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  1. #1
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    DC motor simulation using equivalent circuit

    hello all, i am working on a hardware project, which simulates a DC motor, that is, a hardware circuit replaces a DC motor and performs the function of DC motor. in the equivalent circuit of a DC motor back emf(V_emf) is represented by a voltage source. the back emf depends on load. the real DC motor operates on several amps of (load)current. when i realize the back emf circuit, i am not sure how to design it, so that it can with stand 40amps of load current.


    circuit explanation: motor_+ motor_- is the motor terminal input.
    L1, R6 equates DC motor coil and resistance.
    R1 for series current measurement
    M1,M2,M3 controlled by controller based on current. M2 for rush current and M3,M1 for load current.
    EMF_S+ is the voltage source. through PWM required voltage can be generated. the voltage comes from a regulator.

    is this design correct? how to design/safe guard the back emf regulator from the high load current coming from the motor driver circuit.

    kindly suggest how this concept can be realized.

    refer this post for DC motor current.

    thanks in advance

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    Re: DC motor simulation using equivalent circuit

    I don't understand the concept behind the motor simulation circuit.

    A useful motor model should represent rotor and load moment of inertia, load torque (if any), possibly no-load losses, motor constants that converts between mechanical and electrical side of the model, rotor series resistance and inductance as already mentioned.

    I would start with an equivalent circuit like below. Mechanical quantities are already transformed to electrical. No load-losses are assumed speed proportional.

    Click image for larger version. 

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    Time variable or speed dependent load torque should be modeled by respective modulation of the load current source.



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    Re: DC motor simulation using equivalent circuit

    Also, please note that the current source, in addition to being software or voltage controlled, must be able to sink current (for motor operation) and source current (for generator operation).

    The generator operation will occur whenever there is braking torque, like when the mechanical load's inertia keeps the motor turning as it is commanded to slow down (regenerative braking).
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    Re: DC motor simulation using equivalent circuit

    The generator operation will occur whenever there is braking torque, like when the mechanical load's inertia keeps the motor turning as it is commanded to slow down (regenerative braking).
    My suggestion is to separate inertia and load torque systematically. In this case, only motor/generator power is fed through the current source.



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    Re: DC motor simulation using equivalent circuit

    @ Fvm: the idea is to realize a DC motor as a hardware circuit. circuit is designed with respect to DC motor equivalent circuit.

    operation: when motor signal is given, there will be a rush current flows from motor_+ through the components R6, L1, R1, R8, M2 to motor_-. current is monitored across R1. at peak of rush current, M2 will be turned off and M1 & M3 will be turned ON providing a back EMF voltage to the motor signal. the back EMF signal(EMF_S+) varies in such a way that load current (waveform as given above) flows in the circuit from motor_+ through the components R6, L1, R1, R4, M3, EMF_S+, EMF_S-, M1, R5 to motor_-.

    back emf voltage EMF_S+ is a voltage regulator, through PWM, required voltage is generated which will be always less than motor_+.

    my question is, when load current in the order of 10's amps flow through the regulator, how do i need to give provision in such a way that the motor_+ sees the back emf voltage and the resulting current can pass through motor_- with out affecting the back emf regulator.

    is my explanation of the circuit is correct?
    kindly provide your suggestion.

    thanks



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    Re: DC motor simulation using equivalent circuit

    the current in the circuit is determined by the formula "Ia =(E-Eb)/Ra", where E is motor signal voltage(12v). Eb - generated back EMF voltage( always less than E). Ra -resistance of the coil and other resistance between motor+ and motor- path (around 0.4 phms).

    when we receive the motor signal, Eb will be zero and hence we see a big rush current in the circuit. after rush current, Eb is generated which reduces the flow current to few amps. by varying Eb required current profile(as mentioned before) is achieved.

    my question is how to move this current safely with out affecting the back emf regulator circuit.

    thanks



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    Re: DC motor simulation using equivalent circuit

    Your equivalent circuit misses the point how actual rotor speed and hence EMF is set. In case of a motor, it's a dependent variable. Without modeling rotor and load inertia, you hardly get a physically plausible current profile.



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    Re: DC motor simulation using equivalent circuit

    Hi, please check the attached images. the circuit is simulated using LTspice. the green line is the EMF voltage generated in EMF_S+ line. the blue line is the current flown through resistor R1. the above circuit is redesigned model of a working circuit.

    "the only thing i don't understand is " how this large current flows through M+, L1, R1, EMF_S+, EMF_S- and back to M-. because the EMF_S+ is a voltage from SMPS power supply with a rating of 15v with 3.5A only. the only thing they have extra is a capacitor of 50V with 0.1 Farad parallel to SMPS power supply. is there any chance this capacitor by passes the current going to SMPS.

    Click image for larger version. 

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    Click image for larger version. 

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    thanks



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    Re: DC motor simulation using equivalent circuit

    I have used a DC motor equivalent model as part of a model for an x-y plotter (follower control).
    The DC motor model was a closed-loop system with negative feedback (induced voltage acting back).
    As a result, the model was a second-order system with two time constants
    H(s)=(number of revolutions)/input voltage=kdc/(1+sT1)(1+sT2)



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    Re: DC motor simulation using equivalent circuit

    Quote Originally Posted by welove8051 View Post
    Hi, please check the attached images. the circuit is simulated using LTspice. the green line is the EMF voltage generated in EMF_S+ line. the blue line is the current flown through resistor R1. the above circuit is redesigned model of a working circuit.

    "the only thing i don't understand is " how this large current flows through M+, L1, R1, EMF_S+, EMF_S- and back to M-. because the EMF_S+ is a voltage from SMPS power supply with a rating of 15v with 3.5A only. the only thing they have extra is a capacitor of 50V with 0.1 Farad parallel to SMPS power supply. is there any chance this capacitor by passes the current going to SMPS.

    Click image for larger version. 

Name:	cur1.PNG 
Views:	2 
Size:	11.4 KB 
ID:	141907


    Click image for larger version. 

Name:	cir2.PNG 
Views:	2 
Size:	23.0 KB 
ID:	141908

    thanks

    @FvM: any comments.

    @LvW: didnt understand your comment



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    Re: DC motor simulation using equivalent circuit

    Quote Originally Posted by welove8051 View Post
    @FvM: any comments.
    @LvW: didnt understand your comment
    It was not a comment - it was an ANSWER to your problem.
    Are you familiar with control systems resp. blocks realizing the various transfer functions of a system with feedback?



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    Re: DC motor simulation using equivalent circuit

    Quote Originally Posted by LvW View Post
    It was not a comment - it was an ANSWER to your problem.
    Are you familiar with control systems resp. blocks realizing the various transfer functions of a system with feedback?
    sorry, didn't understand, can you brief it?



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    Re: DC motor simulation using equivalent circuit

    Circuit in post #2 is the DC motor's model, so use it. The back emf is the voltage across the inertia capacitor, like below.

    I have inserted another branch of the circuit for easily determination of the motor's angular position which is the current flowing into the inductor "K".

    angular speed=backEMF/K. If you find out the transfer function of that model , you will get K*angular speed/Input Voltage. Divide by "K" the transfer function you got and you have what is said in post #9.
    Last edited by CataM; 23rd October 2017 at 15:26.



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    Re: DC motor simulation using equivalent circuit

    Quote Originally Posted by welove8051 View Post
    sorry, didn't understand, can you brief it?
    You do not understand the terms"control system" and "feedback"???



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