generating equipments
TRANSCRIPT
DIRECT CURRENT MACHINES
TWO-WIRE GENERATOR- have only two line terminals, one known as the positive terminal and the other as the negative terminal.
THREE-WIRE GENERATOR- a direct-current generator with both slip rings and a commutator used for supplying current to a direct-current three-wire system whose neutral wire is connected to the center point of a high-reactance winding connected across the slip rings and whose two main conductors are connected to the commutator brushes
MOTOR-GENERATOR SET-A motor generator set is a device used for the conversion of electrical energy, typically in the form of frequency or voltage, into another form of energy.
A motor-generator (an M-G set or a dynamotor for dynamo-motor) is a device for converting electrical power to another form. Motor-generator sets are used to convert frequency, voltage, or phase of power. They may also be used to isolate electrical loads from the electrical power supply line.
In the context of electric power generation and large fixed electrical power systems, a motor-generator consists of an electric motor mechanically coupled to an electric generator (or alternator).
ROTARY CONVERTER-A rotary converter is a type of electrical machine which acts as a mechanical rectifier or inverter. It was used to convert AC to DC or DC to AC power before the advent of chemical or solid state power rectification. They were commonly used to provide DC power for commercial, industrial and railway electrification from an AC power source.
MERCURY-ARC RECTIFIER- A mercury-arc valve or mercury-vapor rectifier or (UK) mercury-arc rectifier is a type of electrical rectifier used for converting high-voltage or high-current alternating current(AC) into direct current (DC). It is a type of hot cathode gas discharge tube but is unusual in that the cathode, instead of being solid, is made from liquid mercury and is therefore self-restoring. As a result, mercury-arc valves were much more rugged, long-lasting and capable of carrying much higher currents than most other types of gas discharge tube.
Mercury-arc rectifiers were used to provide power for industrial motors, electric railways, streetcars, and electric locomotives, as well as for radio transmitters and for high-voltage direct current power transmission. They were the primary method of rectification before the advent of semiconductor rectifiers such as diodes, thyristors and gate turn-off thyristors(GTOs).
DC Generator Classifications
Method of Excitation:› Self-excited
SERIESSHUNTCOMPOUND
1. SHORT SHUNT AND LONG SHUNT2. FLAT COMPOUNDED, OVERCOMPOUNDED AND DIFFERENTIALLY COMPOUNDED
Types of Compound Generator
Long shunt - the series field coils are connected in series with the armature coils while the shunt field coils are connected across the series combination.
Short Shunt - the series field coils are connected in series with the load while the shunt field coils are connected across armature coils
Flat-compound – if the series field amp-turns are such as to produce the same voltage at rated load as at no-load.
Over-compound – if the series field amp-turns are such that the rated-load voltage is greater than the no load voltage.
Differentially compound-has an external characteristic similar to that of a shunt generator but with large demagnetization armature reaction. Hence, it is widely used in arc welding where larger voltage drop is desirable with increase in current.
Number of Poles› Bipolar- two poles, one north and one
south› Multipolar- must always be equal to a
multiple of two because it was found necessary to use more than 2 poles; hence, today all DC generators have at least 4 poles.
› Homopolar- designed w/ the armature conductors revolving in a unidirectional field, thusthe emfs generated in them do not alternate during the revolution of the armature;hence, no commutator is needed.
Types of Armature Winding› Multiple or Lap
- contains as many parallel paths as the generator has many poles. -requires as many brush studs as the
generator has many poles. › Series, two circuit or wave
- contains only two parallel paths irrespective of the number of main poles.- requires only two brush studs irrespective of the number of main poles.
Combination of Multiple and Series- also known as “frog-leg winding”-consists essentially of a standard multiple winding and a standard series winding placed together in the same armature slots and connected to the same commutator.
Method of Prime-Mover Drive› Engine Type- has its armature mounted
on a continuation of the engine crankshaft.
› Direct connected to steam turbine- often used as spare exciters in steam power plants.
› Geared to steam turbines› Water Turbines- found in hydraulic
power plants where spare exciter units are direct- connected to water wheels.
Type of Service› General Power and Lighting- are generally
wound for 125 and 250 volts. The voltage regulation should not be more than 2%.
› Railway-have compound-wound fields and are designed for 550 or 600 volts for city railway service, 1200 volts for interurban service, and 3000 volts for trunk-line electrification.
› Electrolytic Works- usually low-voltage machines of large current capacity. Often separately excited from a source higher voltage.
