nike357 wrote:I have the same motors running on a Ramps Board with a 3DPrinter at 12Volts and there they are not getting this hot. (Using same ATX PSU)
I don't have the board so I'm just looking this up, it looks like there isn't support in the board for chopping, see page 16
http://www.gertbot.com/gbdownload/man/Gertbot.pdf , "The controller needs a minimum of 8V to work. If your stepper motor requires less voltage you must add series resistor". If your Ramps Board for your 3DPrinter supports chopping, then the behaviour will be entirely different.
You cannot just hook up a stepper motor to voltage and go to the party, you have to fight the fact that the motor is going to have a lower resistance in the windings than you can allow current through. This getting hot business is the first lesson of stepper motors, well, after the lesson of how you plug in all those crazy wires. So like an LED, that means a current limiting resistor, a big one (or more than one if designed in a parallel configuration to deal with the heat), unless you can live with the low torque you get from running the stepper directly at the rated voltage, which in your case is pretty low.
If you are running an ATX power supply without a normal ATX motherboard load, remember that you need to provide a minimum current draw on at least some of the voltages, look up the specs, in order to get correct functioning, unless the manufacturer is doing this internally somehow. I think usually this means that you need a load on the 5 volt and the 12 volt rails, the 3.3 volt might share with the 5 volt, it all depends.