
- Interval could be after every one hour.
- Again i don't know what amount of data per sample i need,
Actually i need a general overview, that in what ways it'll work and what can be the options.
I really appreciate for your guidance
 

Harald Kapp

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We can help you find a realization, but not your requirements. You'll have to tell us what is required.
That's goi
- I don't know at this time that what amount of data I have to store (how can i know this?).
Thats going to be calculated from the number of samples per time interval (e.g. per second, per minute etc.), the total length of the recording interval, the size of each sample plus any additional information you want to store (e.g. a time stamp when the sampla was taken, a number for which measurement channel the sample is valid etc.). It's up to you to define these parameters.
- Interval could be after every one hour.
What are you going to record and display after that interval? The instantaneous value of the current? the RMS value of the curent? The averaged value of the RMS value of the current within the last hour? Again it's your's to define.
- Again i don't know what amount of data per sample i need,
That depends on what you are measuring, see above, and the resolution required.

The really problem is to take the data form PLC/Arduino (after A/D conversion) and display it at 30 meter distance as well as store it to 1-3 days in PC.
Frankly I think that's the least of your problems. Once you have the sampled data in the PLC/Arduino, it's easy to transfer this information e.g. via RS484 over 30 m (RS485 is good for up to 100 m) to a PC where it can be read by a PC and stored as well as displayed.
I think you'll have to set up a rough sketch of the system setup and most of all you'll have to find/define the parameters of the system as described above.
 

Minder

Apr 24, 2015
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I agree that more information as the purpose of the project and the reasons for taking the measurements, is the load relatively constant or does it fluctuate wildly?
If it is a dedicated system it could display a constant reading, with just the peak and lowest readings stored with a time stamp.
The normal/mean readings may not be worth storing?
M.
 

GunTherapy

Jan 28, 2010
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A three-phase induction motor has three windings, usually connected in either delta or ungrounded-wye. If the motor is operating properly, the current in all three windings (and thus, in the three phase conductors serving the motor) are IDENTICAL. Thus if you want to monitor the motor current you need only monitor ONE phase (i.e., measure the current on one of the three motor phase conductors).

Now if you want to calculate the POWER (real, reactive and/or apparent) consumed by the motor, it's considerably more complicated, and you probably want to use a 3-phase power meter intended for that purpose. Measuring RMS current alone, however, is much simpler.

For the CT (for however many phases you want to meter, though as I said above you only need one), the CT needs to be designed to handle the full-load current of the motor (meaning, when the motor is delivering rated horsepower at the shaft). The manufacturer rates this as Full-Load Amperes, or FLA. For the CT, the motor operating VOLTAGE doesn't matter, only the FLA, since all the CT cares about is current, rather than voltage (technically the insulation on the CT enclosure needs to be rated for at least the motor operating voltage, but this is usually much higher than the operating voltage of most commercial motors).

CT's are typically rated as N:5, where N is the rated primary current (in amperes) that the CT is designed to handle without going into saturation, and 5 is the number of amperes the CT sources from its secondary terminals, when the primary conductor is carrying rated current. For example, with a 300:5 CT, the secondary current will be 5/300 = 1/60 of the primary current that produced it. Thus 36A of primary current (i.e., on one of the motor phase conductors), for example, will produce 6A on the secondary of the CT - this is the current your metering circuit must be designed to handle and measure,

Whatever circuit you design to meter this current, needs to take care of this conversion factor (known as the CT Ratio), so that you're reading a number that corresponds to the current in the motor phase conductor(s), rather than on the secondary of the CT. All this means is your metering circuit multiplies wvatever current it's actually reading, by the CT ratio, so that the current value displayed, is the current in each of the motor windings. Once again, the current is the same in all three phases, so whatever you're measuring on one phase, is the same current that's flowing in the other two phases.

Hope this helps clarify the power systems side of your question.
 

Zee613

Jan 26, 2017
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@ GunTherapy
Thank you so much for comprehensive detail, I got a better insight of my task.
Now the only problem that i have is, How can i display and store (for one day, after every 1 or two hours) the data on PC if i am using PLC?
does SCADA store the value of current (assume that i managed to display it on SCADA)?
And what will be the options for Arduino.
Thanks you so much
 

Minder

Apr 24, 2015
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SCADA is a general description of data collection/monitoring, and is custom programmed to your requirements, usually if interfacing with a PLC then you would most likely have to set up a Modbus interface with an appropriate PLC that has the feature.
If wishing to use Arduino then you would need to write a Modbus interface in the Arduino to PLC.
In N.A., Maple Systems have anything from simple keyboard readouts to dedicated PC terminals etc for using in conjunction with PLC's.
M.
 

TCSC47

Mar 7, 2016
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Is this an educational project or is it intended to be put into a functioning industrial setting (with associated H&S)? What are your budget limitations? Are you working with star or delta and do you want to measure phase current or load current? Do you want to measure any unbalanced current? Do you want to measure power in each phase and power factors?

Still many questions.
 
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