3. ● The Alternator (Three-Phase Synchronous Generator) is mainly a machine absorbing
mechanical power from a prime mover and transforming it into electrical power. An Alternator is
an electrical machine that has two differentiated parts in its construction: the stator and the
rotor.
● The stator includes three-phase windings, shifted of 120 electrical degrees, star or delta
connected. From the stator terminal you can take the outgoing three phase power. The rotor,
moved by the prime mover, at fixed speed, contains the d.c. excitation winding. The excitation
can be provided by a variable independent source or by a d.c. (exciter) generator, coaxial to the
rotor and so moved by the prime mover.
Introduction
Eng. TareQ FoQha PTUK
4. ● The operating principle is based on the rotor magnetic field, that, although fixed (it comes from
a d.c. source), induces some voltages in the statoric windings by effect of the rotation. So, on
each statoric phase, we will have 2 voltages, one induced by the rotor and one called ‘armature
reaction’ that will be produced by the current flowing across the phase. We can sum up these
voltages into a single call “E”, which effective value (RMS) is given by:
Introduction
From equation, it is seen that given a particular
alternator operating at synchronous speed, the single
factor under the user’s control is Φ (field excitation).
Eng. TareQ FoQha PTUK
5. A dc current must be supplied to the field circuit on the rotor. Since the rotor is rotating, a special
arrangement is required to get the de power to its field windings. There are two common
approaches to supplying this dc power:
1. Supply the dc power from an external dc source to the rotor by means of slip rings and
brushes.
2. Supply the dc power from a special dc power source mounted directly on the shaft of the
synchronous generator
Introduction
Eng. TareQ FoQha PTUK
6. The rate of rotation of the magnetic fields in the machine is related to the stator electrical
frequency by the following Equation:
The prime mover of the alternator has two purposes:
1. To provide the alternator with the mechanical power necessary to the electrical request;
2. To keep the speed constant of any electrical change condition of the alternator. This purpose
is given to the automatic speed control operating directly on the prime mover.
Introduction
7. The voltage induced in a given stator phase depends on the flux Ф in the machine, the frequency or
speed of rotation, and the machine's construction by the following Equation:
Introduction
Eng. TareQ FoQha PTUK
8. The per-phase equivalent circuit of this machine is shown in the Figure below:
Equivalent circuit of a synchronous generator
Eng. TareQ FoQha PTUK
9. ● To understand the operating characteristics of a synchronous generator operating alone, the
effect can be obtained by considering the phasor diagram.
Synchronous Generator Operating Alone
If more load is added at the same power factor, then
|IA| increases but remains at the same angle ϴ with
respect to VФ. It is seen that as the load increases, the
voltage VФ decreases rather sharply.
Eng. TareQ FoQha PTUK
10. When the alternator is loaded, the armature flux modifies
the air-gap flux.
● When the load p.f. is unity, the flux in the air-gap is
distorted but not weakened, flux is strengthened at
the trailing pole tips and weakened at the leading
pole tips.
● When the load p.f. is zero lagging, the flux in the air-
gap is weakened, this reduces the generated e.m.f.
● When the load p.f. is zero leading, the flux in the air-
gap is increased. this increases the generated e.m.f
● For intermediate values of load p.f. the effect of
armature reaction is partly distorting and partly
weakening for inductive loads. For capacitive loads,
the effect is partly distorting and partly strengthening.
Armature Reaction in Alternator
Eng. TareQ FoQha PTUK
11. ● The voltage regulation of an alternator is defined as the change in terminal voltage from no-load
to full-load (the speed and field excitation being constant) divided by full-load voltage.
Voltage Regulation
Eng. TareQ FoQha PTUK
12. External characteristics: detection of the phase voltage VФ with variation of the load current.
Tests on the Alternator:
Eng. TareQ FoQha PTUK
Conditions for this test: speed, load power factor and
If (excitation) must be constant.
13. ● The generator excitation system maintains generator voltage and controls the reactive power
flow. The automatic voltage regulator (AVR) takes the fluctuate voltage and changes them into
a constant voltage. The fluctuation in the voltage mainly occurs due to the variation in load on
the supply system. The variation in voltage damages the equipment of the power system.
Automatic Voltage Regulator (AVR)
Eng. TareQ FoQha PTUK
The following figure shows the AVR module that used in the lab, for
inner signaling:
• Maximum voltage (yellow led).
• Minimum frequency (green led).
• Maximum current (red led).
15. ● The AVR senses the generator output voltage and acts to change the field current to maintain
the voltage at its set value.
● AVR maintains the generator O/P voltage + or – 2.5% of its set value over the load range. The
AVR senses and alter field current. A manual/ hand trimmer regulator is fitted on generator
control panel to set voltage level.
Automatic Voltage Regulator (AVR)
Eng. TareQ FoQha PTUK
16. Regulation characteristics: detection of the excitation current If necessary to keep phase voltage
VФ, at variation of the electrical load on the alternator (current IL).
Tests on the Alternator:
Eng. TareQ FoQha PTUK
Conditions for this test: speed and load power factor
must be constant.
17. Most prime movers have a speed drop from 2% to 4%. Most governors have a mechanism to
adjust the turbine’s no-load speed (set-point adjustment).
Terminal characteristics of synchronous generator
Eng. TareQ FoQha PTUK
A typical speed
vs. power plot
A typical
frequency vs.
power plot
18. A similar relationship can be derived for the reactive power Q and terminal voltage VT. When
adding a lagging load to a synchronous generator V decreases. When adding a leading load to a
synchronous generator, its terminal voltage increases
Terminal characteristics of synchronous generator
Eng. TareQ FoQha PTUK
When a generator is operating alone
supplying the load:
1. The real and reactive powers are the
amounts demanded by the load.
2. The governor of the prime mover controls
the operating frequency of the system.
3. The field current controls the terminal
voltage of the power system.
19. Synchronous Generator Ratings
Eng. TareQ FoQha PTUK
Typical ratings on a synchronous machine are voltage, frequency, speed,
apparent power (volt-amperes), power factor. field current, and service
factor.
• For a given mechanical frame size (k) and speed (nm), the higher the
desired voltage, the higher the machine's required flux. However, flux
cannot be increased forever, since there is always a maximum allowable
field current. Another consideration in setting the maximum allowable
voltage is the breakdown value of the winding insulation.
• power limits determined by the mechanical torque on the shaft of the
machine, and the other is the heating of the machine's windings.
• The maximum acceptable armature current determines the rated volt-
amperes
20. Synchronous Generator Ratings
Eng. TareQ FoQha PTUK
The field copper losses are given by
So the maximum allowable heating sets a maximum field current for the
machine.
Since EA = KФω this sets the maximum acceptable size for EA.
• The effect of having a maximum IF and a maximum EA. translates
directly into a restriction on the lowest acceptable power factor of the
generator when it is operating at the rated volt-amperes.
• The angle of IA that requires the maximum possible EA while VФ
remains at the rated value gives the rated power factor of the
generator. Rated PF (Lagging)
For synchronous motor the one major difference is that a large EA gives a
leading power factor instead of a lagging one, and therefore the effect of
the maximum field current limit is expressed as a rating at a leading power
factor
