Split-phase motor — irregular rotating field

1 · Set up the split-phase motor

Teaching preset

Number of poles

Supply frequency

Main winding peak flux density 0.80 T

Auxiliary winding peak flux density 0.56 T

Illustrative calibration: 1.00 internal unit = 0.80 T. The display shows magnetic flux density B in tesla (T). Magnetic flux Φ in webers (Wb) would require the area through which the flux passes. The 0.80 T reference is an assumption, not a measurement.

Current phase displacement 30°

Larger displacement gives a better rotating field. A true split-phase motor usually achieves only a modest displacement.

Rotor slip 20%

This does not change the split-phase field itself. It illustrates rotor speed, rotor frequency and the induced bar currents in the squirrel-cage rotor.

2 · Animate and inspect

Teaching playback speed 0.25 cycles/s

Animation is slowed dramatically for teaching. The displayed synchronous speed is still the theoretical value from frequency and poles.

3 · What should we see?

Show main and auxiliary field vectors Show resultant field vector and trail Show field locus (ellipse / line / circle) Show field strength and speed meters Show induced rotor bar currents (• and ×)

Ask students to predict firstWill a split-phase motor produce a perfectly constant rotating field like a 3-phase motor?

4 · Watch the two winding fields combine

ELLIPTICAL ROTATING FIELD

Main winding fieldAuxiliary winding fieldResultant fieldBackward component

The resultant field rotates, but its length and speed change continuously because the two winding fields are not balanced.

The tip of the resultant field traces an ellipse rather than a circle. That is why a split-phase motor produces starting torque, but not as strongly or smoothly as a balanced polyphase motor.

Rotor current markers: • and × indicate opposite axial directions for the simplified forward-field induction component. The backward component is indicated separately in the readouts. The rotor-current marks are illustrative, not measured currents.

Readouts

Synchronous speed

1,500

Based on supply frequency and poles

Rotor speed

1,200

r/min · (1 − slip) × N_s

Relative speed

300

r/min · synchronous field − rotor

Rotor frequency

10 Hz

Forward field: f₂ = slip × supply Hz

Instantaneous flux density

0.80 T

Illustrative resultant B field

Instantaneous field speed

100%

Relative to ideal uniform synchronous rotation

Locus type

Ellipse

Not a constant-magnitude circle

Forward rotating component

0.78 T

Larger forward field helps starting

Backward rotating component

0.26 T

This opposes the forward starting effect

Slip

20%

Difference between field speed and rotor speed

5 · Instantaneous winding and rotor values

Main field
+0.80 T
horizontal axis

Auxiliary field
+0.48 T
vertical axis

Pole angle
31°
mechanical direction

Phase displacement
30°
between winding currents

Rotor frequency
10.0 Hz
forward-field component

Slip
20%
relative to synchronous speed

Classroom hookAt some moments the field is strong and races ahead; at others it weakens and slows. The large squirrel-cage bars show induced current as the rotor moves relative to the field.

6 · Add the magnetic fields

To scale: blue main field + teal auxiliary field (placed tip-to-tail) = orange resultant field. All three use the same arrow scale in tesla (T). The grey ellipse is the path traced by the resultant arrow tip over a cycle.

7 · What changes when you alter the split?

Main winding only

A single alternating winding does not create a true rotating field by itself. Its air-gap flux pulses back and forth along one axis. That is why a plain single-phase induction motor needs some way to create a rotating field for starting.

8 · Field waveforms

Blue and teal = winding flux densities (T). Orange = magnitude of resultant flux density (T).

9 · How irregular is the rotation?

The curve shows instantaneous field speed during one supply cycle, as a percentage of uniform synchronous rotation. A balanced system would produce a flat line at 100%.

Teaching ideas

Start with auxiliary winding disconnected. Students should see the flux merely pulse back and forth on one axis.
Switch to typical split-phase. Ask: Is it rotating? Yes. Is it smooth? No.
Compare with the ideal 2-phase reference. The field becomes circular and rotates at constant speed and constant magnitude.
Use the forward and backward component readouts to explain why split-phase motors have only modest starting torque.
Ask what happens when the start winding is disconnected after starting. The rotating field collapses back to a pulsating single-phase field, but the already-rotating rotor can keep running.

Model boundaries

This is a conceptual teaching animation, not a finite-element motor model. The stator field is represented by two orthogonal winding flux-density vectors, and the forward/backward components are shown using the standard teaching decomposition. The rotor-bar current markers illustrate only the forward-field component, not the combined forward and backward induced currents. For the backward field the slip frequency is (2 − s) × supply frequency. The flux-density reference (0.80 T per internal unit) is arbitrary and illustrative; no motor geometry or magnetic-circuit parameters have been provided. Magnetic flux in webers would require integrating flux density over a specified area. Centrifugal switch dynamics, saturation and detailed torque-speed behaviour are not solved.

A split-phase motor uses an auxiliary winding with a different impedance to shift its current relative to the main winding current. That creates a starting field that rotates in one direction more than the other, but not uniformly. The locus of the resultant field becomes more circular as the winding magnitudes become equal and the phase displacement approaches 90°.