An induction motor's rotor never quite catches up to the stator's rotating magnetic field — and that small lag, called slip, is not a flaw. It's the entire mechanism that produces torque.
Electrical machines convert electrical energy to mechanical energy (motors) or mechanical to electrical (generators), and the vast majority rely on the interaction between a rotating magnetic field and a rotor. Understanding how a rotating field is created from stationary windings, and why practical induction motors always run slightly slower than that field, explains torque production, starting behavior, and why motor speed isn't simply set by frequency alone.
A three-phase stator winding, fed by three currents 120° apart in time and arranged 120° apart in space, produces a magnetic field whose net direction rotates continuously at a speed set purely by the supply frequency and the number of poles — this is synchronous speed, and it's a mathematical consequence of the phase and spatial relationships, not a moving physical part.
An induction motor's rotor has no direct electrical connection to the supply — current is induced in the rotor bars only because the rotating stator field is moving relative to the rotor. If the rotor ever caught up to exactly match synchronous speed, there would be no relative motion, no induced current, and therefore no torque. Slip — the rotor lagging behind synchronous speed — is exactly what sustains the induced current and torque that keeps the motor turning against a load.
As mechanical load increases, the rotor slows slightly more relative to the field, increasing slip, which increases induced rotor current and therefore torque — a natural, self-regulating feedback relationship that lets an induction motor automatically supply more torque as load demands it, without any external control action needed for basic operation.
No, not while producing torque. At exactly synchronous speed there is zero relative motion between the rotor and the rotating field, so no current is induced in the rotor and no torque is produced — the motor would simply coast down. A small amount of slip is inherent to how induction motors work.
Synchronous speed = 120 × supply frequency ÷ number of poles. A 4-pole motor on 60 Hz supply has a synchronous speed of 1800 RPM; increasing the pole count lowers synchronous speed for the same frequency, which is how multi-speed motors and different base speeds are achieved.
Yes — a synchronous motor's rotor is driven independently (permanent magnets or a DC-excited field) and locks to the rotating stator field with essentially zero slip at steady state, running at exactly synchronous speed. An induction motor has no independent rotor excitation and relies entirely on slip-induced current for torque.
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