What
Is An Asynchronous (Induction) Generator ?
The Asynchronous
Generator is a very reliable generator that was originally designed
as an electric motor but is used as a generator and tends to be comparatively
inexpensive and has some mechanical properties that are useful for wind
turbines. There is a very important element that makes the asynchronous
generator different from the synchronous, that element being the Cage
Rotor. 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. The generator has
a simple mechanical configuration with few wear parts. Apart from the
special shape of the slipping rotor, the machine neither has slip rings
nor brushes. Therefore it requires low maintenance, has a long service
life and a robust design.
Induction generators
work well with single phase or three phase systems that are interconnected
to the utility, as an inducting system requires no governor controls.
Induction generators require excitation to operate thus they are ideally
suited for interconnection with utility applications. In the single
phase operations, it is possible to utilize induction motors as generators
to obtain near three phase efficiency by connecting capacitors to the
other unused leg of the motor. By doing this the generator will run
smoothly operating near one hundred percent (100%) power factor (PF).
The Cage Rotor
The rotor consists of a number of copper or aluminium bars which are
connected electrically by aluminium end rings.
The picture above shows 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, is a 4-pole stator which is
directly connected to the three phases of the electrical grid. The rotor
automatically adjusts itself to the number of poles. The same stator
can therefore be used with a wide variety of poles.
How It
Works
Motor
Operation
When the
current is connected, the machine will start turning like a motor at
a speed slightly below the synchronous speed of the rotating magnetic
field from the stator. There is a magnectic filed that moves relative
to 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 electro-magnetic
force from the rotating magnetic field in the stator. The diagram below
gives insight on the operation.
When operating in conjunction with a large power grid, a standard single
or three phase motor may be used as a generator. In this mode of operation
the motor draws its excitation current from the power company grid.
The nominal speed of the motor, and consequently that of the turbine,
is therefore determined by the line frequency. Driven by the turbine
with positive slip, the motor acts as a generator feeding power into
the grid.
· It is important to note that the operating speed is based on
line frequency and the individual slip factor.
Generator Operation
If the speed of the generator is increased above 1500 Rotation per minute
(rpm) the resultant effect will be that the rotor will move faster than
the rotating magnetic field from the stator. This will then facilitate
the induction of a strong current in the rotor. Power supply transferred
as an electromagnetic force to the stator can be increased by simply
cranking the rotor harder, this will then result in an increase in the
amount of electricity converted.
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 per cent. 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. (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
The asynchronous
generator requires the stator to be magnetized from the grid before
it works.
You can run an asynchronous generator in a stand alone system, however,
it will require that capacitors which supply the necessary magnetization
current be provided. It also requires that there be some left over magnetism
in the rotor iron when you start the turbine. Otherwise you will need
a battery and power electronics, or a small diesel generator to start
the system.
