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capturing dc motor waveform across shunt

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Hello,

 

We have an application where the positive of a Power supply is connected to a shunt and the other end of shunt is connected to the positive of a DC fan motor.

If a CRO is connected across shunt, it shows a sinusoid waveform. We want to replace this CRO with Labview software that can capture this waveform for further anaylsis using some NI hardware preferably a USB based. We have labview 2009 version running on windows 7, 32bit.

 

We are not sure if NI hardware can be used since motor is inductive and once the power supply is turned on, it gives a spike. So we needed some help regarding if any NI hardware can be used directly in the system. 

 

 

 

Thanks

Rohit

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What are the specs of signal you're interested in? Is it high voltage? High frequency? High current? 

Refer to NI-USB DAQ series to config your system based on your measurement specs. 

 

 

Regards. 


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voltage would be in the range of +2V to -2V and current around 7-12 Amps.

Frequency 3-6kHz

 

What I am worried about is, the motor being an inductive load, how will it affect my hardware and in turn my data acquisition

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LabVIEW 2009 or 2009 SP1?  The former will be far more difficult than the latter.  2009 SP1 was the first to be compatible with Windows 7.  You may not have noticed problems yet.  But, I get the impression you haven't tried to install the DAQmx drivers.  Doing this, you'll find the drivers need to get along with both the version of LabVIEW and the version of Windows.  If you're on 2009, this will cause problems.

 

"Spike" is mostly meaningless.  10V on a +/-2V measurement is a spike.  So is 100V.  What is the spike you're worried about?

 

You're wanting to take a voltage measurement.  In order for this to work, the voltmeter needs to have something that qualifies as infinite resistance.  Without meeting this condition, you take voltage from the branch you're trying to read.  Measuring the +/- 2V range is entirely doable.  The real questions you should be aware of are the value of that spike and the version of LV you're working with.  If you meet those, you can go find a DAQ device from the products page.

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I have installed all the required drivers. I am worried about my hardware getting damaged due to spikes or EMF produced by the motor which is essentially inductive. So I was thinking of using a Labview compatible CRO to capture the waveform, but it will be expensive.

I am attaching the photos of the waveform that can be captured on CRO across the shunt.

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Most NI DAQ cards / cRIO modules have some sort of over-voltage protection in them (for some of the modules I've used it's about 30V). Check the maximum level of the spike and you might find you're OK - but any time you over-voltage it you are potentially damaging/reducing the life of the device.

 

There's also the NI VirtualBench which is a scope/DMM/power supply all rolled into one which can be controlled/interfaced with through LabVIEW.

 

Perhaps you could put some sort of surge suppression in your measurement circuit to protect the DAQ device?


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Solution
Accepted by topic author rk_t

A major consideration is such systems is the possibility of ground loops. The motor system ground and the DAQ system grounds may have enough impedance that high ground currents in the motor, especially due to faults, can cause enough voltage difference to disrupt the measurements or damage the measurement equipment.

 

Very rarely is a direct connection to a shunt resistor a good idea with motor loads.

 

Consider using an isolated input DAQ device, an isolated instrumentation amplifier between the shunt and the DAQ system, or an isolated Hall effect current sensor.  I have used the Allegro Microsystems ACS series devices (>2 kV isolation) on a couple of projects.

 

Lynn

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Thank you very much for your replies.. that helped!! 🙂

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