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CM3 Logger slow response

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CM3 Logger slow response

Postby simonwait » Sat Nov 03, 2012 9:17 am

Hi, I have just bought a CM3 with the intention of recording a 3 phase system. With help from Martyn at Pico I have managed to write a code in VB which monitors and logs the data whilst injecting data from another part of the system with descriptions of what is happening at the time.

At present the system seems to idle at around 7A per phase but the system has loads of large inverters in it so as a test we pulled the power and then reapplied about a minute later to see the effects of the inrush current.

What I have found is that the data seems to suggest that when power is removed, rather than the measurement immediately dropping to 0V it steps down over around 40 seconds to about 0.5A. When power is restored we go back to 7A over about 10 seconds with no massive inrush.

This doesn't make sense. Why wouldn't the reading immediately drop and then on powering up immediately go over 7A before settling. I have checked the sample rate seems to be 1ms
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Re: CM3 Logger slow response

Postby Martyn » Mon Nov 05, 2012 11:35 am

7A is your idle current, but what level would you be expecting to measure up to, and what current clamps are you using?
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Re: CM3 Logger slow response

Postby simonwait » Thu Nov 08, 2012 2:31 am

On this occasion maybe under 80A but potentially up to 125A. I am using the supplied TA138
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Re: CM3 Logger slow response

Postby Martyn » Thu Nov 08, 2012 10:59 am

The response time is asymptotic towards zero and if you are only measuring a few amps of current then you are only using a fraction of the full scale deflection of the CM3. From 80A, or higher, towards zero and back up will show a much more rapid response.
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Re: CM3 Logger slow response

Postby simonwait » Thu Nov 08, 2012 12:31 pm

Short of sticking a phantom load and then subtracting this is there any other solution?
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Re: CM3 Logger slow response

Postby Martyn » Thu Nov 08, 2012 2:04 pm

As the input signal level is reduced the averaging time constant increases to maintain the accuracy of the measurements. It is the way the unit has been designed to operate.
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Re: CM3 Logger slow response

Postby simonwait » Fri Nov 09, 2012 12:28 am

So if there was say a 2m/s spike from 7A to 80A would that be seen? Do you have any product which would work in this instance even if not networkable?
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Re: CM3 Logger slow response

Postby simonwait » Fri Nov 09, 2012 2:39 am

Sorry I think I put 1m/s but 720m/s is the shortest cycle time isnt it?
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Re: CM3 Logger slow response

Postby Martyn » Mon Nov 12, 2012 11:47 am

Yes, the conversion time per channel is 720 msecs, and for that reason the CM3 is not designed for looking at spikes. If spikes are of interest then you would need to use the current clamps with one of our oscilloscopes.
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