AC Generators Design and Assembly

A typical AC generator consists of a stationary stator and a rotor mounted within the stator (see below: Typical AC Generator). The stator contains a specific number of coils, each with a specific number of windings. Similarly, the rotor consists of a specific number of field poles, each with a specific number of windings. In addition to the rotor and stator, a generator has a collector assembly (usually consisting of collector slip rings, brushes, and brush holders). DC flows from the exciter, through the negative brush and slip ring, to the rotor field poles. The return path to the exciter is through the positive brush and slip ring.

Rotor - The rotor contains magnetic fields which are established and fed by the exciter. When the rotor is rotated, AC is induced in the stator. The changing polarity of the rotor produces the alternating characteristics of the current. The generated voltage is proportional to the strength of the magnetic field, the number of coils (and number of windings of each coil), and the speed at which the rotor turns.

Stator - The frame assembly is the main component of the stator. Insulated windings (or coils) are placed in slots near an air gap in the stator core. There is a fixed relationship between the unit’s number of phases and the way the coils are connected. The stator in a four-wire, three-phase unit has three sets of armature coils which are spaced 120 electrical degrees apart. One end of each coil is connected to a common neutral terminal. The other end of each coil is connected to separate terminals. Conductors attached to the four terminals carry the current to the system’s switchgear and on to the load.

Collector slip rings - Slip rings are usually made of nonferrous metal (brass, bronze or copper); iron or steel is sometimes used. Slip rings usually do not require much servicing. The wearing of grooves or ridges in the slip rings is retarded by designing the machine with limited end-play and by staggering the brushes. Surfaces of the slip rings should be bright and smooth, polishing can be performed with fine sandpaper and honing stone. Electrolytic action can occur at slip ring surfaces producing formation of verdigris. Verdigris is a greenish coating that forms on nonferrous metals. Electrolytic deterioration can be prevented by reversing the polarity of the slip rings once or twice a year. The stator of the three-wire, three-phase unit also has three sets of armature coils spaced 120 electrical degrees apart. The ends of the coils are connected together in a delta configuration. Conductors are attached to the three connecting points.

References: “Joint Departments of the Army and the Navy, Operation Maintenance and Repair of Auxiliary Generators, 26 August 1996”

Working Procedure of an AC Generator

Operation of power generators is based on the Electromagnetic Induction. whenever a conductor moves relative to magnetic field, voltage is induced in the conductor. If a coil is spinning in a magnetic field, then the two sides of the coil move in opposite directions, and the voltages induced in each side add. The instantaneous value of the resulting voltage (called electromotive force, emf) is equal to the minus of the rate of change of magnetic flux Φ times the number of turns in the coil: V=−N•∆Φ/Δt. This relationship has been found experimentally and is referred to as Faraday's law. The minus sign here is due to Lenz law, which states that the direction of the emf is such that the magnetic field from the induced current opposes the change in the flux which produces this emf. Lenz law is connected to the conservation of energy.

.Since the rate of magnetic flux change through the coil that spins at a constant rate changes sinusoid ally with the rotation, the voltage generated at the coil terminals is also sinusoidal (AC). If an external circuit is connected to the coil's terminals, this voltage will create current through this circuit, resulting in energy being delivered to the load. Thus, the mechanical energy that rotates the coil is converted into electrical energy. Note that the load current in turn creates a magnetic field that opposes the change in the flux of the coil, so the coil opposes the motion. The higher current, the larger force must be applied to the armature to keep it from slowing down. In the animation the coil is rotated by the hand crank. In practice, the mechanical energy is produced by turbines or engines called prime movers.

The production of voltage depends only on the Relative Motion between the coil and the magnetic field. Voltage is induced by the same physics law whether the magnetic field moves past a stationary coil, or the coil moves through a stationary magnetic field. In the animation, the magnetic field is produced by a stationary magnet while the coil is revolving. In AC generators, usually the field is spinning and the power-producing armature is stationary. This armature comprises of a set of coils that form a cylinder. Also, in practice, the magnetic field is usually induced by an electromagnet rather then a permanent magnet.

The electromagnet consists of so called field coils mounted on an iron core. A current flow in the field coils produces the magnetic field. This current may be obtained from an external source or from the system's own armature. Regulation is achieved by sensing the output voltage, converting it to a DC, and comparing its level to a reference voltage. An error is used to control the field in order to maintain a constant output. Most modern AC sources with field coils are Self-Excited: the current for field coils is supplied by an additional exciting winding in the armature.

Wave Winding

In the lap winding, the two ends of a coil are connected to adjacent commutator segments. In the wave winding, the two ends of a coil are connected to the commutator segments that are approximately 360 electrical degrees apart (i.e., 2-pole pitch) and coil span = pole pitch. The result is that the coils under consecutive pole pairs will be joined together in series thereby adding together their e.m.f.s.This way all the coils carrying current in the same direction are connected in series. Therefore, there are only two parallel paths between the brushes, i.e., a=2 ,  independent of the number of poles. This type of winding is used for low-current, high-voltage applications.

Wave winding design procedure :

1. Both pitches YB and YF are odd and of the same sign.
2. Back and front pitches are nearly equal to the pole pitch and may be equal or differ by 2, in which case, they are respectively one more or one less than the average pitch.
3. Resultant pitch YR = YF + YB.
4. Commutator pitch, YC = YA (in lap winding YC = ±1 ). Also YC = (No.of commutator bars ± 1 ) / No.of pair of poles.
5. The average pitch which must be an integer is given by YA = (Z ± 2)/P = (No.of commutator bars ± 1)/No.of pair of poles.
6. The number of coils i.e NC can be found from the relation NC = (PYA ± 2)/2.
7. It is obvious from 5 that for a wave winding, the number of armature conductors with 2 either added or subtracted must be a multiple of the number of poles of the generator.This restriction eliminates many even numbers which are unsuitable for this winding.
8. The number of armature parallel paths = 2m where 'm' is the multiplicity of the winding.

Lap Winding

The windings are connected to provide several parallel paths for current in the armature. For this reason, lap-wound armatures used in dc generators require several pairs of poles and brushes.the finishing end of one coil is connected to a commutator segment and to the starting end of the adjacent coil situated under the same pole.

Following points are consider to design a lap winding 

1. The back and front pitches are odd and of opposite sign.But can't be equal. They differ by 2 or some multiple thereof.
2. Both YB and YF shpuld be nearly equal to the pole pitch.
3. The average pitch YA = (YB + YF)/2.It equals pole pitch = Z/P.
4. Commutator pitch YC = ±1.
5. Resultant pitch YR is even, being the arithmetical difference of two odd numbers i.e YR = YB - YF.
6. The number of slots for a 2-layer winding is equal to the number of coils.The number of commutator segments is also the same.
7. The number of parallel paths in the armature = mP where 'm' is the multiplicity of the winding and 'P' the number of poles.Taking the first condition, we have YB = YF ± 2m where m=1 fo simplex lap and m =2 for duplex winding etc.

• If YB > YF i.e YB = YF + 2, then we get a progressive or right-handed winding i.e a winding which progresses in the clockwise direction as seen from the comutator end.In this case YC = +1.
• If YB < size="1">F i.e YB = YF - 2,then we get a retrogressive or left-handed winding i.e one which advances in the anti-clockwise direction when seen from the commutator side.In this case YC = -1.