• General
  • Health Benefits
  • Rotor
  • Stator
  • Pre-excitation, Self-excitation and Charging current
  • Voltage regulator
  • Alternator Connections
  • Rectifying Diodes
  • Ripple tension
  • Voltage regulator
  • Freewheel Pulley
  • Fan
  • Energy recovery
  • Possible alternator defects
  • Checking the charging voltage and charging current

When the engine is running, the alternator (called “alternator” in English) ensures that the battery is charged and the consumers are supplied with power (such as the radio, lighting, etc.). The alternator is driven by the multi-ribbed belt. The multi-belt drives the alternator pulley, which is connected to the interior on a shaft. The kinetic energy is converted into electrical energy (and heat) in the dynamo.
The speed of the engine affects the voltage of the alternator. The faster the motor spins, the faster the pulley spins, allowing more power to be generated. The voltage must not be too high and is therefore limited by the voltage regulator.
More on the voltage regulator later.

Alternating voltage is generated in the dynamo. DC voltage is applied throughout the entire electronic circuit of the car. The battery can also only be charged with DC voltage. The alternating voltage is converted into direct voltage by means of the diodes in the diode bridge. The magnitude of the voltage generated depends on:

  • The speed at which the conductor and the magnetic field move away from each other
  • The length of the windings
  • The strength of the magnetic field

It is possible to drive without the alternator. For example, if it is defective and no longer supplies voltage, it can be driven until the battery is completely empty. This is of course not recommended because the battery can become defective due to deep discharge, but the car can be driven (a small part) without an alternator and without a multi-belt (to possibly drive on a trailer to be transported).

The current is generated by rotating the rotor in the stator. The rotor is an electromagnet; it only becomes magnetic when a current flows through it. The alternator therefore needs help from the battery before it can start charging. The remaining magnetism in the dynamo is insufficient to allow an electric current to flow through the diodes.

The current to make the rotor magnetic flows from the battery, via the ignition lock and the charging current control lamp to the D+ connection of the alternator. Then the current flows to the rotor. The current flows from the rotor through the regulator to ground. The moment the ignition lock is switched on, the charging current indicator light comes on and at the same time the magnetization of the alternator takes place. When the alternator starts charging, the charging current indicator light will go out.
When the alternator is charging, the north and south poles move relative to the stator. As a result, an alternating voltage is generated in the stator. At one revolution of the magnet, the voltage generated in the conductor has the shape of a sine wave, as shown in the figure.

Because this is an alternating voltage and all consumers in the car only work on direct voltage, rectification still has to take place. Diodes ensure that the alternating voltage is converted into direct voltage.
The charging voltage and charging current must also be limited; when the engine is running at a high speed and there are few consumers on, the alternator only needs to charge a little. The moment more consumers are switched on, the alternator must supply more charging current. At full load, this can increase to 75 to 120 Ampere (depending on the type of car). How this all works is described in the chapters below.

The rotor is not a permanent magnet, but an electromagnet. By allowing current to flow through the rotor, it becomes magnetic and an alternating voltage can be generated. The voltage generated can be controlled by increasing or decreasing the rotor current. This is the job of the voltage regulator.
The rotor has pole claws (north and south poles). Each half with pole claws usually consists of 6 or 7 poles. The other half consists of the same number of poles, so then there are 6 or 7 north poles and 6 or 7 south poles. We are talking about 12 or 14 pole pairs. The number of pole pairs influences the voltage generated in the stator.

The magnetic field in the dynamo is created when the rotor is energized. This already happens when the ignition of the car is switched on. To energize the rotor, a field current is sent through the field windings. This current comes from the battery and is transferred to the field windings via the slip rings and the carbon brushes. It runs from the North Pole to the South Pole, because one slip ring is connected to the North Pole and the other to the South Pole.

When the rotor has been removed, it can be measured to check for defects. The rotor resistance is often around 3 Ohms. For the exact value, refer to the factory data.

The alternator that is used in almost all cars is a three-phase alternator. This means that the alternator consists of three stator coils that are connected to one stator core and a rotor. Each stator coil produces its own generated AC voltage. Since all stator coils are mounted at an angle of 120 degrees to each other, the generated voltages are also phase shifted 120 degrees. These voltages are rectified by the three negative and three positive diodes (so a total of six diodes).

