Demonstrate working principle of A.C. generator and its regulators.

Demonstrate working principle of A.C. generator and its regulators.

INTRODUCTION

The alternator or A.C. Generator is the generator which produces alternating current instead of the direct current as produced by a commutator type generator. This has replaced D.C. generator since early 1960's because of its distinct advantages. However, as the automotive electrical system requires only direct current, the alternating current produced in the a.c. generator has to be converted first into the direct current, which is easily done with the help of diodes.

WORKING PRINCIPLE

When the electromagnet is rotated, the magnetic lines of force cut the stationary Stator loop to produce an induced current. However, as after every half revolution, the magnet reverses its polarities, the current produced in the stator alternates in sign in a revolution.

To convert this alternating current into the direct current, diodes are employed, which allow the current to pass through them only in one direction. Thus it is the d.c. current in the a.c. generator also which is fed to the electrical circuit.

CONSTRUCTIONAL DETAILS OF A.C. GENERATOR

FRAME OR HOUSING

This encloses the entire alternator assembly and is made of cast aluminium in two pieces. Aluminum is light weight, non-magnetic and has higher thermal conductivity so as to keep alternator assembly cool by more efficient transfer of heat. The front part of the frame (called drive-end housing) has ball bearings, while the rear part (called brush-end housing) contains a roller bearing.

ROTOR

In practice the rotor is not a bar magnet as stated above white explaining the principle of working of the alternator. The most common is the claw pole rotor which consists of an iron core around the rotor shaft.

Many turns of copper wire coated with varnish are wound over the core (Fig.), o n both sides of the rotor winding are thick metal plates bent over the winding with triangular fingers called poles. These metal plates are called pole-pieces. The pole-pieces are placed interlacing each other so that when the current flows through the windings, these pole- pieces acquire opposite polarity. This will result in alternating north and south magnetic pole on the pole fingers. This creates magnetic fields between the alternating pole fingers. Thus in contrast to a d.c. generator, an a.c. generator has a rotating magnetic field.

SLIP RINGS & BRUSHES

The current to the rotor winding is carried through the copper slip rings and carbon brushes. The brushes ride the surface of the slip rings on the rotor under spring tension provided by the brush holders. As these brushes carry only the field current (about 2—5 amp) in contrast to the carbon brushes in d.c. generator where all the current produced in the generator has to pass through the brushes, the former have a much longer life.

STATOR WINDING

Three-phase alternator is commonly used in automobiles. Between two halves of the alternator casing, is situated the stator consisting of three sets of winding wound over a laminated iron core.

When the rotor rotates, its moving magnetic field induces current in all the three stator windings. All alternators contain at least six diodes, one pair of a positive and a negative diode for each winding to have full-wave rectification of the a.c. current produced in the alternator. One diode would achieve only half-wave rectification.

Further, three positive diodes are usually included in the rectifier circuit. These are known as the auxiliary, field or excitation diodes. These are smaller than the main diodes and serve to supply a small current back to the field windings in the rotor.

The three windings of the stator are connected together. The two commonly used connections are the star connection and the delta connection.

The current and voltage characteristics of the star- and delta-wound rotors are quite different. Star connection can be considered as a series combination of phases. As such, the output voltage across any two phases will be the vector sum of the phase voltages, while the current output will be the same as the phase current. Thus star-wound stators produce higher voltage.

Likewise, a delta connection can be taken as a type of parallel circuit, which means that the output current will be the vector sum of the phase currents, while output voltage would be the same as the phase voltage.

In case of a star-wound stator, the theoretical voltage at the neutral point of the star is zero. However, in practice, due to small inaccuracies in the construction of the stator and rotor, a potential, known as third harmonic, develops at this point. Its frequency is thrice the fundamental frequency of the phase windings. The energy due to this harmonic is collected by employing an additional pair of diodes, one positive and one negative, at the star point. This results in increase of power output of the alternator by about 15%. This it is seen that full circuit of an actual alternator consists of eight-diode main rectifier and three field diodes.

