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  • Author Author: colporteur
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Blog 6: Now for some initial data

colporteur

With the python library, misaz shared in his blog post, I can continue with my challenge testing.  My initial tests of the sensor are using the locomotive on the bench. I have been using 3VDC, 5VDC and 12VDC to set some expectations before moving to the rail tests.

I still try and follow the process my electronics lab college instructor tried to instill in us 40 years ago. Have some idea of what the measurement should be, before you go poking around with testing equipment making measurements. I had no success getting current measurements using my Fluke VOM so I only have what the sensor is providing.

The python script generates a current & voltage reading every five seconds. The initial voltage applied to the locomotive motor is 5VDC, then changes (highlite) to 3VDC and then to 12VDC.

image

My question to the sharper electronic knives in the drawer than me is, do the current values look reasonable?

The current reading moves between.05 to .09, the average, throwing out the negative values in the calculation, is .06. I was expecting higher current values at the higher voltages. What I interpret from the current reading is a motor that is an active load where the resistance changes to ensure a steady current. That doesn't seem correct?

The motor speed does change with a change in the voltage.

I confess there is a lot of rust and dust accumulated on my electronic theory. Part of my motivation for doing these challenges is to refresh my knowledge. I would appreciate any feedback members have to offer before proceeding with further testing. What do you see in the values so far?

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  • I really appreciate michaelkellett & misaz lending me your expertise. I ran the python script again using the following configuration, to measure both current and voltage.

    max.configure(sample_rate_khz=.5, digital_filter=128, measure_current=True, measure_voltage=True) with the following results. I had to add the sample_rate or the script errored out. This makes it slightly different than what misaz provided.

    image

    The readings are much more stable.

    Break..Break... I could carry this question to another post but I got lazy:o

    Why am I unable to measure the current with a Fluke 77 VOM using the 300mA range?

    I tried using two different meters to read the current. Only when set to the 10amp range on the meters will they produce a reading of 0.07 and 0.06 on the other. If I attempt to use the 300mA on the Fluke meter and 250mA on a second different meter they indicate out of range.

    The reading would suggest the current is within the lower range of the VOM's, Any suggestion on why are they not able to provide a reading?

Comment

  • I really appreciate michaelkellett & misaz lending me your expertise. I ran the python script again using the following configuration, to measure both current and voltage.

    max.configure(sample_rate_khz=.5, digital_filter=128, measure_current=True, measure_voltage=True) with the following results. I had to add the sample_rate or the script errored out. This makes it slightly different than what misaz provided.

    image

    The readings are much more stable.

    Break..Break... I could carry this question to another post but I got lazy:o

    Why am I unable to measure the current with a Fluke 77 VOM using the 300mA range?

    I tried using two different meters to read the current. Only when set to the 10amp range on the meters will they produce a reading of 0.07 and 0.06 on the other. If I attempt to use the 300mA on the Fluke meter and 250mA on a second different meter they indicate out of range.

    The reading would suggest the current is within the lower range of the VOM's, Any suggestion on why are they not able to provide a reading?

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  • in reply to colporteur

    Just a quicky today, the sun is shining and it won't tomorrow so I'm going out on the bike !

    Easy bit first, I suspect the current drawn is really spikey so that the meters are choking on the spikes and only on the 10A range are they happy.

    Next a quick thought about DC motors, I'm assuming your loco has a traditional permanent magnet DC motor with brushes.

    The armature DC resistance will be very low.

    When the motor spins in a magnetic field a voltage is generated in the windings. It doesn't matter why it spins, it still makes that voltage.

    If you apply volts to the motor it will start to spin due to the current in the armature interacting with the mag netic filed from the magnets. As it spins it will make that voltage which tries to stop the current flowing, thats why it's called a "back emf".

    The voltage increases the faster the motor turns.

    The mechnical power in the motor = back emf x current

    The heat going into the motor = current squared x armature resistance

    The force exerted by the armature is proportional to the current.

    The motor will spin faster and faster and the back emf will increase, reducing the force the armature can exert. When this force is equal to the force of friction and load slowing it down the armature spin speed will stabilise.

    If you increase the voltage the motor will spin faster so the current will increase less than you expect.

    With your motor at 5V the input power was about 0.225W and at 12V it's 0.72W.

    With a pure resistive load the increase would have been to  1.296W and the current would have gone up to 0.108A.

    Your measurements are showing just what we should expect for a motor.

    Hope that helps.

    MK