I'm trying to select a motor for a design application that cares mostly about efficiency. Given a required torque and RPM output from the motor, I feel like it should be possible to calculate the efficiency of that operating point given a spec sheet. However, take the spec sheet on page 8 of this pdf for example: https://www.magneticinnovations.com/wp-content/uploads/2020/07/Brochure-Frameless-High-Torque-Motors_MI-F18V1web-std-res.pdf

There doesn't seem to enough information to calculate the efficiency for a given operating point. I feel like you would need the viscous drag (Nm/rpm) and static torque (Nm) as constants to be able to then multiply by the RPM to get the iron losses, and add them to the copper losses which can be found using the resistance (which is included). The total loss can be then compared to the output (torque * rad/s) to get efficiency. However, most data sheets don't have those two constants. Is there another way using the other constants? Is there something obvious I'm missing? This is a PMSM. With some BLDCs, given the no-load current, Kv, and R, you can match a supply voltage to get to the right torque-RPM point, and then find the efficiency that way, using (torque * rad/s)/(V * I).

  • \$\begingroup\$ " for a design application that cares mostly about efficiency" - why is efficiency so important? \$\endgroup\$ Jul 24, 2020 at 23:01
  • \$\begingroup\$ @BruceAbbott Because the motor is used to move batteries that are sized by how much energy is needed to move them before needing recharge...a cycle \$\endgroup\$
    – spaceprops
    Jul 27, 2020 at 3:40
  • \$\begingroup\$ Why do the batteries need to be moved, and why is motor efficiency so important to this operation? As for the spec sheet - you are right there is not enough information to calculate efficiency. You may have to buy a motor and test it. \$\endgroup\$ Jul 27, 2020 at 8:36

1 Answer 1


I would expect some major tradeoffs could be:

  • cost, size, stability margin to load variations, RPM range, Torque max (continuous,peak)
  • cooling{water,air}, sound level, temp rise, MTBF,
  • start current, driver cost
  • and efficiency.

These appear to be all 600Vdc 3 phase, (air or water cooled) high torque 2 to 20Hp range motors.

You can compute power efficiency at max rating which is given as V,I and Watts of dissipation, continuous, max rated. at 160'C max coil temp.

Given magnets tend to weaken with ambient temp rise, I would think you want to keep these cool for reliability reasons and derate 50%. The smallest motors might be maxing efficiency at rated speed, but the bigger heat generators will probably work at best efficiency (hunch) with power de-rated 50% by a few % effic. The water-cooled give more power capacity but also loses 2.5% efficiency or so with water pumping.

I put a few numbers into a spreadsheet for your consideration.

Since these are PMSM you have to interpolate the max current, the voltage at your desired RPM and consider the efficiency of your Variable speed drive (VSD) driver and power source available. enter image description here

  • \$\begingroup\$ I'm still not sure how you calculated the watt losses, and whether the iron losses are accounted for (hysteresis, EM drag, etc.). Do you mind sharing the spreadsheet so I can see the governing equations? Additionally, what about calculating the efficiency at any desired point (e.g. 5 Nm @ 2000 RPM) rather than max rated power like you assumed? \$\endgroup\$
    – spaceprops
    Jul 27, 2020 at 3:39
  • \$\begingroup\$ Watt Loss is in the datasheet, I presume covers everything. I would write the OEM about interpolating, but I would expect the minimum losses maybe half at half speed for optimum efficiency \$\endgroup\$ Jul 27, 2020 at 3:49

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