Cakerawala Generators ABB Generator driven by Taurus 60

The main driven system components are the Gear Unit and Generator which are connected together with a coupling. The Gear Unit is also designed to drive accessories and to accommodate the Starter Motor.

Reduction Drive Assembly

Since the required input speed of the generator is lower than the output speed of the turbine, a reduction drive assembly is necessary.

The reduction drive assembly is an epicyclic, high-speed, star-gear design used to reduce the drive speed from the turbine engine to the generator. The reduction drive assembly is designed for an output speed of 1500 rpm for 50 Hz service.

The reduction drive assembly is located between the engine and the generator. It is bolted directly to the air inlet housing and the oil tank to provide a rigid support. For this reason, the reduction drive assembly does not require alignment with the engine. The firm attachment of the housing provides support to the forward end of the turbine engine.

The reduction drive assembly case consists of a large housing, attached to the air inlet housing, and a smaller output shaft case.

Mounted on the reduction drive assembly is a magnetic pickup which counts the speed of the gear teeth and transmits a signal to the speed monitor control box in the control panel. Another magnetic pickup device directs a signal to the governor to control turbine speed.

The reduction drive assembly gear train is a compound star arrangement with three equally spaced star clusters. The power flows through the input pinion (sun gear) (5) into three first-stage star gears (6), through three second-stage pinion gears (7), and to the second-stage ring gear (4) on the output shaft (9).

The input pinion assembly (5) is supported at one end by a ramp bearing mounted on an adapter on the gear carrier. The other end is supported by the three first-stage star gears (6). Pinion thrust loads are taken by a tapered land thrust bearing. The gear clusters have two sleeve bearings mounted inside their bores. The journal bearing is stationary and is mounted in the carrier to support the gear clusters.

The second-stage ring gear (4) is mounted on a hub with a loose fitting spline, which allows the ring to center itself on the output shaft (9) through a fixed spline.

A sprag-type one-way clutch is mounted on the starter gear shaft. The starter drives through the clutch. When the starter disengages, the sprags lift off the shaft and the clutch overruns continuously.

Each of the first-stage meshes in the power train is cooled and lubricated by three sets of two oil jets directed toward the sun gear between each pair of meshes. Each pinion in the second-stage is cooled and lubricated by two jets on the inboard side. Centrifugal force drives this oil into the ring gear teeth. It is then flung out at the open end of the ring and through holes at the inner end.

Additional oil jets cool and lubricate the accessory pinion gear mesh, the output ball bearing, and the one-way clutch on the starter shaft. All other accessory gear meshes and bearings are lubricated by air-oil mist generated in the housing by the high speed meshes.

The hydrodynamic ramp bearing and thrust bearing on the input pinion assembly and the sleeve bearings on the countershafts are pressure-fed with oil. Pressurized oil is provided by the externally mounted main lube oil pump.

Accessory Drive Assembly

In addition to reducing the speed of the output shaft (Reduction Drive) the gear unit also incorporates the gearing to accommodate Accessory Drives. The only engine-driven accessory used is the Lube Oil Pump (P901) which is mounted on a Drive Pad. The other Drive Pads are not used and are

‘blanked’ by metal plates.

The Starter Motor (M922) is mounted onto a Starter Drive Adaptor which is then bolted to the Starter Drive Pad.

Both the Starter Motor and Lube Oil Pump engage with Spur Gears and then with another spur gear which is attached via splines to the output shaft. The Starter Motor can drive through this gear and the Lube Oil Pump can be driven by it.

Main Lube Oil Pump

Reduction/Accessory

Drive Assembly Generator

Starter Motor

Generator

Introduction

Electricity generation was first developed in the 1800's using Faradays dynamo generator. Almost 200 years later we are still using the same basic principles to generate electricity, only on a much larger scale.

Electricity can be made or generated by moving a wire (conductor) through a magnetic field.

Magnetism

A bar magnet has a north and south pole. If it is placed under a sheet of paper and iron filings are sprinkled over the top of the paper, these iron filings will arrange themselves into a pattern of lines that link the north pole with the south pole of the magnet (diagram 1). These lines show the magnetic field around the magnet.

Diagram 1 Making electricity

If a coil of wire is moved within a magnetic field so that it passes through the magnetic field, electrons in the wire are made to move (as in diagram 2). When the

coil of wire is connected into an electric circuit (at the terminals A and a) the electrons are under pressure to move in a certain direction and a current will flow. This electrical pressure is called voltage.

The amount of pressure or voltage depends on the strength and position of the magnetic field relative to the coil, as well as the speed at which the coil is turning. As the amount of

electricity changes so does its voltage. Diagram 2

Diagram 3.1

Diagram 3.2

Diagram 3.3

Diagram 4

In the diagrams above, the coil of wire is rotating in a clockwise direction. When the coil of wire is in the horizontal position (3.3), the voltage is greatest (diagram 4) because the coil is passing through the strongest part of the magnetic field. At this stage the current flows from 1 to 2 to 3 to 4, out through terminal A, through the globe and back into terminal a. When the coil of wire is in the vertical position (3.2), no electricity is produced because the coil does not cut the magnetic field, and no current flows. When the coil of wire is in the horizontal position again (3.3), the voltage is at its maximum (diagram 3.3), however the current flows in the opposite direction 4 to 3 to 2 to 1, out through terminal a, through the globe, and back into terminal A.The current produced changes direction every half turn (180 degrees). This is called alternating current or AC. The generators at large power stations produce nearly all the electricity we use in this way.

