Steppers and Voltage

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TronCarter
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Steppers and Voltage

Post by TronCarter » Wed Nov 04, 2009 1:00 pm

I have a collection of recycled steppers that I would like to use with the EasyDriver. Some of the motors say 3.9V/PH on the side, but the ED has a minimum of 8v input. I thought that I had read somewhere that the voltage is not as important as the current, and steppers can run in a wide range of voltage. Can someone shed some light on this? I also have some motors with no label, or just part numbers that I find no specs for online.

Tron

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EmbeddedMan
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Post by EmbeddedMan » Wed Nov 04, 2009 3:23 pm

Tron,

That is correct. What's really important is the power dissipation of the coil of the motor. At DC, each coil has a certain resistance. They don't want you to keep cranking up the voltage to get more current to flow through (V=I*R, so if R is constant, you have to raise V to get more I) to get more holding torque, because that will raise the power dissipated and will burn up the motor. So they put specs on the motor for what voltage you should use if you don't limit the current.

The EasyDriver, on the other hand, is a chopper driver. That means that it is constantly monitoring the current flowing through each coil. As soon as the current gets above the set point (set with the current adjust pot on the ED) it cuts back the voltage to the coil (chops) until it dips, then lets it go back up again, over and over, very quickly. This means that no matter what voltage you have going into the ED, it will never let more than <set_current> through the coil.

You're usually very safe to run much higher voltages into the ED, and just set the current adjust real low, then slowly move it up until you feel heating in the motor, then back off. All steppers except for the very, very tiny ones will be able to handle the minimum current that the ED puts out (I think about 150mA/phase) without busting a coil.

The reason running higher voltages is a good thing is because it allows the ramp up time to be much quicker for each step - thus your torque at higher speeds is much better with higher voltages. Many people run their EDs at 24 volts (because it's easy to get power supplies at 24V).

Commercial stepper motor retrofit kits for CNC milling machines will usually run between 80V and 120V DC, even though the motor may only be 'rated' for 5V or so. This is because they use really nice chopper drives (like the Gecko drives) and they want really big torque at high speeds.

Here's what I do when I have a motor with no datasheet (which happens often). Measure the DC resistance of the coil. Make sure it is a bi-polar motor and not a uni-polar one. Pick your input voltage to the ED, and subtract about 1.8V or so. The do the V=I*R calculation to see what the current would be at that voltage with the motor's coil resistance. Make sure to set the current limit of the ED _below_ that value. (otherwise you won't be able to get any microstepping).

I use 12V with my ED, and always start at the lowest current, then adjust it by feel (of the motor shaft) and sound with a 500Hz step rate. After a couple of tries with different motors, you'll get real good at this and won't even look at the motor specs much anymore.

NOTE: Many motors will have a coil resistance so high that you CANNOT set the ED's current limit low enough to be below the max current for the motor. They will still work, but you will not get any (or much) microstepping action. So keep that in mind too. (For example, at 12V input to the ED, any coil resistance above about 60 ohms won't get any microstepping.)

*Brian

bugu1987
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Re: Steppers and Voltage

Post by bugu1987 » Sat Feb 25, 2012 9:18 am

Hello, I am new to stepper motors motors and would like to ask a question. I need to drive an old printer having a stepping motor rated at 3.9V, 1.3A. It has 6 leads. Two are centre taps are the other are ends of the coils.

The easy driver only takes bipolar motors. Am I correct in stating that I can use my motor as a bipolar just by ignoring the centre taps?

The motor is rated at 3.9V. If I operate at 9V, would I be fine because the current through the coil can never exceed 750mA?

Thank you for your help.

Joseph Borg

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EmbeddedMan
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Re: Steppers and Voltage

Post by EmbeddedMan » Mon Feb 27, 2012 3:17 pm

Joseph,

The answer is yes to both of your questions. You can use your motor with the EasyDriver by ignoring the center taps, and your 9V will be just fine even though your motor is rated for less.

Something to consider though - with that low of a voltage, you need a very low coil resistance in order to get the proper microstepping behavior. So you may want to use just half of each coil (use one and and the center) rather than the full coil. This will cut your resistance in half, and allow microstepping at a lower voltage.

Also, please start a new thread when you have a new question rather than replying to somebody else's thread. Thanks!

*Brian

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