Car Alternator: The Claw-Pole Generator That Charges Your Battery

How a Car Alternator Works, Step by Step

A car alternator is a belt-driven three-phase synchronous generator whose rotor is an electromagnet, not a permanent magnet. A ~3:1 pulley ratio spins it at ~2,400 rpm when the engine idles at 800 rpm, and at up to ~18,000 rpm at high engine speed. Five things happen in sequence:

• Step 1: excite the rotor. The voltage regulator feeds ~2–5 A of field current through two carbon brushes on two copper slip rings into a single cylindrical field coil wound around the shaft.
• Step 2: multiply the poles. Two forged-steel pole halves close over the coil, each typically with six tapered claws. The claws interleave, so one coil makes 12 alternating north and south poles: the Lundell, or claw-pole, rotor.
• Step 3: induce three-phase AC. The spinning poles sweep flux through a laminated stator, commonly with 36 slots carrying three windings, inducing an EMF in each by Faraday's law, 120 electrical degrees apart.
• Step 4: rectify. Six silicon diodes in a full-wave bridge turn the AC into six-pulse DC: three pass current out to the battery terminal (B+) and three return it from ground.
• Step 5: regulate. Output rises with speed and flux, so the regulator switches field current on and off to hold ~14 V (≈13.8–14.6 V).

This reverses the older dynamo. The heavy output current is generated in the stationary stator, while the brushes carry only a few amps to the spinning field, so they wear slowly, and a rotor with no output winding on it can spin far faster.

The Claw-Pole Rotor: Twelve Poles From One Coil

Field current drives flux along the steel core inside the coil. At one end the flux turns outward into the first pole half, making every claw on it a north pole. It crosses a sub-millimetre air gap into the stator teeth, runs around the stator's back iron, and returns through the interleaved claws of the other half, which are therefore all south poles. Each stator tooth sees N, S, N, S as the rotor turns: six pole pairs from one winding.

A simple round coil in a compact forged rotor that tolerates heat and speed is why this design has dominated car charging for six decades. The price is leakage flux: claws of opposite polarity sit millimetres apart, so some flux jumps claw-to-claw without ever reaching the stator. Some high-output designs place small permanent magnets between the claws to reduce it, and 14- and 16-pole rotors also exist.

The rotor keeps too little residual magnetism to push current through the two diode drops (about 1.4 V) in its excitation path, so it cannot reliably excite itself at low speed. At key-on a pre-excitation current, classically roughly 0.1–0.3 A through the dashboard charge-warning lamp, magnetises it. Once the machine generates, auxiliary exciter diodes feed the field, both sides of the lamp reach nearly the same voltage, and it goes dark. The lamp therefore reports that the alternator is not producing output, not the battery's state of charge.

The Governing Relations, With the Numbers Worked Through

Frequency. A rotor with p poles completes p/2 electrical cycles per revolution, so f = p × rpm / 120. For 12 poles at idle, f = 12 × 2,400 / 120 = 240 Hz; at 18,000 rpm it is 1,800 Hz, which is why the stator is built from thin insulated laminations to limit eddy currents.

EMF. Faraday's law gives, per phase, E_rms ≈ 4.44 · f · N · Φ · k_w, with N the series turns per phase, Φ the peak flux per pole and k_w a winding factor near 1. Take illustrative values of N = 24 turns and Φ = 0.5 mWb at full field: at 2,400 rpm, E ≈ 4.44 × 240 × 24 × 0.0005 ≈ 12.8 V per phase. A star-connected stator gives √3 × 12.8 ≈ 22 V line-to-line, and an ideal six-pulse bridge averages 1.35 × 22 ≈ 30 V open-circuit, less ~1.4–2 V for the two conducting diodes once current flows. That is about double what a 12 V system needs, so at light load the regulator is already trimming the field at idle. Because EMF is proportional to speed, the same field at 18,000 rpm would in principle give 7.5 times as much.

