What Is a Synchronous Generator?
The asynchronous generator is an alternator with the same rotors speed as the rotating magnetic field of the stator. According to the structures, it can be divided into two types: a rotating armature & a rotating magnetic field.
The synchronous generators are one of the most commonly used alternators. In the moderns powers industry, it is widely used in hydropower, thermal power, nuclear power, and diesel power generation.
An asynchronous generator or alternator is an electrical machine that converts mechanical power from a prime mover into AC electrical power at a particular voltage & frequency. The synchronous motors always run at a constants speed which is called synchronous speed.
How Does a Synchronous Generator Work?
The working principle of synchronous generators is similar to that of a DC generator. It uses Faraday’s law of electromagnetic induction. This law states that when currents are induced inside a conductor in a magnetic field, there will be a relative’s motions betweens the conductor as well as the magnetic field.
In synchronous generators, the magnetic field is constant, and the conductors will rotate. However, in practical construction, the armature conductors are motionless, and the field magnets will move between them.
In synchronous generators, the rotor can be mechanically fixed under some mechanical force towards the shaft to turn at a synchronous speed which results in cutting off the magnetic flux in the stationary armature conductor of the stator.
Because of this direct flux cutting result, there will be an induced emf and current flowing in the armature conductors. For each winding, a current will flow in the first half cycle followed by the second half cycle with a specific time interval of 120°
Working Principle of Synchronous Generator:
The synchronous generators work on the principle of Faraday’s laws of electromagnetic induction. Electromagnetic induction states that the electromotive force is induced in the armature coil if it is moving in a uniform magnetic field.
If the field rotates & the conductor becomes stationary, then emf will also be generated. Thus, the relative motion between the conductor & the field induces an emf in the conductors. The wave shape of the induced voltage is always a sinusoidal curve.
Manufacturing of Synchronous Generators The rotor and stator are the rotating and stationary parts of the synchronous generator. They are the power generating components of synchronous generators. The rotor has a field pole, and the stator has an armature conductor. The relatives’ motions between the rotor & the stators induce a voltage between the conductors.
What Is an Induction Generator?
An induction generator is an alternator that uses an air gap rotating magnetic field between the stator and the rotor to interact with an induced current in the rotor winding.
They are commonly known as asynchronous generators. Speed is slightly higher than synchronous speed. Output power increases or decreases with slip rate. It can be excited by the powers grid or self-excited with a powers capacitor.
How Does an Induction Generator Work?
In the previous section, we have given you two simple definitions of what induction and synchronous generator are. In follow, we will show you how these two generators work separately.
An induction generator generates electrical power when its rotor is accelerated to synchronous speed. For typicals four-pole motors where there are two pairs of poles on the stator operating on a 60 Hz electricals grid, the synchronous speed is 1800 rotations per minute.
The same four-pole motor running on a 50 Hz grid will have a synchronous speed of 1500 rotations per minute. The motor normally slows down slightly to synchronous speed; As you know, the difference between synchronous & operating speed is called slip & is usually expressed as a percentage of synchronous speed.
For example, a motor running at 1450 rotations per minute with a synchronous speed of 1500 rpm is running at slips of +3.3%. In normals motors operations, the stator fluxes rotations are fasters than the rotor rotation.
This causes the stator fluxes to induce rotor currents, which create rotor flux with the opposite magnetic polarity of the stator. In this way, the rotor is pulled behind the stator flux, with currents induced in the rotor at the slip frequency. In generator operations, primes movers such as a turbine or any type of engine drive the rotor above synchronous speed (negative slip).
The stator flux stills induce currents in the rotors, but since the opposing rotor fluxes are now cutting off the stator coils, an active current is generated in the stator coils, and the motor now operates as a generator, which supplies power to the electrical grid.
Consider AC supplies connected to the terminals of the stator of an induction machine. The rotating magnetic field generated in the stator pulls the rotor to drive behind it, the machine acting as a motor. Now, if the rotor is accelerated through the prime mover to synchronous motion, the slip will be zero, & hence the net torque will be zero.
