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Two Phase AC Voltage and current Monitoring: Shunt OR CT based solution.

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Replies: 12

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Hi,

I have an application where I need to Monitor (read and control) the Voltage and Current which is connected to the motors.

I've stuck while choosing between CT and Shunt based solution.

As I read the shunt based solution will have isolators integrated while CT based need an external Isolators. Correct me if I'm wrong. Is that the only difference between those?

Can you suggest a reference for this application?

Thank you.

  • Hello, and thank you for considering TI Sensing products! Our engineers will analyze and respond when we return to the office on Monday.

    Best regards,

    Ian Williams
    Applications Manager
    Current Sensing

  • In reply to Ian Williams:

    Hi Ian,  

    Any updates?

  • In reply to Lakshminarayana:

    Hello Lakshminarayana,

    Here are some possible references:

    http://www.ti.com/lit/ug/tidubk3a/tidubk3a.pdf

    I believe the secondary coil of a current transformer can be used at a low-level voltage.

    Hope this helps,

    Peter Iliya

  • In reply to Peter Iliya:

    Hi Peter,

    What about the Shunt based solution?

    Thank you.

  • In reply to Lakshminarayana:

    Hey Lakshminarayana,

    My apology for the delay.

    For a shunt based solution the most critical aspect is to not violate the input common-mode voltage (VCM) absolute maximum rating. In addition to this you will want the typical operating motor voltage range to be within the device's recommended operating region.

    For example, the INA260 has a recommended operating VCM range of 0-V to 36-V, thus the motor voltage seen by the INA260 input pins cannot exceed this range. However, since this is AC motor I will presume that its voltage will go below 0-V and so INA260 will not work unless its supply voltage is floating with the VCM. Floating the supply voltage is easier to do when the VCM is a DC voltage so choosing a CSA (current-sense amplifier) with integrated isolation is possible option and you can browse through isolated amplifiers here: http://www.ti.com/isolation/isolated-amplifiers/products.html

    Unfortunately, using an isolated amplifier such as AMC1302 also requires a floating supply voltage (Vs) for the isolated input stage of amplifier. While these devices can measure bi-directional (AC) current, the input VCM cannot exceed more than usually a few voltage above/below its floating Vs. Thus using a simple Zener diode that is reverse biased to provide the Vs with a AC VCM will not work since the Zener won't be reverse biased once VCM voltage becomes less than the Zener's reverse break-down voltage. If Zener is not reverse biased, then the device will power down. Maybe using  a full-bridge rectifier + Zener to provide a constant Vs would help.

    Another option is to use a Hall-based, contactless solution. This design could help provide some guidance:

    http://www.ti.com/lit/ug/tidu522a/tidu522a.pdf

    Sincerely,

    Peter Iliya

  • In reply to Peter Iliya:

    Hi,

    thanks for your suggestion.

    I have a few questions on CT based solution. If my CT outputs 0-10V (Voltage type for 0-100A) how should I be able to read with uC analog input? Since most of the example is given are for CT 0-1V Output and then amplify the signal to read through the uC.

  • In reply to Lakshminarayana:

    Hey Lakshminarayana,

    Well you could use you could use an amplifier with a gain < 1 to compress the 0-10V into an appropriate full-scale range of your ADC. Or you could use one of our digital parts such as INA226 that can measure bus voltages from 0V to 28V with an internal ADC. These devices output the data over a digital bus.

    Sincerely,

    Peter Iliya

  • In reply to Peter Iliya:

    Thanks for the reference. Since in my application, there'll be 16 CT use of INA226 doesn't seem to be a good choice since I'll have to use 8/16 Digital bus(I2C lines) to read.

    Any links to reference design?

  • In reply to Peter Iliya:

    Any updates, Peter?

  • In reply to Lakshminarayana:

    Hi,

    I’ll be helping you temporarily. In your last post, links to which reference design did you ask for?

    Regards, Guang