Alternatör V

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Alternator craft

Alternators used to supply communications equipment loads tend to be specified as having a good quality on-load wave form;

From: Telecommunications Engineer's Reference Book, 1993

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Electrical and electronics principles

Charles J. Fraser, in Mechanical Engineer's Reference Book (Twelfth Edition), 1994

2.2.19 Three-phase alternators

Alternators are constructed with a stationary a.c. winding and a rotating field system. This reduces the number of slip-rings required to two, and these have to carry only the field-exciting current as opposed to the generated current. The construction is thereby simplified and the slip-ring losses are minimized. In addition, the simpler arrangement enables heavier insulation to be used and, in consequence, much higher voltages can be generated. The robust mechanical construction of the rotor also means that higher speeds are possible and substantially higher power outputs can be generated with an alternator. A simple form of three-phase generator is depicted in Figure 2.47.

Figure 2.47. Simple three-phase generator

The three coils on the stator are displaced 120° and the rotor, which is a salient pole type, is supplied via the two slip-rings with a d.c. current. As the rotor is driven by some form of prime mover, a rotating magnetic field is established and the e.m.f.'s generated in the coils will be displaced with a phase shift of 120°. The magnitude of the generated voltages are dependent on the flux produced by the rotor, the number of turns on the stator coils and the speed of rotation of the rotor. The rotor speed will also dictate the frequency of the generated voltage.

The no-load and load characteristics of an alternator are very similar to those of the d.c. separately excited generator (Figures 2.28 and 2.29, respectively). In constant speed operation the terminal voltage exhibits a drooping characteristic, where the decrease in terminal voltage is due to 'armature' resistance and reactance effects. For an alternator, the term 'armature' is taken to imply the stator windings.

As the load on an alternator is increased, the speed of the prime mover will drop. This is an unacceptable situation, because the speed controls the frequency of the generated voltage. To maintain a constant frequency, the prime mover must be governed to run at constant speed over the entire range of expected loads. This is particularly important where many alternators are to be run in parallel to supply a distribution system such as the National Grid. In such cases the prime movers are always speed controlled and the output voltage is regulated to comply with the rated values. In the UK, alternators are usually two-pole machines driven at 3000 rev/ min to produce the rated frequency of 50 Hz. In the USA a great deal of the electrical power consumed is generated from hydroelectric power stations. The water turbines used in these installations are fairly low-speed machines and the alternators, which are directly driven, are equipped with multiple poles to produce the rated frequency of 60 Hz. An alternator running at 240 rev/min, for example, must have 30 poles to give the rated output frequency.

The production of the rotating magnetic field may also be activated using three, 120° displaced, rotor coils supplied with three-phase current. The rotational speed of the field is related to the frequency of the currents, i.e.

(2.80)Ns=f×60Number of pole pairs

where Ns is the speed of the field (rev/min) and f is the frequency of the supply currents. The speed of the rotating field is termed the 'synchronous speed' and for an equivalent single pair of poles (i.e. three coils) this is 3000 rev/min when the frequency of the supply currents is at 50 Hz.

The use of a.c. excited rotor coils to produce the rotating magnetic field simplifies the mechanical construction of the rotor and greatly facilitates the dynamic balancing of the machine. An added advantage is that the waveform of the generated voltage is improved. The a.c. method of exciting the field is used extensively in large alternators. Salient pole rotors are normally restricted to the smaller machines.

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Telecommunication power systems

Clive R Nightingale Dip. EE CEng FIEE, in Telecommunications Engineer's Reference Book, 1993

24.2.5 Alternators

Alternators used to supply communications equipment loads tend to be specified as having a good quality on-load wave form; a maximum total harmonic distortion of 5% is not unusual. Associated regulators capable of maintaining close limits of voltage, and control to within plus or minus 1% or 1 1/2% of the nominal voltage are readily obtainable under steady state load conditions. Built in overload protection and discrimination can be obtained, and to limit transient output variations, and maintain the best possible compatibility with the normal mains supply, low impedance machines are desirable.

