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Measuring the varying Resistance of a DC motor

I have a DAQ that uses LABVIEW. I need to calculate the changing current going to a DC motor. I cannot measure the current directly because it is too high for my DAQ to handle. Basically I need to continuously monitor the varying resistance of the motor, so that I can calculate the current the motor is pulling. Does anybody have any idea how I could do this? I was considering breaking open an ohmmeter, but I'm not sure how an ohmmeter works. Does the ohmmeter send a certain voltage with certain resistances?

Thanks in advance

Reply to
troyrivera
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It isn't motor resistance that changes. It's back emf. As a motor is loaded the back emf decreases, drawing more current.

Reply to
Rich256

Search the web for 'current shunt' -- that should help. Basically you want to measure a small voltage across a very small resistance with a big current flowing through it. You keep the resistance small so your measurement doesn't disturb the system, which means you need a good low-offset amplifier in the DAQ or out.

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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Reply to
Tim Wescott

Perhaps even searching for 'current measurement' would be helpful.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/
Reply to
Tim Wescott

Motor resistance does change, as the coils heat up.

Back-emf doesn't depend on load, but on motor speed and - to a lessor extent - motor temperature. The magnetic field generated by a permanent magnet decreases as the magnet gets hotter, and with it the back-emf per unti motor speed.

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Bill Sloman, Nijmegen
Reply to
bill.sloman

thanks. a current shunt seems to be exactly what i'm looking for.

Reply to
troyrivera

Which comes first, load or speed? Put on a load and the motor slows etc, etc. I would think the motor resistance change is negligible. Stall a motor and it probably burns due to lack of back emf.

Reply to
Rich256

Back emf is created by the 'moving motor winding cutting the magnetic field generted by the fixed magnet/excitation coils. Speed thus coomes first.

If you've got a servo-controlled motor that maintains the same speed, you see the same back emf independent of the torque being generated.

Small - +0.3% per degree Celecius.

No, it burns out due to escessive curren through the coil, which intelligent driver design can prevent.

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Bill Sloman, Nijmegen
Reply to
bill.sloman

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