1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

BLDC pump suddenly runs dry...what happens next?

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Flyback, Apr 13, 2014.

  1. 4pyros

    4pyros Well-Known Member Most Helpful Member

    Joined:
    Oct 22, 2010
    Messages:
    4,368
    Likes:
    282
    Location:
    Pennsylvania, USA
    Can you measure the voltage across the current sensing resister in the power supply when the pump runs dry?
     
    • Like Like x 1
  2. Flyback

    Flyback Well-Known Member

    Joined:
    Jan 5, 2007
    Messages:
    2,010
    Likes:
    34
    yes when its built
     
  3. 4pyros

    4pyros Well-Known Member Most Helpful Member

    Joined:
    Oct 22, 2010
    Messages:
    4,368
    Likes:
    282
    Location:
    Pennsylvania, USA
    How about during the testing you are doing?
     
    • Like Like x 1
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. tcmtech

    tcmtech Well-Known Member Most Helpful Member

    Joined:
    Feb 22, 2009
    Messages:
    6,703
    Likes:
    432
    Location:
    Sawyer, North Dakota, USA

    So this whole thing is mostly theory and cobbled together parts based on an incomplete and unfinished concept that someone else left for dead and not an actual in use system? :arghh:

    (I really should have be able to guess that given who started the thread.:banghead:)
     
    • Like Like x 1
    • Agree Agree x 1
  6. alec_t

    alec_t Well-Known Member Most Helpful Member

    Joined:
    Jul 10, 2011
    Messages:
    9,325
    Likes:
    1,231
    Location:
    Cardiff, Wales
    Loop 1 yes. Loop 2 sort of.
    Loop 2 in 'normal' mode is a phase-locked loop primarily for the commutation (although it does provide the SpeedFB signal used in loop 1). See Fig 7 of the datasheet. Your unconventional use of the IC is to incorporate loop 2 into an extended loop including the tacho and the converter, for controlling speed by controlling/throttling coil current.
    As I understand your intended system, if the current were at a set level and the pump load were suddenly reduced the motor would speed up, increasing the tacho output. To counter the speed increase you would use the tacho signal to reduce the current from the converter significantly. That reduction would be your indication of dry-running.
     
    Last edited: Apr 19, 2014
  7. 4pyros

    4pyros Well-Known Member Most Helpful Member

    Joined:
    Oct 22, 2010
    Messages:
    4,368
    Likes:
    282
    Location:
    Pennsylvania, USA
    Maybe, I can't tell, its been like working on something in a incubator with those long gloves with the lights out.
     
  8. tcmtech

    tcmtech Well-Known Member Most Helpful Member

    Joined:
    Feb 22, 2009
    Messages:
    6,703
    Likes:
    432
    Location:
    Sawyer, North Dakota, USA
  9. Flyback

    Flyback Well-Known Member

    Joined:
    Jan 5, 2007
    Messages:
    2,010
    Likes:
    34

    I don't see how the motor would speed up, because we are talking about the pump "suddenly" running dry ("sudden" being the operative word here)....ie complete and sudden, severe load removal......the current loop wouldn't be able to react immediately, so at the instant of sudden load removal, there would be miles too much current flowing in the motor coils.....this would literally slingshot the rotor round at a heck of a speed......and the control loop wouldn't be able to keep up with this, and so it would pulse motor coils at the wrong times, and so the motor would just end up running unsmoothly, rather than actually speeding up.........and so the current loop wouldn't detect the increase in speed that it needs to detect in order to reduce the current from the buckboost.....and so the system would in fact, end up increasing current....obviously not wanted.

    I can assure you that the essential method that we are using here is the method used by all high power BLDC motors.....that is, not PWM'ing the bridge transistors in order to control motor current......if you do it like that, then that is ok for small low power motors, but its no good for big motors because you have to have a current sense resistor downstream of the motor coils which gets used to control the current in the motor coils....and you oviously have to wire to this sense resistor, and wiring down to sense resistors is not practical with high power motors, as you get too much wiring inductance, therefore, with high power motors, you have an upstream variable current or voltage source, and you vary that in conjunction with the motor speed feedback.

    I say the upstream regulator can either be variable voltage or current, but either way, you have to put current limiting circuitry in there, so why not just use that to regulate the current and make it a current regulated upstream power source?

    I guarantee all that doing high frequency PWM of motor coils to control coil current is never done with high power BLDC motors...thus the method we are doing is not at all unusual.
    do you agree?

    I am still not sure how the RVCO pin voltage affects the commutation frequency?
    I thought the VCO was to do with "Locking" the coil commutation gate pulses to the back-EMF signals?.....ie, not to do with speed setting?, which is surely the job of the SPEEDSET pin of the ML4425?
     
    Last edited: Apr 21, 2014
  10. shortbus=

    shortbus= Well-Known Member

    Joined:
    Mar 23, 2008
    Messages:
    1,231
    Likes:
    77
    Location:
    youngstown, oh
    I may be missing something in all of this. But it is my understanding of a BLDC, that current has to do with the torque out put and the speed has to do with the pulse frequency.

