So just to clear the air, I am a total noob when it comes to electronics, but willing to learn. Here goes the question.
I plan to use a small 24v DC motor, 315rpm free spin, 220rpm under load, 4Ncm stall torque. When spinning freely, it draws less than 50mA current, at max torque it draws 120mA.
My goal is to make the dc motor stop spinning when it reaches specific load, torque, external resistance, lets say at about 3.5Ncm, it stalls at 4Ncm.
My assumption is that the dc motor itself draws from 50mA up to 120mA current dependind on the load, the biger the external load, the more current it draws. So if I need it to stop just about before its max torque (120mA), I should set it to stop spinning when the current reaches around 100-110mA. Stronger current - bigger load, right?!? The motor will otherwise be controlled by an on/off switch.
As a bonus, it would be nice if I could make a 3-way switch, to have 3 preset points at which the motor would stop spinning, i.e. at 70mA, 90mA and 110mA, but thats highly optional.

I hope I presented all the necessary specifications, if I am missing something, please let me know.
So, is this doable, if it is, how can I achieve it?

Thanks,

Alex

 

I can't go into a lot of the circuit specifics, but you need to be aware of a few things that I can quickly mention.

1: a "3-way switch" is a specific device which is not very well named or understood by many. You'd probably be wanting a 3 (or more) position switch.

2: the motor may fail completely if it is permitted to run near its stall point for very long, or very often.

3: "stall speed" or "stall point" relating to operation of electrical motors also is a very specific term, and typically refers to the point where the current increase becomes disproportionate to the change of speed or applied load.

You are asking good questions, and may be delving off track only with your "is this doable" question. Of course it is possible. It may well be feasible or even practical for you, but only you can determine that.

 

Welcome to AAC!
A switch-adjustable constant-current supply may suffice.

 

Thanks for your reply. Just quick thoughts related to your 3 things pointed out
1 - as I wrote off the bat, total electric noob, didnt know 3way switch meant something else, what I meant a switch that could shift/alternate through 3-4 previously completed circuits
2 - no matter the load, the motor wil be running at 5-6 second runs at most, separated by 10-20 second breaks, so no endurance issues here. Furthermore, if the circuit is done right, it should prevent it from getting close to its stall point
3 - didnt know that

 

The motors torque, speed, and current-draw curves will not coincide. You should find that information. You make no mention of acceptable load speed range either, so presumably it is inconsequential?

A clutch may work better for your application than a direct drive, but I suspect you've considered that.

 

Welcome to AAC!
A switch-adjustable constant-current supply may suffice.

Honestly, have no idea what you are talking about. Does that switch also reduce the motor rpm? If that is the case then its a no-go.

 

5 sec runs, 20 sec apart probably will kill it, if it will be doing that for over 2 hours at a time.

 

The motors torque, speed, and current-draw curves will not coincide. You should find that information. You make no mention of acceptable load speed range either, so presumably it is inconsequential?

A clutch may work better for your application than a direct drive, but I suspect you've considered that.

What do you mean by acceptable load speed range? Speed as in rpm at near-stall load? If thats the case, it would be ideal to keep constant rpm until it reaches the threshold resistance/load/friction and then comes to a stop. Clutch is not an option because of space constraints in the assembly itself

 

5 sec runs, 20 sec apart probably will kill it, if it will be doing that for over 2 hours at a time.

Wow, that was unexpected. In real-life scenario, it would do these 5/20sec cycles for about 3 minutes, then about 2-3 minutes complete rest, then again 5/20sec cycle for about 3 minutes...in total for about 40mins. Didnt know it would put such strain on the motor :-(

 

That is a very small motor!
Is the 315rpm at 24vdc with or without gearbox?
If you want that type of control, you are getting into the area of microprocessor etc.
I assume the 4N*cm and 120ma is the rated continuous stall torque?
Max.

 

That is a very small motor!
Is the 315rpm at 24vdc with or without gearbox?
If you want that type of control, you are getting into the area of microprocessor etc.
I assume the 4N*cm and 120ma is the rated continuous stall torque?
Max.

Yes you are absolutely correct. I think it has its own internal gearbox, the ratio is 10:1, its completely sealed as a whole. 315rpm at 24vdc. You are right about the Ncm continuous stall torque. So what does this micropocessor are thing mean? Arduinos or is there something simpler than that?

 

I assume you are running the motor with no controller right now, just on/off control.
To sense current limit, you will need some kind of current detect circuit, generally there is a little leeway as often the motor will enter the peak torque level, which should only be experienced briefly, otherwise if too long a duration, motor burn out occurs.
Max.

 

Actually, it will have a potentiometer to reduce the current/voltage, which will (I hope) linearly lower the rpms and stall torque values. I need this stop feature for only when the potentiometer is at its max position, i.e. motor running at full speed. These current detect circuits you speak of, are they readily availible off-the-shelf products or I have to assemble/engineer them myself?

 

Generally most applications are unique so it usually entails putting something together.
Most methods are based on sensing current without due influence on motor performance, i.e. non-invasive.
A potentiometer is usually a bad way of control as this affects speed with load.
There are simple PWM circuits such as http://www.discovercircuits.com/DJ-Circuits/simplepwm2.htm that will do it a little more efficiently, the Mosfet for controlling the motor can be changed as well as the motor voltage supply.
Max.

 

Generally most applications are unique so it usually entails putting something together.
Most methods are based on sensing current without due influence on motor performance, i.e. non-invasive.
A potentiometer is usually a bad way of control as this affects speed with load.
There are simple PWM circuits such as http://www.discovercircuits.com/DJ-Circuits/simplepwm2.htm that will do it a little more efficiently, the Mosfet for controlling the motor can be changed as well as the motor voltage supply.
Max.

I understand. Will look harder for some current detecting circuits then. Thanks

 

Since this seems way to complicated, can I instead have a buzzer on the circuit to buzz/beep when the current is at around 100mA, when the motor is struggling the hardest. Would that be simpler to do?

 

It wouldn't be much simpler. You'd still have to sense current, compare the value with some limit and then switch the buzzer on/off accordingly.