Alternators in Automobiles

The demand for power supply in automobiles has been increasing steadily over the years. It is estimated that the power output from an alternator has increased 5 times in a span of 30 years from 1950 to 1980. The demand for power supply is going to increase at a higher rate in the future. This need will rise due to the new technologies which rely completely on electronics such as the ECUs, safety equipments, navigation system, etc.

Alternator (also known as generator) is the main power generating device. Alternators should not only withstand the higher load demand, but also be quieter in operation and have a long life. The electronic voltage regulators are a must to withstand the fluctuations in loading and also engine speed changes.

Alternators are an energy generating device that meets the constant energy demands of the fuel injection system, ECUs, safety equipments, lighting, etc. Alternators are also used to recharge batteries. It is a highly reliable source of energy which supplies energy at anytime, provided the engine is running.

Generation of Electrical Energy in Automobiles:

When the engine is stopped, battery acts as the energy source. Whereas, when the engine is running, alternator is the energy source. Alternators function is to supply electrical energy to the systems necessary for operation. Energy supply can be based on the driver’s needs (for e.g. lighting) and also systems which require continuous supply irrespective of the driver’s command (for e.g. ECUs and sensors).

It is necessary that the alternator power output is optimally matched with the battery capacity, starter motor requirements, and other electrical loads. For example, in a normal driving condition, the following points should be considered for smooth operation:

• Battery should always have enough charge at any given time to supply power to the starter motor to crank the engine, irrespective of temperature.
• The ECUs, sensors and actuators should always be ready for operation. For example, ignition, Fuel injection, Anti-lock Braking System (ABS), Traction Control System (TCS).
• Vehicle safety system must operate immediately, such as air-bags, ABS.
• Lighting system to operate at nights and in foggy conditions.
• When the vehicle is parked, a number of electrical loads must continue to operate without draining the battery too much so that there is enough charge to start the engine again. For instance, Anti theft system is mostly in operation when car is parked.

Electrical Loads:

Based on the requirements, there are 3 different types of electrical loads:

• Permanent loads such as ignition, fuel injection, ECUs.
• Long duration loads such as lighting, music system, Air-conditioners.
• Short duration loads such as horn, turn indicators.

Vehicle Electrical System Layout:

The layout of wiring of electrical equipments, alternator and battery can make a significant difference to the voltage output of alternators and also the state of charge of the battery.

• If all the electrical equipments are connected to the battery, then there is a reduction in charging voltage to the battery due to high voltage drop. This is due to the cause that both the battery charging current and electrical load current pass through the same charging line, thereby resulting in voltage drop.
• If all the electrical equipments are connected to the alternator side, then charging voltage to the battery is higher and at the same time voltage drop to the electrical equipments is lesser. But this can harm electrical devices which cannot withstand high voltage peaks.
• The best way to deal with the above problems is to connect the high voltage sensitive equipments to the battery and the insensitive equipments to the alternator.

Working Principle of Alternators:

The availability of power diodes paved the way for the series production of alternators. Bosch started with the series production of alternators around 1963. The working principle of alternator makes it far more efficient than the DC generators.

Alternators work on the principle of electromagnetic induction. When an electric conductor (Wire or a wire loop) cuts through the lines of magnetic forces of DC magnetic field, a voltage is induced in the conductor. In the case of alternators, magnetic field rotates inside the stationary conductor.

Electromagnets are used to generate magnetic field inside the conductor. Electromagnetism is based on the fact that when an electric current flows through a wire or winding, it generates electromagnetic field around them. The number of turns in the winding and the magnitude of current flowing through the winding determines the strength of magnetic field. An iron core is used to further strengthen the magnetic field, which when rotates induces an alternating voltage in the winding. Single phase alternators have only one winding in the armature.

Principle of operation of a 3-phase Alternator:

3 phase alternators work on the same principle as the single phase alternators, except that they have three windings in the armature separated at an angle of 120° from each other. According to the law of induction, voltage is induced in each of the three windings of same magnitude and frequency. The only difference is that these sinusoidal voltages are 120° out of phase with each other. Therefore, a 3 phase alternator develops constantly recurring 3 phase alternating voltage.

It would normally require 6 wires, 2 wires each for a winding to transfer the voltage generated. To reduce the number of wires to 3, the 3 windings are interconnected forming a ‘Star’ connection or ‘Delta’ connection.

In an automotive alternator, the 3 phase winding with iron core acts as the stationary component, so it is also known as ‘stator’ winding. Whereas, the electromagnet or the excitation winding acts as a rotor. When the rotor rotates, its magnetic field induces a 3 phase alternating voltage in the stationary winding.

Rectification of Alternating Voltage:

All the electrical components of a car can operate only on direct current (DC). Even the battery cannot store Alternating current (AC). Therefore, the current from the alternator has to be rectified with the help of power diodes (Zener diodes) that can operate over a wide range of temperature.

The power diodes or rectifier diodes have forward and reverse direction. It works like a non-return valve, which allows current in one direction but blocks in the reverse direction. The diodes allow only positive half waves to pass through, suppressing the negative half waves. These results in a pulsating DC, also known as half wave rectification. Full wave rectification can be applied wherein both positive and negative half waves are rectified.

Bridge Circuit for 3 phase AC rectification: 

3 phase AC generated in the 3 phase winding is rectified with the help of 6 diodes. Two diodes for each phase, one on the positive side and the other on the negative side. The positive half waves pass through the positive side and the negative half waves pass through the negative side, and rectification takes place.  

