I have a 10 Hp, 1770 rpm rated motor on an air compressor. The air compressor started sounding different the other day, and seemed to be turning too slow. I measured the motor shaft speed with a stroboscope and it was turning at 1184 rpm. The voltage and amperage on each phase is good and power supply is at 60 Hz. What could my problem be? It is a 600 VAC 3 Ph 60 Hz motor, induction type.

 

If current/voltage is normal then not much to worry about, the thing that often causes low rpm and low current is one of the rotor bars has lost connection, many modern motor have a cast-in rotor bars and do not suffer from this, but older or different style can be the cause of this.
Years ago rotor bars were copper and silver soldered ends, when the solder over heated and flung off, the motor turned into a 3ph choke and never rotated at all!
Max.