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  1. #1
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    speed control of dc motor using scr

    Hi,
    I'm developing Motor Speed Controller for Permanent Magnet Synchronous Motor(PMSM).I don't know why people love PWM so much for controlling the speed of the DC/AC motor.Every where I see is the PWM method for speed control, mostly in H-bridge manner equiped with transistors.I have my university final year project that is SCR based DC motor(1-phase) speed control in closed loop.So I'm desiging this project and I'm controlling the speed by the varying the firing angle of SCRs.The motor drive which I'm developing is AC-DC, that is it takes the Alternating Current to get Power.This motor operates in the 1st quadrant where both Voltage and Current is positive

    Any comments,suggestions and view points will be highly appreciated :)

    I bought this Permanent Magnet Synchronous Motor:
    =======================================
    D.C Motor 5 Horse Power
    240 V, 16A, 2050 RPM
    Single Phase





    I am using 8051 microcontroller at this time for controlling the speed of the motor and this motor only has armature control.The speed will be set by entering the RPM through Keypad and speed would be control by Proportional Intergral controller, I will develop PI algorithm in the code, so I don't need to make extra analog circuit for this and I will have the LCD for the output of LCD and for feedback I am using bicycle dynamo for the feedback( I'm using this as RPM to Voltage converter-> the higher the wheel spins, it's corresponding voltage would be generated, so I can know with what speed the wheel is spinning).My circuit has zero cross detection circuit, I'll connect this output with interrupt of the 8051 controller, as soon as the (AC sine wave) 0 Voltage is reached, I'll come to know about the position of the sine wave and then fire the corresponding SCRs from 0-180 degrees ranging depending on speed required.



    This tire will also contains the braking mechanism, as soon as I apply the brake the controller would sense the increasing of load and will reduce the firing delay for SCRs to generate more power to keep the wheel spining at constant speed in NO-LOAD to FULL-LOAD region.
    The initial circuit which I have designed is this, I didn't get the time to make the proper schematic, I'll replace them as soon as I can.

    That's the initial design



    Feedback portion



    The bicycle dyamo voltage is feed into ADC by which controller can sense the speed of the spinning tire.

    It's power circuitry



    I am also thinking of replacing the optoisolators with the pulse transformer as a better substitute( I don't know whether I'm correct or not) and to add the snubber circuit in paralled to Silicon Control Rectifier and Diodes for protection.My friend advised me to first design this circuit by using the smaller DC motor like 12V, because there is less likelihood for destroying the SCR's while making the circuit, because of high voltage/current and back emf.Once the coding for microcontroller is done and other necessary things, just simply replace the motor and few components with higher Voltage/Current ratings.

    So I bought another small DC motor 12V and some BT151 SCRs which has current rating of 10A.






    For 220V DC motor 5 HP, BT152 SCR will be suitable which has current rating of 15-20A as my motor need 16A.

  2. #2
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    scr speed controller

    PWM is easy to control


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    scr speed control

    I don't know why people love PWM so much for controlling the speed of the DC/AC motor.Every where I see is the PWM method for speed control, mostly in H-bridge manner equiped with transistors.
    Well, PWM it's easier than controlling SCR's to get the DC Voltage you want. Also more efficient. And with the H Bridge you can reverse the motor and stop it. Just a thought.

    For 220V DC motor 5 HP, BT152 SCR will be suitable which has current rating of 15-20A as my motor need 16A.
    Mmmhhh I'm not sure, but remember that a motor can use 3 times the nominal current at startup. Maybe you need >48amps SCR's. You should check it before you buy them 'cause they're expensive.



    •   Alt27th December 2007, 18:21

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    dc motor speed control using scr

    Mmmhhh I'm not sure, but remember that a motor can use 3 times the nominal current at startup. Maybe you need >48amps SCR's. You should check it before you buy them 'cause they're expensive.
    So do I have to buy the SCRs with more rating or is there any other effective method for controlling the inrush current at startup?



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    scr based project

    for your project you need to divide your project in following parts
    1. Synchronising network (For detection of Phase(+))
    2. Control Network (Usually Microcontroller)
    3. SCR Firing Network (Pefr a Optocoupler MCT2E ).
    4. ADC Network

    since our power system is on 50/60 Hz.
    thus each time you need to find out the startinf of phase via a ZERO CROSSING NETWORK.

    Reply for mosre confermation

    Regards,
    Prashant
    prashant_hcet@yahoo.com



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    scr control dc motor

    Very correct prashant, this is how I also had divided my circuit.I was using the 8051 but I'm thinking to replace this one with the PIC16F877A and unfortunately last night I discovered that the PIC was dead.Right now I'm heading toward the local market for some semiconductor shopping...and I'll also purchase components to make the Incircuit debugger for the PIC too.

