Electrical Machines - Generators

Electrical Machines - Generators

The primary supply of all the world's electrical energy is generated in three phase synchronous generators using machines with power ratings up to 1500 MW or more. Though the variety of electric generators is not as great as the wide variety of electric motors available, they obey similar design rules and most of the operating principles used in the various classes of electric motors are also applicable to electric generators. The vast majority of generators are AC machines (Alternators) with a smaller number of DC generators (Dynamos).

Voltage and Frequency Regulation

Most generator applications require some way controlling the output voltage and in the case of AC machines a method of controlling the frequency. Voltage and frequency regulation is normally accomplished in very large machines carrying very high currents, by controlling the generator excitation and the speed of the prime mover which drives the generator.

·         Stand Alone (Island) Systems

In smaller, stand alone systems particularly those designed to capture energy from intermittent energy flows such as wind and wave power the voltage and frequency control may be carried out electronically. In principle these control systems are similar to Motor Controls and the the various components are outlined in that section.

·         Grid Connected Systems

In grid connected systems the generator voltage and frequency are locked to the grid system. Changing the energy output from the prime mover does not affect the frequency and voltage but will cause the output current to increase resulting in an equivalent change in the generator output power. When connecting a generator to the grid, it's speed should be run up so that it's output frequency matches the grid frequency before the connection is made. See more details about, and examples of voltage and frequency regulation of grid connected systems on the Wind Power page.

Generator Power Handling

The mechanical shaft power P in Watts applied to a generator is given by:

P  =  ωT

Where ω is the speed in radians per second and T is the torque in Newton metres.

As with electric motors, the maximum power handling capability of the generator is determined by its maximum permissible temperature.

Generator Load

Voltage and frequency regulation correct for minor deviations in the generator output as noted above but large changes in the load demand (current) can only be accommodated by adjusting the torque of the prime mover driving the generator since generally, in electric machines, torque is proportional to current or vice versa.

Generator Types

AC Generators (Alternators)

·         Stationary Field Synchronous AC Generator

In a stationary field generator, the stator in the form of fixed permanent magnets (or electromagnets fed by DC) provides the magnetic field and the current is generated in the rotor windings.

When the rotor coil is rotated at constant speed in the field between the stator poles the EMF generated in the coil will be approximately sinusoidal, the actual waveform being dependent on the size and shape of the magnetic poles. The peak voltage occurs when the moving conductor is passing the centre line of the magnetic pole. It diminishes to zero when the conductor is in the space between the poles and it increases to a peak in the opposite direction as the conductor approaches the centre line of the opposite pole of the magnet. The frequency of the waveform is directly proportional to the speed of rotation. The magnitude of the wave is also proportional to the speed until the magnetic circuit saturates when rate of voltage increase, as the speed increases, slows dramatically.

§  Generator Speed and Frequency

The output frequency is proportional to the number of poles per phase and the rotor speed in the same way as a synchronous motor. 

The alternating current output generated in the rotor can be connected to external circuits via slip rings and does not need a commutator.

Typical applications are portable AC generators with output power up to 5 kilowatts.

Small low cost applications such as domestic wind turbine generators are usually designed to run at high speed. For a given power handling requirement, the higher the speed, the lower the required torque. This means that the generator can be smaller and lighter. Furthermore, the high speed generator needs fewer poles, simplifying the design and reducing the costs.

·         Rotating Field Synchronous AC Generator

The power handling capacity of a brushed machine is usually constrained by the current handling capability of the slip rings in an AC machine (or even more by the commutator in a DC machine). Since the generator load current is generally much higher than the field current, it is usually desirable to use the rotor to create the field and to take the power off the generator from the stator to minimise the load on the slip rings.

By interchanging the fixed and moving elements in the above example a rotating field generator is created in which the EMF is instead generated in the stator windings. In this case, in its simplest form, the field is provided by a permanent magnet (or electromagnet) which is rotated within a fixed wire loop or coil in the stator. The moving magnetic field due to the rotating magnet of the rotor will then cause a sinusoidal current to flow in the fixed stator coil as the field moves past the stator conductors. If the rotor field is provided by an electromagnet, it will need direct current excitation fed through slip rings. It does not need a commutator.

If instead of a single coil, three independent stator coils or windings , spaced 120 degrees apart around the periphery of the machine, are used, then the output of these windings will be three phase alternating current.

§  Series Wound Generator

Classified as a constant speed generator, they have poor voltage regulation and few are in use.

