In South Africa, the word “Generator” isn’t unknown. In fact, with the continuous bouts of load shedding, generator and inverter systems bring about a sigh of relief if you have access to them.
We look at a different type of generator, specifically generator excitation control systems. Excitation systems can be defined as a system providing direct current (DC) to the rotor windings of a generator in order to control the strength of its magnetic field.
The four common excitation methods include the following:
- Shunt or Self Excited
- Excitation Boost System
- Permanent Magnet Generator (Three phase synchronous generator)
- Auxiliary Winding
Huebner Speed Monitoring provides three-phase synchronous generators; let’s take a closer look at this popular excitation method in more detail.
What is Excitation?
There are two main sources of energy conversion: a motor and a generator. Generators convert mechanical energy into electrical energy by moving the electrical conductors in a magnetic field. In order for this conversion to take place, excitation creates the electromagnetic field by means of an electric current to enable the energy conversion.
An electric generator or motor consists of a rotor spinning in a magnetic field. The excitation process provides direct current (DC) to the rotor windings to control the magnetic field’s strength.
Excitation current (DC) is supplied using either brushed or brushless excitation means. However, most modern generators today use brushless excitation since it requires less maintenance.
Three-Phase Synchronous Generator
Three-phase synchronous generators are used in nearly all commercial power plants. The least common type is the induction generator. Three-phase synchronous generators are also known as alternators because they convert the mechanical energy from the prime mover (gas turbine, steam turbine, water turbine, etc.) into electrical energy, as explained above. They are highly efficient thanks to their permanent excitation.
The synchronous generator is a type of AC generator. Popular applications include energy generation in wind turbines, steam turbines, and hydro turbines. A static or rotating magnetic field constructs the synchronous generators used in our industries.
Working Principle of Synchronous Generator
- The working principle of a synchronous generator is based on Faraday’s Law of electromagnetic induction.
emf= dΦ/dt
- This law says that the rate of change of flux in any device will produce EMF in that device. If a device is static and the field is rotating, it will also produce a field in the device.
- In the case of a synchronous generator, the rotor is rotating, producing a field in the stator.
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Synchronous Generator Construction
- There is no residual magnetism in a synchronous generator to produce self-excitation.
- The external DC supply is provided to the rotor, which produces a magnetic field in the rotor. When the rotor is rotated by means of mechanical energy, its field links with the stator windings to produce a voltage in the stator.
- There are two terms commonly used to describe windings in a machine, namely: armature windings and field windings.
- The windings that produce the main field in a machine are called field windings, and the windings that produce voltage are called armatures.
- In the application of a synchronous generator, the field windings are the rotor windings, and the stator windings are the armature windings.
Let’s break down some of these elements making up a three-phase synchronous generator.
Stator Core and Windings
[1] Manufactured from laminate steel, the stator has milled slots along its entire length that are filled with copper conductor windings. Each winding slot is fitted with a wedge to prevent the copper windings from being displaced due to the centrifugal forces created during operation. An alternating current is induced in the stator windings as the rotor’s magnetic fields intersect with the windings.
Rotor and Windings
[2] The rotor consists of solid steel with slots milled along its entire length; the slots are filled with copper conductor windings. Direct current (DC) is supplied to the rotor windings to produce an electromagnetic field around the rotor. As the rotor rotates, the north and south of its magnetic fields will induce alternating current (AC) flow in the stator windings.
Bearings
[3] Bearings support the weight of the shaft when the generator is idle and in operation. Small units may use anti-friction roller bearings, but larger units use plain/sliding bearings. Where plain bearings are used, the lubricating oil pressure must be maintained when the generator is in operation. A reduction in lubrication oil pressure will damage the bearings and shaft.
Cooling System
[4] Cooling is achieved by air, hydrogen, or water. Smaller generators can be air-cooled, but larger units are often hydrogen cooled, while the very large units can be water-cooled.
Frame
[5] The frame houses the stator, rotor, bearings, and cooling channels used by the generator. It provides the structural strength needed to mount all of the components and is designed to absorb vibration.
Bushings
[6] Bushings are used to pass electrical current through the enclosure of the generator without raising the electrical potential of the generator casing. Bushings usually consist of an aluminium or copper central conductor and a porcelain insulator.
Retaining Ring
[7] Retaining rings place residual compression stress upon the rotor windings; this counteracts the centrifugal forces the windings are subjected to when the generator is in operation.
Cooling Fans
[8] Air is drawn through the generator casing using single-stage axial fans; it removes waste heat from the generator casing.
Choosing Huebner Speed Monitoring for Your Three-Phase Synchronous Generator
Well-designed excitation systems provide reliability of operation, stability, and fast transient response. Huebner Speed Monitoring offers just that; a DSG P synchronous generator that is robust, permanently excited, and highly efficient.
Contact Huebner Speed Monitoring for all your questions, orders, and enquiries. Alternatively, click here to learn more.
Resources:
[1] [2] [3] [4] [5] [6] [7] [8] https://www.theengineeringknowledge.com/introduction-to-synchronous-generator/
https://savree.com/en/encyclopedia/three-phase-synchronous-generator