The difference between asynchronous and synchronous generator

A synchronous generator is called “synchronous” because the waveform of the generated voltage is synchronized with the rotation of the...


A synchronous generator is called “synchronous” because the waveform of the generated voltage is synchronized with the rotation of the generator. Each peak of the sinusoidal waveform corresponds to a physical position of the rotor. The frequency is exactly determined by the formula f = RPM x p / 120 where f is the frequency (Hz), RPM is the rotor speed (revolutions per minute) and p is the number of poles formed by the stator windings. A synchronous generator is essentially the same machine as a synchronous motor. The magnetic field of the rotor is supplied by direct current or permanent magnets.

The output frequency of an asynchronous generator is slightly (usually about 2 or 3%) lower than the frequency calculated from f = RPM x p / 120. If the RPM is held constant, the frequency varies depending on the power level. The peaks of the waveform have no fixed relationship with the rotor position. An asynchronous generator is essentially the same machine as an asynchronous or induction motor. The magnetic field of the rotor is supplied by the stator through electromagnetic induction.

The output frequency of a synchronous generator can be more easily regulated to remain at a constant value. Synchronous generators (large ones at least) are more efficient than asynchronous generators. Synchronous generators can more easily accommodate load power factor variations. Synchronous generators can be started by supplying the rotor field excitation from a battery. Permanent magnet synchronous generators require no rotor field excitation.

The construction of asynchronous generators is less complicated than the construction of synchronous generators. Asynchronous generators require no brushes and thus no brush maintenance. Asynchronous generators require relatively complicated electronic controllers. They are usually not started without an energized connection to an electric power grid, unless they are designed to work with a battery bank energy storage system. With an asynchronous generator and an electronic controller, the speed of the generator can be allowed to vary with the speed of the wind. The cost and performance of such a system is generally more attractive than the alternative systems using a synchronous generator.

Asynchronous (induction) generators


Introduction
Most wind turbines in the world use a so-called three phase asynchronous (cage wound) generator, also called an induction generator to generate alternating current. This type of generator is not widely used outside the wind turbine industry and in small hydropower units, but the world has a lot of experience in dealing with it anyway. The picture illustrates the basic principles in the asynchronous generator, much similar with the synchronous generator presented before. In reality, only the rotor part looks different.
The curious thing about this type of generator is that it was really originally designed as an electric motor. In fact, one third of the world's electricity consumption is used for running induction motors driving machinery in factories, pumps, fans, compressors, elevators and other applications where you need to convert electrical energy to mechanical energy.
One reason for choosing this type of generator is that it is very reliable and tends to be comparatively inexpensive. The generator also has some mechanical properties which are useful for wind turbines, like the generator slip and a certain overload capability. 



The cage rotor
  The key component of the asynchronous generator is the cage rotor (it used to be called a squirrel cage rotor but after it became politically incorrect to exercise your domestic rodents in a treadmill, we only have this less captivating name).
It is the rotor that makes the asynchronous generator different from the synchronous generator. The rotor consists of a number of copper or aluminium bars which are connected electrically by aluminium end rings, as you see in the picture. In the picture is shown how the rotor is provided with an "iron" core, using a stack of thin insulated steel laminations, with holes punched for the conducting aluminium bars. The rotor is placed in the middle of the stator, which in this case, once again, is a 4-pole stator which is directly connected to the three phases of the electrical grid.



Motor operation
When the current is connected, the machine will start turning like a motor at a speed which is just slightly below the synchronous speed of the rotating magnetic field from the stator.If we look at the rotor bars from the previous picture, there is a magnetic field which moves relative to the rotor. This induces a very strong current in the rotor bars which offer very little resistance to the current, since they are short circuited by the end rings. The rotor then develops its own magnetic poles, which in turn become dragged along by the electromagnetic force from the rotating magnetic field in the stator.


