Voltage Drop Compensation

Discussion in 'General Electronics Chat' started by viju, Nov 7, 2009.

  1. viju

    Thread Starter Senior Member

    Sep 28, 2008
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    I have a set up as in the attachment.When I try to compensate the drop by connecting the remote sensing terminal of the power supply unit to the motor terminals after the shunt resistance my current reading becomes erratic.I understand that it is due to voltage compensation which affects the voltage drop across the shunt.

    I am measuring the current by converting the drop across the shunt through a 1:100 gain amplifier which is then fed to the PLC analog input module.

    How can I measure the terminal voltage at the terminals without affecting the current measurement?
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Tell us about the shunt amplifier (x100 gain). Is it differential input?
     
  3. Ron H

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    Apr 14, 2005
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    Sometimes power supplies are shipped with the outputs strapped to the sense inputs. Are these jumpers present in your power supply?
     
  4. viju

    Thread Starter Senior Member

    Sep 28, 2008
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    1. Gain amplifier is not differential one.It's just a non - inverting amplifier with isolation.

    2. In remote sensing terminals I have Source+ , Sense+, Source- and Sense -.I short (Source+ and Sense+) and (Source- and Sense-) when I do not use remote sensing. When I use remote sensing I need to remove the short and connect the remote sensing terminals to the motor terminals appropriately.
     
  5. someonesdad

    Senior Member

    Jul 7, 2009
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    Have you looked at the various signals with a scope? I'm certainly no expert, but I'd hypothesize that noise across the motor's terminals is getting sent back to the remote sensing circuit, which tries to compensate quickly, causing the erratic behavior. Perhaps you could insert an RC low pass filter in the remote sense leads to effectively "short out" these unwanted transients. This is, however, messing with a feedback loop, so you may want to consult with the experienced control folks if you get unexpected behaviors.

    Other thoughts:

    • Increase the wire size of the voltage supply leads -- you may be able to minimize the voltage drop and remove the need for the remote sensing.
    • A low pass filter somewhere in the current sensing circuitry might help.
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
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    It looks to me as though by tying the sense line AFTER the shunt, you are trying to make the power supply compensate for current changes that the shunt is seeing. Try tying the + sense line to the supply side of the shunt and see if you get more stable operation.
     
  7. mik3

    Senior Member

    Feb 4, 2008
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    It would be helpful if you can post a graph with the voltage across the shut with and without remote sense to see the difference.
     
  8. Ron H

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    The sense lines compensate for I*R drop across the resistance in the wires between the power supply and the load. The sense resistor should just look like a little more resistance in the wire.
     
  9. BillB3857

    Senior Member

    Feb 28, 2009
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    Agreed that the remote sense is to compensate for I*R drop of the feed line but having the sense after the shunt presents unequal IR drops for the+ and - lines. Depending upon the internal sense circuit of the supply in use, that could or could not be an issue. It is a simple change to try. If it worked, great. If not, not much lost.
     
  10. someonesdad

    Senior Member

    Jul 7, 2009
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    Ron is right. Consider three resistors R1, R2, and R3 in series and put across the output terminals of the power supply. Put the sense lines across R2 and the power supply will source whatever current it needs to in order to get the desired voltage across R2 as measured by the sense lines. R1 and R3 can be anything (even very unequal) as long as the total resistance of the three resistors is within the compliance range of the power supply. For that matter, put the sense lines across any one of the resistors and you should get the same behavior (again, assuming you're operating within the limits of the power supply).
     
  11. viju

    Thread Starter Senior Member

    Sep 28, 2008
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    I tried it.But my voltage drop after the shunt is around 0.5 Volts.Length of the wire is around 2 meters for each +ve & -ve.Current drawn by the motor under load is around 4.5amps @ 13.0 Volts and the speed is 2900 rpm at 80Nmm.I need to maintain constant voltage at the motor terminals.

    With this present setup I am not able to compensate.Guide me.


    I need to take the picture of graphs for both the combination.
     
  12. someonesdad

    Senior Member

    Jul 7, 2009
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    Hi, viju: As mentioned previously, you may be able to do without the sense lines. If you use 12 gauge (AWG) (= 3.3 mm^2) wire, the voltage drop on each leg at 4.5 A and 20 °C will be about 50 mV for copper.

    However, you haven't given us any details about what you are trying to accomplish (i.e., an engineering statement of your goal(s)), so if the motor load is changing and you're trying to keep the motor speed constant, this might not be sufficient.

    I recommend you post some pictures of scope traces. Put two scope probes across the motor, then subtract the signals to show us the voltage across the motor (or use a differential amplifier).
     
  13. mik3

    Senior Member

    Feb 4, 2008
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    In addition to the signal graphs, which will help a lot, post more details about the power supply. Maybe we can find the internal circuit diagram of it.
     
  14. viju

    Thread Starter Senior Member

    Sep 28, 2008
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    I work for an auto industry as an Automation Engineer. I have built a motor performance test setup for testing the PMDC motor in the production line.It is the end of line test.

    In this test setup I test and measure No load (speed, Current, Voltage), terminal resistance and Load ( Speed, Current,Torque and Voltage).Each test is carried individually in an indexing round table.

    I have no problem in reading and measuring other parameters except voltage at terminals after connecting remote sensing wires.If I connect the abovesaid wires I get erratic current measurement which is drawn by the motor.But current drawn by motor remains same. I hope as I am amplifying the millivolt drop across the shunt , connecting remote sense wire back feeds the compensation thus cancelling the drop with regard to the current seen by the shunt.

    I am yet to take the graphs.Extremely sorry
     
  15. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    The erratic signal may be instability in the power supply when it uses negative feedback to try to stablize the voltage. There may be a compensation circuit that you could add. Sometimes more resistance in the circuit can be stabalizing. Capacitance can sometimes make it better sometimes worse. Do you really need the voltage across the motor to be constant?
     
  16. viju

    Thread Starter Senior Member

    Sep 28, 2008
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    Yes I need to maintain constant voltage at the terminal end.
     
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