Hi, I am using a toy DC motor with dual shaft like this one (below) for a project and have been facing issues with starting the motor when it is slowed down. Seems like the initial torque is sometimes insufficient to get the motor going.
I had these motors lying around so I cannot trace it back to finding a datasheet to further understand what could be going wrong. I am happy to replace my motor it with this motor: DC Motor with Jumper Wire | Buy in Australia | CE09245 | Core Electronics if I can go through the datasheet. Could you please help me find it?
With the motor that you’ve linked there, it wouldn’t be geared down at all and would spin incredibly fast and is would be what is inside the yellow TT motor that you have there without the gearbox (you should be able to remove the clear strap and pull it out).
If you’re having issues with the motors not spinning you may need to alter the code you’re using to run the motor to have a kick start where it would go to full speed for ~0.5 seconds when below a certain motor speed and starting from not moving. If this still doesn’t fix the issue you may need to look into a more powerful motor, we have a category dedicated to micro gear motors that should have something that will work for your project.
You would be replacing the plastic yellow motor with the Metal Gear Motor, it will not fit in place of the original motor and adapt to the yellow gearbox.
The motor that you’ve linked has no gearing at all and will spin at 10,000+ RPM compared to the ~200-300 RPM of the original TT motor, it will have very limited torque capabilities.
So the advantage of those motors is that they have metal gears rather than nylon, so they will be a bit more durable. It would also appear that they have a higher gear ratio that you run of the mill TT motor, so they will be more torquey at the cost of speed.
Great to hear that the little trick with kick starting the motor worked for you.
Hi Dan, thanks for confirming! I have ordered a bunch to test. I have been looking for more durable options anyway so coming across this extra-geared motor is perfect!
Not really a trick and is easily explainable. When you use one of the electronic “H” bridge type controllers you cannot “creep” a motor from standstill. The reason is to do with the flywheel diode(s). If you care to analyse the circuit you will see (for most of them anyway) that there is never a diode DIRECTLY across the motor. One always has to connect via the motor supply.
If you connect an oscilloscope directly across the motor you will see the positive PWM pulsed but also a negative pulse which at low speeds cancels out the wanted PWM with a net result of zero volts at the motor. This situation remains with the negative pulse matching the PWM until a point is reached where the PWM is slightly wider. This is when the motor starts to move and once started the negative pulse reduces in width rapidly and all of a sudden the motor is spinning at about 25 or 30% of full speed. Once moving the PWM can be reduced to slow the motor to a very slow speed but due to all of this the motor cannot be started very slowly.
I did post this a few years ago and offered to follow up in detail with oscilloscope pics but the post did not generate any interest so I just assumed that everyone knew all about this problem and so I did not bother further.
Some application like a motorised focus control or remote control golf buggy need to creep a motor slowly from standstill so the reversing is done with a relay where a flywheel diode can be directly connected.
Cheers Bob
Hi Prithul271176
Just an add on to the above reply.
Do you run this motor in both directions or is it used in one direction only.
If it is one way only there are 2 avenues to explore.
Ditch the controller and just use the PWM to switch a low side Mosfet with the flywheel diode connected directly to the motor.
Leave the controller there and just connect a diode across the motor. You could do this as an experiment anyway to check my statements above but be careful the diode is the correct way around and only drive the motor in one direction.
Use a shortly diode here that will withstand many times the operating voltage and current of the motor. A tried and proven type here is MBR20100CT. A bit of an overkill for a small motor but WILL work.
Cheers Bob
[Core Forum page has not been loading for me since yesterday. I’m replying through email, hope you get this]
Yes, I am running the motor in only one direction. I don’t have a diode handy right now but as soon as I do, will give this a shot! Sounds like a fun experiment!
Hi Prithul271176
Yes got it OK.
It will work (worked for me anyway). Establish which side of the motor is positive and connect diode cathode to here, anode to other side. be aware though that should you reverse the polarity for any reason this diode will be a short circuit.
If you need to start the motor slowly from standstill the usual motor driver will not work for the reasons I outlined above. These sorts of “H” bridge drivers are very useful if you want to start a motor at some speed but if you need to start slowly a low side Mosfet switch and a relay work every time. The down side here is this dreaded 3.3V these days (I personally hate it) as you would need to find a Mosfet to work RELIABLY at this level or find a way to drive a 5V type with the lower drive voltage (opto coupler ??)
In your case it should be easy, just fit the diode.
Cheers Bob
Hi Prithul271176
I got your last post you posted by Email.
If you have been unable to load the Forum pages you may not have seen the reply I posted to Dan so I will copy the text in this post to you and it might arrive in Email.
Copy:
Hi Dan and All.
Not really a trick and is easily explainable. When you use one of the electronic “H” bridge type controllers you cannot “creep” a motor from standstill. The reason is to do with the flywheel diode(s). If you care to analyse the circuit you will see (for most of them anyway) that there is never a diode DIRECTLY across the motor. One always has to connect via the motor supply.
If you connect an oscilloscope directly across the motor you will see the positive PWM pulsed but also a negative pulse which at low speeds cancels out the wanted PWM with a net result of zero volts at the motor. This situation remains with the negative pulse matching the PWM until a point is reached where the PWM is slightly wider. This is when the motor starts to move and once started the negative pulse reduces in width rapidly and all of a sudden the motor is spinning at about 25 or 30% of full speed. Once moving the PWM can be reduced to slow the motor to a very slow speed but due to all of this the motor cannot be started very slowly.
I did post this a few years ago and offered to follow up in detail with oscilloscope pics but the post did not generate any interest so I just assumed that everyone knew all about this problem and so I did not bother further.
Some application like a motorised focus control or remote control golf buggy need to creep a motor slowly from standstill so the reversing is done with a relay where a flywheel diode can be directly connected.
Cheers Bob
End copy.
Cheers Bob
