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For my application I need a torque of \$\tau=0.87 \, Nm \$ and a speed of \$ w=140 \, RPM \approx 14.65 \, rad/s \$. This gives me a mechanical power \$ P_m=12.75 \, W\$. $$ P_m=\tau w $$ I have selected this stepper motor, which gives me the desired speed torque characteristics. enter image description here Now, I want to double check that the motor will generate enough electrical power to match or exceed the mechanical power requirements (\$P_m<=P_e\$). $$P_e=UI$$ From the motor spec \$I=6.37\$, \$R=0.3\$. As recommeded here, I obtain \$ U=IR\$; this gives me a value \$U=1.91 \, V\$.

Now this gives me an electrical power of \$P_e \approx 12 \, W\$, which is lower than my \$ P_m\$. Looking at the graph I would expect a \$P_e\$ considerably higher than my \$P_m\$ requirement. Question: why is \$P_e\$ not higher?

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  • \$\begingroup\$ 12w is the electrical power consumed by one coil when operating as a space heater - what you might call the "parasitic resistance loss". \$\endgroup\$ – Chris Stratton Nov 18 '14 at 13:49
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To much text for a comment.

  • Your link leads to 6 differnt moters, none of them is listed with 0.3 Ω
  • You can not use the DC resistance to make power calculations
  • The inductance is not stable, it changes with speed and load
  • Stepper motors are supplied with a pulsed DC voltage/current, that makes power calculations a lot more difficult than with DC or AC currents
  • Stepper motors have a bad efficiency, if the diagram above has no bugs, it shows a situation with Pm = 46.2 W and Pe = 305 W (Eventually this is peak and not average power)

I don't think that i'm wrong when i concluded that you will need a lot of profound electrotechnical and mathematical knowledge the make reliable calculations with stepper motors.

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  • \$\begingroup\$ All I need is a reasonable ballpark estimate to: (i) know that the motor will have the desired mechanical performance (I guess the torque -- speed characteristic graph is sufficient for that?) (ii) choose the correct power supply. \$\endgroup\$ – A.L. Verminburger Nov 18 '14 at 14:59

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