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Power Quality Solutions for Renewable Energy Systems

Mohammad A.S. Masoum, Ewald F. Fuchs, in Power Quality in Power Systems and Electrical Machines (Second Edition), 2015

11.3.2.2 Generator Operation

The alternator or regenerative braking operation occurs in quadrant II of characteristic 4 (see Figure 11.12). Once the machine has gained about the base speed the only stringent requirement is an efficiency of η = 75-95% which depends upon the output power rating of the drive. At regenerative operation of the alternator the pulse-width-modulated (PWM) inverter acts as a PWM rectifier providing AC excitation for the (induction) generator if necessary, and delivering generator power to the battery or the power source at a DC voltage of VDC. Alternatively the excitation of the induction generator can be provided by an appropriate capacitor bank.

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Space Power Systems Engineering

R.W. Powell, in Progress in Astronautics and Rocketry, 1966

Turbine-alternator assembly:

The turbine-alternator assembly (Fig. 11) is designed to produce 35 kw of usable electric power in addition to the auxiliary power required for system control and component operation.

Fig. 11. SNAP-8 turbine alternator assembly in the clean room.

The turbine is an overhung, four-stage, impulse machine. The first two stages are split-path, partial-admission stages, while the remaining two are full-admission stages.

The alternator is a homopolar-inductor type. Dynamic seals at each bearing of the alternator prevent the lubricant-coolant fluid from flooding the rotor cavity. The alternator is cooled by a cooling jacket utilizing the system lubricant-cooling fluid, ET-378. Turbine alternator testing in a rated power loop will demonstrate the capability of this unit to produce 45 kwe using a prototype boiler and condenser at rated operating conditions. In addition, detailed performance maps will be obtained.

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Experience of the Manufacture and Testing of Small Aerogenerators and a Stand-by Combustion Engine

J.R. White, A.A. Pinney, in Energy for Rural and Island Communities: Proceedings of the Third International Conference Held at Inverness, Scotland, September 1983, 1984

CURRENT MODELS

A more appropriate alternator, one with a lower cut-in speed, and costing over £200 new, was fitted to the newest machine to be installed at Aberfeldy, at Easter 1983. This alternator was 9.5 mm wider than previous ones, but the casing was easy to modify, by the addition of one sheet of plywood to the pattern. The cut-in speed appears to be much better. An anemometer and a mercury-bath DC Ampere-hour meter were included. Readings were listed by hand each day.

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Road Transport

JS Davenport, ... MJH Chandler CEng, FIEE, FICE, FIHT, in Electrical Engineer's Reference Book (Sixteenth Edition), 2003

Control

Since the alternator is designed to be inherently self-regulating as to maximum current output and since the rectifiers eliminate the need for a cut-out relay, the only form of output control necessary is a voltage regulator in the field circuit.

The conventional type of electromagnetic vibrating-contact voltage regulator, with either single or double contacts, has been used. Also employed to enable higher field current to be used and to give longer contact life has been a vibrating-contact regulator with the contacts connected in the base circuit of a transistor, the transistor being protected by a field discharge diode against inductive voltage surges.

To eliminate all maintenance and wear problems, it is now general practice to use solid state components in all parts of the voltage control unit.

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Magnetic Particle Testing

Ramesh Singh, in Applied Welding Engineering, 2012

Alternating Current

When an alternator produces an alternating current voltage, the voltage switches polarity over time, in a very particular manner. If this polarity over time wave trace is graphed, it is seen to change rapidly but smoothly over the cross over line (point zero). The shape of the curve so produced is called a sine wave.

One cycle of this reversal is termed a wave cycle, and the rate of this alternation is called the frequency, which is measured in Hertz (Hz). In the USA, the electrical supply from the grid is at 60 Hz, while in most of Europe the frequency is 50 Hz.

Due to the sine wave pattern of current, the current can only penetrate the surface, thus only the surface of the metal is magnetized by alternating current. This method is effective for locating discontinuities that extend to the surface, such as fatigue or service cracks. Similarly, in a weld, a surface opening can be detected by AC current magnetization. As stated in the introduction, this type of magnetizing is not able to detect subsurface discontinuities.

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Magnetic Particle Testing

Ramesh Singh, in Applied Welding Engineering (Second Edition), 2016

Alternating Current

When an alternator produces AC voltage, the voltage switches polarity over time in a very particular manner. If this polarity over time wave trace is graphed, it is seen as changing rapidly but in a smooth transition over the cross-over line (point zero). The shape of the curve so produced is called a sine wave.

