Looking fo a high torque dc motor speed controller solution .

Discussion in 'General Electronics Chat' started by LETITROLL, May 14, 2014.

  1. LETITROLL

    Thread Starter Member

    Oct 9, 2013
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    I am currently working on a project and i need to control the speed of a DC motor that is turning a relatively heavy mechanism , the motor i got is rated at 220 Vdc/2.4A ,

    I can turn the motor at 24 v but when i connect it with the mechanism it does not move at all due to lack of torque current .

    An electronic solution handling at least 10 A would be really helpful .
     
  2. pwdixon

    Member

    Oct 11, 2012
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    56
    Is the voltage supply to the motor collapsing due to power supply limitations or has the drive output got too high an impedance? Need to see something of the circuit you are driving the motor with.
     
  3. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
    2
    I am not using any circuit to drive the motor , just a bridge rectifier between wall outlet and DC motor connections .

    The motor moves the mechanism really fast when supplied by 300 V dc going from wall 220 V ac outlet through rectifier bridge .

    i just need to vary the speed by lowering the voltage and having a sufficient current for the torque power .

    here is a picture of the mechanism :
     
    Last edited: May 14, 2014
  4. pwdixon

    Member

    Oct 11, 2012
    488
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    Sounds really dangerous. But why are you even feeding 300V if the motor will run at 24V and the motor is only rated to 220V?

    I guess you could try a variac to set the input voltage, at least then you might be able to identify what the minimum voltage is required to start the motor running. Then you can at least gauge what you really need to drive this thing.
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    That is because you have added the inertia of the mechanism to the inertia of the rotor. The torque of a motor is not related at all to the voltage, it is related to the current. To control speed you want a 220VDC supply with an adjustable current up to say 10A. You can achieve this via Pulse Width Modulation (PWM). Once the motor starts at 2 to 3 times the steady state current you will find that the faster it goes the less torque there will be. This is due to the back EMF generated by the rotor windings moving in the magnetic field of the stator.
     
  6. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    This sounds like you need some gearing in the mix.:) Figure for the maximum speed you want on the roller when the motor is at full RPM. Then you can slow the roller down by changing the motor RPM, without loosing too much torque. Very few pieces of machinery use a motor directly with out some form of gearing involved.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You can pick up KB/Baldor SCR or PWM controllers on ebay that will offer speed control.
    If you do not require maximum rpm you could use the 120v versions on your motor.
    If the load changes significantly, or you need precise control, you may need feed back control.
    Max.
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    You can buy $15 China quality motor speed controllers on ebay too (SCR type).

    He already has the bridge rectifier to put after the speed controller. :)
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    There are KB's on ebay right now for $15.00 251456093276
    Max.
     
    THE_RB likes this.
  10. LETITROLL

    Thread Starter Member

    Oct 9, 2013
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    I have managed to get a speed variator based on a 10 A thyristor , and i can see the speed of the motor changing .

    But when i attach the motor to the mechanism , it works only on the highest speed , in lower speeds the motor hangs.


    Any Ideas
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    That is because the speed controller is essentially a velocity type so the available current to operate the load is not present at lower RPM, for this type of drive you would need a tach feedback type.
    Or a larger motor or G.B. reduction could be used.
    Max.
     
  12. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    See post #6
     
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