Generator operation
If we manually crank this rotor around at exactly the synchronous speed of the generator, e.g. 1500 rpm, as we saw for the 4-pole synchronous generator on the previous page nothing will happen. Since the magnetic field rotates at exactly the same speed as the rotor, there will be no induction phenomena in the rotor and it will not interact with the stator.
If speed is increased above 1500 rpm then the rotor moves faster than the rotating magnetic field from the stator, which means that once again the stator induces a strong current in the rotor. The harder is cranked the rotor, the more power will be transferred as an electromagnetic force to the stator, and in turn converted to electricity which is fed into the electrical grid.



Generator slip 
The speed of the asynchronous generator will vary with the turning force (moment, or torque) applied to it. In practice, the difference between the rotational speed at peak power and at idle is very small, about 1%. This difference in per cent of the synchronous speed, is called the generator's slip. Thus a 4-pole generator will run idle at 1500 rpm if it is attached to a grid with a 50 Hz current. If the generator is producing at its maximum power, it will be running at 1515 rpm.
It is a very useful mechanical property that the generator will increase or decrease its speed slightly if the torque varies. This means that there will be less tear and wear on the gearbox, because of lower peak torque. This is one of the most important reasons for using an asynchronous generator rather than a synchronous generator on a wind turbine which is directly connected to the electrical grid.



Automatic pole adjustment of the rotor
The clever thing about the cage rotor is that it adapts itself to the number of poles in the stator automatically. The same rotor can therefore be used with a wide variety of pole numbers.


Grid connection required
An asynchronous generator is different, because it requires the stator to be magnetised from the grid before it works.
However, an asynchronous generator in a stand alone system can be used if it is provided with capacitors which supply the necessary magnetisation current. It also requires that there be some remanence in the rotor iron, i.e. some leftover magnetism to start the turbine. Otherwise a battery and power electronics will be needed, or a small diesel generator to start the system.

A diagram of asychronous motor convert to generator

The increasing importance of energy efficiency has brought electric motor makers to promote a variety of schemes that improve motor performance. Unfortunately the terminology associated with motor technologies can be confusing, partly because multiple terms can sometimes be used interchangeably to refer to the same basic motor configuration. Among the classic examples of this phenomenon is that of induction motors and asynchronous motors.
All induction motors are asynchronous motors. The asynchronous nature of induction-motor operation comes from the slip between the rotational speed of the stator field and somewhat slower speed of the rotor. A more-specific explanation of how this slip arises gets into details of the motor internals.
Most induction motors today contain a rotational element (the rotor) dubbed a squirrel cage. The cylindrical squirrel cage consists of heavy copper, aluminum, or brass bars set into grooves and connected at both ends by conductive rings that electrically short the bars together. The solid core of the rotor is built with stacks of electrical steel laminations. The rotor contains fewer slots than the stator. The number of rotor slots must also be a nonintegral multiple of stator slots so as to prevent magnetic interlocking of rotor and stator teeth when the motor starts.
It is also possible to find induction motors containing rotors made up of windings rather than a squirrel cage. The point of this wound-rotor configuration is to provide a means of reducing the rotor current as the motor first begins to spin. This is generally accomplished by connecting each rotor winding to a resistor in series. The windings receive current through some kind of slip-ring arrangement. Once the rotor reaches final speed, the rotor poles get switched to a short circuit, thus becoming electrically the same as a squirrel cage rotor.
The stationary part of the motor windings is called the armature or the stator. The stator windings connect to the ac supply. Applying a voltage to the stator causes a current to flow in the stator windings. The current flow induces a magnetic field which affects the rotor, setting up voltage and current flow in the rotor elements.
A north pole in the stator induces a south pole in rotor. But the stator pole rotates as the ac voltage varies in amplitude and polarity. The induced pole attempts to follow the rotating stator pole. However, Faraday’s law says that an electromotive force is generated when a loop of wire moves from a region of low magnetic-field strength to one of high magnetic-field strength, and vice versa. If the rotor exactly followed the moving stator pole, there would be no change in magnetic-field strength. Thus, the rotor always lags behind the stator field rotation. The rotor field always lags behind the stator field by some amount so it rotates at a speed that is somewhat slower than that of the stator. The difference between the two is called the slip.
The amount of slip can vary. It depends principally on the load the motor drives, but also is affected by the resistance of the rotor circuit and the strength of the field that the stator flux induces.
A few simple equations make the basic relationships clear.
When ac is initially applied to the stator, the rotor is stationary. The voltage induced in the rotor has the same frequency as that of the stator. As the rotor starts spinning, the frequency of the voltage induced in it, fr, drops. If f is the stator voltage frequency, then slip, s, relates the two via fr = sf. Here s is expressed as a decimal.
When the rotor is standing still, the rotor and stator effectively form a transformer. So the voltage E induced in the rotor is given by the transformer equation
E = 4.44 f N фm
where N = the number of conductors under one stator pole (typically small for a squirrel-cage motor) and фm = maximum magnetic flux, Webers. Thus, the voltage Er induced while the rotor spins depends on the slip:
Er = 4.44 s f N фm = s E

