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Limiting the current without changing the voltage?

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gloriouss89

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Hey guys!
I want to limit the current of DC motor and i used LM317's current limiting feature to do this ..Well i was successful when i hold the motor to see if the current exceeds the 0,3 A..it stays at this value after a while(i examined this as giving different voltage from my power source)..But while also measuring the voltage across the motor i realized it also limits the voltage when it reaches 0,3 A..i mean i hold the motor with my hands, the current increases to the certain value and the voltage decreases to the certain value..

My question is can i prevent the decrease of voltage while limiting my current ? i want my voltage steady or close to being steady..
Thanks in advance...
 
Hi,

Are you sure, maybe you want to regulate the speed?

One of the main things about a motor is its internal resistance.
Although this equation isnt complete, the motor current is RELATED to the
voltage like this:
I=(Va-Vb)/Rm
where
I is the current
Va is the armature voltage
Vb is the 'back emf' voltage
Rm is the armature resistance

Also, Vb is related to the speed like this:
Vb=w*Kb
where
w is the angular speed of the shaft
Kb is the back emf constant for the motor

This gives us:
I=(Va-w*Kb)/Rm


and from this we can see that the current depends on the armature voltage,
the speed of the shaft, and the internal resistance, and a motor constant.

Since Kb and Rm are constant, this equation basically says that
current I is related to armature voltage minus speed, so that if you change
the speed you change the current (while holding Va constant).

This means that you can not regulate voltage and current at the same time
unless you dont change the speed.

You can regulate speed however, if that's what you are really after.
 
My question is can i prevent the decrease of voltage while limiting my current ? i want my voltage steady or close to being steady..
Thanks in advance...

No. Current and voltage across a load is related by Ohm's law. If you can, Mr Ohm would be turning in his grave.
 
Hi,
Are you sure, maybe you want to regulate the speed?
...
...
This means that you can not regulate voltage and current at the same time
unless you dont change the speed.

No, it's not about the speed. I just don't want to pull so much current from the power source in case of other important components' getting low power performance in the whole system..Actually, this is some kind of unsure project assigned to me by my supervisor in my internship company which might end up unsuccesfully..They know this doesn't seem possible but they got me searching :confused:

Ok, i completely understand what you're saying about controlling the current and voltage at the same time is not possible related to those equations..
However when i connect the power source directly to the motor then i hold the motor, this time voltage decreases very slighty (to 9 V with 10V power source) ..With my limiting circuit it decreases to about 5-6 V with the same power source..What do you say about this?
 
No, it's not about the speed. I just don't want to pull so much current from the power source in case of other important components' getting low power performance in the whole system..Actually, this is some kind of unsure project assigned to me by my supervisor in my internship company which might end up unsuccesfully..They know this doesn't seem possible but they got me searching :confused:

Ok, i completely understand what you're saying about controlling the current and voltage at the same time is not possible related to those equations..
However when i connect the power source directly to the motor then i hold the motor, this time voltage decreases very slighty (to 9 V with 10V power source) ..With my limiting circuit it decreases to about 5-6 V with the same power source..What do you say about this?

Its Ohms Law.!
 
Hi again,

Let me quote your original question again here first...

My question is can i prevent the decrease of voltage while limiting my current ? i want my voltage steady or close to being steady..
Thanks in advance...


If you want the voltage to stay at a certain level then you have to
make sure you have enough power to supply the motor to begin with.
If you limit the current, that means you have to use a
'current limiter circuit' (which i assume you have done already)
which, by its very nature, controls current by lowering the voltage.
Thus, there is no way to keep the voltage the same because the
current limit circuits (all of them) limit current by means of lowering
the voltage.
To say this another way, as the circuit kicks in and
starts to limit the current, the circuit forces the voltage to go to
a lower level than it was previously and this is actually what
causes the current to limit. If you dont allow the voltage to go down,
you cannot limit the current. It's as simple as that with a DC motor.

I guess an interesting question would be, why do you want the
voltage to stay the same?
 
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I guess an interesting question would be, why do you want the
voltage to stay the same?

i don't exactly know why my supervisor want me to make the voltage steady..Maybe it's because they were afraid of abrupt changes in load consequently the voltage..And this might somehow (via EMI or smt else) affect the other components of delicate system of rocket components (company is in the missile development field so they are careful about every trivial thing which might affect other components)..As i said, it's just my unsure guess.. Anyway today he told me it might not be possible to do what they're asking for ..they just wondered if there is a way to do this and gave me this research project..I was sent to internship program too early (i'm not even a sophomore yet) so i couldn't realize the absurdity in what they're asking for at first, while hoping and searching for a certain solution for weeks..
Thank you for making it clear there is no solution for this..;)
 
gloriouss89 said:
(company is in the missile development field so they are careful about every trivial thing which might affect other components)

Which country do you live in? I want to know if I should be building a bomb shelter.
 
