Alternator FAQ list
Alternator function :
The automotive alternator is an electricity generation plant which provides power to recharge the vehicle starter battery
and also supply electricity for other 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 ) which is converted to DC
( direct current) by a rectifier (diode pack)
Alternators , Dynamos , and
Starter Motors are often referred to as
ROTATING ELECTRICS .
Where is my alternator located ?
The engine driven alternator will be located in the engine bay of the vehicle and normally
bracket mounted to the engine block. Not
always accessible. Sometimes other components
may have to be removed before the alternator can be accessed.
How is the Alternator driven
Normally belt driven from a drive belt
connected to the crankshaft pulley . In a few
instances the alternator may be shaft driven or
gear drive. 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 performance.
How long does an alternator last ?
Usual life expectancy of a motor car alternator is 100,000 to 200,000 miles, though this depends on the operating conditions
of the vehicle such as environment, temperature, driving cycle, maintenance, etc.
Premature alternator failure can be caused by
engine oil and fluids leaking on to the unit.
I have fitted extra electrical accessories such as amplifiers and driving lamps do I need a
higher output alternator
for more amps ?
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. Some
special purpose vehicles with split charge
dual battery systems may be fitted with an additional alternator or battery charging management system.
How does an Alternator work ?
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.
basic vehicle alternator consists of a ROTOR
with magnetic poles and an excitation winding
which spins as it is driven from the engine .
The ROTOR is an electromagnet which when energised creates a magnetic field.
The housing of the Alternator consists of
Stationary wires forming a "winding" of coils
of wire known as a STATOR.
The stator normally consists of a soft iron laminated core around which reasonably heavy gauge copper wire is wound
to form usually three windings connected in Star or delta formation.
Charging voltage output of the alternator is generated
in the stator.
Power is supplied to EXCITE the rotor so the magnetic field is created. (Without excitation current there will not be
a magnetic field) 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. This excitation
control is used to regulate the 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 the
vehicle battery 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.
What are the symptoms of a faulty alternator ?
The first sign of alternator charging
system fault is often indicated to the
driver by the battery
charge warning light on the dash illuminating
whilst the engine is running followed by
gradual loss of electrical power until the
engine cuts out and fails. The discharged
battery can prevent 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. Start the
engine and run at around 2000rpm whilst monitoring battery voltage which 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
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
and causes internal component damage .
On modern alternators controlled by an electronic control unit an incorrect battery type may cause problems.