    About the optocoupler, I'm getting little confused.Some people are telling me to use pulse transformer and some about optocouplers.Well I'll also purchase few optocoupler MCT2E and judge at the end, what would be the better substitute.



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    dc motor speed control schematic

    I have a DC motor with the same ratings: 240 V / 16A
    It has a Tacho on the back of it. Basically i'm looking for a controller (or schematic) for it. Can someone point me in the right direction?



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    Re: SCR based DC motor Speed Controller

    [size=3]hi man i had constructed this ckt before.but this does not have any dV/dt and dI/dt protection for that reason this might cannot be applicable.the thing with pulse T/F is not so recomended where the area is compact, reason for this is, if use it we have to provide the over voltage protection ckt for the gate(R,Cand a zener diode) and it T/F has phase inversion i.e. 180° of o/p voltage and,also it consumes more area than a opto-coupler.[/size]
    "Tell whether we can use driver IC (ULN28XX series of drivers) or an darlington opto-coupler to drive the gates of SCR from the µController ."
    hope the reply will as early as possible.
    Dilip



    •   Alt6th November 2009, 05:42

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    Re: SCR based DC motor Speed Controller

    I haven't seen ULN28xx series being used to drive the gate of the SCR. They are sink circuit blocks. They sink current, you need to come up with the idea that you need to keep the gate of the SCR as reverse biased by keeping the driver on. Once they are off, the gate transistor inside the SCR must be forward biased. In other words, yes you can use them also.

    It's not correct to say, that pulse transformer gives 180' out of phase signal. You can find inphase pulse transformer also.

    When you talk about dv/dt or di/dt. You cannot give protection for dv/dt, di/dt. You need to see the SCR rating for that. When you turn the SCR ON at 90' sinusoid, it gives the highest dv/dt and di/dt across the SCR. For inductive load, you can add a leakage network or you call it as a snubber, you add this protection only for the zero crossing, because you want to turn your SCR off. It would help your SCR to turn off by reducing it's latching current of anode.



    •   Alt6th November 2009, 06:00

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    Re: SCR based DC motor Speed Controller

    there are pulse T/F which does not gives 180° but their terminals should be be reversed.the direction of induced EMF in a T/F can be known by Lenz`s Law.



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    Re: SCR based DC motor Speed Controller

    Hello shadil,
    You looked very confused by your posts. The model of pulse transformer can be thought of as a differentiator circuit with gain stage output.If you want to understand the 0' in phase output or 180' out of phase output. You must know the square hysterisis of pulse transformer and behaviour of it.Apart from it, there's no degree you can in between them.What you mean by forward and reverse terminals?



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    Re: SCR based DC motor Speed Controller

    hi....i have the same project in final year now...can u mail me all the circuit diagram or any help in mail pls
    Regard,
    ketan(jidiyakettan2gmail.com)



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    Re: SCR based DC motor Speed Controller

    @jidiya
    I had turn the design to analog one, didn't go for microcontroller.
    Attach are the files of Presentation, Final Year Report.
    My project mate did the documenation, circuit diagrams are correct and verified. In youtube, video has also been uploaded of this project running with these circuits.


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    Re: SCR based DC motor Speed Controller

    As for MCU I think you may consider some dspic33 from Motor Control family. They are generally more meant for motors like BLDC (in general: ones which need extensive commutation logic) but will don decent job in your case. 8051 is kind of stone-age these days.

    As for startup current of the motor you may consider forming some kind of soft-start circuit to limit the current. I think the simplest solution would be high power resistor which can be shorted with electromechanical or solid state relay. This way you can limit current when motor starts and then remove limiter by closing the relay.



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    Re: SCR based DC motor Speed Controller

    thank you very much!!
    i search but didin't find ur yourube link....can u say it?



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    Re: SCR based DC motor Speed Controller

    at 0:44 you can see.


    If this helps click help me.


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    Re: SCR based DC motor Speed Controller

    @umery2k75
    i have to do prjct in digital way.....u post the circuit diagram of microcontroller and all this steps is right or it has any modifiacation?????i have to do that way....so pls if yes then help

    regards,
    jidiya



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    Re: SCR based DC motor Speed Controller

    Hi, I am having a final year project with control the firing angle of thyristor to vary the output voltage. Can u mail me your circuit diagram and all the information??
    thank you..^^ (fexx288@gmail.com)



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    Re: SCR based DC motor Speed Controller

    Files have already been uploaded in this forum on request of jidiya. Check in above posts.



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