Difference between the rotor's physical rotation speed and the stator's rotating magnetic field is called what?

• A. Lag
• B. Phase shift
• C. Synchronous speed
• D. Slip

Answer: (D)

Slip is the difference between how fast the stator’s rotating magnetic field turns (the synchronous speed) and how fast the rotor actually turns. In an induction motor, the stator creates a magnetic field that would spin at a fixed speed set by the electrical frequency and the number of poles. The rotor, due to induction, can’t quite keep up, so there’s a lag. That lag is the slip, and it’s what enables rotor currents to be generated and torque to be produced. The slip is defined as s = (Ns − Nr)/Ns, often expressed as a percentage. For example, at 60 Hz with a four-pole machine, the synchronous speed Ns is 1800 RPM. If the rotor runs at 1720 RPM, the slip is (1800 − 1720)/1800 ≈ 4.4%. When load increases, rotor speed drops further and slip increases to produce more torque; when there’s no load, slip approaches zero.