Hopefully I am posting this in the right area.

I am designing a system where I have to limit the maximum torque that a DC motor is able to provide.
The torque should be directly proportional to the current, so if I can limit the current to a fixed maximum value then the torque should be limited.

The controller calculates a target motor torque based on various input and translates that into a target current. It then uses current feedback signals to do closed loop control of the current.
The motor is run on a 48V supply voltage using PWM control.

I need to create a separate current limiting circuit which will prevent the motor ever drawing more than the defined safe current, regardless of what the main controller does. This is a separate protection circuit and should not include any components which require any form of programming/software.

Are there any common circuits which can do this task?

 

I see two options, the one option commonly referenced as an "electronic fuse", the other often called a "current limited power supply"
or a "foldback current limiter."The difference being that the electronic fuse would typically interrupt the circuit in the event of the limit being exceeded.
The current limited supply is typically a voltage regulated system up to the current limit, at which point the voltage is reduced to hold the current at some level.
Both of these approaches are quite mature and so there are a lot of both circuits and products available. And now you have the keywords for searching.

 

Isn't that three options?
Fold back being the third.
That collapses the current to a foldback value. (Which obviously reduces the voltage to whatever is required to maintain that current....)
The end result usually being similar to a fuse.

 

I need to create a separate current limiting circuit which will prevent the motor ever drawing more than the defined safe current,

How much is this "safe" current?

 

Actually, power supplies with both voltage adjust and current limit adjust have been around for a very long time. The linear supplies like that seem to last forever, some brands, and about a month,another brand. Of course that is in a machine about 120 degrees F all the time.
AND, the "Two Choices" were an "electronic fuse" or "another kind"

 

How much is this "safe" current?

It will depend on the application as the safe torque limit is dependent on the physical application. The one I am currently looking at would be about 4.5 amps

 

Actually, power supplies with both voltage adjust and current limit adjust have been around for a very long time. The linear supplies like that seem to last forever, some brands, and about a month,another brand. Of course that is in a machine about 120 degrees F all the time.
AND, the "Two Choices" were an "electronic fuse" or "another kind"

OK, maybe I need to give some more info. The motor is being controlled by a controller running off a 12V DC battery supply. So I cannot change the power supply to the Controller. I am ideally looking for a circuit I can place between the controller and the motor which will limit the maximum current draw.

 

I see two options, the one option commonly referenced as an "electronic fuse", the other often called a "current limited power supply"
or a "foldback current limiter."The difference being that the electronic fuse would typically interrupt the circuit in the event of the limit being exceeded.
The current limited supply is typically a voltage regulated system up to the current limit, at which point the voltage is reduced to hold the current at some level.
Both of these approaches are quite mature and so there are a lot of both circuits and products available. And now you have the keywords for searching.

Ideally I'd prefer a circuit which caps the current allowing the motor to continue operating, rather than cutting the power completely.
I would like something I can put between the controller (Which already exists and I can't modify), and the motor.

 

Also you have digital fuses, usually DIN-mounted.. on which you can set the max current, and are resetable with click or even from remote control (PLC signal).
This is if you are concidering the "fuse" option...

EDIT: I did not read your last reply, that you dont want to cut the motor operation, just to limit it. Are you sure that will not mess up the mechanical operation, or cause damage?
What kind of controller are you using? Most programmable controller have this parameters availible to edit.

 

Isn't that three options?
Fold back being the third.
That collapses the current to a foldback value. (Which obviously reduces the voltage to whatever is required to maintain that current....)
The end result usually being similar to a fuse.

As the motor is in PWM control and the current is not constant but has a profile that follows the PWM drive, will a foldback current limiter work?

 

Foldback limiting is rather agnostic regarding the load. Depending on the response speed it might cut the voltage during each pulse, although it makes more sense to just reduce the voltage to hold the current at the preset limit.
For a battery powered system that could be a bit complex, because usually the foldback is integral with the voltage regulator circuit. THAT is a realm I have not explored.

 

It sounds like to me that what You need is a very "Low" Frequency PWM-Current-Limiter-Circuit.

The low-Frequency will cause the Motor to buzz or vibrate when the Current gets too high.

For maximum effectiveness the Frequency should probably be around ~20Hz ,
this should help to "un-clog" whatever is overloading the Motor, unless it's something solid, like a rock.

The Motor may stop, but it is not likely to overheat,
even during a long-term jamb-up, because of the Current-Limiting function.
But, there will be no cooing Air circulation, so it will probably get somewhat hotter than normal.

Does this sound like something You would be interested in ?
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