How to measure high voltage (60-70V) and current (75-80A) using a PCI DAQ or USB DAQ

[Reena]

Hi

I am working with a system which works on approximately 5kW. The voltage output of the system may go maximum upto 60-70V and hence the corresponding current somewhere around 75-80A. I have 10 such systems which I want to read one by one continuously for long periods of time.

I am trying to design the best appropriate automated system for this and is looking for best hardware that would be appropriate for this purpose. On searching for some options, I found that an Attenuator SCC-A10 may be used for getting the voltage down by a factor of 10. But I am confused whether the high current will pose a problem and also how to measure this high current. 

I need to measure the voltage and current simultaneously. Please suggest what would be the best appropriate hardware system for the same(preferably PCI or USB)

Hoping for an early reply. Thanks and Regards

Reena Sharma

Learning Made Easy

[johnsold]

Reena,

Before recommending a DAQ device, more information is required.

1. Are the voltages and currents DC or AC, and, if AC, what frequency?

2. Do all the systems have a common connection or are they independent?

3. How fast do you need to sample? How fast do the signals change?

4. What is the error introduced if the voltage and current samples are not exactly simultaneous but are separated by a small, known amount?

5. Are there interfering signals which will make the measurements more difficult?

At that power level you generally should avoid direct connections to the computer.  It is very easy for a ground loop or other transients on the power system to damage the computer or DAQ devices.

Lynn

[Reena]

Hi Lynn

Thanks for the immediate reply. The answers to your queries are:

1. The currents and voltages will be DC.

2. All the 10 units are independent and provide different terminals for output.

3. The voltages and current do not vary at a very fast speed. Changes are always very slow for each unit somewhere in seconds. So, the sampling rate does not need to be high.

4.I dont think there are any other signals that will be interfering with the acquisition.

I hope this helps. Please suggest what hardware should be used for such measurement.

Thanks.

[Henrik_Volkers]

To monitor the currents you needs some type of current transducers (shunt resistor or a LEM sensor ... ) to get voltages.

What type of sources are you monitoring?  A lot of profesional DC sources in this class already provide current monitoring outputs or a communication connector.

[Reena]

Hi Henrik

I am actually monitoring the performance of a solar panel array that is giving me an output of maximum 140 V as open circuit voltage and 80A as maximum short circuit current. The power is always constant at 5kW. I have been going through NI website and came across a few voltage attenuators and wish to know whether it will work for my application. I am planning to tap my open circuit voltage directly and introducing a resistance divider at its output. Measuring the voltage across one of these can provide me with the current in terms of voltage. (Though i will have to take resistances that has atleast 5kW power rating). Using 10 such modules can work for a 10 array system according to me but I am not sure.

Please guide if this solution will be efficient enough.

Thanks

Reena

LME

[johnsold]

Reena,

The way to do it correctly is to use a voltage divider followed by an isolation amplifier for each array.  Allegro makes current sensors using the Hall effect.  They can meaure up to 100 A with 500 V of isolation. (ASC758 series). Put one of these on each array also.

Then any DAQ device (or combination of devices) which has 20 or more analog input channels would work.

Some people have had problems getting USB devices to run for very long periods of time, usually because the OS turns off the power to the USB port.

I think you might be better off with the compact DAQ series or a PXI chassis.  It might cost a little more at the beginning, but may give you improved reliability in the long run.  You may also be able to get the isolated inputs in those form factors as well.

Lynn

[JÞB]

Good point Lynn

5kW is just not safely brought into a Computer Environment.  Proper test equipment designed for high power monitoring applications is essential.  You won't find this in a USB or PCI DAQ device.

ALTHOUGH- you might want to peek at this case study

[Reena]

Hi Everybody,

Thanks a lot for all the inputs. I have forwarded the issues to the higher management and I hope they decide to opt for a Caompact DAQ system. Also, I have one more query that if somehow by using a DC electronic load or something the voltage levels are brought down to a level 0-10 V and the current is converted to voltage within levels using the current sensor, will using a card like USB-6211 be appropriate. Does USB or PCI acquisition for long periods of time always tend to give problems irrespective of the type of signal we are measuring?

Thanks and Regards

Reena

[Reena]

Hello All

There is a good news that the idea for using a compact daq has been accepted by the authorities in the company. I will be really grateful, if you could suggest me with some of the hardware modules appropriate for my application and the way i can use them to the best.

Thanks a lot

Reena

[JÞB · Accepted solution]

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@Reena wrote:

Hello All

There is a good news that the idea for using a compact daq has been accepted by the authorities in the company. I will be really grateful, if you could suggest me with some of the hardware modules appropriate for my application and the way i can use them to the best.

Thanks a lot

Reena

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I could make some suggestions but do not have the time to completely understand your needs and the forums are not the best solution. 

Your Local NI representitive actully gets paid to do that kind of thing.  a google search suggested LME was in Pasadena. Zack Collins would be the rep to contact