§  Shunt Wound Generator

Classified as a constant voltage generator, the output voltage can be controlled by varying the field current. They have reasonably good voltage regulation over the speed range of the machine. 

§  Brushless Excitation

Rotating field machines are used for the high power generating plant in most of the world's national electricity grid systems. The field excitation power needed for these huge machines can be as much as 2.5% of the output power ( 25 KW in a 1.0 MW generator) though this reduces as the efficiency improves with size so that a 500 MW generator needs 2.5 MW (0.5%) of excitation power. If the field voltage is 1000 Volts, the required field current will be 2500 Amps. Providing such excitation through slip rings is an engineering challenge which has been overcome by generating the necessary power within the machine itself by means of a pilot, three phase, stationary field generator on the same shaft. The AC current generated in the pilot generator windings is rectified and fed directly to the rotor windings to supply the excitation for the main machine.

§  Cooling
The efficiency of a very large generator can be as high as 98% or 99% but for a 1000 MW generator, an efficiency loss of just 1% means 10 MegaWatts of losses must be dissipated, mostly in the form of heat. To avoid overheating, special cooling precautions must be taken and two forms of cooling are usually employed simultaneously. Cooling water is circulated through copper bars in the stator windings and hydrogen is passed through the generator casing. Hydrogen has the advantages that its density is only about 7% of the density of air resulting in fewer windage losses due to the rotor churning up the air in the machine and its thermal capacity is 10 times that of air giving it superior heat removal capability.

·         Permanent Magnet AC Generators

Smaller versions of both of the above machines can use permanent magnets to provide the machine's magnetic field and since no power is used in providing the field this means that the machines are simpler and more efficient . The drawback however is that there is no simple way to control such machines. Permanent magnet synchronous generators (PMSGs) are typically used in low cost "gensets" to provide emergency power.

The voltage and frequency output of the permanent magnet generator are proportional to the speed of rotation and though this may not be a problem for applications powered by fixed speed mechanical drives, many applications such as wind turbines, require a fixed voltage and frequency output but are powered by variable speed prime movers. In these cases, complex feedback control systems or external power conditioning may be required to provide the desired stabilised output.

Generally the output will be rectified and the varying output voltage fed through the DC link to a buck - boost regulator which provides a fixed voltage coupled with an inverter which provides a fixed frequency output.

·         Variable/Switched Reluctance Generators

Similar in construction to the switched reluctance motor, the generator is a doubly salient machine with no magnets or brushes. As the inert, iron rotor poles of the reluctance generator are driven past the stator poles, the changing reluctance of the generator's magnetic circuit is accompanied by a corresponding change in the inductance of the stator poles which in turn causes a current to be induced in the stator windings. A pulsed waveform therefore appears at each stator pole. In polyphase machines the outputs from each phase are fed to a converter which switches each phase sequentially on to the DC Link to provide a DC voltage. The system needs position sensing on the rotor shaft to control the timing of the triggering of the converter switches. These position sensors also enable the current to be controlled by varying the turn on and turn off angles of the output current depending on the rotor position. As with the permanent magnet generator, buck - boost regulators are also used to provide control over the output.

The machine unfortunately is not inherently self exciting and various methods have been adopted to enable start up, including the provision of a DC excitation current from a backup battery through the stator windings during start up, or the use of small permanent magnets embedded in some of the rotor poles.

Characteristics

Compact, robust designs.

Variable speed operation.

The generator phases are completely independent.

Inexpensive to manufacture.

Because they have simple, inert rotors with no windings or embedded magnets they can be driven at very high speed and can operate in high ambient temperature conditions.

Suitable for designs up to megawatt capacity and speeds of more than 50,000 rpm.

§  Applications

Hybrid electric vehicle (HEV) drive systems, automotive starter generators, aircraft auxiliary power generation, wind generators, high speed gas turbine generators.

See also Integrated Starter Generator

·         Induction Generators

Induction generators are essentially induction motors which are run slightly above the synchronous speed associated with the supply frequency. See an explanation of how induction motors work on the AC Motors page. Induction generators however have no means of producing or generating voltage unless they are connected to an external source of excitation. The squirrel cage construction is used for small scale power generation because it is simple, robust and inexpensive to manufacture.

