VAC measurements

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Guest

VAC measurements

Hello,

I use an ADC11/12. In the software, you can select under measurements Voltage AC (besides Voltage DC, frequency and db) as a measured value.
Can a VAC source connected and measured direct to the terminal board?
Is 2,5 VAC/channel then also the max allowable voltage?

Second question, what does a measured value mean of 0b236? It''s the b between the numbers that bothers me.
This is also displayed in the recorded graph. It seems that the decimal point is missing somehow. What can be the reason of this?
Connecting a 4-20mA source doesn't give this problem.

Thanks
Peter

picojohn
Posts: 382
Joined: Mon Jun 11, 2007 1:10 pm
Hello Peter,

You may not connect an ac voltage directly to the unit as the input range is 0 to 2.5v dc. You may however, connect a dc voltage that represents an ac voltage.

With respect to your second question, what type of signal are you monitoring and how is it connected to the ADC-11?

If you send a copy of the .plw file for the recording session in question for my attention to support@picotech.com, I will take a look to try and understand what you are seeing and why.

Regards
John

Guest

John,
Is the option 'Voltage AC' then a fault in the software?
Or can an AC Voltage be connected to another unit?

We've connected a current clamp (1mV/A) to the unit. There's now a readout of 0b425 which should represent 4.25 Amp.

This afternoon, I installed a bridge rectifier on the output and a Capacitor over it to make a more flattened DC voltage. But we still see a value of 0b....
I will save a plw file of this and send it to you.

Peter
Belgium

Guest

Problem with the 'b' is solved. I changed the country settings under Windows from France to France Belgium where another setting of the keyboard is used.
The only problem still left is that we have to set the sampling interval to 1 second - which is to long - otherwise the displayed reading doesn't stay at a less or more fixed value.
Even if we reduce to 999 ms instead of 1 sec, the problem starts. The value overshoots and goes for 100 % and drops down to 0 for a certain measured fixed value. (Current probe measured over a lamp of 1000 W)

Can you explain me the difference in scan time and sampling interval?
Am I right that the scan time is the time the converter reads the incoming values on a channel, then takes the average of this measurements during the scan time and then display or store this average at the speed given in at the sampling interval?

picojohn
Posts: 382
Joined: Mon Jun 11, 2007 1:10 pm
Hello Peter,

With reference to your last two messages;

I am pleased that you have resolved some of your issues; however, it would help if you could clarify some points for me please.

My understanding is that you are using an AC current clamp to measure the current drawn by a 1kw lamp, presumably powered by 220VAC? If that is correct, how and where are you placing the current clamp?

Secondly, you are rectifying the current clamp output before applying it to the ADC-11? If that is correct, have you accounted for the forward voltage of the rectifier compared with the voltage from the clamp?

I will be happy to look at how best to achieve what you need using our products if you could explain your ultimate objective.

Regards
John

Guest

John,

We want to use this application on-site to measure the current of a motor (screw compressor)
There are problems with it and we can't pinpoint the problem.
We've 2 broken compressors and the repairs costs for its exceeds now already 25.000 â‚¬, so a lot of money.

Running current is in order of 60 A but starting current can be as high as 400A.

The clamps we're using gives us 1mV/A. This small voltage is of course AC.
We're trying it now with a lamp and 4 heaters of 500 W because those gives us the correct current and we can simulate this in our shop.

The idea was using a small safety transformer with a ratio of 25 to increase the mV voltage of the clamp from 100mV (100A) to 2,5 V and then rectify it with a bridge rectifier with a small capacitor over it.
I suppose the bridge rectifier will reduce the voltage with 0.6 V, so the rectified voltage should be (2.5 -0.6) x 1.4 for a reading of 100A.

I'm also a little bit afraid that the high starting current (400A) could destroy the converter because we're transforming the signal.

Am I looking to far perhaps?

picojohn
Posts: 382
Joined: Mon Jun 11, 2007 1:10 pm
Peter,

My suggestion would be that you use a current clamp such as our PP266 (http://accessories.picotech.com/current_clamps.html) connected to a PicoScope 2202 (http://www.picotech.com/picoscope-2202.html), which can be run using PicoLog data logging software or PicoScope.

As the clamp can take up to 600A, it will cope with both the peaks and standard running current of your motor.

The entire setup will cost a fraction of the repair cost for your compressors.

Regards
John

Guest
John,

Can the ADC20 or ADC24 measure the sine wave generated by the current clamp?
Am I right that one can't run the EL040 together with an ADC-11 (with Picolog) so that they're displayed on the same graph?
We have to measure 3 currents while measuring also the voltage.

Overlapping graphs or importing them from other loggers should be very useful. This feature is available with the Gemini loggers which we use as stand alone temperature loggers.

With the smallest Picosope, I only can measure 2 current clamps because there are only 2 channels.

The problem we're facing now with the screws is a problem for which they've called me. I will use this setup once and then it will be in my closet for another year.

PEter

picojohn
Posts: 382
Joined: Mon Jun 11, 2007 1:10 pm
Peter,

The ADC-20/24 is not in my opinion best suited to paticular your needs.

You are correct that the EL040 will not work alongside an ADC-11, as the EL040 Current Converter is an EnviroMon product (http://www.picotech.com/enviromon.html).

From our product range, I believe that the units suggested in my last response are the most suitable for your needs. If you require more channels, then you could use two PS2202 units or, you could use our four channel PS3424 (http://www.picotech.com/oscilloscope_kits.html). Using either of these combinations will allow you to view all channels in a single graph using the PicoLog software application.

Regards
John

Guest