Variable speed of DC motor

Discussion in 'General Electronics Chat' started by Alexvatt, Dec 13, 2013.

  1. Alexvatt

    Thread Starter Member

    Aug 24, 2013
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    I have a pump that is driven by DC motor. The pump has to work in a pulsed mode: it pumps for 2s and then it has to be idle for next 2s. I release the excess pressure with a solenoid valve but I also want to reduce the motor speed as well. I’m planning to run the motor at max speed for 2s and then reduce its speed by 50% for 2s. Will pulsed mode of operation burn the motor quickly?
    Thank you.
     
  2. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Depends on the motor and the load. Best to ask the manufacturer.
     
  3. MaxHeadRoom

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    Jul 18, 2013
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    One of the KB/Baldor drives should do it, they are available on ebay, they have settable accel/decel so it is easier on the motor, if under constant load it is better rather than applying full voltage at that frequency of switching.
    If it is an impeller pump, the current/load will be lower under pressure.
    Max.
     
  4. Alexvatt

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    Aug 24, 2013
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    I use PWM to control speed.
     
  5. MaxHeadRoom

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    It may pay to ramp up to maximum at the first 2 sec if possible.
    What is the frequency of operation?
    Max.
     
  6. Alexvatt

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    Aug 24, 2013
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    I use Arduino UNO to generate PWM signal and default frequency is 500Hz. In theory I can increase it to max of 60kHz but I’m not sure if it would make any sense.
     
  7. MaxHeadRoom

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    No I meant what is the frequency/period between the switching of the 2sec hi and 2sec low.
    Or is this a constant repetition?
    Max.
     
  8. GetDeviceInfo

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    if, at max speed, your motor is rated for the load, and you then run it at some reduced loading for some duty, you'll be fine. Note however that reduced speed, may not necessarily mean reduced load. A simple approach would be to monitor the temp rise, possibly employing a thermistor if there isn't one already.
     
  9. Alexvatt

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    Aug 24, 2013
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    It's a constant repetition. It will run for 10 min once or twice a day.
     
  10. MaxHeadRoom

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    Just keep in mind that when applying full voltage to a stationary DC motor, maximum current will flow until the motor produces enough counter EMF via RPM.
    If it is started at full voltage over a short period of time, it may not produce sufficient to heat the motor to any degree, but worth monitoring.
    Max.
     
  11. Alexvatt

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    Aug 24, 2013
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    In the first 2 s it pumps into a pressure vessel and it’s a pretty heavy load (although it will never reach max rating). In the next 2s it pumps into the atmosphere so a load variation is quite significant.

    It’s a miniature diaphragm pump and the motor is really small. It consumes around 7W with the max load. It will run for 10 min at a time and I don’t think it will ever overheat. I can check it without a temperature sensor.

    Actually I’ve got two reasons why I want to reduce the speed. One is to reduce the load variation, second to reduce pressure. I have one valve that controls pressure and the flow rate that it provides is not enough to reduce the pressure in the system to a desired level. I have two options: to install another valve in parallel or to reduce pump output.
     
  12. MaxHeadRoom

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    What about an adjustable pressure relief valve to accurately control and probably the most precise method?
    Max.
     
  13. Alexvatt

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    I monitor pressure with enough precision via pressure sensor. I'm not sure if there is and advantage in employing a pressure relief valve. I think it will be just an unnecessary expense when additional solenoid valve costs only$15 and I got it anyway.
     
  14. MaxHeadRoom

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    What type of pressure sensor? Is it monitored by the Arduino?
    If so you could control the motor accordingly?
    Without knowing all the details and facts of the system and the desired results, this is all guess work.
    Max.
     
  15. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    modulate your diaphragm armature.

    Oops, motorized, I missed that.
     
    Last edited: Dec 14, 2013
  16. strantor

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    As Max has pointed out, starting the motor, especially with high load, is the worst possible scenario; it is the time of the highest current draw, and therefore the highest heating. And this is what you're going to be doing every 4 seconds. The heat will be cumulative, and on such a small motor, it won't have a very high thermal mass. It should be easy to test out. If it doesn't burn up in the first couple of minutes, then it probably won't ever burn up. Can you afford to lose a motor as a test?
     
  17. Alexvatt

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    Aug 24, 2013
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    I know that it will work. I run it for about 5 min at a time many times and did not have problems with overheting.
    My question was about long-term impact of varaiable speed operation of DC motor’s longevity.
     
  18. MaxHeadRoom

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    DC motors/servo's have traditionally been in motion control & CNC control etc for decades, in applications where the motor is in constant and variable rpm many hours at a time.
    There are two rating, continuous torque rating and peak torque, naturally the peak torque should only occur very briefly as in applying full power to stationary motor or high load etc.
    In extreme conditions, applying full power to a locked rotor for a period of time can de-magnetize the field, this is why I suggested a soft start from zero rpm.
    The maximum continuous torque usually occurs at zero rpm for a DC motor.
    Max.
     
    Last edited: Dec 15, 2013
  19. strantor

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    When you say you ran it 5 min at a time, do you mean continuous, or in your pulsed mode?
     
  20. Alexvatt

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    Aug 24, 2013
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    Thank you for the advice. I can do soft start and just pump into the atmosphere during this period. What would be an optimal time for acceleration form 0rpm to max?
     
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