slip

Slip is the percentage difference between the rotating magnetic field inside an induction motor and the actual speed of its rotor. In a three-phase motor running at 60 Hz in North America, the magnetic field rotates at 1800 rpm (synchronous speed). If the shaft turns at 1750 rpm, the slip is about 2.8 percent. This lag is not a flaw, it is the mechanism that allows the motor to produce torque.

The physics is straightforward. The rotating magnetic field induces current in the rotor bars only when there is relative motion between field and rotor. If the rotor reached synchronous speed, no current would flow, no magnetic force would act, and the motor would stop. Slip must exist for the rotor to experience the force that keeps it turning. Under full load, most industrial induction motors run at slips between 2 and 5 percent.

At no load, slip is minimal, perhaps 0.5 to 1 percent. As mechanical load increases, the rotor slows down, slip rises, and more current flows through the rotor. This increased current creates stronger magnetic braking force, which delivers the extra torque needed to turn the load. A motor wound for 1800 rpm might actually turn at 1700 rpm under rated load. If a motor is stalled (rotor locked), slip reaches 100 percent and full rotor current flows, causing rapid heating.

Slip varies by motor design. Standard motors have slip around 3 to 4 percent at rated load. High-slip motors, used in applications requiring high starting torque, may have slip of 5 to 10 percent or more at full load. These motors use a higher-resistance rotor bar material to increase the torque curve. Low-slip motors, used where energy efficiency and constant speed are critical, have slip of 1 to 2 percent and are common in pump and fan drives.

Measuring slip requires knowledge of both synchronous speed (determined by line frequency and pole count) and actual speed (measured with a tachometer). The formula is simple: slip in percent equals (synchronous speed minus actual speed) divided by synchronous speed, times 100. A rise in slip above normal may signal bearing wear, rotor bar fracture, or excessive load. In utility and industrial settings, slip is monitored to predict motor failure and to calculate actual power consumption.