Crankshaft position sensor scenario: if the steel tab stops in the magnet’s air gap, what happens to the induced voltage?

Introduction / Context:
Many automotive sensors (reluctor, variable reluctance, and some Hall-effect arrangements) rely on changing magnetic flux caused by rotating toothed wheels or tabs. The induced voltage depends on motion through the magnetic field. When motion stops, diagnostic expectations for the sensor voltage must be clear.

Given Data / Assumptions:

• The disk has stopped with the tab fixed in the air gap.
• We are dealing with a sensor that relies on changing flux linkage (typical VR sensor behavior).
• No external excitation that would create a separate active output is considered.

Concept / Approach:
By Faraday’s law, induced emf equals the negative rate of change of flux: E = - dΦ/dt. For a stationary tab and magnet, Φ is constant in time (even if its absolute value is high or low), making dΦ/dt = 0. Therefore, the induced voltage is zero when the rotor stops.

Step-by-Step Solution:

Verification / Alternative check:
Oscilloscope observation on a VR sensor shows the output collapses to zero when the wheel stops, regardless of tooth position. Hall sensors with separate supply may hold a logic level, but the question concerns induced voltage from changing flux, which becomes zero without motion.

Why Other Options Are Wrong:

• Increases / decreases / will remain constant: These assume a nonzero induced voltage persists; it does not when there is no change.
• Reverses polarity: Polarity changes with direction of movement, not at rest.

Common Pitfalls:

• Confusing magnetized static field presence with induced voltage generation; static fields alone do not induce emf.
• Assuming electronic sensors always produce voltage; passive inductive pickups need motion.

Final Answer:
is zero