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High currents monitoring

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High currents monitoring

Postby Alessandro Mandelli » Wed Jul 14, 2004 7:22 am

Just registered in the forum and browsed through the archive. It doesn't seem my question has ever been asked. If I'm wrong, I beg your pardon.

I would like to test my supplier's data for locked rotor amps and running current amps. I usually buy very big squirrel-cage three-phase 400/3/50 start-delta start refrigerant-cooled motors, sometimes in excess of 200 horse power.
By following my supplier's advice, I protect the motor with magnetothermal switches varying from 125 to 300Amps nominal and 10x starting current.
Sometimes these data proved to be on the wrong side of the road, forcing replacement of the switch caused by unexpectedly high LRA or MRA.

I would like to check the LRA myself using an analog oscilloscope like the ADC-40, coupled with a current clamp or similar.
My sampling times would be very low, usually around 4/5 seconds just around the Y/D switching and expected resolution doesn't need to be excessively high (8 bit I think will suffice).
I don't need to measure ripple. Just the current sine wave at 50Hz.

Is this application feasible with the ADC40? What accessories do I need in order to read a max locked rotor current of let's say 3000A and convert it to the analog input of the ADC-40? What if I'd like to sample voltage as well? Do I need a transformer 230:5? Do I have to ground the pc to the same earth of the motor?

Sorry for the numerous questions. Hope to be your customer.
Alessandro Mandelli
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Postby markspencer » Wed Jul 14, 2004 8:25 am

Hi,

Thank you for your questions and the great detail you have provided.

The ADC-40 will be able to sample at and be able to look at the 50Hz sine wave, this will be achieveable in both Picolog and Picoscope.

To measure the 3000 amps you will need to source a current clamp that will convert the current to voltage, the voltage should be kept within the voltage range otherwise you will have to do further signal coinditioning. In the software Picoscope you would then use a custom range to convert the voltage in the an Amp unit. If you where to use Picolog then a scaling file or calculated parameters could be used to convert the voltage into a unit of ampage.

To measure the voltage you will need to use an isolating probe, or contact free attenuator as there is no opto isolation between the ADC-40 and the PC.

If you wish to measure the Voltage and Current at the same time then you will require either two ADC-40's or a dual channel scope, I would (recommend that you consider the ADC-212/3).

However, the 2 ADC-40's could only be used in one set up, that is Picolog's 'Real time continuous' mode as Picoscope only supports one unit at time as does Picolog in 'Fast Block' mode. If you are considering using Picolog in real time continous mode I would recommend that you use the USB to Parallel port adapter, to control the timing for taking sampling.

If these anseers have producted further question or you would like more information please do not hestitate to ask.

Best regards,
Regards,

Mark Spencer
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Postby Alessandro Mandelli » Wed Jul 14, 2004 8:46 am

Thanks for your fast answer.
Is your company providing current clamps with analog output already in the acceptable range of the ADC-40 input?
Is there some accessory your company can provide for reading, reducing and isolating 400V or 230V voltage down to the input range of ADC-40?
If not, would you kindly suggest me where to look at?

And No, I don't think to measure voltage and current at the same time. I'm considering to sample three phases at the same time though, that would complicate the matter :lol: and bankrupt my tight budget.

Cheers
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Postby markspencer » Thu Jul 15, 2004 7:43 am

Hi,

Thank you for the additional information.

We do have an isolating probe that will allow you to measue up to 700 vAC. This is the x10/x100 Differential Probe details available here:

http://accessories.picotech.com/oscillo ... robes.html

We do have some current clamps but the highest only goes to 2000 amps, not the 3000 amps that you require. You could try these distributors:

www.farnell.com
www.rswww.com

To measure the three phases at once your will require three ADC-40's one for each phase and a current clamp for each unit, this will mean that you will alos need three parallel ports in your PC, or use the USB to Parallel port adapter. It also means that you will need to use Picolog in Real time continuous mode, therefore the fasted time interval obtainable is 1ms, and then only accurately if used with the USB to Parallel port adapter. The sampling will also not be taken at exactly the same time either.

Please do not hesitate to ask me any other questions you may have regards our products and your requirements.

Best regards,
Regards,

Mark Spencer
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