How can I tell the difference between a single phase and three phase electric motor?
Look at the the electric motor’s power cord plug: if there are 4 pins (3 phase and 1 ground wires), it is a 3 phase; if there are 3 pins (1 phase, 1 return, and 1 ground), it is a single phase. One more double-check clue, if the motor has one or a bank of large capacitors mounted on its body, it is a single phase (provided it has an RPM of 1800 or higher: an assumption very likely for a few-HP motor of this compressor size). This starter cap creates a phase-offset voltage which drives a startup stator coil. This coil gives an itermediary jump point between the 180-degree-apart stator field increments. Without this capacitor the starting rotation direction would be ambiguous and the starting torque very weak. Weaker single phase brushless motors that are wound to turn at less than 1800 RMP have the stator field jump in 90 degree or smaller angle increments and hence do not need the phase offset voltage of a startup capacitor. A 3-phase motor has the field rotate in 120 degree increments
