Understanding Torque and H-Bridges for a DC Motor

Thread Starter

BarryBozeman

Joined Apr 1, 2016
41
I recently acquired one of these motors:
https://www.bmisurplus.com/products/37425-johnson-hc-970-motor

There are a couple of different varieties, but the general principle seems to be that they're 12v/2a.

I'm driving it with a LM350 - a 3-amp adjustable regulator. The LM350 is powered by a 12v/2 amp power supply.

It seems to have pretty negligible torque. I can stop it with my finger, at which point it has trouble getting started again.

I can't use PWM for this application because it involves audio circuitry and the oscillation is too loud.

Can anyone help me understand how to get this motor to produce a robust response? Do I need to build an H-Bridge? Can anyone suggestion something for this particular application?
 

djsfantasi

Joined Apr 11, 2010
5,591
Looks like a small hobby motor. H-Bridge, PWM and the like isn’t going to give you much more torque. Which is what I think you mean by more robust response.

How is this motor going to be used? That’s important to know. I csnt comment further until I know how you’re going to use it and you quantify what you mean by robust.
 

mvas

Joined Jun 19, 2017
538
That is a good sized motor ... you can get more torque by adding gears that reduce the speed
and / or Increase the frequency of the PWM above 20Khz to make it inaudible.

If you apply 12 volts directly to the motor and it has a hard time "self-starting" then it is BAD.
Do not stall the rotor, as that can overheat the coils.
 
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drc_567

Joined Dec 29, 2008
710
The specifications indicate the maximum efficiency is at 3308 rpm. So in order to produce the most power, you need to drive a larger diameter gear with a smaller one at that speed. ... A slow rotation speed may not be practical due to low torque.
 

crutschow

Joined Mar 14, 2008
23,333
Is the voltage staying at 12V when it starts.
A 2A motor will likely draw more than 2A when starting and may be pulling down you 2A supply.

If you have a 12V supply, why do you need the LM350?
 

MaxHeadRoom

Joined Jul 18, 2013
19,029
The specifications indicate the maximum efficiency is at 3308 rpm. So in order to produce the most power, you need to drive a larger diameter gear with a smaller one at that speed. ... A slow rotation speed may not be practical due to low torque.
Actually the typical torque curve for a DC motor is maximum at zero RPM, with a fairly flat but slightly down sloping at max rpm.
I have one of these motors on the bench right now and they are a very popular motor in the hobby world, and display a very high torque..
However they are fairly low pole count and consequentially are not as quiet as their higher pole versions.
Max.
 

drc_567

Joined Dec 29, 2008
710
... the OP states that there seems to be negligible static torque ... restart is hesitant. The photo shows that some wear and use has occurred. Maybe the internal commutation contacts or brushes have oxidation or some other repairable condition.
 

Thread Starter

BarryBozeman

Joined Apr 1, 2016
41
How is this motor going to be used? That’s important to know. I csnt comment further until I know how you’re going to use it and you quantify what you mean by robust.
I need to rotate a can of Coca Cola.

Increase the frequency of the PWM above 20Khz to make it inaudible.
Would a PWM make a difference, as opposed to a voltage regulator, with regards to torque?

If you have a 12V supply, why do you need the LM350?
I would like to have variable speed control.
 

Thread Starter

BarryBozeman

Joined Apr 1, 2016
41
I have not measured the terminals yet - I will try this tomorrow.

In the meantime, I am hoping to understand how to implement this project. If I were to buy an off-the-shelf motor/new parts for this project, what would you recommend?
 

drc_567

Joined Dec 29, 2008
710
... for the purpose mentioned above, there should not be a large torque magnitude requirement to achieve the objective. However, the comment that the motor does not rotate immediately may indicate that there is some sort of internal problem ... poor electrical contact between the rotor commutator due to residue build-up, whatever. If it is not possible to disassemble the motor and examine it, then a different motor must be obtained. A suitable rotation speed may be obtained by utilizing a turntable ... as a platform for the can, ...something like a 'lazy susan' turntable plate as a large diameter gear, while the 1/4" diameter motor shaft would be the small diameter gear ... wrapped with a layer of friction tape or something similar. The ratio of the two different circular diameters would produce the necessary rpm reduction for your application ... an 8" to 1/4" ratio would yield a speed reduction of 32. The idea is that friction contact between the small motor shaft and the large turntable plate outer diameter could be practically arranged somehow.
 
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djsfantasi

Joined Apr 11, 2010
5,591
You definitely have to gear down that motor. Or replace it.

I recommend that you take a look at Hankscraft motors. They are a specialty company, making geared motors and other resources for retail displays. I’ve used several of their motors in various products.

BTW, is the Coke can full or empty?
 

MaxHeadRoom

Joined Jul 18, 2013
19,029
In the meantime, I am hoping to understand how to implement this project. If I were to buy an off-the-shelf motor/new parts for this project, what would you recommend?
Here is a DIY method, plenty more out there like it, also ebay for about $2.50.
http://www.discovercircuits.com/DJ-Circuits/simplepwm2.htm
Test it on a 6v battery, should turn with still lots of torque.
If the end desired result it slow rotation, maybe look for a geared version.
The smoothest/quietest is the motors made for servo applications.
Max.
 

Thread Starter

BarryBozeman

Joined Apr 1, 2016
41
My goal is to *not* use PWM, as stated above - unless there is a really convincing reason.

I just tested the motor with a direct 12v and it did not seem to run at all.
It still "works" when being fed from the voltage regulator, but presumably there is some loss there. Also, I am noticing that the regulator gets quite hot. That might be normal behavior, but I don't know. Either way, it's still a pretty weak turn.
 

MaxHeadRoom

Joined Jul 18, 2013
19,029
Something amiss, if it does not run with straight 12vdc. Particularly if off-load.
Mine runs around 3+krpm at 12vdc.
Max.
 

djsfantasi

Joined Apr 11, 2010
5,591
What’s the purpose of the speed control? Does the can turn at various speeds at various times? Or were you considering speed control in order to get a slow speed rotation for the can?
 

shortbus

Joined Sep 30, 2009
6,979
My goal is to *not* use PWM, as stated above - unless there is a really convincing reason.

I just tested the motor with a direct 12v and it did not seem to run at all.
It still "works" when being fed from the voltage regulator, but presumably there is some loss there. Also, I am noticing that the regulator gets quite hot. That might be normal behavior, but I don't know. Either way, it's still a pretty weak turn.
The convincing reason is that voltage regulators are not the thing used to change a motors speed but PWM is. If the voltage regulator was really "working" you wouldn't be here asking the question. :)
 

Thread Starter

BarryBozeman

Joined Apr 1, 2016
41
While the PWM does "work", the frequency bleed is a problem. The question is more about why this PWM works and the voltage regulator does not. Saying "it's not the thing used to change a motor's speed" is a little bit broad.

I should clarify the situation:
the motor does not run on 12vdc directly.
it does run on a voltage-regulated 12vdc, but it ends up being closer to 10v by the time it reaches the terminals. at this point, it runs weakly.
the motor does run somewhat strongly with PWM.
 
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