OpenEnergyMonitor Community

Measure the current of 3 phase induction motor

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
    -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)

Is this a college project?

What do you mean by “what type”? Have you looked at manufacturers’ catalogues to see what they offer?

What are the advantages and disadvantages of measuring each phase current? What do you know about the three currents?

How do you intend to transmit the data? What options are available to you? What factors might influence your choice?

These are related. Can you think why? What might make one or both of the choices impractical? What else might you need?

thanks for your reply,
No, it’s not a collage project. I am working in a Printing company as a Trainee Engineer and my Manager assign me this task.

  • Sorry for this silly question, I am going to choose ACS758LCB 50A for it because the total current at each phase is approx. 25 amp (at running).
  • The only advantage i know is that in a unbalance system current is not same at each phase that’s why i am using three CT’s at every phase.
  • it can be through Wifi or TCP/cat 5 (Ethernet), i think the distance and noise environment may influence (and sorry someone misguided me, the approx. distance b/w machine and display office not more that 30 meter).
    I am exploring the ways that how it can be done.
    thanks for your guidance

OK, sorry about that, but your questions read just like you were a student who wanted us to do his project for him. I’m ready and willing to help and steer a student, but I won’t do the project for him.

The first thing to check is inrush current. When a large motor starts up, the current initially is much higher than the normal running current. The motor manufacturer should be able to tell you what the starting current will be. Even if you have a “soft start” or star-delta starter to reduce the starting current, it will still be something to check.
If your normal running current is 25 A rms, the peak current is about 35 A, that only leaves about 40% for inrush or overcurrent. You must get the details and do the maths, but I don’t think that’s enough.

You won’t be using CTs with the ACS758LCB. You will be making a direct metal-to-metal connection in each wire to the motor. And that means that you must be very careful how you mount the ACS758LCB on a circuit board. You MUST have a large air gap between the high voltage terminals connected to your motor and the low voltage side connected to your electronics, and you must make sure that no dirt etc can bridge that gap. That air gap needs to be good to at least 2500 V for the safety of everyone concerned.

I doubt whether you will need to measure all 3 phase currents. If it is only the one motor that you are measuring, then the three currents should be the same to within a small amount. However, if they are not the same, it could indicate damage inside your motor and knowing that damage was happening could allow you to replace the motor before it fails completely.

If it is a factory, I would choose wired every time unless it is too difficult or expensive to run cables.

For your electronics, I think you probably need to use one of the Arduino range that has Ethernet built in or available as a Shield. Our emonTx Shield could be used as well, but it was designed for CTs and it offers very little if you are using the ACS758LCB.
From the data sheet, I think you can connect the ACS758LCB directly to the Arduino - GND & VCC directly to the Arduino’s GND and +5 V, and VOUT to one of the analogue inputs. With a 5 V supply, the output of the ACS758LCB appears to be ±2 V centred on 2.5 V, so that is perfect.
For software, you can use one of our sketches and emonLib. You will need to remove the parts for the RFM radio module and instead add in an Ethernet library and the appropriate lines to send the data. EmonLib has a filter to remove the 2.5 V offset and will perform the rms calculation. You will need to calculate the calibration coefficient. If you are using the 100 A version, its output is 20 mV/A, and I think the calibration coefficient will be 50. (For a voltage applied to the analogue input, it is the current that gives 1 V at the input.)

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Thanks you so much for you guidance, I got better insight of my task now.


My project is somewhat similar can u explain to me what u did to measure motor current ?
I want to measure 5hp motor current.

Thank you

Hello. Recently, I was trying to figure out how to measure and monitor an inrush current for a 50Hz high voltage motor. It’s a general question, but it kind of confused me. Thanks in advance.

If you’re specifically looking at the inrush current, then emonLib itself won’t help you much, because it averages current over a long time (long in terms of the duration of the peak, that is).

I’d suggest you want to take the basic current and voltage measurement routines out of emonLib, and over maybe each half-cycle (using the voltage signal as a reference) record the peak and calculate the average current. I think you won’t be able to send the data in real time, so I’d suggest you save a few cycles’ worth of data to detect when you do have a startup, and then store it to send the result later. I’ve no idea what the duration of the inrush will be - how big the motor is will determine that to a large extent - so that strategy might not work.

Thanks for your answer. However, I do not have experience using EmonLib software and I was thinking to somehow use a clamp meter instead. Generally speaking, the motor I am trying to test is a submersible motor. I am just trying to figure out methods on how to monitor and measure inrush current precisely.

What did you mean by this? To me “high voltage” means greater than 415 V. But if it is submersible, that is unlikely.