Explanation of synchronous motors

A synchronous motor has a special rotor construction that lets it rotate at the same speed — that is, in synchronization — with the stator field. One example of a synchronous motor is the stepping motor, widely used in applications that involve position control. However, recent advances in power-control circuitry have given rise to synchronous-motor designs optimized for use in such higher power situations as fans, blowers, and to drive axles in off-road vehicles.
There are basically two types of synchronous motors:
• Self-excited — Using principles similar to those of induction motors, and
• Directly excited — usually with permanent magnets, but not always
The self-excited synchronous motor, also called a switched-reluctance motor, contains a rotor cast of steel that includes notches or teeth, dubbed salient poles. It is the notches that let the rotor lock in and run at the same speed as the rotating magnetic field.
To move the rotor from one position to the next, circuitry must sequentially switch power to consecutive stator windings/phases in a manner analogous to that of a stepping motor. The directly excited synchronous motor may be called by various names. Usual monikers include ECPM (electronically commutated permanent magnet), BLDC (brushless dc), or just a brushless permanent-magnet motor. This design uses a rotor that contains permanent magnets. The magnets may mount on the rotor surface or be inserted within the rotor assembly (in which case the motor is called an interior permanent-magnet motor).
The permanent magnets are the salient poles of this design and prevent slip. A microprocessor controls sequential switching of power on the stator windings at the proper time using solid-state switches, minimizing torque ripple. The principle of operation of all these synchronous-motor types is basically the same. Power is applied to coils wound on stator teeth that cause a substantial amount of magnetic flux to cross the air gap between the rotor and stator. The flux flows perpendicular to the air gap. If a salient pole of the rotor is aligned perfectly with the stator tooth, there is no torque produced. If the rotor tooth is at some angle to the stator tooth, at least some of the flux crosses the gap at an angle that is not perpendicular to the tooth surfaces. The result is a torque on the rotor.Thus, switching power to stator windings at the right time causes a flux pattern that results in either clockwise or counterclockwise motion.
One other type of synchronous motor is called a switched reluctance (SR) motor.

Its rotor consists of stacked steel laminations with a series of teeth. The teeth are magnetically permeable, and the areas surrounding them are weakly permeable by virtue of slots cut into them. Thus the rotor needs no windings, rare-earth materials, or magnets.
Unlike induction motors, there are no rotor bars and consequently no torque-producing current flow in the rotor. The absence of any form of conductor on the SR rotor means that overall rotor losses are considerably lower than in other motors incorporating rotors carrying conductors. Torque produced by the SR motor is controlled by adjusting the magnitude of current in the stator electromagnets. Speed is then controlled by modulating the torque (via winding current). The technique is analogous to the same way speed is controlled via armature current in a traditional brush-dc motor.
An SR motor produces torque proportional to the amount of current put into its windings. Torque production is unaffected by motor speed. This is unlike ac-induction motors where, at high rotational speeds in the field-weakening region, rotor current increasingly lags behind the rotating field as motor rpm rises.

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Electrical for Us: The difference between asynchronous and synchronous generator
The difference between asynchronous and synchronous generator
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Electrical for Us
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