AC induction motor speed from poles and frequency, slip from the nameplate, and what happens to speed, torque, power and voltage when a frequency inverter (VFD) runs the motor slower or faster.
Synchronous and typical full-load speed for each pole count at the selected mains frequency, and at common VFD frequencies.
Three-phase induction motor on a V/f frequency inverter. p is the number of poles (not pole pairs), f the frequency, s the slip. Base frequency is the mains frequency on the nameplate.
Rules of thumb for the warnings: a self-cooled motor at full torque below about 20–25 Hz gets too hot, because its own fan slows down; above about 1.5 × base frequency check the bearings, balance and the maker's mechanical speed limit; for fans and pumps the power rises with the cube of speed, so running faster than rated quickly overloads the motor.