› Electric Furnace Work- shunt or differentially compound excitation is desirable, since such generators will not maintain large current on short circuit.
› Boosters- are DC generators connected in series with the line to raise or lower the voltage. They are generally low voltage machines of large current capacity and are usually driven at constant speed by shunt motors. They may be shunt, series, or compound-wound.
› Exciters- generally designed for 125 to 250 volts normal. They may be either shunt or compound-wound.
› Balancer- consists of two similar DC machines directly coupled to each other and connected in series across the outer conductors of a 3-wire system of distribution. Can be either shunt or compound wound.
Special Feature of Construction› Noncommutating-pole Machines› Commutating-pole Machines› Compensating Wound Machines-
are essential on rolling-mill motors and generators subjected to sudden changes of load, as, for example, in railway service.
› Diverter-pole Machines
Diverter Pole Generators
Compound wound direct-current generator with the series winding of the diverter pole opposing the flux generated by the shunt wound main pole; provides a close voltage regulation.
Voltage Regulation and LossesBy voltage regulation of a generator is meant the change in its terminal voltage with the change in
load current when it is running at a constant speed. If the change in voltage between no-load and full load is small, then the generator is said
to have good regulation but if the change in voltage is large, then it has poor regulation.
The voltage regulation of a d.c. generator is the change in voltage when the load is
reduced from rated value to zero, expressed as percentage of the rated load
voltage.
Stray Losses› Mechanical
Windage Bearing Friction Brush Friction Hysteresis Loss Eddy- current Loss
Iron
Copper Losses› Shunt-Field Copper Loss› Series-Field Copper Loss› Armature Copper Loss› Commutating-Field Copper Loss Load Losses
Alternating Current (AC) Generator- A machine that converts mechanical power into alternating-current electric power.
Types of AC Generator Synchronous AC Generator-used
because they offer precise control of voltage, frequency, VARs and WATTs. This control is achieved through the use of voltageregulators and governors. A synchronous machine consists of a stationary armature winding (stator) with many wires connected in series or parallel to obtain the desired terminal voltage.
INDUCTION GENERATOR-The induction generator is nothing more than an induction motor driven above its synchronous speed by an amount not exceeding the full load slip the unit would have as a motor.
PHASE CONVERTER-A phase converter is a device that produces three-phase electrical power from a single-phase source, thus allowing the operation of three-phase equipment at a site that only has single-phase electrical service.
FREQUENCY-CHANGER SET--A frequency changer or frequency converter is an electronic device that converts alternating current (AC) of one frequency to alternating current of another frequency. The device may also change the voltage, but if it does, that is incidental to its principal purpose.
Classifications of an Synchronous AC Generator
Frequency› 60 cycles per second› 25 cycles per second-railway service.
Number of Phases Single phase Two Phase Three Phase
Wye connected Delta connected
Reasons why all generators are three phase
More capacity can be obtained out of a given armature than when wound single or two phase.
It is more economical to transmit power over a 3-wire 3-phase circuit than over a 4-wire 2-phase circuit.
Relative Motion› Revolving Armature Type› Revolving Field Type
Type of Prime Mover› Engine-Type Alternators› Steam Turbine Type› Water-wheel Type Alternators
› Vertical› Horizontal
Main reasons why revolving field has entirely superseded the revolving armature of construction in the case of alternators
The armature end windings can be more satisfactorily braced in the case of a stationary armature.
With the terminal voltages obtained from present-day alternators, it is entirely out of the question to use rubbing contacts to collect the current from the armature.
Cheaper construction
Engine-Type Alternators Designed for
operation w/ reciprocating steam engines and internal combustion engines, which may be either gas or oil engines.
Water-wheel Type Alternator Units are
driven by water turbines, which may be of the impulse, reaction, or propeller type.
Steam-Turbine Type Alternator
Such generators are designed for operation with steam turbines.
In the early development of such units vertical mounting was considered feasible, but it was soon realized that the horizontal method of mounting was superior in many details.
Important Features of Turboalternators
Armature Yoke Armature Core-forms the part of the magnetic
circuit in the stator. Armature-Coil Supports-use to withstand the
large stresses produced under load, short-circuit conditions.
Rotor Field Windings- generally composed of strap
copper laid flatwise in the rotor slots. This bring pressure to centrifugal force on the flat side of the copper; hence there is no possibility of the insulation between turns becoming chafed or cut.