As with an induction motor, when the stator coils of a multi-phase induction generator are connected to an alternating current grid, by transformer action a voltage is induced into the rotor windings, or the conducting bars of a squirrel cage rotor, with the frequency of this induced voltage in the rotor being equal to the frequency of the applied stator voltage. When the individual rotor windings are short circuited, or connected together through an external impedance, (the conducting bars of the squirrel cage rotor are already short-circuited together), a large current flows through the coils creating a magnetic field, which by Lenz's Law has a polarity opposite to stator field. This causes the rotor to rotate, being dragged along by magnetic attraction behind the rotating field created by the stator. The magnitude of the torque on the rotor depends on the magnitude of the relative speed between the rotating rotor and the rotating field created by the stator, commonly called the slip. The rotor thus accelerates up towards the synchronous speed set by the frequency of the grid suppy reaching a maximum when the magnitude of the induced rotor current and torque balances the applied load, while at the same time, the frequency of the currents induced in the rotor windings are reduced, keeping in line with the slip frequency. But the faster the rotor rotates, the lower is the resulting relative speed difference between the rotor cage and the rotating stator field, or the slip, and thus the voltage induced into the rotor winding. As the rotor nears synchronous speed, its torque decreases in line with the slip reducing the acceleration as the weakening rotor magnetic field is insufficient to overcome the friction losses of the rotor in idle mode. The result is that the rotor remains rotating slower than synchronous speed. This means that in motor mode, an induction machine can never reach its synchronous speed because at that speed there would be no current induced into the rotors squirrel cage, no magnetic field and thus no torque.

In generator mode however, the stator is still connected to the grid providing the necessay rotating field, but the rotor shaft is driven by external means at a speed faster than the synchronous speed so that the electromagnetic reactions are reversed since the rotor will rotate faster than the rotating magnetic field of the stator so that the polarity of the slip is reversed and the polarity of the voltage and current induced in the rotor will likewise be reversed. At the same time, by transformer action, the current in the rotor will induce a current in the stator coils which now supply the generator's output energy to the load. As the rotor speed is increased above the synchronous speed, the induced voltage and the current in the rotor bars and the stator coils will increase as the relative speed between the rotor and the stator's rotating field and hence the slip increases. This in turn will require a higher torque to maintain the rotation.

The output voltage of the generator is controlled by the magnitude of the excitation current.

The following diagram illustrates the characteristics of a multi- phase induction machine when configured as either a motor or as a generator.

Since the rotor current is proportional to the relative motion between the stator's rotating field and the rotor speed, known as the "slip", the rotor current and hence the torque are both directly proportional to the slip within the stable operating region around the synchronous speed of the machine and the frequency of the rotor current is the same as the slip frequency.

At the synchronous speed the slip is zero, and no electricity would be consumed by the motor or produced by the generator. Though both machines operate at speeds within a few percent of the synchronous speed they are asynchronous machines.

Increasing the load on the generator reduces its speed and hence its output frequency, while increasing the torque on the drive shaft increases its speed and output frequency, Reducing the load and the driving torque have he opposite effect.

§  Fixed Speed Induction Generator

Fixed speed induction generators like the one described above actually run over a small speed range associated with the generator slip. They receive their excitation from the electricity supply grid and can only be run in parallel with that supply. When used on line, they are fine for returning power to the grid from which they derive their excitation current but useless as standby generators when the electric grid goes down. Their limited speed range restricts the possible applications.

§  Variable Speed - Self Excited Induction Generator(SEIG)

Small scale electricity generating systems are quite often stand alone applications, remote from the electricity supply grid, utilizing widely fluctuating energy sources such as wind and water power for their source of energy. The fixed speed induction generator is not suitable for such applications. Variable speed induction generators need some form of self excitation as well as power conditioning to be able to make practical use of their unregulated voltage and frequency output.

§  Operation

Self excitation is obtained by connecting capacitors across the stator terminals of the generator. When driven by an external prime mover, a small current will be induced in the stator coils as the flux due to the residual magnetism in the rotor cuts the windings and this current charges the capacitors. As the rotor turns, the flux cutting the stator windings will change to the opposite direction as the orientation of the remanent magnetic field turns with the rotor. The induced current in this case will be in the opposite direction and will tend to discharge the capacitors. At the same time the charge released from the capacitors will tend to reinforce the current increasing the flux in the machine. As the rotor continues to turn the induced EMF and current in the stator windings will continue to rise until steady state is attained, depending on the saturation of the magnetic circuit in the machine. At this operating point the voltage and current will continue to oscillate at a given peak value and frequency determined by the characteristics of the machine, the air gap , the slip, the load and the choice of capacitor sizes. The combination of these factors sets maximum and minimum limits on the speed range over which self excitation occurs. The operating slip is generally small and the variation of the frequency depends on the operating speed range.

