Are there safe ways to run an induction motor rated for 50/60 Hz at a lower frequency?
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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