Gauss meter

Discussion in 'General Electronics Chat' started by viju, Mar 1, 2009.

  1. viju

    Thread Starter Senior Member

    Sep 28, 2008
    120
    0
    I need to measure the magnetic flux of a magnet after magnetizing the ferrites of the brushed DC motor.What hall effect sensor should I use?
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
  3. viju

    Thread Starter Senior Member

    Sep 28, 2008
    120
    0
    I have built a circuit for measuring flux density using 3503 hall effect sensor.

    I get a reading of 500G and -500G when I place this sensor inside the magnetized motor.From this how can I calculate flux?

    Which area should I take to calculate flux from the flux density I get?
    Is it the sensor area or the Magnet area?

    Kindly guide me.
     
  4. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    Wouldn't the area be dependent on what you are trying to do?

    What do you need the flux value for? This will tell you what area to consider. For example, if you want to know the voltage generated by a coil wrapped around the magnet when moving, the coil area would be of interest.

    By the way, it generally is not that easy to calculate flux from spot readings with a Gauss meter, over large areas, since the magnetic field changes value and direction significantly.

    You mentioned a motor, so perhaps you are interested in the area of the air-gap. This can make things easier since the iron will control the direction of the magnetic field in the airgap, but there is still an approximately sinusoidal distribution of the field strength. Can you explain what you are trying to do?
     
  5. viju

    Thread Starter Senior Member

    Sep 28, 2008
    120
    0
    Thankyou for the reply.

    I need to measure the flux of the magnet after magnetization for a two pole PMDC motor. I use flux meter with pick up coil with some turns wound on a armature.Measuring flux is an offline check in our production system where I work. Normal value of flux for the motor produced is in the range of 21000 ~23500 maxwells.

    My intention is to certify all the products produced with the proper flux levels by making online check without compromising the production output capacity.

    How do I measure flux or measuring Gauss is sensible or not?

    Kindly suggest.
     
  6. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469

    I do think what you are doing is sensible, but I'm not sure how to correlate the measurement reading of flux density with the motor flux value. One question is what is the meaning of the motor flux value. My guess is that this is the typical magnet flux constant used in the torque equation. If so, this is dependent on the geometry, number coil turns, number of poles and magnetic flux density in the airgap. I believe that this value would be approximately proportional to the peak value of magnetic flux density in the airgap. This would mean that your measurement of magnetic flux density (in Gauss) is a good indicator of that motor flux constant (in Maxwell). However, it is not foolproof. If somebody puts the wrong number of coil turns, the value or flux constant would not come out correctly.

    But, I'm not sure, so you will want to verify this.
     
    Last edited: Apr 1, 2009
  7. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    You have to consider the sensor area but not the case area. You need to know the sensor's size insight the hall sensor case.

    Also, this kind of measurement will give you the flux at the point where the sensor is placed. If the magnetic field is not even then you have to measure the flux at many points and make a graph of how the flux varies in space.
     
  8. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    I can elaborate a little on my previous post. I looked up a formula for total flux (\lambda) from the permanent magnets. This formula is for a linear motor and would clearly not apply to your rotary motor. However, this will give you a feel for what you are dealing with.

    From "Linear Electric Actuators and Generators" by I. Boldea and Syed A. Nasar. page 98

     \lambda={{2}\over{\pi}}K\; N\; \tau\; L \;B

    where

     \tau is the pole pitch
     K is the primary winding factor
     L is the primary stack width
     B is the fundamental of the permanent magnet normal airgap flux density averaged over the primary slot height
     N is the number of coil turns

    This is pretty much in line with what I said before. You can use the flux density measurement from your hall sensor as an indicator that the magnets are correctly magnetized (and that probably everything is ok), but the motor flux constant is dependent on geometry and coil turns etc., so this measurement alone is not proof that the motor flux is within specification.
     
    Last edited: Apr 2, 2009
  9. viju

    Thread Starter Senior Member

    Sep 28, 2008
    120
    0
    Thanks members for your valued suggestions. I am working on that now.Let you know the feedback once I crack the solution.

    Thanks again.
     
Loading...