Hi I’m interested in building up an experimental setup for motor current signature analysis. Since I’m not from an electrical engineering background I do have some doubt regarding the equipment like sensors and DAQs. Many works of literature are showing current transformers (1A-15A)and Hall effect sensors used as current sensors. Will it be enough to use a 4-20mA current transformer along with a DAQ (https://www.dataq.com/products/di-1110/) with up to 160 kHz maximum throughput sampling rate and 4-20-mA to get current waveform required to do motor signature analysis using discreet wavelet transform? Regards -rustum
Modern day Motor frequency inverters use (0/4)-20 mA for motor current analysis.
I suggest you to look schematics and manual one of the best inverters, like from Siemens or ABB.
I think you will get more detailed information from there.
If something, I will try to help in another way.
You need to calculate the phase current of the motor under normal and abnormal condition. The maximum of these values along with the specifications of your ADC define the ratio of the current transformer. You also need to read the datasheets of the selected current transformer and wires to see their performances under high frequency transients.
Lukasz's question is pertinent here: What sort of motor are you looking to measure?
Transformers can be used for isolating (and scaling) AC signals. The AC current in the transformer primary (which is the "load" current being measured) induces an AC current in the transformer secondary and this is monitored (typically by loading it with a low-value resistor and monitoring the voltage signal across the resistor).
Current transformers cannot be used to monitor DC currents. Hall effect and some other technologies are able to monitor AC or DC currents. If you need to measure current in a DC motor, these may be more appropriate.
The 4-20mA current loop inputs on data monitoring equipment are designed to operate with DC signals over this range. They typically connect to an active transducer (that could be powered by powered from the minimum 4mA loop current). The transducer converts a monitored quantity into a current signal. The 4mA minimum is often used to indicate a system fault: If the current drops below 4mA, this may indicate a calibration error; that the transducer has failed or the cable has been disconnected. This usually causes an alarm indication that the system requires maintenance.
It appears that you are planning to perform a high-speed analysis of current transient conditions. Depending on the capability of the measuring device (the DAQ system), it might be possible to do this simply by monitoring the voltage across a current shunt (a low-value resistor with an accurately-known resistance) connected in series with the motor. This is probably only appropriate if you do not need complete (galvanic) isolation between the motor and the monitoring system.
If you require isolation, this might be provided by a suitable transducer that can perform the high-speed current monitoring and convert this to an equivalent 4-20mA signal and is also designed to provide galvanic isolation. Your problem then becomes one of identifying a suitable transducer to suit the kind of device you are monitoring. Note that transducers may be available that can provide other signal ranges (such as 0-20mA, -20 to +20mA, -1V to +1V, etc.) and various types of DAQ equipment can monitor these signal ranges.
If you are monitoring current on an AC system and need galvanic isolation, then a current transformer might be appropriate: This will produce a bipolar signal (one that swings both positive and negative) and the DAQ must be selected to suit this. If this is what you need, then a 4-20mA current input is not the right choice.
Keep in mind that the accuracy of your measurements depend on each part of the monitoring system: If using a current transformer, this has an accuracy rating that can vary with the signal magnitude (for example, instrument-grade current transformers might offer 0.5% accuracy within a specified current range and frequency range but become less accurate if operate outside this rating). Transducers and DAQ devices will similarly have an accuracy specification over a limited range of signal amplitude and frequency. You might need to make some initial estimates about what these ranges are before you can even make initial selections about how to perform the monitoring. Possibly some "low tech" initial approach such as monitoring voltage across a series current shunt with a voltmeter or oscilloscope to gauge the signal magnitude will assist with this initial selection.
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