Alternator Ripple Frequency Calculator

Every alternator produces current ripple at frequencies set by three things: engine speed, pulley ratio, and pole count. This calculator shows where those frequencies land for any combination — the stator (electrical) frequency, the rectifier ripple fundamental, and the engine firing frequency that drives torsional vibration at idle.

Further reading: this ripple-voltage tutorial (PDF) shows oscilloscope patterns for a healthy alternator and seven distinct rectifier/stator failures, measured healthy-ripple amplitudes across a dozen alternator models, and why amplitude-threshold "ripple testers" misjudge wye-tap machines.

Formulas:

f_electrical = (engine RPM / 60) × pulley ratio × (poles / 2)
f_ripple     = rectifier pulses × f_electrical
f_firing     = (engine RPM / 60) × (cylinders / 2)        [4-stroke]

Parameters

Engine speed RPM

Typical: small marine diesels idle at 600–900 RPM; cruise 1800–2800; redline 3000–4000. Full range covers cranking (~50) through race engines (10,000).

Pulley ratio (crank : alternator) : 1

Typical 2:1–3:1 (alternator overdriven so it charges at idle); marine auxiliaries commonly around 2.5:1. 1:1 = direct drive; below 1:1 = underdriven (unusual); above 4:1 risks overspeeding the alternator at engine redline.

Pole count poles 2 (turbo-alternator / lab machine) 4 (high-speed wound machine) 6 (small wound machine) 8 (small frame, uncommon) 10 (uncommon) 12 (standard claw-pole — the overwhelming default) 14 (some large-frame units) 16 (large frame / high output) 18 (large PM machines) 20 (large PM machines) 24 (low-speed PM machines) 32 (very low-speed PM machines)

Claw-pole (Lundell) alternators — nearly all automotive and marine units — are almost always 12-pole (6 pole pairs). Large-frame high-output units are sometimes 14 or 16. Counts outside 8–16 are for simulating other machine types (permanent-magnet, wound-field industrial).

Rectifier pulses per electrical cycle 1 (single-phase half-wave — simulation only) 2 (single-phase full-wave) 3 (3-phase half-wave, antique) 6 (3-phase full-wave — standard) 12 (dual-stator / 12-diode, rare) 24 (dual 6-phase — simulation only)

A standard 3-phase full-wave bridge (6 diodes) produces 6 ripple pulses per electrical cycle regardless of wye/delta winding. Extra avalanche-diode packs do not change the pulse count. Other values are for simulating non-standard rectifier topologies.

Engine cylinders (4-stroke) 1 2 3 4 5 6 8 10 12 16

Sets the firing frequency — the dominant torsional excitation at idle.

Results

Quantity	Value
Alternator shaft speed	RPM
Alternator shaft frequency	Hz
Stator tap / electrical frequency a failing rectifier diode shows up here and at low multiples of this	Hz
Rectifier ripple fundamental	Hz
Ripple 2nd harmonic	Hz
Engine firing frequency torsional excitation source at idle	Hz

Current-sensor bandwidth limit (10 kHz): the ripple fundamental is at these settings.