Alternator FAQ list
Alternator function :
The automotive alternator is an electricity generation plant which provides power to recharge one or more vehicle batteries as well as
supplying electricity for vehicle electrical systems such as lighting, wipers, heater motor, car audio ,sat nav, etc whilst the vehicle engine is running.
Alternators generate AC (Alternating current ) converted to DC (direct current) by the alternator rectifier (diode pack)
p>The term Rotating Electrics refers to Alternators, Dynamos, and
The Alternator is generally engine driven by drive belt connected to the crankshaft pulley , or coupling to shaft drive or gear drive.
If drive belt driven correct belt tension is essential.
The alternator must be mounted
securely to enable it to be driven efficiently.
Broken or loose mounting brackets can result in loss of drive belt tension and subsequent loss of alternator charging performance.
Alternator lifespan -
Alternator lifespan of a motor car alternator is 100,000 to 200,000 miles, dependant upon operating conditions
such as environment, temperature, driving cycle, maintenance, etc.
Alternator premature failure causes
Engine oil and fluids leaking on to the unit.
Incorrect or poor electrical connection
Voltage Spikes eg from jump starting
A higher output alternator
may be required when additional accessories are fitted
The vehicle manufacturer calculates the power output of the alternator required for each vehicle with regard to original equipment
fitment. Increasing the load on a vehicles electrical system may require an alternator system upgrade including cable.
Special purpose vehicles with split charge dual battery systems may be fitted with an additional alternator or battery charging management system.
Alternator operating priciple
The alternator generates electricity by "electromagnetic induction". When a conductor such as a wire moves through
the lines of force of a magnetic field a voltage is induced in the conductor.
Whether the conductor moves in relation to the
magnetic field or vice versa the principle is the same.
The Alternator Rotor has magnetic poles and an excitation winding which rotates as it is driven from the engine.
The ROTOR becomes an electromagnet which when energised creating a magnetic field.
The stator normally consists of soft iron laminated core around which reasonably heavy gauge copper wire is wound to form usually three windings connected in Star or delta formation.
is sandwiched between the alternator housing.
Charging power output of the alternator is generated in the stator.
Excitation : Power is supplied to EXCITE the rotor to create a magnetic field.
(excitation current is required to create a magnetic field unless permanent magnets are used )
Some alternators are SELF-EXCITED, this means after the rotor is initially excited a wire connected to the
stator taps some of the power generated
to keep the rotor excited and power generation in progress whilst the rotor is spinning.
The brush pack or brushes form the means of connecting the spinning rotor to the excitation power source.
Alternator Voltage Regulation
By controlling the excitation current of the rotor the magnetic field strength is proportionally controlled.
power control is used to regulate alternator output by means of the Voltage regulator.
With ECU regulated systems, the duty cycle of the excitation current can be reflected as a feedback signal to measure
the amount of alternator capacity being used via the DFM signal terminal (DF Monitor).
The initial current produced by the alternator
is as its name suggests alternating (AC) .
Unfortunately AC cannot be used to recharge vehicle batteries so has to be converted to DC.
The process of converting AC power to DC power is known as rectification.
The rectification is done by the Diode pack (a diode acts like a one way valve in an electrical circuit).<
Car alternators are Three Phase AC generators with rectification to DC.
The DC voltage level produced by the alternator must be slightly higher than battery voltage to allow the battery to charge.
The Diode pack has a secondary function when the ignition key is off to prevent the power draining from the battery via
the alternator windings causing the battery to discharge.
Symptoms of a faulty alternator
The first sign of alternator charging
system fault is often indicated to the driver by illumination of the battery charge warning light
on the dash whilst the engine is running followed by gradual loss of electrical power until the
engine cuts out and fails.
Low battery voltage may mean the starter motor clicks or does not operate preventing the vehicle engine from
being started .
Have the vehicle battery tested.
A visual check with the engine stopped should be carried out to ensure the alternator is secure and drive belt correctly
tensioned. Also check for oil ,fuel or coolant leaks which may have caused the alternator to fail.
Check all connections are tight and wires are in good condition. Check all heat shields are in place .
A quick check for a failed alternator is to measure battery voltage with the engine off using a voltmeter.
engine and run at around 2000rpm whilst monitoring battery voltage using a voltmeter. Voltage reading should have increased slightly
if the alternator is charging the battery. B+ terminal voltage at the alternator should also be checked.
Incorrect alternator output voltage ( alternator overcharging or
undercharging ) can affect sensitive vehicle computer systems and related systems.
Typical battery voltage for a 12 volt system
would be :
- Engine off : 12.5 volts
- Engine running at 2000 rpm : 13.2 to 14.6 volts at battery(regulated voltage)
- A battery voltage indicated above 15 volts with the engine running
on a standard 12v charging system often indicates a fault
though vehicle manufacturers specification should be checked to verify.
- Some modern vehicles have alternators with dynamic electronically controlled output which is affected by the embedded software in the vehicle engine control computer.
These require special equipment to test and verify as output
varies according to demand.
Alternator Noise possible causes:
Excess drive belt tension
Insufficient drive belt tension
Stator winding fault causing phase imbalance
What causes Alternator output to fail ?
Common causes of alternator failure are:
Broken or defective wiring to the alternator.
Wear and tear ; Worn out brushes ,sliprings, bearing failure, diode pack failure.
Excessive drive belt tension causing premature bearing failure.
Insecure mounting bolts / bracket causing insufficient drive belt tension.
Poor electrical connection causing high resistance and leading to overheating.
Electrical overload due to a defective battery or fitment of vehicle accessories, or vehicle wiring faults.
Inadequate ventilation and cooling possibly caused by fitment of vehicle body kits.
Voltage surge caused by boost starting using jump leads without surge protection. (Diode failure)
Oil, fuel or coolant leaks onto the alternator.
Water leaks on to the alternator : Water
entering an air cooled alternator can cause problems particularly in Winter when at
freezing temperatures the water turns to ice
causing internal component damage .
On modern alternators controlled by an electronic control unit an incorrect battery type may cause problems.