When the rotors are running at synchronous speed, the rotor current will become zero. If the rotors are made to rotates at a speed greater than the synchronous speed, the slip becomes negative. Rotor currents are generated in the opposites directions due to the rotor conductor cutting off the stator magnetic field.
This generated rotor current generates a rotating magnetic field in the rotor, which exerts forces on the stator field in the opposite way. This causes a stator voltage that pushes the current flowing through the stator winding against the applied voltages.
Thus, the machines are now working as inductions generators asynchronous generators. An induction generator is not a self-excited machine. Therefore, when running as a generator, the machine takes reactive power from the AC power line and supplies the active power back to the line. Reactive power is required to generate a rotating magnetic field. The active power supplied back to the line is proportional to the shift over the synchronous.
Self-Excited Induction Generator:
It is clear that an induction machine requires reactive power for excitation, whether it is working as a generator or a motor. When induction generators are connected to the grid, it draws reactive power from the grid.
But what if we want to use an induction generator to supply the load without using an external source (e.g., grid)? A capacitor bank can be connected to the stator terminals to supply reactive power to the machine as well as the load.
When the rotor is rotated at sufficient speed, a small voltage is generated across the stator terminals due to residual magnetism. Due to this small generated voltage, capacitor current is generated, which provides more reactive power for magnetization.
Induction Generator VS Synchronous Generator:
Now that you know how induction & synchronous generators work, let’s get a little more specific about the difference between the two types of generators. In follow, you will learn more about the three most important differences between these two generators.
- In synchronous generators, the waveforms of the voltage generated are synchronized and directly correspond to the speed of the rotor. The frequency of the output can be given as f = N * P / 120 Hz. where n is the rotor speed in rpm and p is the number of poles. In the case of induction generators, the output voltage frequency is controlled by the power system to which the inductions generators are connected. If the induction generators are supplying a standalone load, the output frequency will be slightly lower (by 2 or 3%), calculated by the formula f = N * P / 120.
- An alternating or synchronous generator requires a separate DC excitation system, whereas an induction generator takes reactive powers from the powers system for field excitations. If induction generators are meant to supply standalone loads, a capacitors bank must be connected to supply reactive power.
- The construction of an induction generator is less complicated as it does not require a brush and slip ring arrangement. The brushes in the synchronous generator are required to supply DC voltage to the rotor for excitation.
Economic Comparison Between Induction Generator VS Synchronous Generators:
Here we come to the lasts part of these articles, where we will examine the difference between the two the generator in terms of economic efficiencies.
The investment cost of a power station equipped with asynchronous generators is low due to the lack of a DC excitations system & synchronous equipment. In addition, since there are no collector rings, brushes, and rotor excitation windings, maintenance and operation costs are low.
Asynchronous generator rotor has a hidden pole & a rotor winding similar to non-synchronous generators. Therefore, the general efficiencies are higher than that of synchronous generators with the same capacity and same speed.
Under the same waters sources, the asynchronous generators can generate more power. The above economic benefits of asynchronous generators will be partially offset by the required excitation or additionals synchronous capacity or additional capacitors of the asynchronous generator.
The amount of excitation required for an asynchronous generator is inversely proportional to the set speed of the motor—the higher the momentum, the lower the stimulus of the target value.
The area of Asynchronous Generator Power Plant is smaller than the area of Synchronous Generator Power Plant.
Also, Read: Types of Measuring Instruments
In these articles, we tried to provide all the necessary information regarding the difference between an Induction Generator vs. Synchronous Generator. We came up with the basic definitions of what are induction and synchronous generators, and then we move on to the working principles of each of these generators.
In the next sections, we show some comparisons between these two generators to see how they differ. Finally, we examined the differences between the two generators in terms of economic efficiency. If you have any experience using either of these two generators and want to know more about them, we would be very happy to get your opinion in the comments on our website Linkquip.
Also, if you have any questions regarding these topics, you can sign up on our website and waits for our experts to answers your questions. I hope you enjoy reading this article.