Steppers and resistor-question.

[viktorcnc]

Hello you all.

I have tried the steppercalc to find the correct value for my limiting resistors. I just need somebody to confirm that I'm on the right way and that my values are correct. I have a Slo-syn 53oz 200step pr. rev. stepmotor (3 actually) with the following specs: 1.25v & 3,8A. I will drive this with a FET-controller and a current limit resistor i series with the powersupply rated 14,7v. (I've tried to measure the resistance in the windings and found 0.4-0.8-0.4 ohms, but steppercalc doesn't ask for that). According to the programme the limit value is 1,77ohms ca.100 watts. I plan to put 14 resistors rated 27ohms & 9watts i parallell to get a total about 1.9ohms in series with the powersupply. Am I doing the right thing here guys? I will repeat this for each axis and I'm planning to buy the resistors soon. Any feedback is much appreciated. Keep up the good work.

[Al_The_Man]

I make it 3.54 ohms to drop 13.45 volts at 3.8amps the wattage would be just over 50watts.
Al

[H500]

Can your power supply put out the 23 amps that you will need for 3 axis? Why not use a chopper drive instead? 50 watt resistors might be expensive.

[viktorcnc]

First of all I want to thank you both for the replies. I trust Al The Man with his tip on the resistance value and will go with that. Thanks Al! For the next reply from H500 I will consider your suggestions, but at a later time as I already have bought and installed the FET drivers. My powersupply will do about 42 amps and the resistors I can get cheap. I have one question that pops up in my mind: When the label on the stepmotor says 3,8A that means each winding and not the total amount right. So I will have to multiply 3,8A x 2 x 3 motors to get the total current flow from the supply?

[H500]

I don't know for sure, but typically the rating is for 2 coils energized at the same time.

[ger21]

The rating is per phase, so yes, 3.8x2x3 = 22.8a.

If you use a chopper, like a Xylotex or Gecko, they only use at most 2/3 of 1 phase, or 2/3 x 3.8 x 3 = 7.6a. Much more efficient.

[extreme]

hello,
If i run my motors in full step than i need only half of that current?
when full step, then only one coil is energized at a time right ?

[H500]

hello,
If i run my motors in full step than i need only half of that current?
when full step, then only one coil is energized at a time right ?

Full stepping is usually done by energizing both coils to maximize the torque.

[extreme]

if two coils are energized in same polarity then it's a halfstep? if in different polarity then it is a step. Right?
so for 1.5 amp/phase I need atleast 3 amp supply? if i use more voltage than rated and resistors than i need even more current or not?

Thanks
Robert

[H500]

In half stepping, you alternate between energizing one and two coils. Full stepping can be done energizing one or two coils.

3 amps is correct. If you use more voltage with resistors, the current remains the same. You need to size the resistors to limit the current to 1.5 amps per phase.

[extreme]

Thanks for help. Anyone knows a good chopper schematic ? I've made the one from free source and it doesn't work like it should . I'm aiming at 30 volts.