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  1. #1
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    how to match calculated and measured current of a brush DC motor ?

    hello dear forum,

    I have a full bridge driving a 24 V DC brush motor

    I have measured the max current of H bridge with ACS sensor with STM32 uC
    and wrote down for four different duty of PWM
    I also measured the Vsource

    I measured the L and R of the motor with LCR meter

    I locked the rotor of the motor

    however the calculated I max and measured I max are very different from each other

    I have attached an excel file for calculating the I max of RL circuit from power electronics book of Muhammad Rashid

    I tried with diferent R and L however still not a match

    what am I doing wrong ?

    please advise

    thank you

  2. #2
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    Re: how to match calculated and measured current of a brush DC motor ?

    Hi,

    I assume you just donīt take waveform into account.

    Donīt use "RMS" measurement here, neither for voltage nor for current
    Donīt use "max" measurement method.
    --> use "average" measurement method.


    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.


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  3. #3
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    Re: how to match calculated and measured current of a brush DC motor ?

    hello ,

    Where can I find the formulas for calculating the average current ?

    my final goal is to find the speed of the motor from current measurement

    thank you



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    Re: how to match calculated and measured current of a brush DC motor ?

    Difference between peak and average current isn't very large in this circuit, the observed differences between calculation and measured currents can't be explained this way.

    There are however unclear points:

    - what's the H-bridge switching pattern?
    - is it correct to neglect switch voltage drops?
    - does the H-bridge driver involve dead time respectively asymmetrical delay?

    Without an actual motor voltage waveform, it's mostly guessing.

    my final goal is to find the speed of the motor from current measurement
    Speed is primarily set by applied voltage. Current measurement may be used to correct for the torque related speed reduction.


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  5. #5
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    Re: how to match calculated and measured current of a brush DC motor ?

    hello ,

    gate signals of one leg ( the other leg Low MOS always ON )

    Click image for larger version. 

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    IR2183 driver has 500 ns deadtime

    motor voltage and current waveforms (ACS sensor output)

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    MOSFET data
    STP100NF04
    N-channel 40V - 0.0043Ω - 120A - TO-220 - D2PAK
    STripFET™ II Power MOSFET

    how to calculate the current and speed of motor correctly ?
    which formulas are valid ?

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    thank you for your help

    - - - Updated - - -

    this is the current signal when motor locked - other waveforms same
    Click image for larger version. 

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  6. #6
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    Re: how to match calculated and measured current of a brush DC motor ?

    You are apparently showing the D=0.35 case, calculated and measured peak current is only 5% apart here. If you are asking why the difference is so big for D=0.14, you should show the respective measurements.

    As for your general questions, it's useful to determine the average current over a PWM period, average current is proportional to motor torque. Speed is in a first order proportional to motor voltage, or more exactly motor emf, terminal voltage - voltage drop at winding resistance.


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    Re: how to match calculated and measured current of a brush DC motor ?

    I locked the rotor and give different duty and measured average , max and min motor current
    I have 750 W transformer 220 / 24 V which I rectify and apply to circuit

    I took 30 measurements in 100 uSec ( PWM frequency is 10 Khz )
    I quicksort algorithm the measured data
    I took average of 30 measuremets for average
    took average of 3 most high measurements for I max
    took average of 3 most low measurements for I min

    as mentioned above I calculated R of motor from average measurements
    the R is not constant ( I need constant R value for calculating speed when motor is running - isnt?)

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    I dont understand - I am stuck
    neither average calculated R is constant
    nor I max calculation from formulas of Muhammad Rashid power electronics book
    gives same values as measured data



  8. #8
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    Re: how to match calculated and measured current of a brush DC motor ?

    D=0.141 waveforms?


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    Re: how to match calculated and measured current of a brush DC motor ?

    0.141 duty

    gates
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    motor Voltage and current
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    I have one question ; should not the current waveform be like this ?

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    thank you



  10. #10
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    Re: how to match calculated and measured current of a brush DC motor ?

    Hi,

    are you sure you gave the system enough time - before each measurement - for the values to stabilize.

    Mind:
    the first PWM periodīs average current will be low.. then it will increase...after a certain time it will not increase (noticably) anymore. Then itīs time to start the measurement.


    Mind:
    that the coil (copper) will become warm... the warmer the higher the copper resistance.

    Klaus

    - - - Updated - - -

    Hi,

    I have one question ; should not the current waveform be like this ?
    coil waveform is different form driver waveform
    * coil: triangle shape
    * driver: like your hand drawn picture.

    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.


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  11. #11
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    Re: how to match calculated and measured current of a brush DC motor ?

    should not the current waveform be like this ?
    No. The motor current should be triangular (integral of the rectangular motor voltage). The current through an inductor can only change continuously. The shown waveform clarifies that the motor doesn't behave purely inductive.

    My first question is if there are additional components between H bridge and motor, e.g. filters, clamping diodes, saturated inductors. What's the exact motor type, does it involve eddy currents?


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  12. #12
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    Re: how to match calculated and measured current of a brush DC motor ?

    @Klaus: I wait 3 seconds until circuit stabilize
    the ACS circuit is at the input to the H bridge

    @FvM:
    there is an electrolytic capacitor parallel to H bridge C18 1000uF
    U10 is ACS current sensor
    D37 D38 are not present
    component values are not real
    other than that it is a standart H bridge

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    the motor label
    the motor is standart DC brush motor with 24 V brake
    Its current can be 50 Amperes when the wheelchair is against an obstacle

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    thank you



  13. #13
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    Re: how to match calculated and measured current of a brush DC motor ?

    O.K. that's unexpected. You are measuring H-bridge DC link current rather than motor current. The deviations from expected waveform in post #9 can be mainly explained by capacitor C18.

    If you are interested in motor current, I would measure it directly. Peak currents will be always different, motor average current can be probably calculated by putting in bridge duty cycle.

    Imot,avg = IDC,avg/D


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  14. #14
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    Re: how to match calculated and measured current of a brush DC motor ?

    @FvM
    thank you for your answer

    //// Imot,avg = IDC,avg/D \\\\\

    is there a webpage or book, where I can find this equation and theoretical explanation of this ?



  15. #15
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    Re: how to match calculated and measured current of a brush DC motor ?

    Hi,

    a paper, a pencil, drawing both current waveforms, estimating the average of both currents, estimating the relationship of both average currents....
    --> maybe you can safe the cost for the book.

    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.


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  16. #16
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    Re: how to match calculated and measured current of a brush DC motor ?

    @Klaus @FvM

    hello thank you for your advise
    however I didnot take pencil and paper but I opened MS Excel
    and calculated R_motor with FvM's formula
    and it worked really

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    now I have another question
    The IR2183 gate driver has 500 ns dead time
    the motor current can reach 50 Amperes max at 24 V

    my question ; is this deadtime enough ?
    or what value does it have to have ?



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