Robin2:
Usually the specifications include the coil resistance which, with the voltage, would allow you to determine the current.
No, no , no!
Stepper motors are NOT light bulbs. High power steppers have a low resistance but are NOT* designed to have their current set by their internal resistance and the supply voltage. Being made of huge coils, they are very inductive, and therefore you need to apply a high voltage in order to make the current change rapidly (dI/dt=V/L). Since you need to invert the current in one coil to achieve a full step, the rate at which you can step is limited by your supply voltage and the motor's inductance. The voltage you have available does NOT define the current that the motor should run at.
The current limit is basically thermal, P=I^2*R losses. If the motor gets too hot, that's too much current.
You must use a feedback loop in your current chopper circuit to control the current. More load on the motor or higher operating speeds means greater PWM duty cycle is needed to reach the same current.
- the exception being low-power motors, of which this MIGHT be one, in which case the above post is kind of right but it's a really really bad approach in general. If your current limit is defined by your supply voltage and the motor's internal resistance, be aware that you will get very little torque and speed out of the motor because all the power is being dissipated in the motor's windings and there is basically none left over to actually provide any motive force.