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Automotive Alternator (AC Circuits)

The advent of modern automobile industry has lead to the innovation of electrical circuits and engineering. Today’s modern cars are equipped with sophisticated electronics and many automated high end devices that can ease the way of driving and providing lots of relief to the driver and those who are travelling.

Automotive Alternator

This modernization in car industry is basically the advancement in “automotive alternators”.  In this article we shall learn the basics of automotive alternator. Basically an Alternator is a electro-mechanical device that converts mechanical energy into electrical energy in the form of alternating current.

As we are all aware that almost all the accessories used in car electronics is basically DC operated. Means that these parts require Direct Current (DC) power supply source. Now the alternator is generating AC current from the rotational movement of engine crankshaft. So this AC current is converted into DC current by means of an electronic component called “Diode”.

What is a Diode..?

A diode is a silicon semiconductor device that works unidirectional. It converts the alternating current to direct (unidirectional current). When positive half cycle of AC current occurs, diode will pass through it and turns itself “forward bias” dropping around 0.7V to 1V across it. As the negative half cycle occur, it “reverse bias” hence blocking the negative cycle. Thus generating a DC current.

What is Automotive Alternator

What is Automotive Alternator:

The automotive alternator is the alternator that has built in 3 phase diode bridge circuit that converts the AC current into DC current in order to power the DC loads used in car (automobile) like head lights, back lights, speakers, music player, automated electric wipers, rear and front camera and sensors

This DC current (DC power) is used to charge the car battery and turn ON these above mentioned loads when engine is running. The diagram on the right shows the basic electrical circuit of a common automobile alternator.

Today, modern cars use voltage regulator circuit to turn ON or OFF the battery power to the rotor coil to maximize/optimize the battery charge and to regulate output voltage.

The voltage regulator also deals in charging circuit for the battery. As soon as more DC load in the vehicle is turned ON, the ECU (Engine Control Unit) sends signal to the alternator to start battery charging. With variable electrical loads in the modern vehicle, the alternator has to deal with it by adjusting its charging rate accordingly.

3 Phase Bridge Rectifier:

The three phase full wave bridge rectifier is the main part that generates the DC voltage required by power electronics of a car. This bridge rectifier consist of 6 diodes. Two are connected in series and three pairs of those two are connected in parallel.

These three pairs are directly in connected with three stator windings. These stator winding generate AC voltage by means on electromagnetism. The stator winding is magnetically coupled with the rotor winding or field winding or field coil.

How Automotive Alternator Works:

First, we must know that an alternator cannot charge the dead battery. This is because at initial startup, the alternator itself needs a battery to energize the rotor coil to kick start the rotor winding and generate magnetic field.

Now when the car ignition is turned ON, the engine starts. The engine crankshaft start to rotate and then the sheave is also rotated by means of a belt connecting the crankshaft and sheave.

As the battery has energized the rotor coil, a metallic weight would become “electromagnet”. This electromagnet will rotate pass the three phase stator winding. The three stator windings are connected in Y configuration.

As a result these two stator and rotor gets mutually coupled and induce electricity (AC current) on stator winding. This AC voltage (current) is the passed to the 6 diode network of bridge rectifier to convert from AC to DC. The rotating field coil is connected to the battery by means of “slip rings” and “carbon brushes”. The two carbon brushes are fixed and held by means of pressure spring making firm contact with copper slip rings.

Now as the AC power start to generate and converted to DC power, it is then feedback into the rotor coil and cutoff the battery connection to the rotor to save energy/charge of battery. The battery will then start to charge from rectified DC source from diode.

Ball bearings are used to aid in the smooth rotation of rotor shaft.

Three common faults in alternator:

1- Leaky diodes:

The diode can fail in the way that it start to produce AC ripple voltage. This ripple can be sensed as noise in your car’s electrical system. The best way to identify this problem is to perform this small test.


Take digital multimeter DMM, and turn to lowest AC volts reading. Connect the red positive lead of DMM to the BAT terminal on your car’s alternator. Now connect black negative lead to the body where the alternator is mounted. Now start the engine and run at 1500 RPM. The reading on DMM must show 0.5V to 1V. Anything greater than 1V indicates bad alternator due to ripple problem of diode.

2- Short Diodes:

If you feel that your car battery is draining very fast then it is time to check for “shorted diode”. Any diode out of 6 diodes is shorted to ground could cause the rapid drainage of battery. In this case do this.

Using you DMM at “DC mA” ammeter mode, connect the DMM in series with the battery negative terminal and alternator BAT terminal. This is done by removing the BAT terminal wire from alternator and removing the battery negative wire. Make sure that ignition and all car electronics is OFF and engine is OFF. Now check the reading at DMM. It should be less than 0.5mA. If it is not then wait 30 minutes so that computer devices in car electronics goes in “sleep mode”. Now if it still shows 0.5mA or higher then it is bad alternator or faulty diode causing battery depletion.

3- Open Diode:

If the alternator output is not generating DC voltage (DC power) at all, then it is likely that diode is gone “open”. Hence it is time to replace that diode.

Overview of Basic Parts of Automotive Alternator:


It is the static part of alternator that do not rotate. It is made of iron core and consist of 3 w    inding connected in “Y” “Wye” fashion.

Rotor: A spinning mass that has field coil energized by car battery.

3 Phase Bridge Rectifier: To convert AC voltage to DC voltage to power car electronics and accessories.

Slip Rings: Copper rings to make contact and supply DC power to rotor

Carbon Brush: Fixed firmly and makes contact with slip rings to connect with rotor.

Voltage Regulator: To control the output voltage independent of rotor speed. Connect / disconnect battery to the rotor in order to regulate output.

An Advance electrical system of typical Automotive Alternator is shown on right side.