Three-phase induction motor — Rotating field & rotor induction

1 · Set up the motor

Phase sequence

Number of stator poles

Supply frequency

Slip 5%

Adjust slip to represent a different steady operating condition, not a motor acceleration test.

2 · Animate and inspect

Teaching playback speed 0.25 cycles/s

Animation is slowed dramatically. Supply frequency and displayed RPM remain the actual theoretical values.

3 · What should we see?

Rotating air-gap flux and poles Stator phase groups and currents Induced rotor bar currents (• and ×) Electromagnetic torque arrow

Ask students to predict firstAt 5% slip, can the rotor ever catch the magnetic field completely?

4 · Watch the field and rotor

FORWARD ROTATION

Phase APhase BPhase CField / N poleRotor / torque

Follow the explanation

Three alternating currents, spaced by 120°

Three stator winding groups carry currents separated by 120 electrical degrees. Their magnetic effects add together; the resultant is not stationary.

5 · Observe the measurements

Synchronous speed

1,500

r/min · 120 f / poles

Rotor speed

1,425

r/min · (1 − slip) × Ns

Relative speed

r/min · field − rotor

Rotor frequency

2.5 Hz

f₂ = slip × supply Hz

A +1.00 pu

B −0.50 pu

C −0.50 pu

Rotor current is induced while slip exists.The rotor is slightly slower than the rotating field, so a changing flux linkage remains in the rotor frame.

6 · Instantaneous 3-phase currents

Currents shown in per-unit values. The moving marker follows the rotor-field animation; reversing phase sequence swaps B and C.

7 · Why increased slip changes torque

Illustrative steady-state torque–slip shape, not a motor manufacturer's characteristic or a measured torque value. The curve depends on rotor resistance, reactance, voltage and other factors.

Classroom prompts

Pause at 0° and step 30° at a time. Do the individual winding currents rotate, or does their resultant field rotate?
Set slip to 0%. What happens to induced rotor current and torque?
Set slip to 100%. Why is the rotor frequency then 50 Hz on a 50 Hz supply?
Change from 2 poles to 4 poles. What happens to synchronous speed?
Swap phase sequence. Which direction does the field rotate?

Model boundaries

This is a conceptual teaching animation, not a transient electromagnetic or motor-design solver. The flux distribution, rotor bar currents and torque are stylised. The slip setting instantly selects an assumed operating state; no acceleration, voltage drop, saturation, rotor leakage or real dynamic reversing is calculated. Torque behaviour at a given slip varies significantly with motor design.

A squirrel-cage rotor has conductive bars shorted together by end rings. The dot (•) and cross (×) symbols indicate opposite axial current directions. Relative field motion induces rotor voltage and current; the magnetic interaction produces torque. In steady motoring, rotor speed stays below synchronous speed.