Power Stations Generators

With large power station generators, the coils actually remain stationary, and the magnetic field rotates. This still produces the same effect as described above. The magnet rotates as the turbine to which it is attached rotates.

When only one of the coils of wire is connected in the stationary part of the generator (known as the stator), the electricity circuit is said to be one phase (or one circuit). Diagram 5.1 shows what a rectangular coil or winding may look like. It is mounted inside the stator as in diagram 5.2 and has terminals A and a. When the magnet rotates the voltage is produced as shown in diagram 5.3.

Diagram 5.1

Diagram 5.2

Diagram 5.3

Diagram 5.4

Diagram 5.5

It is more cost efficient and technically better to connect three sets of coils in the stator.

Diagram 5.4 shows how these coils are mounted. Each of these coils will be connected as separate electrical circuits. When the magnet rotates an identical voltage is produced in each coil and circuit, but each is staggered or delayed from one another (diagram 5.5). The electricity circuit is said to be three phase.

Relative Motion between conductor and magnetic field (Condition 3 above), will result in a similar effect if the conductor is stationary and the magnetic field ‘moves’.

In practice the generator output voltage and current will be high and require good connections between the ‘conductors’ and the load circuit cables to minimise resistance and losses and therefore the generation of heat. These results in the conductors being stationary and the magnetic field being rotated in practical generators. The following diagrams will examine the construction and operating principles of a typical generator.

The turbine is called the prime mover and the shaft, magnet and copper windings make up the alternator.

An alternator consists of a rotor and a stator. The rotor is directly connected to the prime mover and rotates as the prime mover turns.

The rotor contains a magnet that, when turned, produces a moving or rotating magnetic field. The rotor is surrounded by a stationary casing called the stator, which contains the wound copper coils or windings. When the moving magnetic field passes by these windings, electricity is produced in them.

By controlling the speed at which the rotor is turned, a steady flow of electricity is produced in the windings. These windings are connected to the electricity network.

The structure of the alternator usually stays the same regardless of the type of energy being used to produce electricity.

The prime mover can be a turbine driven by steam, water, wind or burning gases. The prime mover can also be an engine (like a car engine) that uses fuel to turn the generator.

Generator Output Cables (3Ф 50 Hz) Exciter Field (DC)

Permanent Magnet Generator (AC 250-300Hz) Other control wiring (Metering, Temperature Sense) Typical Generator arrangement

The diagram above shows that the generator rotor shaft is supported by bearings at the Drive and Non-Drive ends and is rotated by the torque transmitted from the engine through the gear unit and coupling. Attached to the rotor shaft is a fan and the rotor which is located within the stator casing so that it aligns with the stator windings. Also attached to the rotor shaft are the rectifier, exciter armature and the PMG (Permanent Magnet Generator). There will be interconnecting conductors between the rotor, rectifier and the exciter armature, shown above as .

The following descriptions have been taken from the Installation and Maintenance Manual.

Functional Discription

During generator set operation, the three-phase ac power generated in the exciter armature is applied to the rectifier where it is converted to direct current power. The dc output from the rotating rectifier is then applied as field excitation current to the generator rotating field coils. It should be noted that, with this arrangement, the main generator field coils rotate and its armature is stationary, while the exciter field is stationary but its armature rotates with the main generator rotor shaft. As a result, a single rotating assembly, consisting of exciter armature, exciter rectifier, and main generator field coils is formed, greatly simplifying all electrical connections within the generator assembly.

A sensing transformer supplies the bus potential signal to the regulator. The main generator output is controlled by the generator field current. The generator field current is in turn controlled by the brushless exciter circuit. The power transformer through the regulator, furnishes the excitation to the exciter field. Variations in bus potential, then, will be sensed and subsequently corrected by this

All ac generators require that direct current (excitation) be applied to the rotor windings (field coils) in order to set up the magnetic flux necessary for generator operation. Because the amount of dc current going into the field of the exciter will determine the output voltage of the exciter, the exciter output, being applied to the generator field, will therefore control the output voltage of the main generator.

Upon proper voltage buildup, the generator accelerates to 100 percent speed and excitation and voltage control are assumed by the voltage regulator.

A crosscurrent-compensating transformer provides the proper signals to the regulator to accommodate reactive loadsharing between multiple units in parallel.

COMPONENT DESCRIPTIONS and around the rotor for cooling. The rotors have layer-wound field windings, which are then cemented with a high-strength resin and baked.

The rotor is in electrical and mechanical balance at all speeds, up to 125 percent of rated speed.

Stator

The stator is built with high-grade silicon steel laminations, which are precision punched and individually insulated. Windings, form-wound in lined slots, are repeatedly treated with

thermosetting synthetic varnish and baked for maximum moisture resistance, high dielectric strength, and high bonding qualities. The windings are also braced to withstand shock loads such as motor starting and short circuits. Space heaters can be supplied to minimize condensation during shutdowns.