Anybody Teach me about BUCK step down module

[gr_monk]

Hi, I want to run my 12V DC motor with 24V SMPS power supply.

here's devices/modules I have now

1. 12V dc motor
   FIT0403 DFROBOT - Motor: DC | with encoder,with gearbox; 12VDC; 7A; Shaft: D spring; DF-FIT0403 | TME - Electronic components
2. Cytron motor driver module
   10Amp 5V-30V DC Motor Driver
3. BUCK step down module
   
   

From Catalog, here's specification of this module :
Input Voltage : 4.0V ~ 38V|
Output Voltage : 0.25V ~ 35V |
Output Current : 5A (max) (4.5A recommended)|
|conversion efficiency|96% (max)|
|Output Power|75W (max)|

4. 24V SMPS (600W) power supply

What I did was like:

1. Connect Power -> Step downer -> motor driver -> dc motor
2. adjust variable resistor, so that when power is on and motor is off, step down module output voltage goes to 12V.
3. turn on dc motor by sending PWM signal to the driver.
   (4). I expected that dc motor would rotate with proper speed.

What actually happened :
4. step down module's output voltage drops to 3.8~4.1V, when I send signal to the motor driver.
5. because of that low voltage, dc motor runs very slowly.

Here's my question:

1. Is this a correct way to use step down converter module? Is this module malfunctioning?
   (actually, I've tried to power my Arduino Uno board with this module, (power24V->9V), and it seems work properly.)
2. any better way to use 24V DC power to run 12V DC motor?

[PaulRB]

Motor:

Max. operating current 7A

Buck converter:

@gr_monk

Output Current : 5A (max) (4.5A recommended)|

I see a problem there.

[gr_monk]

thank you for reply, nice suggestion. I thought it would be ok because nominal current of DC motor is around 400mA. Is there a possibility that large starting current have caused BUCK module permanantly broken?

[PaulRB]

@gr_monk

possibility

Yes, it's possible. Get a buck regulator with 10A capacity. You don't want to go beyond 80% of the max rating of components. All motors are "stalled" at the moment they are switched on and will draw their maximum current, even if for only a split second

[gr_monk]

Alright. Last question : how about using 2(or more) buck converter parallelly to power 1 motor? I have many of that 5A buck, but no 10A capacity large one.

[PaulRB]

Hmm, not sure about that idea. If not connected, two buck converters will never have exactly the same output voltage. One will always output, or try to output, a slightly higher voltage than the other. If you connect 2 together, their output voltages will of course be identical, but one module will be trying to increase that voltage a little, and the other module will be trying to reduce it a little. Maybe that will mean one module will output more current than the other.

@Paul_B will know the answer, I imagine.

[Hutkikz]

@PaulRB

Hmm, not sure about that idea. If not connected, two buck converters will never have exactly the same output voltage. One will always output, or try to output, a slightly higher voltage than the other. If you connect 2 together, their output voltages will of course be identical, but one module will be trying to increase that voltage a little, and the other module will be trying to reduce it a little. Maybe that will mean one module will output more current than the other.

Yes that is the problem. One converter will carry the majority of the current( and likely overheat) while the other does very little.

There is also the issue that the frequencies will not be synchronized causing ripple currents.

There are ways to correct this but not all converters can be paralleled.

[Paul_B]

@PaulRB

@Paul_B will know the answer, I imagine.

Eh? What?

I don't think it is accurate to say "the other module will be trying to reduce it a little".

The regulator at the higher voltage will supply all the current until it current limits (if indeed it does) at which point the voltage will fall, the first will supply its limiting current and the other will supply the balance. Will it overheat? If it is not appropriately designed (for the current limit).

The only other concern about paralleling two regulators is the possibility they may oscillate. This is possible, and would demonstrate that their design was faulty.

In general, the possibility (likelihood) that they will not actually evenly "share" the load makes it a poor idea.

@Hutkikz

There is also the issue that the frequencies will not be synchronized causing ripple currents.

Methinks ripple would be worse if they were synchronised!

[PaulRB]

The consensus of opinion is to get a 10A buck converter.

