| Synchronous speed The synchronous speed of an AC motor is the rotation rate of the rotating magnetic field created by the stator. It is always an integer fraction of the supply frequency. The synchronous speed ns in revolutions per minute (RPM) is given by: where f is the frequency of the AC supply current in Hz and p is the number of magnetic pole pairs per phase. When using total number of poles, use 120 as constant instead of 60.;For example, a small 3-phase motor typically has six magnetic poles organized as three opposing pairs 120° apart, each powered by one phase of the supply current. So there is one pair of poles per phase, which means p = 1, and for a line frequency of 50 Hz the synchronous speed is 3000 RPM. |  |
| Slip Typical torque curve as a function of slip (slip is represented by g here, which is proportional to s in the formula at left). Slip s is the rotation rate of the magnetic field, relative to the rotor, divided by the absolute rotation rate of the stator magnetic field where is the rotor rotation speed in rpm. Slip is zero at synchronous speed and 1 (100%) when the rotor is stationary. The slip determines the motor's torque. Since the short-circuited rotor windings have small resistance, a small slip induces a large current in the rotor and produces large torque.At full rated load, typical values of slip are 4-6% for small motors and 1.5-2% for large motors, so induction motors have good speed regulation and are considered constant-speed motors |  |
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