Electromagnetic induction and rotating fields
Faraday's law and how a changing field, not a moving one, is what actually induces voltage.
- Explain electromagnetic induction and rotating fields at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real drive unit
Overview
Faraday's law of induction is the second pillar of motor physics alongside the Lorentz force, and it explains everything from why induction motors have slip to why transformers exist.
In depth
Induced voltage is proportional to how fast the enclosed flux is changing, times the number of turns linking that flux — the minus sign captures Lenz's law, that the induced current always opposes the change that caused it.
Rotating fields without a rotating magnet
Three fixed windings fed with time-varying, phase-shifted currents produce a magnetic field pattern that rotates in space even though no physical part is moving — this is the trick that lets a stator, entirely stationary, create a spinning field for the rotor to follow.
An induction motor's rotor only experiences a changing field, and therefore only generates torque, when it rotates slower than the stator field. Catch up exactly and induction — and torque — stops.
Key takeaways
- Faraday's law: induced voltage depends on the rate of change of flux linkage.
- Lenz's law explains why induced currents always oppose the change causing them.
- Rotating fields can be created by stationary windings fed with time-shifted currents.
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