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Which country do you live in? I want to know if I should be building a bomb shelter.

I take this as a joke..My country is written in my profile (Turkey). You don't need to worry if you're not a terrorist. Besides, why did you react like this..doesn't your country have a some kind of military-tech organization? Having a military or related companies doesn't mean you attack everywhere. It's just a precaution if any threat occurs against you..
 
I understand the joke and was thinking the same thing.

Rocket science is an endeavor for smart knowledgeable experienced companies to work at. If immature, reckless people (worst case, not your company) are shooting off rockets we want to be prepared...meaning, far away.

Terrorists have nothing to do with the joke.
 
Ok now i understand..i'm sorry i misunderstood the point..
I thought; although Oblong saw my country in my profile; he was trying to imply something bad about us because my country is a middle eastern country..
Sorry again for my misperception about the point..sometimes i might get oversensitive about this topic because i had a bad memory about this topic recently..
 
Sorry I wasn’t being insulting but when I read Rocket scientist I laughed, I couldn’t help myself.

flat5 said:
Rocket science is an endeavor for smart knowledgeable experienced companies to work at. If immature, reckless people (worst case, not your company) are shooting off rockets we want to be prepared...meaning, far away.

Terrorists have nothing to do with the joke.

I’m glad I’m not the only one who finds this amusing.

Did your senior actual have a straight face when he said this to you. I think they are pulling your leg;)

gloriouss89 said:
Anyway today he told me it might not be possible to do what they're asking for ..they just wondered if there is a way to do this and gave me this research project..

This isn’t meant to insult you in anyway I like most everyone has asked my share of obvious questions when starting out. Just when combined with rocket scientist and the above quote I’m laughing as I write this.

Again sorry no harm no meant!

On the Contrary thanks for the laugh.
 
yeah i admit this situation is very funny indeed..:D they just put me into an endless research for this limiting thingy.. i guess they didn't know what else to give me as a research when i came here for an internship program..Actually in my department nobody is electronics engineer like my supervisors :rolleyes:.. they are almost all aeronautical engineers..So that's the reason why they don't care much about this thing..they mostly struggle with MATLAB and their main concern is navigation..i can say i'm in the wrong people's hand as an electronics engineering student..
There was much more funny stuff about my department in my first day..i came to my supervisor and said i was sent here as an intern..he told me he wasn't even expecting me..i explained that i just finished my freshman year and i don't know much more than basic physics, java and autocad..he thought for a while which topic to give me as research..So here it is :D..
I understood from the beginning they don't really care about interns..
I was trying to find a faultless solution for this limiting to take their attention..
what can i say!! i wish they had sent me to the department where electronics engineers exist :)
 
Maybe what you need to do is have a current limited voltage supply come off of your main supply so that if the motor draws too much current it will not affect the main supply, only the motor's supply.
 
Reminds me the story I read about a popular joke major league baseball players would play on brand new bat boys. After the game was over one would tell the new bat boy to go out and role up the foul line. ;)

Anyway don't feel bad about the situation. There is a lot of misunderstanding around current limiting and current regulation, both can operate only within the fixed limits of the maximum source voltage and source current available to the input of the device.

Think of it is these terms, how would a current regulator work/respond if you shorted the output load connection? How would a current regulator work/respond to an open circuit on it's load connection? A current limiter is only a current regulator once the demand load rises to a certain defined value, under that value it just follows ohms law.

Good luck with your studies

Lefty
 
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Noggin has a good point. If the concern of your superiors is the effect that motor current may have on the other systems using the common power supply, then they may be interested in isolation. This is a different approach than simple current limiting, but it would have the desireable effect. Isolation of the motor can be approached in several different ways. The most obvious is to build a "local buffer power supply" (for want of a better phrase). This would be a DC circuit with its own local temporary power storage. The motor would draw from this local storage as it likes, with spikes and variations in current as demanded by the loading on the motor. However, the local storage unit would be charged from the main power supply very steadily and carefully so as not to disturb that main supply. The amount of charging current could be set to a limit (hence a current limited power source) and be constant. The limit would be calculated by determining the average motor power drain over a long period. Averaging would be calculated over start/stop cycling, different loads and so on. This average power would then be used to calculate the constant charging current between the main supply and the local buffer supply.

Such a system could be very simple depending on what voltages are involved. A local buffer supply would use a battery to power the motor, and the constant current feeding new charge into the battery would be done by a battery charger circuit. The charger would be a constant current design. You would have to be careful about what kind of battery to select since the different chemistries (lead-acid, NiMH, NiCad, etc.) all have different charger design requirements. In a missile, weight and size and environmental tolerance will be critical, so spend time on your battery selection.
 
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