School of Physics Lecture Demonstration Manual

Ee-3 The Three Phase Induction Motor

Published: Tuesday 14 March 2006 - Updated: Tuesday 29 March 2011


To demonstrate the rotating magnetic field inside a three phase induction motor



Click on pictures to enlarge.

3 Phase Motor Pic 1

3 Phase Motor Pic 2

3 Phase Motor Pic 3


3-Phase Induction Motor A

3-Phase Induction Motor B


The apparatus consists of a car alternator as the source of three phase electricity which is applied to two stators from identical units.The alternator requires a DC source to power its stator.

The rotating magnetic field is demonstrated directly by placing a small compass needle in one of the stators. The alternator is rotated by hand and the compass needle rotates in sympathy. Rotating in the opposite direction causes the needle to change rotation direction.

An induction motor may be demonstrated by replacing the compass needle with the mounted can (see above photo). The rotating field induces eddy currents in the can. The interaction between the two fields causes rotation.

The final demonstration requires the hand crank to be replaced with a drive shaft connected to a portable electric drill. In this demo the stator has been mounted horizontally and an aluminium ring is supported on a low friction point support.

Safety notes

Electrical safety 


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