Estimating power need of motor drive system

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Estimating the need of power supply wattage for motor drive can be a complex task and may require experimentation. The needed electrical power depends on the actual mechanical load and use of motor. The power demand is proportional to the product of motor speed and torque. The following table summarizes the relation between speed and torque vs power.

Moving slowly Moving fast
Producing low torque Very low power consumption Low to medium power consumption
Producing high torque Low to medium power consumption High power consumption

Calculations[edit | edit source]

Instantenous electrical power demand of motor can be estimated from the equation:

$ P_{electrical} [W] = \frac{Torque [Nm] * Speed [RPM]}{0.095488*efficiency [%]} $


  • we assume efficiency of motor being 80% and drive efficiency 97%, which makes total efficiency of 0.80*0.97 = 0.776 = 77.6%.
  • motor rotates at 1000 rpm and during maximum instatenous load it outputs 2 Nm peak torque

Instantenous input power becomes:

$ P_{electrical} [W] = \frac{2 Nm * 1000 RPM}{0.095488*77.6}=259.9 W $

PSU sizing guide[edit | edit source]

In overall, PSU should be sized so that it does not overload or overheat during any condition in machine use. Since servo systems typically have greatly varying load, to find minimum sufficient PSU size might require measurements from the actual machine. To esimate minimum sufficient PSU size, measure RMS power consumption of the system during at least 10 second period of heavy use.

If measurements can't be done, then maximum RMS power load can be estimated roughly by summing rated power values of motors in the system. However, in typical machines the average power consumption is significantly less than summed motor power.

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