    When your motor 'unloads' the rotor and stator will lose synchronization. The motor will sit there and vibrate, and not rotate. This is the biggest problem with "sensorless" BLDC drives. A BLDC that uses Hall or optical sensors, doesn't have this problem, since the sensors are responsible for commutation.

    And why would the pump "pump "suddenly" run dry"? Even when the tank gets low, it's not going to be a "sudden" thing, unless there is a shut off valve involved.
     
  11. Flyback

    Flyback Well-Known Member

    Joined:
    Jan 5, 2007
    Messages:
    2,010
    Likes:
    34
    it would suddenly run dry because the tank is long, thin and tall, and the pump sits at the bottom, and the 650W pump will literally suddenly run dry...this wouldn't be the case if it was a big wide tank.

    ..it sounds like you agree with my conjecture about "Pump suddenly running dry"...and it looks like our fears are real
     
  12. alec_t

    alec_t Well-Known Member Most Helpful Member

    Joined:
    Jul 10, 2011
    Messages:
    9,325
    Likes:
    1,231
    Location:
    Cardiff, Wales
    Torque is proportional to current, assumed quasi-constant. Load balances torque. Hence reduced load = torque tending to accelerate motor until frictional/windage load again balances torque.
    What is the loop time costant?
    If you are talking about the internal phase-locked commutation loop (rather than your external speed-control loop) then I doubt that would happen. The commutation frequency is good up to ~2kHz according to the datasheet. 2kHz clocking a six-state commutation cycle = 2000/6 rps = 20,000 rpm. Is your motor really going to reach that speed before your speed-control current loop catches it? If so, I reckon you need to re-design that loop.
    Should be ok. The control loop time response time will be key.
    It sets Kv. See page 8 of the datasheet.
    That's my understanding. When locked, the VCO frequency will be 6 times the motor rotation rate.
    But you're over-riding that!
     
    • Thanks Thanks x 1
  13. 4pyros

    4pyros Well-Known Member Most Helpful Member

    Joined:
    Oct 22, 2010
    Messages:
    4,368
    Likes:
    282
    Location:
    Pennsylvania, USA
    How big is this motor in hp?
    Any pics of the setup, motor and pump, tank?
     
  14. Flyback

    Flyback Well-Known Member

    Joined:
    Jan 5, 2007
    Messages:
    2,010
    Likes:
    34
    Sorry no pics, don't know HP, its 650W
     
  15. 4pyros

    4pyros Well-Known Member Most Helpful Member

    Joined:
    Oct 22, 2010
    Messages:
    4,368
    Likes:
    282
    Location:
    Pennsylvania, USA
    According to an online converter;
    650 watts equals .87 hp
    Its not that big.
    Do you have a part number or speck sheet for the motor pump combination? or was it put together from separate components?
     
  16. Flyback

    Flyback Well-Known Member

    Joined:
    Jan 5, 2007
    Messages:
    2,010
    Likes:
    34
    separate components, sorry
     
  17. 4pyros

    4pyros Well-Known Member Most Helpful Member

    Joined:
    Oct 22, 2010
    Messages:
    4,368
    Likes:
    282
    Location:
    Pennsylvania, USA
    Remind me again why you are not using an off the shelf pump?
     
  18. Flyback

    Flyback Well-Known Member

    Joined:
    Jan 5, 2007
    Messages:
    2,010
    Likes:
    34
    we are developing this for use in water tanks which have become standard throughout the agricultural world, and we want a small pump so that we can go into business and sell to other agricultural places..not just any off the shelf thing, which are too large for use and storage, and don't run off the typical agricultural dc bus
     
  19. 4pyros

    4pyros Well-Known Member Most Helpful Member

    Joined:
    Oct 22, 2010
    Messages:
    4,368
    Likes:
    282
    Location:
    Pennsylvania, USA
    What is the typical agricultural dc bus?
     
  20. Flyback

    Flyback Well-Known Member

    Joined:
    Jan 5, 2007
    Messages:
    2,010
    Likes:
    34
    it is 18 to 32vdc
     
  21. tcmtech

    tcmtech Well-Known Member Most Helpful Member

    Joined:
    Feb 22, 2009
    Messages:
    6,703
    Likes:
    432
    Location:
    Sawyer, North Dakota, USA
    Never heard of an agricultural DC bus around here. In fact got zero hits on internet searches as well. :(

    120/240 VAC @ 50/60 Hz is the standard in every part of the world until you go to three phase power.

    The closest agricultural DC standards would be for equipment designed to run in and off of agricultural equipment electrical systems which would be either standard 12 or 24 volt power and very little of that exceeds a few hundred watts draw for a single item.

    The majority of everything after that tends to follow the common 12/24/48 VDC system voltages that the solar and AE power industry uses which BTW already has a market filled with low cost high quality pump systems of every shape size configuration and capacity you can name that actually work.:rolleyes:
     

Share This Page