One cycle of this reversal is termed the wave cycle. The rate of this alternating is called frequency, and it is measured in Hertz (Hz). In the United States, the electrical supply from the grids is at 60 Hz, but in most of the Europe, the frequency is at 50 Hz.

Because of the sine wave, the penetration of the current to magnetize the material is only surface deep; thus, only the surface of the metal is magnetized by AC. The method is effective for locating discontinuities that extend to the surface, such as fatigue or service cracks. Similarly, in welds, the surface opening can be detected by AC current magnetizing. As stated in the introduction, this type of magnetizing is not able to detect subsurface discontinuities.

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Main Equipment

Swapan Basu, Ajay Kumar Debnath, in Power Plant Instrumentation and Control Handbook (Second Edition), 2019

4.1.4 Cooling of Stator and Rotor

Generators/alternators are supposed to deliver electric power at a certain output voltage and current. For large thermal power stations, those values may be very high, especially the current, which is reduced afterward by the use of the generator transformer by raising the output voltage, thereby reducing the current flowing through the grid. Therefore, the generator windings should be capable of sourcing the high-density currents that in turn produce enormous heating effects. To overcome/reduce this undesirable heating, the stator windings are made of hollow copper conductors through which the pure quality cooling water circulates. With this cooling water, the problem often experienced is the formation of scale depositions on the internal walls of the hollow conductors. These deposits lessen water flow as well as heat transfer; both are detrimental to generator stator cooling while also reducing the load delivering capacity and forcing frequent downtime. To get minimum scale deposition, maximum heat transfer, and the lowest possible electrical conductivity of the cooling water, a small portion of the normal stator cooling water flow (approximately 1%–10%) is bypassed for rejuvenation through an MB demineralization (DM) plant and returned to the original stream. Another way is to take advantage of the station shutdown and go for chemical cleaning of the hollow conductors to remove the scale. However, cleaning operations are a longstanding process and invite corrosion of metals of hollow conductors.

The rotor is also subjected to carrying high field excitation current and the heat generated due to a high centrifugal force. For that reason, pressurized hydrogen is introduced inside the generator housing, which enables cooling of both the stator core and rotor elements and taken out from a suitable point for reentry after heat release through an external cooling arrangement.

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Main Equipment

Swapan Basu, Ajay Kumar Debnath, in Power Plant Instrumentation and Control Handbook, 2015

4.1.4 Cooling of the Stator and Rotor

Generators or alternators are supposed to deliver electric power at a certain output voltage and current. For a large thermal power station, those values may be very high especially the current, which is reduced afterward by the use of a generator transformer by raising the output voltage, thereby reducing the current flowing through the grid. Therefore the generator windings should be capable of sourcing high-density currents that in turn are responsible for producing enormous heating effects. To overcome/reduce this undesirable heating, the stator windings are made of hollow copper conductors through which the cooling water circulates in a loop, comprising the cooling circuit with redundant coolers.

With this cooling water, the problem often experienced is the formation of scale deposits on the internal walls of the hollow conductors. This deposits decrease water flow as well as heat transfer, both of which are detrimental as far as cooling of the generator stator is concerned; the result is reductions in load delivery capacity and frequent downtime. To get minimum scale deposition, maximum heat transfer and the lowest possible electrical conductivity of the cooling water, a small portion of the normal stator cooling water flow (∼1–10%) is bypassed for rejuvenation through a mixed bed demineralization plant and returned to the original stream.

Another way, as a last resort, to solve this problem is to take the opportunity of the station shutdown and do a chemical cleaning of the hollow conductors using a suitable scale-removing solution. However, the cleaning operations are long-standing process resulting in outage of the machine vis-à-vis revenue loss as well as corrosion of metals that make up the hollow conductors.

The rotor is also subjected to carrying high field excitation current and heat generated due to high centrifugal force. For that reason, pressurized hydrogen is introduced into the generator housing; this enables cooling of both the stator core and rotor elements. It is taken out at a suitable point for reentry after heat release by an external cooling arrangement.

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