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The hidden power in electric motor rotors: Why modern generators don't rely on carbon brushes?
As technology advances, the design and operation of generators are also evolving. The change from early generators that relied on carbon brushes to today’s increasingly common brushless technology has not only improved generator performance but also reduced maintenance costs. Many readers may be curious about why modern generators do not rely on carbon brushes. What technological innovations are hidden behind this?
Excitation in Electromagnetism
In electromagnetism, excitation is the process of generating a magnetic field using electric current. A generator or motor consists of a rotor rotating in a magnetic field. The magnetic field can be generated by permanent magnets or electric field coils. For machines that use electric field coils, current must flow through the coils to generate (excite) the magnetic field, otherwise the rotor cannot transfer power.
Electric field coils offer the most flexible form of flux regulation, but they consume current.
Excitation in generators
For many large generators, an electric current must be established before the generator can generate electricity. Although some of the generator output can be used to maintain the magnetic field once started, an external current source is still required during starting. Controlling the magnetic field is very important as this will maintain the voltage in the system.
Amplifier Principle
Except for permanent magnet generators, the output voltage of the generator is proportional to the magnetic flux. The sum of the magnetic flux consists of the magnetization of the structure and the magnetic field generated by the excitation current. If there is no excitation current, the magnetic flux is very small and the armature voltage is almost zero. By controlling the field current, the voltage of the generating system can be regulated to eliminate the voltage drop caused by increasing the armature current.
A generator can be viewed as a current-to-voltage amplifier.
Use of independent incentives
For large generators, it is common practice to use a separate excitation generator connected in parallel with the main generator. This is a small permanent magnet or battery excited generator that is designed to provide the necessary current for the larger generator.
A new era of self-motivation
Modern generators are usually self-excited, that is, part of the rotor output power is used to drive the electric field coils. When the generator is turned off, the rotor core will retain a certain amount of residual magnetism. When starting the generator, do not connect any load first. Then its initial weak magnetic field will induce a weak current in the rotor coil, thereby strengthening the magnetic field and eventually establishing a strong voltage.
Startup and other variants
Self-excited generators must be started without external load. There are various types of self-excited designs, ranging from simple shunt designs that use power from the main winding to excitation boost systems that provide a temporary energy boost to cope with load changes.
When the residual magnetism of the generator is not sufficient to achieve full voltage, there are usually provisions to inject current from another source.
The rise of brushless incentives
Brushless excitation technology enables the magnetic flux in the motor to be generated without the need for carbon brushes. This technology, developed from advances in semiconductor technology, uses a rotating rectifier to collect the induced AC voltage on the shaft of the synchronous machine and rectifies it to provide it to the field winding of the generator. While brushless excitation has historically been deficient in terms of fast flux regulation, this is improving as new solutions become available.
The future of technology
Today's brushless technology is more sophisticated, using high-performance wireless communications to achieve full control of the magnetic field, such as thyristor rectifiers and switching interfaces, making the operation of the generator more flexible and efficient.
As technology continues to evolve, whether generators can completely get rid of carbon brushes remains a challenging issue. What new technologies will emerge in the future to solve these challenges?