The star connection is more common. With this connection, alternator is able to start charging at lower speeds and has lower maximum output than alternator with delta connection stator. Delta connections for stators are used for those alternators where high output at high alternator speed is required. Such alternators can produce about 73% more current than the same alternators with star connections. These, however, produce lower current at low speeds and, therefore, have to be operated at high speed to obtain maximum output.

ALTERNATOR REGULATION

In case of alternators, diodes are employed in the stator output circuit, which allow the current to flow in one direction only. Thus the cutout relay used in d.c. generator regulators is not required here because diodes do not allow the current to flow from the battery to the alternator when it stops or slows, although the connection between the two still remains.

Moreover, the magnetic field produced by the current in the Stator windings opposes in polarity and approaches in magnitude the magnetic field provided by the rotor as the generator output increases. This causes the alternator to limit its own output to a safe maximum value. Thus alternators also do not require current limiters as they do not produce enough current to cause damage to the alternator itself. However, an alternator could produce more than 250 V if not controlled. Thus alternator regulators require only the voltage to be controlled.

The output voltage of an alternator depends upon:

1. Speed of rotation of the rotor

2. Number of turns in the stator windings

3. Strength of the rotor magnetic field which itself is dependent on the following: 

(a) Number of turns and type of wire used for the rotor winding.

(b) Air gap between the stator and the rotor poles. Increasing the gap decreas es the field strength.

(c) Battery voltage applied to the rotor.

Out of the above, it is not practical to keep the rotor rpm constant at different engine speeds. Moreover, the number of turns in the stator and the rotor windings are fixed in a given alternator. Therefore, the maximum output voltage of the alternator is controlled by the control of the field (rotor) current. This is done by means of regulator.

However, whenever the output voltage is below the maximum limit, the voltage regulator does not come into action to limit the field current. Rather at low alternator speeds, the field current is maintained at its maximum so as to allow the alternator output to be as high as possible.

Further, when the output voltage is below the regulator setting, it is controlled by the battery. If the battery is discharged, its voltage will be lower than that of the fully charged battery. Therefore, the alternator will charge it at lower voltage but high current. As the battery becomes charged, the battery voltage would increase till the regulator setting is reached when the regulator will start operating to limit the maximum charging voltage. Thus below the maximum limit, the battery acts as the voltage regulator of the alternator.

That is the reason why testing of the charging system must be done With the battery in fairly good condition, say at least 75 percent charged, so as to obtain accurate results. Moreover all connecting wires and grounds must be in good condition and tightly secured, otherwise the voltage regulator may sense a battery voltage which is lower than the actual and hence may allow the alternator output to exceed the normal charging voltage, thus causing overcharging of the battery.

The alternator regulators may be of two types :

1. Mechanical, i.e., the Vibrating contact point type.

2. Electronic, i.e., the Transistorised type.

The vibrating contact point regulators are similar to the point type voltage regulator for dc. generator. The transistorised regulators do not employ any vibrating con tacts which increases their life. Due to this, these are now almost universal on modern cars. In practice the above basic circuit is improved by using various resistors and capacitors. Besides alternator regulators using integrated circuits (IC's) are being increasingly used.'Integrated' implies that the transistors and circuits are made together in a single part. Such a regulator has the following advantages:

1. Outside wiring of the alternator is simplified

2. Number of moving parts are reduced,

3. Better resistance to vibration.

4. Voltage fluctuations are reduced.

A hybrid IC voltage regulator circuit is shown in Fig. The hybrid system uses both the IC chip as well as the connection of discreet components on a ceramic plate using film techniques. The l.C. forms the main part of the regulator and contains the sensing elements and temperature compensation components. The IC controls an output stage such as a Darlington pair. This method results in a compact and reliable device. The latest in the charging system control is the use of computer.

In such a computer-controlled charging system, the voltage regulator is placed inside the power module and is controlled by the logic module, which also senses battery temperature and battery voltage at the alternator, besides the alternator output voltage. A duty cycle proportional to the difference between the desired control voltage and the alternator output voltage energizes the rotor field.