Excitation Current: 

The excitation current required to magnetize the rotor is initially drawn from the battery. The current passes through 3 exciter diodes and then towards the rotor. Once the magnetized rotor starts rotating inside the 3 phase winding, AC is induced in the stator. Now the excitation current can be tapped from the winding and the battery is no longer a source of current for excitation.

Reverse Current Block: 

The rectifier diodes not only rectify AC, but also prevent the battery from discharging through the 3 phase winding. The diodes prevent the current flow from battery to alternator. When the engine is stopped or running at low speed, the current in the battery would discharge without the diodes in their place.

ALTERNATOR DESIGN:

Claw pole alternator is the most commonly used alternator in modern vehicles. The construction of this type is as follows:

• 3 phase stator winding with a solid laminated iron core pressed together. The turns of the windings are embedded in the grooves of the iron core.

• Rotor on which two claw shaped poles are mounted, and excitation winding is enclosed between the 2 poles. A fan is mounted on both the ends. Two collector rings are provided on the shaft which draws excitation current from the diodes via carbon brushes. Fans must be designed based on clockwise rotation or anti-clockwise rotation.

• Pulley is mounted on one end of the rotor shaft. Rotors can rotate in both the directions.

• Rectifier has at least 6 power diodes which are embedded in the heat sink.

• Collector end shield is provided at the end where rectifier is placed. It acts as a cover.

• Drive end shield is provided at the pulley end. Stator is enclosed between the two end shields.

• Electronic regulator along with Carbon brush holders form a single unit.

Design Criteria: 

An alternator is designed based on the following criteria:

• Vehicle type
• Operating conditions
• Engine speed range
• Power requirements of the electrical equipments in a vehicle
• Battery voltage
• Available Space

Features of a Claw Pole Rotor:

• The claw pole rotor has excellent heat dissipation quality.
• The claw shaped poles face each other in rotor shaft, where the pole fingers mesh with each other to form alternating north and south pole which envelop the excitation winding.
• Lower number of poles means lower efficiency. Higher number of poles results in more amount of magnetic leakage.
• Alternators have 12 or 16 poles based on energy demand.

WORKING OF AN ALTERNATOR:

When the rotor (12 pole rotor) is excited, magnetic flux is induced in the left hand claw pole and its fingers. The magnetic flux flows through the air gap to the stator winding, and then returns to the right hand claw pole side, hence completing the circuit. This magnetic field of force cuts through the 3 phase stator winding and after one complete revolution (360°), 6 sinusoidal waves are generated in each phase.

The generated current is divided into primary current and excitation current. The primary current is rectified and then supplied to the battery and other loads. The excitation current is sent back to the rotor as rotor requires continuous supply of current for excitation.

Pre-Excitation Circuit: 

When the engine is started, it runs at a low speed. There is not enough residual excitation current to build up magnetic field in the rotor. Therefore, self excitation doesn’t take place and there is no output current from the alternator.

Battery is used as the source for the excitation current. The current (I_B) flows through the charge indicator lamp, and then to the rotor. It generates magnetic field in the rotor which in turn generates output current in the stator proportional to the rotor speed.

Once the engine gains speed, the self excitation current from the alternator should exceed the voltage drop across the excitation diodes to enable self excitation of the rotor. Once self excitation occurs, the supply from the battery is blocked with the help of charge indicator lamps. Once the generated voltage from the alternator exceeds the battery voltage, the charge indicator lamp resists the flow if current from the battery to the rotor.

Excitation Circuit: 

It is the duty of the excitation current to magnetize the rotor during alternator operations. The self-excitation current (I_err) is tapped from the 3 phase stator winding. The self excitation current flows through the excitation diodes, carbon brushes, collector rings, and to the excitation winding in the rotor to generate magnetic field. It flows further to the terminal DF of the regulator and flows out through (D-), and further to the ground (B-).

Actual Alternator Current Flow Circuit:

The output AC voltage generated in the stator winding is rectified with the help of power diodes (B+) and converted into DC current (I_G) which flows to the battery and other loads. Simultaneously, the self excitation current (I_err) used to generate magnetic field in the rotor is tapped from the stator winding. The cycle is repeated again and again.

VOLTAGE REGULATORS:

A regulator maintains the alternator output voltage over a wide range of alternator speed, independent of engine speed and vehicle loads. Maintaining a constant alternator voltage will prevent the damage of high voltage sensitive devices and also prevent the battery from overcharging. The alternator voltage may vary slightly based on temperature. In cold conditions, the alternator voltage is slightly higher because it is difficult to charge batteries at low temperatures.

The voltage output from the stator depends on the magnetic field generated by the rotor. The magnetic field strength can be varied by varying the flow of excitation current flowing to the rotor. Voltage regulator helps in varying the flow of excitation current to prevent the voltage output to exceed a set value.

Contact regulators have a movable contact point which is pressed against a fixed contact point by a spring. This contact regulator is of a single element type. It comprises of a regulating contact, an electromagnet and a regulating resistor. When the output voltage is beyond a set value, the electromagnet pulls in the armature and opens the circuit. This excites the resistor which restricts the flow of excitation current to rotor and thus the output voltage drops. When the output voltage falls below the set limit, the electromagnetic force lessens and the spring presses the movable contact towards the fixed contact, thus closing the circuit and allowing the excitation current to flow through.

Voltage regulator characteristics:

• Shorter switching time
• Insensitive to shock and vibration
• No wear
• Compact in size