Autopsy: Alternator
Theo strips and explains this vital but little understood component.
[A] STATOR
The principles underlying the alternator’s function are the same as a dynamo’s: when a conductor is passed through a magnetic field, a current is induced in the conductor, its direction changing when magnetic field’s direction changes. Instead of rotating the conductor within a magnetic field like the dynamo, the alternator’s magnet (rotor) rotates within static windings (stator). Three separate conductors are wound around a laminated soft-iron core. When the magnet rotates inside the stator, each winding produces an alternating current (AC) created by the rotor’s 12 poles, hence the name, alternator.
[B] RECTIFIER
The battery cannot be charged with an alternating current so the stator’s output is converted to direct current (DC) by a bridge rectifier. For every revolution of the 12-pole electromagnetic rotor, 36 sinewaves in total are induced in the stator’s three phases, each wave producing a positive and negative voltage as the conductors are cut by the magnetic fields. The three windings are each soldered to three negative (earth) diodes and three positive (output) diodes, which are semiconductors that act like electrical one-way valves. The diodes generate heat due to inefficiencies in their operation so they are mounted on heat-sink plates which also act as electrical connectors, the plate on the right being connected to earth via the central mounting spigot; the middle plate being the electricallyinsulated positive connection, two brassed 9.5mm Lucar connectors making a permanently live connection to the battery.
[C] SLIP-RING- AND DRIVE-END BRACKETS
Made from aluminium alloy, the end brackets contain the rotor bearings, provide a platform for the electronics and brushholder, and clamp the stator using long through-bolts.
[D] COVER
The plastic cover is screwed to the end bracket with two screws and prevents dirt from entering the alternator but also electrically insulates the exposed live components.
[E] ROTOR
An alternator’s electromagnet is created by winding copper wire around a softiron core pressed onto the shaft (rotor), which is rotated by the pulley. The soft-iron claws focus the magnetic flux, the claw at the slip-ring end having six north poles and the drive end having six south poles.
[F] SLIP RINGS
The ACR alternators use a face-type slip ring pushed onto the end of the rotor, one end of the field winding being soldered to the ring and the other to the centre contact, both being set in an electricallyinsulated moulding.
[G] BRUSHES
Made from graphite, each brush is kept in contact with its slip ring by a compression spring. The brush in contact with
The dynamo and control box combination provided electrical power for cars for most of the 20th century until the alternator took over in the Sixties, when electrical equipment became more plentiful and power-hungry, and production costs became increasingly important. The Lucas ACR alternator has an internal voltage regulator, superseding the AC range with external regulation, and was available as the 28amp 15ACR, through to the 66amp 20ACR in essentially the same package, contrasting with the 20 or so amps from a standard dynamo. We’ve stripped a generic ACR facsimile to see what’s inside. the outer ring is connected to the ‘IND’ (indicator light) terminal on the rectifier, and the brush in contact with the centre is connected to the output regulator.
[H] OUTPUT REGULATOR
The output of the alternator is regulated by increasing or decreasing the strength of the rotor’s electromagnet by increasing or decreasing the average current flow through the field windings. There are many different iterations of output regulator configuration, this example having the later 14TR regulator. When stationary, there is insufficient residual magnetism in the rotor’s field windings but the magnet is energised when the ignition is turned on and current flows to the ‘IND’ terminal on the rectifier via the charging lamp bulb, which in turn is connected to one end of the field windings.
The other end of the field windings is connected to the output regulator. The regulator is essentially a solid-state switch, controlled by an integrated Zener diode/input transistor (input device), rapidly making and breaking the field windings’ circuit to earth. In simple terms, either a dedicated wire from the battery (battery sensing) and/or a wire from the field diodes (machine sensing) is connected to a powertransistor (output device) via a resistor that connects the field windings to earth, turning on the electromagnet. When sensed voltage exceeds the regulated 14.2 volts, which is the threshold breakdown voltage of the Zener diode, current flows to the base of the integrated input transistor, which in turn switches off the main power transistor, turning off the electromagnet. The rotor is switched on and off about 30-60 times a second.
[I] SURGE PROTECTION DIODE
This is an avalanche diode connected between the ‘IND’ terminal and earth that shorts the live side of the field windings to earth if a high voltage is experienced, protecting the regulator.
[J] PULLEY
The pulley is positively located on the rotor by a Woodruff key and retained by a nut and spring washer. The pulley is driven by the engine’s crankshaft by an A-section 'vee' belt.
[K] FAN
Heat is generated by the solid-state electronics and by the windings. The centrifugal fan creates a low pressure at its centre when rotated, drawing air from the rear of the alternator through to the front.
[L] BEARINGS
Deep-groove sealed-for-life ball bearings are used at either end of the rotor. Low-output models have 6202 bearings at each end but later and higher output models use a larger 6302 drive-end bearing. Drive-end bearings can experience lateral loads of up to 1,800N; the slip-ring end bearing runs the hottest.