The stator core is made up of stacked plates, which are separated from each other by insulating material. The stator core amplifies the magnetic field in the alternator and thus increases the generated voltage. The stator coils can be connected in two ways; by means of a delta connection (recognizable by 3×2 connections) and a star connection (4 connections, of which 3 separate connections and one connection where the 3 ends of the coils are connected to each other. The star connection is most common, because it allows a faster connection high voltage can be achieved The delta connection is used with dynamos that have to deliver a lot of power.
The moment a stator coil makes contact with the stator core (ground fault) or if one of the coils is interrupted (wire break), the stator no longer works properly. A multimeter can be used to check whether there is a ground fault or wire break. Well under one condition; the stator coils must be disconnected; both ends must not make contact with other components. Often unsoldering is sufficient. The resistance of the coils should be very small; about 0,05 ohms. The resistance between the stator coils and the stator core must be infinite. When a resistance is present (if it is very high) then there is a connection.

The picture below shows a disassembled stator and a rotor. In reality, the rotor rotates in the stator and they just don't touch each other.

Pre-excitation, self-excitation and charging current:

The engine is stopped and the indicator light is on. The pre-excitation current goes through the battery, ignition lock, rotor and regulator to ground. This is possible because in the voltage regulator the Zener diode is blocked and the base current T1 is made conductive because T2 stops conducting.

When the motor has started, the rotor has been made sufficiently magnetic to switch to self-excitation. The self-exciting current then goes via the rectifier diodes (minus side) to the stator coil, then via the field diodes to the rotor and via the regulator to ground.

Charging current:
An alternating voltage is generated in the stator coil as the rotor rotates through it. The green line marks the path that the current flows from stator coil V. The current is rectified by a rectifying diode (from AC voltage to DC voltage) and goes to the battery and consumers via terminal B+.


The charging current that goes to the battery and consumers via the alternator connection B+ provides the entire power supply for the car. When the engine is turned off, the alternator does not supply power. All consumers will therefore use power from the battery.
When the engine is running, the alternator must be able to supply enough current to supply all consumers. When the engine is running, it is therefore never the intention that power from the battery is used. The charging current of an alternator depends on the number of consumers and the state of charge of the battery. The maximum charging current is stated on the alternator (usually between 60 and 90A).

The charging voltage of the alternator can easily be checked, if there is any doubt whether the alternator is charging properly or not. By measuring the positive and negative poles of the battery with a voltage meter (multimeter) with the engine running (the voltage from the dynamo is directly on this), you can check whether the dynamo is charging properly:

  • If the voltage with the engine running is around 14,2 volts, the alternator works as it should
  • If the voltage is then 13,8 volts, the battery is almost full and the consumers are switched off. The alternator does not have to supply much voltage and therefore does not. The charging voltage is just right
  • If the voltage is 12,4 volts or lower, you know that the alternator is not charging properly. This is because this is the voltage that a full battery also has. So there is a problem with the alternator.
  • If the voltage is lower than 12,4 volts, the alternator will no longer charge. The battery will continue to discharge until the voltage is 8 volts. Then the engine will stall and nothing will work.

In the latter case, i.e. when the dynamo no longer charges, you can choose to replace the dynamo. This is often very expensive and it is cheaper to look for a reconditioned alternator. There are many overhaul companies that completely disassemble the alternator and make it like new again. This can save (more) than half of the new price.
Always make sure that when you are going to replace the alternator, you disconnect the negative terminal of the battery! If you don't do that and the B+ connection (which you remove from the dynamo) touches the bodywork or the metal engine block, you will get sparks due to a short circuit. In that case, expensive electronic control units can break down, for example.

Voltage regulator:
When the voltage rises above the regulated voltage, the Zener diode (in the diagram above) becomes conductive, causing the base of T1 to be ground through T2. T1 is blocked, the magnetic field drops, causing the alternator voltage to drop.
As a result, the rotor current is lost, so that the dynamo does not recharge for a short time. By continuously switching T1 on and off, the voltage is adjusted.

The picture shows a loose rotor with a loose voltage regulator held against it. The voltage regulator is mounted between the D+ and DF connections of the alternator and drags its carbon brushes over the rotor. When a consumer is switched on (eg the lighting), the charging current will drop briefly from 14,4 to 13,8 Volts. The voltage regulator catches this and will quickly adjust the voltage higher to 14,4 volts.

