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 (IB)
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 (Ierr) 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 (IG) which flows to
the battery and other loads. Simultaneously, the self excitation current (Ierr)
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
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ReplyDeleteis their any alternator attach with car compressor if yes then please shear some knowldge on how alternator works with compressor
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