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Topic: Question: voltage vs current in a stepper motor (Read 1 time)previous topic - next topic

Bill2k

Jan 10, 2013, 06:37 pm
I have a few stepper motors that l'd like to run at a higher voltage. They are 1.75volts at 3.5 amp. I plan on making a current controller for them. My question is, if I run them at 12 volts, do I still supply them with 3.5 amps?

cmagagna

#1
Jan 10, 2013, 07:26 pm
Correct. Stepper motors are current driven. It's very common to drive stepper motors at 12-30 volts when using stepper controllers like the A4988 (used on the Big Easy Driver).

Good luck!

larryd

#2
Jan 10, 2013, 10:21 pm
In the old days, we use power resistors to do this. They get HOT.
This had the result of increasing the pickup power of the stepping motor.
No technical PMs.
The last thing you did is where you should start looking.

MarkT

#3
Jan 11, 2013, 12:13 am
The real reason for doing this (it might seem bizarre at a first glance) is to get high speeds out of stepper motors.

The huge number of effective poles (100 in most steppers) means the back-EMF is large even at low RPM.  Thus
although the windings are rated for 1.75V and 3.5A, as soon as the thing starts turning the back EMF rapidly
increases (so if you drive it at 1.75V quickly the back EMF dominates and the current and torque falls rapidly
(at perhaps 100 RPM even).

However if you drive the current and have a supply that can counter the back-EMF to 50V then you can get
the motor turning 30 times quicker (and extend the torque range 30 fold too).

Using low impedance windings (thus low-inductance) means the current can be rapidly changed allowing
fast step rates (also needed for rapid speeds).  Again a high supply voltage enables faster rates-of-change of
current.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Bill2k

#4
Jan 11, 2013, 03:16 am
Thanks for the responses. They helped explain a lot. I found what seems like a good schematic to throw together for a controller board that will drive my stepper motor.

http://www.arjan-swets.com/cnc/e99ch09.pdf

larryd

#5
Jan 11, 2013, 03:26 am
Sure it looks good.
Note: IC1 L297.  You will have to investigate the I/P to O/P control functions as opposed to using the stepper motor sketches that are out there.
No technical PMs.
The last thing you did is where you should start looking.

Bill2k

#6
Jan 11, 2013, 03:44 am
I am fairly new to electronics. What do you mean by I/P to O/P control functions?

Grumpy_Mike

#7
Jan 11, 2013, 04:52 am

What do you mean by I/P to O/P control functions?

I/P = input
O/P = output

That is rather an old chip. I have used these:-
http://www.pololu.com/catalog/product/1182

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