Below are 2 scope images that were measured on the DF connection of the dynamo. These signals are passed on to the engine control unit. For clarity, the rotor is magnetic to the bottom of both statues.

The signal in the graph was measured while few or no consumers were switched on. The rotor is thus minimally magnetic. The duty cycle here is approximately 10%.

The signal in the graph below was measured while many consumers were switched on. The rotor is energized much more here to achieve the 14,4 volt charging current. The duty cycle here is approximately 50%.

Alternator connections:

  • B+ goes to the battery; this is where the charging voltage and the charging current pass through.
  • D+ is the control voltage of the rotor for adjusting the alternator voltage.
  • D- is the mass of the alternator.
  • W is a connection that used to be used for rev counters of old diesel engines. Today it no longer exists.
  • DF or LIN are the possible connection for the control for the energization of the rotor from the engine management system.

Rectifying Diodes:
The alternator supplies alternating voltage, but because only direct voltage is used in the car, the alternating voltage (AC) must be converted into direct voltage (DC). This is done by the rectifying diodes. Diodes allow current to flow in only one direction. The positive part of the alternating current is used, the negative part is lost.

The picture shows a disassembled diode bridge. The red probe points to one of the three mindiodes.
On the other side of the diode bridge are the plus diodes. The stud is the B+ connection, on which the thick cable is mounted that goes to the battery.

This is the principle of a single-phase alternator. In the image above (on the right) you can see that the phase is constantly interrupted, there is no voltage for a while, and then there is a phase again. No voltage is thus generated in the part between the phases. To prevent this, three-phase alternators use star and delta connections. This gives the result below.
The image below shows 3 different colors; black, red and blue. These are all separate phases. The image shows that there is a lot of space between, for example, the black phases. By connecting the other phases, this space is bridged. This creates a gradual power supply.

Ripple tension:
After rectifying the voltage by the rectifying diodes, there is always a small ripple left. The signal is never nicely flat. The ripple voltage must never exceed 500 mV, as this could cause malfunctions or defects in the car electronics.
The image shows a scope image measured on the battery. This picture will change when the engine speed is changed or when consumers are switched on.

Voltage regulator:
The voltage regulator switches the magnetic field on and off by switching the current through the rotor on and off. The voltage regulator ensures that the charging voltage remains constant (between 13,2 and 14,6 volts). The level of the charging voltage is, among other things, dependent on speed. The faster the crankshaft turns, the faster the rotor will turn. If the voltage were not adjusted, it could rise to 30 volts at a high speed. This is prevented by the voltage regulator. The picture shows a separate voltage regulator. This is in most cases visibly attached to the dynamo.

The generated voltage depends not only on the motor speed, but also on the number of stator turns and the strength of the magnetic field of the rotor. The number of stator turns is determined when the alternator is designed, but the magnetic field strength of the rotor can be controlled. This can be reduced by switching the rotor off and on very quickly. When the voltage becomes high, the rotor switches off. If the voltage is too low, the rotor is switched on again. By doing this very quickly in succession, an average field strength is created. The charging voltage thus remains as constant as possible.

When the voltage on the positive terminal of the alternator (D+) is lower than the adjustment voltage, a current flows from D+ through the rotor to D- (minus terminal) and a voltage is generated in the alternator. The generated voltage is set to D+ again. The moment the voltage on D+ is higher than the adjustment voltage, the Zener voltage is reached (see figure below), making transistor T2 conductive. The transistor T1 then does not conduct, so that no more current can flow through the rotor. The magnetic field is thus switched off, so that the charging voltage drops. This voltage continues to fall until the Zener voltage is no longer reached. Subsequently, transistor T2 will be non-conducting and T1 will conduct again. This cycle is constantly repeated.

Freewheel Pulley:
Today many alternators are equipped with an overrunning pulley (see picture below). These pulleys can only be driven in one direction. When the V-ribbed belt has been removed from the pulley and you turn the pulley by hand, you will notice that the interior of the alternator only rotates in one direction and remains stationary in the other direction. This system is to protect the multi-belt. When the engine is running at a high speed and the throttle is released in one go, the engine speed will drop rapidly. A heavy-duty dynamo can slow down a bit less quickly. This speed drops more slowly than the engine speed. The consequence of this is that the multi-belt is loaded more heavily and in the worst case is cut in half, because the multi-belt then has to slow down the alternator. With a freewheel pulley, the dynamo will move with it when accelerating, but will turn out at its own speed when decelerating.

