Lesson Sources and induction · Induction and spikes
Electromagnetic induction
Electromagnetic induction: whenever a conductor cuts magnetic flux (or flux cuts the conductor), a voltage is induced. That is the reverse of “current makes a field.” Direction of motion, field polarity, and relative speed set polarity and magnitude. Generators, alternators, transformers, and many motor actions depend on this — if you can picture flux being cut, you can predict induced voltage.
Current makes flux — flux cutting makes voltage
Known law: current in a conductor → magnetic field around it (strength ∝ current; direction from current direction).
Induction in reverse: move a conductor through lines of flux → voltage induced in the conductor. Connect a meter — pointer swings from zero-center; reverse motion → reverse deflection.
No relative motion → no cutting → no induced EMF (even if flux exists).
Relative motion — wire or magnet can move
Three ingredients always:
- Conductor
- Magnetic field
- Relative motion between them
Either move the wire through the field or move the magnet through a stationary coil — alternators commonly spin the field past fixed windings. The physics cares about cutting rate, not which part you label “moving.”
Polarity — motion and field direction
Reversing conductor motion reverses induced voltage polarity and meter deflection.
Reversing magnetic field polarity (swap N and S) with same motion also reverses induced polarity.
Conclusion: induced polarity depends on field direction relative to motion direction.
Fleming’s left-hand generator rule
To find induced current direction (generator convention):
Hold left hand with thumb, forefinger, middle finger mutually at 90°:
| Finger | Represents |
|---|---|
| Thumb | Thrust — conductor motion |
| Forefinger | Flux — field (N → S) |
| Middle finger | Current — induced in conductor |
Change field polarity or motion direction → induced current direction flips. Use this before guessing “which way the meter goes” on a lab loop or generator polarity check.
How much voltage — three factors
Induced voltage magnitude grows when you increase:
- Number of turns of wire (more turns → more series conductors cutting flux)
- Strength of magnetic field (flux density)
- Speed of cutting (relative velocity or rate of flux change)
Quantitative relationships used in the field:
- Conductor moving perpendicular to field:
e = B × l × v
(B = flux density, l = conductor length in field, v = velocity)
- Coil / flux change:
e = N × (ΔΦ / Δt)
(N = turns, ΔΦ/Δt = rate of flux change in webers per second)
One weber per second cut → 1 volt induced (for one path). Faster spin on a generator → higher output voltage before regulation.
From single loop to machine thinking
A one-turn loop between magnet poles: rotate → alternately cuts flux up and down → AC voltage at brushes/slip rings.
Add turns → voltages add in series → higher EMF per rpm.
Stronger magnets or faster rpm → more lines per second → higher EMF.
That is the skeleton of AC generators, transformers (changing flux links another coil), and motor/generator action in machines you will service later.
Field measurement units — weber recap
Flux is measured in webers (Wb). Cutting 100,000,000 lines of flux per second (1 Wb/s) induces 1 volt in a single conductor path. In practice you reason with turns × flux change × speed rather than counting lines — but the unit ties math to the picture of “how fast flux is being cut.”
Field case
Situation. Apprentice spins a portable generator slowly by hand, reads low AC voltage, and declares the unit “weak magnets.” At rated rpm on the engine, output is nominal.
Lesson. Induced voltage scales with speed of cutting and turns. Slow hand-crank = low ΔΦ/Δt — not necessarily failed excitation. Always compare at nameplate rpm before opening the machine.
In the field
Symptom
Low or zero generator output
Where to look
rpm, excitation, brush contact, turn count (rewinds), drive belt
Likely causes
- Underspeed, open field circuit, worn brushes, shorted turn
What to measure
- Output V vs rpm curve
- field current
- resistance of windings
What not to do
- Conclude “dead magnets” at hand-crank speed
Checklist
- I state induction = conductor cutting flux → voltage
- I list conductor, field, relative motion
- I use Fleming left-hand rule (thumb/fore/middle)
- I name three factors: turns, field strength, speed
- I cite e = Blv and e = N(ΔΦ/Δt) conceptually
- I explain magnet-moving vs conductor-moving equivalence