I am driving a nema 23 stepper with a dm542t driver. I'm using a DC power supply 30V 10A.
When i power the driver, the stepper turns but the current delivered by the power suplly (shown on it) is around 0.3A, while I have set the driver to output 4A.
I don't think the driver is outputing 4A because the stepper can't hold a 450g mass at 20cm (=0.88 Nm) while it's holding torque is 2.4Nm from the datasheet.
Can someone explain me why please.
Please show the datasheet.
Think of the driver as a power adapter that plugs into your 30V 10A dc power supply and converts 30V*0.3A=9W of power into about 4A*2.25V=9W of power for your underspecified stepper.
IS your power supply current regulated? Did you set the current to MAXIMUM?
I do not control the current, I can only control the voltage, there's no difference between 10V and 30V on the holding torque.
Ok, now i understand, thank you for this precision.
But still, i don't understand why it is not capable of holding a 450g mass at 20cm from the axle ?
Ah. Look at the specification you linked to!!! The motor has TWO phases with how many amps per phase? Do the math to get the current setting for a controller. The current for one phase is 4 amps. Times 2 is 8 amps. No wonder the motor has no torque.
Ok so i measured the current through a phase and it reaches 2A in amplitude at one moment while moving.
The system is a beam perpendicularly connected to the horizontal rotor of the stepper with a mass at the opposite side of the beam. So that the torque generated by the mass changes as a function of the angle of the beam (like a cosine). When the beam is about +/- 45° from vertical, the stepper holds still with around 1A through a phase.
But under over +/-45° from vertical, the stepper drops because apparently the holding torque isn't enough anymore. Actually the beam is 20cm long, the mass 450g so max torque when the beam is horizontal should be mgl = 0.459.810.2 = 0.88 Nm which is less than 2.4 Nm specified by the datasheet ??
From the datasheet, you can read the 0.65ohm resistance of a coil, and then apply ohms law to see that at one of the motor coils, 4A*0.65ohm=2.6V -- the driver converts it's input power to that, and manages the stepping/current between both coils.
Maybe it is a driver misunderstanding/misconfiguration.
and:
What is your driver set for? How did you "set the driver to output 4A"? What type of amps?
Wait, it's just this ? I've just misred the datasheet. Yeah you're right it's 4A per phase.
So right now my stepper is powered at 50% current, so it should hold 50% of its maximum holding torque i.e 1.2 Nm which is not the case. I'm assuming everything is linear though.
A bad assumption! Your power supply is not capable of supplying that current. Monitor the supply voltage while running the stepper. Should not vary while the motor is running.
Actually i'm not using a dm542t sorry. i'm using a TB6600 
HALJIA TB6600 4A Stepper Motor Driver Controller 20KHZ CNC Single Axis 2/4 Phase Hybrid Stepper Motor Driver Board Controller : Amazon.fr: Commerce, Industrie et Science
i've set at 32 microstep and 4.0 Peak current. I know that microstepping decrease holding torque but it stabilize my system. I have connected the stepper as A+ A- B+ B- following the link from stepperonline i sent you.
Looking at the dip-switch table, the highest you could set it is 4.2A peak, which with the text for the wiring of the two phases gives 4.2A/1.4= 3A per coil, and then perhaps the standstill current setting might de-rate that by 50% to 1.5A per coil.
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I do you know that my power supply can't provide 8A ? It is a 30V 10A, so 300W and as @DaveX said, the driver will need to provide 4A0.65ohm=2.6V per phase so 4.2V in total, leading to P = UI = 4.2*8 = 33.6 W
I'm not going any deeper than a snapshot of an image from your link:
Still, that looks like maybe 4.0A/1.4=2.8A per coil.
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MY personal bad experience with Chinese power supplies is that specifications are "OR". It will supply a rated voltage or it will supply a rated current, BUT, it will not do both at the same time. I hope that is not your case.
It doesn't need to. @Paul_KD7HB is wrong in this aspekt. The stepper driver works like a buck converter. You must consider the power, not the amps to select a PSU.
And the selected amps at the driver is always the current for one coil.
The TB6600 is too weak for your stepper. And the holding torque in the datasheet is the torque in standstill at fullstep. To get the full torqe of the datasheet you must not use microstepping.
Maybe not always.
Per this bit from the DM542T manual, it looks like the marketers of these drivers might advertise their current handling with a √2 inflation factor:
Also, it looks like some of those drivers (DM542T optional, the other one hard-coded automatic) halve the power when they aren't moving. (0.4sec after the last step)
Assuming they deliver 4A to a coil, the two phases aren't in series with those drivers, so the 4A through 0.65ohm is just a plain 2.6V. And the microstepping drivers do some partial-energization, so for two coils they could take 4+4 amp or chop it down so both coils share the 4A. So 4A*0.65ohm= 2.6V or P=(I^2)*R= 4^2*0.65=10.4W and either doubled to 21.8W or not, depending on the mode.