[stevemj]

@gr_monk

I thought it would be ok because nominal current of DC motor is around 400mA. Is there a possibility that large starting current

In th epast I have used a series current limiter to protect the motor's brushes and the power supply at motor start. This just caused a slower start - but imperceptibly. The current limit was created by a two bipolar transistor circuit and a couple of resistors.
It saved my client a lot of cost, about 3/4 million were fitted to their machines.

[gr_monk]

Maybe I should try soft-start my dc motor and regulate overcurrent.

[stevemj]

I think that would be one way, if you can arrange a soft start.
Another technique I have seen only once in commercial equipment (a Xerox stapler) was a humungeous capacitor that could store sufficient energy to provide the start up current.
Let us know what you do and how you get on

[JCA34F]

I would have spent the money you blew on the buck on a 12V, 150W power supply.

[gr_monk]

@stevemj I tried soft-starting the dc motor, and here's the result:

void screw_open(float screw_distance)
{
  long targetPos = 16*360*screw_distance/5;
  
  encoderPos = 0;
  
  long error = targetPos - encoderPos;
  long control = Kp*error;
  int i= 0;
  int speed_acc = 0;
  timestamp = millis();
  lastError = 0;
  cumError = 0;
  //variable = (condition) ? optionA : optionB;
  do{   
    control = computePID(targetPos, encoderPos);
    now = millis();
    
    if ((now - timestamp) >= PWMinterval)
      {          
        timestamp = now;
        speed_acc += 1;
        speed_acc = constrain(speed_acc, 0, 254);
      }
    big_motor_current = read_and_report_DC_current(CURRENT_SENSOR);
    setMotor(min(abs(control), speed_acc), (control>=0)?true:false, DIR_SMALL, PWM_SMALL_PIN );
    i++;
    if(i>700)
    {
      Serial.print("iteration over\n");
      break;      
    }
  }while(abs(lastError)>10);
  setMotor(0,false,DIR_SMALL,PWM_SMALL_PIN);
}

with this, I measured DC motor's current when Starting and Stopping the motor.
and here's the result (vertical axis : current[A])

and the dc motor smelt like burning. and the temperature went very high, such high that I can't grab it with my bare hand.

within a few days I'm gonna try with 10A capacity BUCK converter and another dc motor, but I'm not sure whether this is a proper way to do so.

[stevemj]

Hi again,

I do not have expertise in software coding, so I am not the person who can help you with that.

The graph X-axis are not scaled; I presume it is mS. The shape of the graph is not what I would anticipate. I would expect to see a short period of constant current while the motor accelerates and then the current drop to the running current of 400mA. The running current does depend on the mechanical load of course.

I do not see how the buck converter’s output is current limited. How did you do that?

Do you have a reservoir capacitor on the converter output? If so what value?

Did you try to combine soft start and current limit concurrently. I am thinking that the soft start PWM is causing large current to be drawn from the converter. These substantial current drawn in pulses is causing heating – as you have found, but the pulses are too short to cause movement.

When I used a current limit it was like this:

Current limiters using transistors and diodes

[TomGeorge]

Hi, @gr_monk
Can you please post a drawing of your circuit please?
Include component names, pin labels and ALL power supplies.

Thanks.. Tom....

[PaulRB]

I checked the data for your motors again

FIT0403 DFROBOT - Motor: DC | with encoder,with gearbox; 12VDC; 7A; Shaft: D...

Max. operating current
7A

@gr_monk

I measured DC motor's current when Starting and Stopping the motor.
and here's the result (vertical axis : current[A])

That graph shows currents exceeding 20A! That's 3x what the link shows and 4x what your buck converter can supply. Are you sure you measured the current correctly? Are you sure you posted the correct link for your motor?

[Paul_B]

@PaulRB

Max. operating current
7A

You could argue that "operating" means "not stalled" and it would be hardly surprising that the stall current is three times that.

However it also cites:

Operating current 350mA

Which sounds very modest.

[gr_monk]

@PaulRB @JCA34F @TomGeorge @Hutkikz @Paul_B @stevemj for those who are concerned with this question, I solved this issue. and the main problem was dc motor I used. used one was broken (short circuit) : shortly after I replaced dc motor, everything works fine.

[stevemj]

What a shame such a major part was faulty Plesed that you found a problem that could be resolved.

What was you arrangement to drive the good replacement motor? What power supply? With what output current capability? Did you add a substantial capacitiy resivoir?