# How to calculate my power supply voltage and current?

My stepper motor specifications:

• Phase = 2
• step angle = 200 step/rev
• voltage = 12 V
• current = 0.33A/phase
• resistance = 34ohm/phase
• Inductance = 46omh/phase
• holding torque = 23N.cm min
• detent torque = 4.6N.cm min

I am making a CNC machine so I need the power supply to power 3 of my stepper motor with these ratings. Can someone calculate the power supply voltage and the current I should get to power my CNC stepper motors?

The given current per phase is an approximate value which you could have calculated by Ohm's Law - it is just the voltage (12V) divided by the resistance per phase (34 ohms):

12 / 34 = 0.353A

This is the maximum current that the motor windings can handle without overheating (which is why they're rounded it down to 0.33A/phase).

How much current the stepper motor will actually draw depends on how you intend to use it. However, stepper motors draw the most current when they are standing still. This means that Ohm's law (above) can to used to calculate the current requirements of the driver.

You have 2 phases, and a current per phase of 0.33A, so your total current shouldn't exceed 0.66A per motor.

If you're using 3 motors, and a 12V power supply, your total current should not exceed 0.66A per motor x 3 motors = 1.98A.

• This isn't quite right. The peak current listed in the datasheet is only achieved for a very brief period during the charge cycle. The average current, which is what matters for sizing the power supply is more like 1/3 of this peak. See geckodrive.com/support/step-motor-basics/… – jcoffland Sep 5 '18 at 6:54

The average current consumed by the stepper motors is what is most important for sizing the power supply. The peak current listed in the datasheet is only achieved for a brief period during the motor coil charge cycle. Not only that, the charge on one motor coil will recirculate and end up charging the other coil. Back EMF is also captured in the driver's capacitors which also ends up charging the motor coils.

Rule of thumb:

Power_supply_current = number_of_motors * 1/3 * motor_peak_current


for typical motors wired bipolar series. However, if you have an 8 wire motor and have wired it bipolar parallel then you will need to double the value calculated above. There is no need to go above this value. The estimate already contains a healthy margin of error.

In testing, I have found that stepper motors often draw an average current about 1/4-1/5 of the rated peak current when idle or running slowly. This goes up at midrange speeds and then drops off at higher speeds. The current goes up in the middle, not because the motor is getting more power, but because of energy losses. It goes down at high speeds as increasing back EMF cancels out the incoming voltage from the power supply.

So finally, to answer the original question. You should only need about 0.33A to run all three motors, assuming they are wired bipolar parallel and your driver does not have any gross inefficiencies.

Current of Font = ((Vmotor / Vfont) * (Itotal) * Ms

Ms= margin of safety (can be set 2)

If you power the driver PWM with 24v

((12/24) * 0.66) * 2 = 0,66A

Its 24-volt supply should provide 0.66A for each motor

If you use 3 step motors: 0.66 x 3 = 1.98

Its electrical source should be 24v x 2A