Ripple. In the bridge the most positive phase conducts through its upper diode and the most negative through its lower diode, so each diode conducts for roughly 120° per cycle and there are six pulses per electrical cycle: 6 × 240 = 1,440 Hz at idle. Unfiltered, the output dips only ~13% below peak (1 − cos 30°), and the low-resistance battery smooths it further.

Self-limiting current. Each phase has an inductive reactance X = 2πfL. EMF and reactance both rise in proportion to frequency, so once E is well above battery voltage the maximum current approaches a ceiling set by E/X that no longer depends on speed. The output curve therefore climbs steeply above cut-in (roughly 1,000–1,500 alternator rpm) and flattens near its rating, ~70–200 A for cars, with no current limiter needed. The diodes also stop the battery discharging back through the stator, eliminating the cut-out relay a dynamo required.

Regulation, Efficiency and What It Costs the Engine

The regulator is a closed-loop switch: below its setpoint a transistor connects the field winding; above it the transistor turns off and field current decays through a freewheeling diode. The coil's inductance averages the pulses, so some 30–70 W of field power (2–5 A at ~14 V) steers kilowatts of output, falling to a fraction of an amp at high speed and light load. The setpoint is temperature-compensated, higher when cold and lower when hot, to suit lead-acid charging. Many modern cars let the engine computer command the voltage over a LIN bus (“smart charging”), charging harder while coasting, and a load-response feature ramps the field over a few seconds when a big load switches on so the idle does not stumble.

Efficiency is only ~50–65%. Losses come from stator copper (I²R), iron loss in the stator and claws, field power, fan windage, bearings and the rectifier. Current always flows through two diodes in series, so at 150 A and ~1 V per diode the bridge alone dissipates about 2 × 1 × 150 ≈ 300 W. That is why the diodes sit in finned heat sinks, and why some premium units use MOSFET “active rectifiers” instead.

Worked example. Delivering 150 A at 14 V gives P = 150 × 14 = 2,100 W, or 2.1 kW. At 60% efficiency the belt must supply 2,100 / 0.6 = ~3.5 kW from the crank (about 4.7 hp), and 1.4 kW becomes heat inside the alternator. At 2,000 engine rpm the alternator turns at 6,000 rpm (ω ≈ 628 rad/s), so its shaft torque is 3,500 / 628 ≈ 5.6 N·m, or about 17 N·m at the crankshaft through the 3:1 pulley. If the engine converts fuel to work at ~25–30%, that is ~12–14 kW of fuel energy, roughly 1.3–1.6 litres of petrol per hour.

Ratings, Standards and How Alternators Are Tested

An alternator is specified by its output current versus speed at regulated voltage, measured on a test bench; ISO 8854 covers test methods and general requirements for alternators with regulators. Bosch-style type labels quote two points: “14V 60/120A” means about 60 A at 1,800 alternator rpm, a conservative stand-in for engine idle, and 120 A at 6,000 rpm. At a night-time idle with every load on, demand can exceed the low-speed figure, and the battery covers the gap.

ISO 16750-2, which took over the pulse from ISO 7637-2, defines load dump. If the battery connection breaks while the alternator is charging hard, the field flux cannot collapse instantly, and an unsuppressed 12 V system can see roughly 79–101 V for 40–400 ms. Zener-type (avalanche) rectifier diodes, used by Bosch and others, clamp that surge to a few tens of volts.

Workshop testing follows the energy path:

• Voltage: a rested, fully charged lead-acid battery reads ~12.6 V; with the engine running the system should sit at ~13.8–14.6 V.
• Output: a carbon-pile load and an inductive current clamp at ~2,000 engine rpm show whether the unit approaches its rated current.
• Voltage drop: under load, the B+ cable and the case-to-battery-negative path should each drop only a few tenths of a volt. Corroded grounds are a classic “bad alternator” that isn't.
• Ripple: an oscilloscope on B+ should show six even humps per electrical cycle; an open diode or phase leaves a regularly repeating missing or deeper hump.
• Resistance: the rotor coil reads a few ohms across the slip rings and open-circuit to the shaft; each stator phase reads a small fraction of an ohm.

