Synchronous generator physical description

In this subsection the physics of synchronous generators will be discussed since most electric power today is produced by synchronous generators [107]. In addition, the Lagerwey LW-50/750 is equipped with a synchronous generator. Synchronous generators are synchronous machines used to convert mechanical power into electrical power.

Construction

Physically, most synchronous generators consist of a stationary part, called the stator, and a rotating part driven by an external torque, called the rotor. The stator is, in general, connected to the utility grid, and consists of a three-phase winding on a ferromagnetic core. The core is constructed of thin laminations to reduce eddy current losses. Mostly, the rotor has a winding through which a direct current is flowing: the field winding. The field winding produces a rotor magnetic field. This rotating magnetic field induces a three-phase set of AC voltages within the stator windings of the generator. The magnetic field created by the stator windings reacts with the rotating field thereby producing an electromagnetic torque. This torque is the mechanism through which the synchronous generator converts mechanical to electrical energy.

Since the rotor is rotating, a special arrangement is required to get the DC power to the field windings. There are two common approaches: either via an external DC source by means of slip rings and brushes or via a special DC power source mounted directly on the synchronous generator shaft. Slip rings and brushes were used on all smaller synchronous machines, while on larger generators and motors the DC power is provided by the latter approach [44]. The DC power required for excitation takes approximately one to a few percent of the rating of the synchronous generator [63].

The DC excitation of the field winding can also be provided by permanent magnets. Permanent magnet excitation brings the following benefits: i) higher torque or output power per volume, and ii) elimination of rotor copper losses due to the absence of electrical excitation, implying a substantial increase in the efficiency [92, 286]. The main disadvantages are: i) the field is uncontrollable [108], and ii) the assembly is a tricky job.

Depending on the rotor construction, a synchronous machine may be either a round-rotor (or cylindrical), or a salient-pole type. Figure 3.23 shows the cross-sections of both types. The salient-pole construction is mostly used in low-speed applications where the diameter to length ratio can be made larger to accommodate the higher pole number (20 up to 120 poles). This results in ring-generators. An example of this type is the synchronous generator used in waterwheel turbines. The round-rotor construction is favoured in high-speed applications where the diameter to length ratio has to be kept small to keep the mechanical stresses from centrifugal forces within acceptable limits. Examples are synchronous generators driven by steam or gas turbines.

Field flux

Field flux

Field winding

Field flux

Field winding

Round rotor

Field flux

Salient Pole Machine Definition

Field winding

Salient-pole rotor

Field winding

Round rotor

Salient-pole rotor

Figure 3.23: Cross-section of a two-pole round, and on a four-pole salient-pole rotor.

Synchronous speed

Synchronous generators are by definition synchronous, meaning that the produced electrical angular frequency with constant rotor field excitation is locked in or synchronized with the mechanical speed of the stator magnetic field, or equivalently the generator shaft speed. The produced frequency is completely determined by the generator shaft speed and the number of pole-pairs f = — (3.69)

where f the produced frequency in hertz, p the number of pole-pairs, and n is the speed of the generator shaft expressed in rotations per minute (r.p.m.). For example, to generate 50-Hz power in a four pole machine, the rotor must turn at 1500 r.p.m. This relationship is depicted in Table 3.2.

p [-]

1

2

3

4

5

6

30

60

75

100

n [r.p.m.]

3000

1500

1000

750

600

500

100

50

40

30

Table 3.2: Relationship between number of pole-pairs p, and rotational speed n of a three-phase synchronous generator linked to a grid with a fixed 50-Hz frequency.

Table 3.2: Relationship between number of pole-pairs p, and rotational speed n of a three-phase synchronous generator linked to a grid with a fixed 50-Hz frequency.

Large wind turbines have relatively low rotor shaft speeds, typically from 50 to 15 r.p.m. in the power range from 300 to 1500 kW, while wind turbine generators typically have 2, 3, or 4 pole-pairs (with base speeds of 1500, 1000 and 750 r.p.m. respectively at 50-Hz grid frequency, see Table 3.2). Consequently, if we want to link such a wind turbine driven generator to the grid with a fixed 50-Hz frequency, a mechanical transmission with a speed ratio between 10 and 100 is required to increase the angular velocity of the rotor shaft in order to come to an angular velocity well-suited for the (high-speed) generator. Most wind turbines are equipped with a rotor-transmission-generator drive-train where the combination of generator base speed and transmission ratio is selected such that the drive-train costs are minimized [106].

The interest in low-speed generators (which are directly connected to the shaft of the turbine and hence eliminating the transmission) has increased significantly in the past decade [92]. These so-called direct-drive generators necessarily have a large diameter, because it follows from basic generator theory that the torque that can be produced is directly related to the volume of the generator [298]. This construction offers a number of advantages in comparison to its counterpart with transmission. The most important advantages are a reduced noise level and a reduction of installation costs [186]. In addition, when combined with efficient, variable frequency power supply allowing the rotational speed to vary with the wind velocity, the aforementioned advantages can be obtained. The main disadvantages are the relatively high generator mass and the present purchase price.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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Responses

  • callie
    Why does synchronous motor used in making an improvised waterwheel as generator of electricity?
    2 years ago

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