RepRap kit buying

I'm new to the forum so I apologize if this post is posted on the wrong folder.

I nearly bought this "RepRap complete kit" from Amazon.com:

Reprap complete kit Ramps1.4,LCD2004,heatbed MK2a,hotend2.0,Nema17 stepper motor

I'd like to know if this has been a good choice or a too fast buying (maybe I should have wrote here before ;P ).
Particularly I'm not sure about Nema 17 stepper because I read somewhere that "low voltage" stepper are better than "high voltage" ones.

The kit came with no datasheets and also on internet I only found few specification on this site (for steppers):

Skidproof Nema17 Stepper motor,4-Lead,1.8 degree

I'm also new to electronics so...don't blame me too much if I made a very bad choice

Thank you and I'm sorry if my english could seems strange.

Your motors should have some markings on them, but sometimes its hard to identify coz there are lots of versions of exact same looking motor, sometimes even bipolar and unipolar look identical and even have nearly similar codes / identification marks.

Alternatively, just to be sure you can use a multimeter to measure at least the coil resistance, e.g. if its 2-3-5 ohms its ok, if its 10-12 ohms or more then at 12v and approx 1A the rising edge will be slow coz it gets near the ohms law which is the final target and that takes a long time to reach. The rise edge is a function of time which rises fast at first, after which it flattens and in the end needs a long time to reach target. So it makes sense to use it in its first interval when it rises fast, so coils can energize fast and motor can produce power fast. The variables that can be changed for different motors are coil resistance and inductance, these make the time constant, so its good to have motors with low coil resistance, it makes the final value further away and then the usable part of function is in the interval when it rises fast.

Ppls use wordings like "high inductance" or "low voltage" or else, but i think it can be understood better as "high current", e.g. 1-2A instead of 0.25A or better as "low coil resistance" instead, coz inductance only matters for time constant, while resistance also gives the value at which the function aims at (aims at ohm's law given current tho wont reach it, just aims at that value). The voltage written on the motor is nothing but the ohms law = curent rating of wires * coil resistance. The maximum voltage that can be actually used depends on the wire insulation, e.g. 400v or so, and in practice a motor rated at 12v if you try use it at 12v, you would have to wait like an impractical period of time to actually reach the rated current. The current will be cut off coz of frequency and then the power given by the motor will be very much under its rated torque because the current didnt had enough time to rise to a value high enough.

Thats the theory. Practically you could try overcome this with a 24V supply instead of 12V, but its more likely that you wont really need it. You will probably settle for those motors and 12v, since you have them. Under certain work regime, it may give somewhat less torque, but most probably will be enough for reprap. Good luck and have fun !

Thank you for the answer. I will read it very carefully and if I'll have more questions I hope you could answer me again.

Also this are my stepper data:

Weight:0.4kg
2 Phase
Skidproof Shafts
Step Angle:1.8±5% degree/step
Voltage: 12V
Current: 1.2A/phase
Resistance:10.0±10% Ω/phase
Inductance:20±20%mH/phase
Lead style:AWG26 UL1007
Comes with 4-Lead Wire
Holding torque:48N.cm Min
Detent torque:2.6N.cm Max
Insulation cllass:B
Rotor torque:68g.cm2

Here I have a new question. How much time would be a reasonable time to reach the working current value?

Edited 3 time(s). Last edit at 09/08/2013 07:39AM by Peppna.

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...but I will learn...

If i am not mistaking for your values and 12v psu it would take ~0.0010779930014654s for current to rise to a set level of 0.5A. On the other hand the motors from this post datasheet would need ~0.0005995044. Latter is a bit lower, but not *that* much, but still it may show when the motor needs to rotate fast. So the torque applied to the rotor depends on this current. Though once it reaches the set level if we use a constant current driver then the voltage will be chopped for the current to stay steady. Therefore if driven this way it means its not important when holding position or at slow moves, but it can become an issue at high speeds - only. Bottom line, its not ideal, but since you bought them already, lets hope for the best. Probably will work coz normally should be room between the torque of the motors and what the printer *really* needs, or otherwise you could decrease acceleration parameters. In the extreme case you want to actually change something, then you may increase the voltage from 12v psu to 24v psu and then with 24v it will take ~5.93595*10^-5 to reach 0.5A, thats even faster than the other motor @12v. Also lets say that the current setting should be really more than 0.5A though, so the numbers above (and their proportions) arent nailed to the board, just the ideea.