It was shown in Unit 1 that an alternating voltage having a sine-wave pattern is induced in a single conductor, or armature coil, rotating in a uniform magnetic field with station- ary field poles. Similarly, an emf is generated in stationary armature conductors when the field poles rotate past the conductors. A voltage will be induced in the armature conductors whenever there is relative motion between the armature conductors and the field.
DC generators have stationary field poles and rotating armature conductors. The alter- nating voltage induced in the armature conductors is changed to a direct voltage at the brushes by means of the commutator.
AC generators are also known as alternators because they supply electrical energy with an alternating voltage. These machines do not have commutators. Therefore, the armature is not required to be the rotating member.
Alternators are classified into two groups, depending on the type of construction. One group consists of the revolving armature type of ac generator. This machine has stationary field poles and a revolving armature. The second group consists of the revolving field type of ac generator, which has a stationary armature, or stator. The field poles rotate inside the stator for this type of ac generator.
REVOLVING ARMATURE-TYPE ALTERNATOR
Figure 12–1 shows a revolving armature-type alternator. The kilovolt-ampere capacity and the low-voltage rating of such an alternator are usually rather small. This
machine resembles a dc generator but has slip rings rather than a commutator. An ac generator cannot supply its own field current. Thus, the field excitation is direct current and is supplied from an external direct-current source.
REVOLVING FIELD-TYPE ALTERNATOR
Revolving field alternators are used for most applications. In this type of machine, the revolving field structure (rotor) uses slip rings and brushes to take the excitation current from an external dc source. The stationary part of the generator (stator) is a laminated core. This core consists of thin steel punchings, or laminations, securely clamped together and held in place in the steel frame of the generator. The armature coils are placed in slots in the stator. The field voltage is usually in the range between 100 and 250 V. The amount of power delivered to the field circuit is relatively small.
The rotating field alternator has two advantages over the rotating armature ac generator:
1. Voltages can be generated as high as 11,000 and 13,800 V. These values can be reached because the stationary armature windings do not undergo vibration and centrifugal stresses.
2. Alternators can have relatively high current ratings. Such ratings are possible because the output of the alternator is taken directly from the stator windings through heavy, well-insulated cables to the external circuit. Neither slip rings nor a commutator are used.
The Revolving Field
Two different types of revolving field structures are commonly used. The first type to be described is the salient pole rotor. The second type of structure is known as the cylindrical rotor. The salient pole rotor has projecting field poles. It is used with alternators operating at speeds below 1800 r/min. Prime movers for such slow-speed alternators include diesel units and waterwheels. (The prime mover supplies the mechanical energy input to the generator.)
Figure 12–2 shows a salient pole rotor, which is used on a slow-speed alternator. Each pole has a laminated steel core to reduce the eddy current losses. The field coil windings are mounted on the laminated poles. The windings are connected in series to give alternate north and south polarities. The field poles are magnetized by low-voltage dc taken from an external source by two slip rings. Each field pole is bolted to the fabricated steel spider. In some cases, poles are dovetailed to the spider. This type of construction is used as a safeguard against the centrifugal force of the rotating members causing the structure to fly apart. The spider is keyed to the generator shaft. The salient pole rotor in Figure 12–2 has slots on each of the pole faces. A damper winding, also known as an amortisseur winding, is placed in each of these slots. The purpose of this winding will be described later.
Large steam turbine-driven alternators normally operate at speeds of 1800 r/min and 3600 r/min. At these speeds, large salient pole rotors are impractical. As a result, most steam turbine-driven alternators have cylindrical rotors. If a steam turbine-driven alternator is rated at less than 5000 kW, then frequently this machine is a 1200-r/min salient pole type. A speed reduction gear is used with this machine, as well as 5000- or 6000-r/min turbines. The higher-speed turbine is more efficient than a 3600-r/min turbine. The entire unit costs less and is more efficient than a direct-driven, 3600-r/min motor–generator set using a cylindrical rotor generator.
The field coils of cylindrical rotors are embedded in slots. They are not wound on protruding or salient poles. Slip rings conduct the low-voltage dc excitation current to the revolving field circuit. Generally, the exciter is mounted directly on the generator shaft. A cylindrical rotor is shown in Figure 12–3.
The separately excited field of an alternator can be disconnected from the dc supply by a two-pole switch. As the switch is closed, a momentary voltage is induced in the field windings. This voltage arises because the collapsing lines of flux cut the turns of the field windings. The induced voltage is large enough to damage equipment. To eliminate this voltage, a special field discharge switch is used.
Figures 12–4 and 12–5 show the connections for the field circuit of a separately excited alternator. In the closed position, the field discharge switch acts like a normal dou- ble-pole, single-throw switch. When the field discharge switch is in the closed position, an auxiliary switch blade is in an open position.
As the switch is opened, the auxiliary blade closes just before the main switch blades open. When the main switch blades are fully open, a circuit path still exists through the auxiliary switch blade. This path goes through the field discharge resistor and bypasses the field rheostat and the ammeter. As a result, the field discharge resistor is connected directly across the field windings.
The voltage induced in the field coils by the collapsing magnetic field dissipates quickly as a current through the field discharge resistor. This arrangement eliminates any danger to persons opening the circuit using a two-pole switch. In addition, the insulation of the field windings is protected from damage. All types of alternators use such a field circuit or one that is very similar. A larger machine may use a field contactor or field circuit breaker for the same purpose. Each of these devices will have two normally open main poles and one overlapping normally closed discharge pole.
The Brushless Exciter
Most large alternators use an exciter that contains no brushes. This is accomplished by adding a separate three-phase armature winding to the shaft of the large alternator rotor. The brushless exciter armature rotates between stationary wound electromagnets, as shown in Figure 12–6.
The dc excitation current is connected to the wound stationary magnets. The amount of voltage induced into the brushless exciter armature can be controlled by the amount of dc excitation current applied to the electromagnets. The output of the three-phase armature winding is connected to a three-phase bridge rectifier mounted on the same rotating shaft, as shown in Figure 12–7.
The rectifier converts the three-phase ac produced in the armature winding into direct current before it is applied to the main rotor windings. Because the brushless exciter armature winding, fuses, rectifier, and main rotor windings are all mounted on the same shaft, they rotate together, eliminating the need for brushes or sliprings. A rotor with a brushless exciter winding is shown in Figure 12–8.