From Dynamo to Alternator: A Short History

Until the 1960s most cars used a DC dynamo, whose armature spun inside fixed field poles while a commutator and brushes rectified the output mechanically. Every amp passed through those sliding contacts and the wound armature limited safe speed, so dynamos were geared up only modestly and gave little or no charge at idle. They also needed a cut-out relay, to stop the battery discharging back through the dynamo at low speed, plus voltage and current regulation. As radios, heaters, power accessories and stop-and-go traffic grew, outputs of a few tens of amps fell behind.

Heavy-duty alternators already served buses, police cars and military vehicles, but the compact, affordable silicon power diode of the late 1950s made them practical for ordinary cars. The 1960 Chrysler Valiant is credited as the first car with an alternator as standard. GM, with its Delcotron, and Ford followed within a few years, and most European makers switched during the late 1960s and 1970s. Later steps moved the regulator inside the case (Delco-Remy's 10SI, introduced around 1970, is the best-known example), added compact designs with two internal fans, and handed voltage control to the engine computer. Claw-pole machines with a transistor inverter in place of the diodes now also serve as belt starter-generators in some stop-start and 48 V mild-hybrid cars.

Failure Modes, Misuse and Look-Alikes

• Bearings and belts. Heat and over-tensioned or misaligned belts wear out the bearings, heard as a growl that follows engine speed; a glazed or slack belt squeals and undercharges. Many engines, especially diesels, fit an overrunning alternator pulley, a one-way clutch that stops the heavy rotor's inertia from whipping the belt as crankshaft speed pulses.
• Brushes, regulator and diodes. Brushes eventually wear out of contact. A failed regulator either leaves the field off, draining the battery to a no-start, or stuck on, overcharging to 16 V or more and boiling the battery. An open diode or stator phase cuts maximum output and often adds a whine heard through the radio.
• Misuse. Pulling a battery cable with the engine running to “test” the alternator invites a load-dump spike. Reversed jump leads forward-bias an upper and a lower diode in series straight across the battery, which can destroy the rectifier or blow the main fusible link. And an alternator maintains a battery rather than recovering a flat one: a deeply discharged battery can hold it near full output, and hot, for a long time.
• Look-alikes. The dynamo is the machine it is most often confused with, but a dynamo spins its output winding inside fixed poles and rectifies with a commutator. Bicycle hub “dynamos” are really small permanent-magnet alternators delivering AC, usually rated 6 V, 3 W. A power-station generator uses the same synchronous principle at hundreds of megawatts, locked to 50 or 60 Hz instead of following engine speed.

The car alternator compared with the machines it is most often confused with

Machine	What spins inside what	How output is delivered and controlled	Typical output
Car alternator (claw-pole)	Electromagnet rotor fed ~2–5 A spins inside fixed three-phase stator windings	6 diodes in a full-wave bridge give DC; regulator varies field current to hold ~14 V	~70–200 A at ~14 V; up to ~18,000 rpm
DC dynamo (cars before the 1960s)	Output winding (armature) spins inside fixed field poles	Commutator and brushes carry the full output; needs a cut-out relay plus voltage and current regulation	Tens of amps; little or no charge at idle
Bicycle hub “dynamo”	Permanent-magnet ring spins around a fixed coil	Raw AC to the lamp; winding reactance limits current, no field to control	6 V, 3 W
Motorcycle permanent-magnet alternator	Magnet rotor spins around fixed stator coils	Rectifier-regulator usually shunts surplus current to ground	A few hundred watts
Power-station synchronous generator	DC-excited field rotor spins inside a fixed stator (the same principle)	Speed locked to the grid; delivered as 50 or 60 Hz AC through transformers	Hundreds of megawatts