The pulley is threaded onto the shaft of the rotor (see picture above). The outer part of the pulley only takes the inner part with it in one direction of rotation. The blocking device ensures that the inner part is clamped against the outer part. The complete pulley will then be locked, so that the alternator is driven by the multi-ribbed belt. When the accelerator pedal is released, the inner part rotates at a higher speed than the outer part; the engine speed has dropped faster than the rotor speed. The blocking device is then not in operation, whereby the ball bearings allow the rotor to have a different speed than the crankshaft.

The image shows a dynamo fitted with an overrunning pulley.

The dynamo heats up when it has to supply energy. To prevent it from overheating, it must be cooled. The fan inside the dynamo provides the cooling. Today, there are also alternators that are connected to the engine's cooling system. The coolant takes care of the cooling.

Energy recovery:
If the alternator is charging at its maximum capacity (with many consumers switched on), additional fuel consumption will occur. This is because the alternator will turn more heavily because the magnetic field in the stator will then be greater. The magnetic field will cause the rotor to spin more heavily and the crankshaft will have to pull harder on the v-ribbed belt to turn it. Today, car manufacturers have found a convenient solution for this. The dynamo always charges, but will not just recharge its maximum capacity while driving (unless the battery is really empty). Maximum recharging occurs when the car brakes on the engine. So when the driver takes his foot off the accelerator and lets the car roll out (eg at a traffic light or at the exit of a motorway). At such a moment the car does not consume any fuel anyway and the kinetic energy (kinetic energy) of the vehicle ensures that the car continues to roll. The battery is now charged to the maximum until the gas is accelerated again. At that moment, the dynamo ensures that the voltage supply remains stable.
This charging method leads to lower fuel consumption.

Possible alternator defects:
There may be some typical problems or defects in the alternator. The technician often knows what he or she can then check or measure. Below are some typical complaints:

  1. The charge current indicator light normally illuminates during pre-assist, but only goes out when the engine is running at a higher speed; faulty alternator (probably a faulty field diode).
  2. The same complaint as above, only also burns weakly with a running engine with a high speed or with many consumers switched on; faulty alternator (probably a faulty diode).
  3. The charge current indicator light comes on weakly during pre-assist, but only goes out when the engine is running at a higher speed; (probably a defect in the alternator or a defect in the cabling or its connections).
  4. The charge indicator lamp does not illuminate during pre-assist or when the engine is running; (defective alternator, bad cabling/connections or a defective charging current indicator light).

Checking the charging voltage and charging current:
The amount of energy that the dynamo supplies depends on the capacity and on what consumers and the battery demand. For example, the dynamo must be able to supply 100A to supply all consumers and to be able to charge an empty battery at the same time. The amount of energy supplied by the dynamo drops to almost zero when the battery is full and no consumers are switched on. The maximum capacity of the alternator is often stated on the type plate or on a sticker on the alternator. This is often between 65A and 120A. This is often displayed as follows: 14V 17/85A. This means: regulated voltage (14V), charging current (17A) at 1800 rpm and charging current (85A) at 6000 rpm of the alternator (so not the crankshaft speed).

If there is a defect in the alternator or in the cabling, the maximum capacity may not be achieved at maximum load. This can be checked by checking the charging current. This can be done by loading the alternator as high as possible with a special test equipment when the engine is running or by switching on as many consumers as possible (such as the seat heating, rear window heating, all lighting, fan motor on the highest position, etc). The value of the charging current can be determined using a clamp meter be checked. The measured value must correspond to the value stated on the alternator.
The regulated voltage can be checked by pressing the multimeter measure the voltage between the B+ terminal and ground at an increased engine speed (2000 rpm). The regulated voltage should be between 13.8 volts and 14.5 volts.
To check whether the wiring is in order, the voltage difference between the positive terminal of the battery and the B+ connection of the alternator can be measured; the voltage must be less than 0,3V. If not, there is a problem with the cable or cable connections.
If the ground circuit is not good, you will not only have problems with the charging system, but also with other systems. The ground circuit can be checked by running the engine at 2000 rpm and connecting the voltmeter between the negative terminal of the battery and the housing of the alternator. This voltage must also be less than 0,3V.

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