If the generator is overloaded the voltage will collapse rapidly (see diagram above) providing a measure of built in self-protection.

§  Control

In variable-speed operation, an induction generator needs a frequency converter to adapt the variable frequency output of the generator to the fixed frequency of the application or the electricity supply grid. During operation the only controllable factor available in a self excited induction generator to influence the output is the mechanical input from the prime mover, so the system is not amenable for effective feedback control. To provide a controllable output voltage and frequency, external AC/DC/AC converters are required. A three-phase diode bridge is used to rectify the generator output current providing a DC link to a three-phase thyristor inverter which converts the power from the DC link to the required voltage and frequency.

See also examples and description of an asynchronous Doubly Fed Induction Generators (DFIG) and in- line frequency control of a fixed speed synchronous generator, both used to provide regulated frequency and voltage output from variable torque, variable speed drives in wind turbine generator applications.

DC Generators (Dynamos)

Direct Current (DC) Generator

The stationary field AC generator described above can be modified to deliver a unidirectional current by replacing the slip rings on the rotor shaft with a suitable commutator to reverse the connection to the coil each half cycle as the conductor passes alternate north and south magnetic poles. The current will however be a series of half sinusoidal pulses just like the waveform from a full wave rectifier as shown below.

The output voltage ripple can be minimised by using multipole designs.

The construction of a DC generator is very similar to the construction of a DC motor.

The rotor consists of an electromagnet providing the field excitation. Current to the rotor is derived from the stator or in the case of very large generators, from a separate exciter rotating on the same rotor shaft. The connection to the rotor is through a commutator so that the direction of the current in the stator windings changes direction as the rotor poles pass between alternate north and south stator poles. The rotor current is very low compared with the current in the stator windings and most of the heat is dissipated in the more massive stator structure.

In self excited machines, when starting from rest, the current to start the electromagnets working is derived from the small residual magnetism which exists in the electromagnets and surrounding magnetic circuit.

Automotive Alternators

The automotive generator is a variable speed AC machine delivering a fixed level DC output.

The typical generator is a self excited alternating current machine. By using an alternator rather than a DC generator the use of a commutator and its potential reliability problems can be avoided. However, direct current is required for all the loads in the vehicle including the battery and furthermore, the DC output voltage must be constant regardless of the engine speed or the current load. The charging system must therefore include a rectifier to convert the AC to DC and a regulator to maintain the generated voltage within design limits independent of the engine speed.

The rotor is driven by the engine and provides the field excitation. Its speed is directly related to the engine speed and depends on the ratios of the gearing or pulleys driving it. The output current is taken from the stator.

Automotive alternators are usually three phase machines to enable a compact design and at the same time a reduction in the current in the stator windings by spreading it between three sets of windings. This also gives a reduction in the potential voltage ripple after rectification.

·         Construction

The rotor is a claw pole rotor in which the two ends of the rotor form the north and south poles of an electromagnet. The "claws" extend between each other effectively producing alternate north and south poles as they pass the stator poles. The rotor current energising the electromagnet is fed from the stator windings via three auxiliary diodes which rectify it, before passing it through two slip rings to a single rotor coil.

The moving magnetic field associated with the rotor poles causes a current to flow in the stator windings as the field passes over the stator conductors.

The three phase current produced by the alternator is rectified in a full wave, diode bridge circuit to produce a DC output. The alternator EMF is directly proportional to the alternator (or engine) speed. The alternator is however designed to deliver full voltage, normally 14.2 Volts for a 12 Volt nominal lead acid battery, at idle speed and to maintain the output voltage constant at this level as the engine speed increases.

·         Voltage Regulator

To prevent the battery from being overcharged the DC output voltage must be kept below the 14.2 Volts maximum charging voltage specified for the battery. This is the function of the regulator which senses the alternator's output voltage and if it is greater than the 14.2 Volts reference voltage, provided by a Zener diode, it interrupts the current to the field (rotor) coil. Without a field current the alternator voltage begins to fall. When the alternator voltage falls below the reference voltage, current will be supplied to the field coil once more maintaining the output voltage at the desired level. The rotor thus receives a pulsed DC current over the engine operating speed range, smoothed somewhat by the rotor winding inductance.
Alternative designs monitor the load current on the alternator and provide a feedback mechanism using pulse width modulation to control the stator current to provide a constant output voltage regardless of the load.

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