Current Limiting For Steppers

[waleed alhadidi]

Hi
i am waleed ,
I'm sorry for the stupid questions that i will ask, but that's better than living in ignorance
i am building a 3-axis stepper driver and need to know why should i use current limiting for my motors ? it is well known that the motor winding will (pull) the current it needs from the power supply, so what is the current limiters used for ?
Another point, some IC's are said to have choppers, does that mean that i don't need current limiting circuits when using these ICs ?

Thank you for your patience with me

[martinw]

Dear waleed alhadidi,

I'm no expert, but these people post some quite useful stuff. It may be a bit confusing to begin with, but their data sheets were really useful to me when I started.

http://www.st.com/stonline/products/...re/an/1650.pdf

Best wishes,

Martin

[martinw]

also try and Google

"stepper motor basics"

Best wishes,
Martin

[waleed alhadidi]

Thank you very much martin for the rapid reaction , that pdf is very helpful
but i still need someone to simplify the theory for me more than that (i feel very stupid indeed)
Well , in the pdf they said that the L297 contains chopper , does this mean that i do not need current limiting circuitry if i used this ic ? well that will be a great victory for me and i won't ask no more silly questions then.

[waleed alhadidi]

i tried the "stepper motor basics", they say that

Small stepping motors, such as those used for head positioning on floppy disk drives, are usually driven at a low DC voltage, and the current through the motor windings is usually limited by the internal resistance of the winding. High torque motors, on the other hand, are frequently built with very low resistance windings; when driven by any reasonable supply voltage, these motors typically require external current limiting circuitry.

but why ?
if we want high voltage to get higher rising speed for the current, then why would we use a limiting resistor that decreases voltage across the winding, i mean what i we don't use that resistor and let the full voltage across the winding and let the winding choose its current which is (V/R) ?

best wishes,
waleed

[martinw]

Originally Posted by waleed alhadidi Thank you very much martin for the rapid reaction , that pdf is very helpful but i still need someone to simplify the theory for me more than that (i feel very stupid indeed) Well , in the pdf they said that the L297 contains chopper , does this mean that i do not need current limiting circuitry if i used this ic ? well that will be a great victory for me and i won't ask no more silly questions then.

Dear waleed,

As I said, I am no expert. The ST website has loads of good information, and there is probably a lot more here on CNCZone. I have never had much luck with the L297 +L298 chips, My fault no doubt.

Best wishes,

Martin

[waleed alhadidi]

thank you martin i appreciate your help, i will try the st website

best wishes
waleed

[martinw]

Originally Posted by waleed alhadidi thank you martin i appreciate your help, i will try the st website best wishes waleed

Dear waleed,

I think that you should ask your questions from better men than me, and they are here on CNCZone.

I 'm sorry, but people here will be far better teachers.

Good luck with your project,

Best wishes,

Martin

[gar]

071110-2111 EST USA

waleed:

Consider one coil of your motor only. In the motor this coil consists of both inductance and internal resistance. One equivalent circuit for this is a series inductor and resistor. Also note that the time constant of the series LR circuit diminishes as the ratio of resistance to inductance increases. So if you add external series resistance, then the time constant becomes less.

If the stepper motor rotor is not moving, then the steady state current is determined by the applied voltage and the internal coil resistance. If you raise the applied voltage the current will be increased and you will overheat the coil. Thus, you add external series resistance as you raise the voltage so as to limit the steady state non-rotation current to its rated value. By adding the external resistance you shorten the time constant and the current rises more rapidly.

Using a chopper circuit you can eliminate the external series resistance, work at a higher source voltage, maintain current limiting, and eliminate most of the external resistor's power loss.

.

[waleed alhadidi]

martin, gar, thank very much my friends i appreciate your help and i think i got it
thank you for explaining, thank you for your time

best wishes
waleed

[Khalid]

Gar, good explanation:-)