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October 4, 2006

How do I protect against in-rush current?

Filed under: Stepper Motors — mota @ 3:20 am

Looking to size a fuse when the maximum input current is 5.0A. You will need a 5.0A slow blow fuse. This will protect against possible in-rush current.

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Can I modify the shaft of a stepper after it’s received?

Filed under: Stepper Motors — mota @ 3:09 am

Making changes to a stepper‘s output shaft is not recommended.  A couple things could happen: there is only a hair’s worth of space between the rotor and and the stator – any shaving in the motor will arc; also because of the limited space the shaft has to be isolated so that it doesn’t vibrate into the stator during the machining; the shaft/rotor cannot be removed b/c it will de-magnetize the motor and all torque will be lost.

Specials are available: www.motadistribution.com

October 3, 2006

Can I write a conditional statement in the SC8800E to test the various IO, such as the home limit status?

Filed under: Stepper Motors — mota @ 6:16 pm

You can individually, or globally, test for only the 4 programmable inputs.  See page 55 of the manual.  You cannot directly test the parameter, “IO”, within the SC8800E, such as the case where the Home limit I/O bit changes from 0 to 1.  You could, however, send an ASCII string from an external source [IO, then CR (carriage return)], and then the SC8800E would reply with a 15 character status.  You could then parse this information to determine the status of I/O bit D, which is the Home limit status.  See page 57 of the manual for more info.

Q: Can VS can be a stored value?

A: No.  VS itself is a volatile parameter.  VS stays the same value to which it was set in either immediate mode or program mode, until it is changed.  If you ever lose power, it will default back to 5, unless a program (CONFIG or some other) changes this value.  Program variables (W, X, Y, Z), however, can be stored, even when power goes off.

Program Home
VS=X
MGH
END

With this program, you could send the ASCII string, X=200, run the program Home, and the VS value then becomes 200.  Subsequent changes in X would then change the VS value.

Stepper Motor Sizing

Filed under: Stepper Motors — mota @ 4:20 pm

Load Torque + (Acceleration Torque x Inertia) will give you the correct result for choosing a stepper.

Inertia = (Rotor Inertia + Load Inertia).  Formulas for calculating load inertiaFormulas for calculating torque.

A rule of thumb for choosing a stepper based on inertia ratios is to choose a rotor inertia that is not less than 1/10 of the load inertia (10:1 ratio, load to rotor) and not less than 1/3 (3:1 ratio, load to rotor) of the load inertia for quick acceleration/deceleration moves.  Use a gearhead if your load inertia ratio is too high.  Formula for choosing a gearhead for inertia ratio: Inertia Ratio = Total Inertia / Rotor Inertia * Gear Ratio^2. 

There is one exception: the AlphaStep series uses a resolver for closed-loop control so you can choose motors with 30:1 or 9:1 load to rotor inertias without losing stalling.

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