I have an induction motor that is rated for 50/60 Hz, with full-load amperages of 1.85A / 2A respectively. It is not rated for any other operating frequency.

If I connect it to a VFD, will it be safe to run at say 40 Hz, assuming a linear relationship between the frequency and full-load amperages?

i.e.: for full-load amperages (assuming average RMS value between three phases), if 60 Hz = 2A, and if 50 Hz = 1.85A, so perhaps 40 Hz = 1.70A?


An induction motor (when run at a lower speed) should have the RMS voltage reduced as frequency gets below a certain threshold. This is to stop the magnetics saturating at low frequencies. Saturation isn't necessarily reflected in the amount of current drawn being above (say) full-load current so, deal with the problem in a direct and predictable way by lowering the applied voltage as frequency drops.

It's called keeping the V to F frequency constant: -

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Picture source

  • \$\begingroup\$ So looking at an induction motor nameplate, its rated for 208V for 60 Hz (Y-connected, low voltage) and 190V for 50 Hz, so from your graph, and assuming a linear relationship, it should be 172V for 40 Hz? \$\endgroup\$ – plu Mar 7 '18 at 19:58
  • \$\begingroup\$ 208V 60Hz = 3.47V/Hz, so at 40Hz the V should be 40 x 3.47 = 139V. But really, when you set up a VFD and tell it that the motor is rated for 208V 60Hz, the VFD will automatically determine the V/Hz pattern is sends to the motor. \$\endgroup\$ – JRaef Mar 7 '18 at 20:14
  • \$\begingroup\$ Oh, so that's how it works, thanks. Actually, my motor has a rating from 208 - 230V @ 60 Hz, so taking the high estimate, 230V / 60 Hz = 3.83V/ Hz, which translates to 191.66 V / 50 Hz (close to nameplate value of 190V @ 50 Hz). And so that hypothetical 40 Hz should operate at ~153V RMS. \$\endgroup\$ – plu Mar 7 '18 at 20:19
  • \$\begingroup\$ @plu there's a flat part to the graph and 60 Hz could be enough along that graph so that at 50 Hz the graph is still flat so you do need to be wary about making predictions. \$\endgroup\$ – Andy aka Mar 7 '18 at 20:47
  • \$\begingroup\$ @Andyaka To help further answer the original question, assuming that we know the correct operating voltage, would one still be able to infer a "maximum loading condition" based on extrapolating the rated full-load amps? i.e.: (from original post) "60 Hz = 2A, and if 50 Hz = 1.85A, so perhaps 40 Hz = 1.70A?" \$\endgroup\$ – plu Mar 27 '18 at 1:08

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