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BLDC drive help

Discussion in 'General Electronics Discussion' started by sid2286, Nov 2, 2014.

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  1. sid2286

    sid2286

    102
    1
    Aug 24, 2011
    Hello,

    I have a 24vdc bldc motor, the speed of the motor can be controlled by trimpot that can be connected to its speed wire, the variation is 0rpm at 0.5v and maximum speed at 5v.
    This above works fine, however when I connect a load then the motor starts at 0.9 v instead of 0.5v.
    How should I make the motor drive at 0.5v when load is connected.

    Thanks in advance,
    Sid
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,391
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    Nov 17, 2011
    This is a property of the motor. The BLDC controller will provide power to the motor as a fixed ratio of input voltage (to the controller) to output voltage (to the motor). Under load an electrical motor will require a higher power to start running than without load. With other loads the start voltage will vary and can be even higher than 0.9V for higher loads.

    option 1: life with the behavior as is.
    option 2: use an operational amplifier to generate 0.9V...5V from 0.5V...5V

    For option 2: use a summing amplifier with a fixed offset and a gain slightly less than 1 according to y = 0,9111x + 0,4444 (x=input voltage, y=output voltage).
     
  3. sid2286

    sid2286

    102
    1
    Aug 24, 2011
    Thank you for your reply,

    Will the attached circuit solve the problem?
    So this is for a specified load, what will happen if the unknown load is used?
    What can be done in that case?

    Thanks
    Sid
     

    Attached Files:

  4. Harald Kapp

    Harald Kapp Moderator Moderator

    10,391
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    Nov 17, 2011
    Sorry, this circuit will not work for two reasons:
    1. The output voltage of an inverting amplifier will go negative for a positive input voltage. Your opamps do not have a negative supply, only ground and 5V. You will need e.g. +-5V (you're correct in choosing a rail-to-rail amplifier MCP604 for this - other wise youd have to use an even higher supply voltage)
    2. The input of your circuit is not correctly configured for a summing amplifier which typically looks like this. As you have drawn the circuit, Vin will override the offset from R1/R2. And R3 R4 are unnecessary because the voltage at the center node of this divider is also overridden by Vin.

    You'd need a controller with a feedback mechanism that ensures a stable relation between input voltage and rpm of the motor. As it looks, your controller ensures a stable relation between input voltage and ouput voltage, but the output voltage does not have a stable relation to the rpm of the motor. Typically a sensor (e.g. a tachometer generator) is required to create a feedback signal for the controller to verify the correct rpm of the motor and adjust the control signal accordingly.
     
  5. Bluejets

    Bluejets

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    Oct 5, 2014
    How big is the motor, ie. how much current does it draw and also what are you using to power it.??
     
  6. sid2286

    sid2286

    102
    1
    Aug 24, 2011
    So should I continue with the same opamp i.e MCP604 or should I use LM324?

    So to achieve above equation, what should be the circuit like?

    Its a medium size 24Vdc motor, the specification on it 24Vdc Speed 3000RPM and Power 50W.
    it has another 4 wires other than the supply wires(24V and Grd)
    the 4wires are 1. speed, 2. direction, 3. 5Vdc and 4. Grd.
     
  7. Bluejets

    Bluejets

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    Oct 5, 2014
    OK...but what are you using to power the motor..??
     
  8. sid2286

    sid2286

    102
    1
    Aug 24, 2011
    SMPS: 24Vdc/1.5A
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    The problem here is not the opamp but the power supply. The MCP604 is a goof choice, the LM324 is not, because it can't deliver +5V atthe output when powered by a 5V supply. But you will need a +/- 5V supply (bipolar) for the MCP604, too.

    I gave you the link to a typical summing amplifier configuration. This is a better source.
    You need to realize Vout = 0.9111*Vin + 04444V
    The transfer function of this type of summing amplifier is Vout=-(Rf/Rin1*V1 + Rf/Rin2*V2). Comparing the coefficients you get
    Rf/Rin1=0.9111 and
    Rf/Rin*V2=0.444V
    You then need a second stage to invert the negative output of the summing amplifier to create a positive control signal for the BLDC controller.
    A simplified circuit can use only one opamp, mainly the summing amplifier. Replace the positive input voltages V1 and V2 by negative voltages (remember, you will need a negative supply anyway). The summing amplifier will invert the sign of the sum (see above equation) and deliver the positive control voltage as required.
     
  10. sid2286

    sid2286

    102
    1
    Aug 24, 2011
    Thank you for your reply!

    I can arrange for +/-5vdc using IC 7660 , however from the datasheet of MCP604 it suggest that its a single supply IC 2.7V to 5.5V, so +/-5V supply to the IC wont damage the IC?
     
  11. Bluejets

    Bluejets

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    Oct 5, 2014
    Don't see how you can expect a 50W motor to run on a 36W power supply.
     
  12. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    You're right, the MCP604 allows max. 7V between V+ and V-. It is tehrefore not suitable in this application. An LM324 could be used, but with at least +/-7V supply voltage. The LM324 does not have rail-to-rail outputs, the output is at least V+ -1.5V.

    Bluejets is right about the power supply. You will need a supply that is capable of delivering all the power the motor needs. Otherwise the supply will at least go into current limit mode and reduce the output voltage on overload,
     
  13. sid2286

    sid2286

    102
    1
    Aug 24, 2011
    Oh so assuming that it might be a supply issue, I also can get 24v/2.5A smps, but will that solve my problem or I will still have to make opamp arrangement.?
     
  14. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Impossible to say from afar.
    What is the current when the motor is starting up under load (at 0.9V control voltage)? What is the 24V supply's voltage under these conditions? Does it break down or is it still stable?
    If the current is less than 1.5A and the voltage is a stable 24V, you'll encounter the same problems with a more powerful supply.
    If current is more than 1.5A and the voltage breaks down, a more powerful supply could alleviate your problems. Whether is completely cures the observed symptm needs to be tested.
     
  15. Bluejets

    Bluejets

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    Oct 5, 2014
    Depends on loading, but motors in general tend to draw large hunks of current to initiate rotation.
    Hence the possibility of your problem.
     
  16. sid2286

    sid2286

    102
    1
    Aug 24, 2011
    Hi,

    Sorted out the problem, it was as Bluejet suggested, the power supply was the problem., I tired using 5A SMPS and it worked fine.

    Thank you Harald and Bluejet for your advices.
    -Sid
     
  17. Harald Kapp

    Harald Kapp Moderator Moderator

    10,391
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    Nov 17, 2011
    Great news.
     
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