Chapter 5Chapter 5

SYNCHRONOUS GENERATOR SYNCHRONOUS GENERATOR EXCITATIONEXCITATION

Reporter: CRISELLE O. LLEVAReporter: CRISELLE O. LLEVA

BSEE-5BBSEE-5B

EXCITER CAPACITY AND VOLTAGE

A. CAPACITY

-The induced voltage of an alternator is proportional to the product of the number of conductors in series upon the armature, the speed of rotation of the field, the number of the field poles, and the magnetic flux per field pole

GENERAL CONCLUSIONS

- The excitation required varies considerably for different machines, depending upon the size, the number of poles, the speed, and the regulation desired.

- For alternators of different capacities but otherwise similar, the relative excitation naturally decreases as the size of the alternator increases.

- In general, it may be said that small-capacity machines with many poles require a proportionally large excitation, and large-capacity machines with a few poles a proportionally small excitation.

- A small alternator of many poles may require as much as 3 to 5 percent exciter capacity, while a large machine of a few poles may require only as much as 0.3 percent exciter capacity.

B. VOLTAGE

-Exciters are built for either 125 or 250 volts, these two values being found best suited for all the exciter demands.

-In case of very large alternators it is desirable to use 250-volt exciters in order to decrease the current, and thereby use small copper sections in the entire field windings, in both the exciter armature and alternator field, as well as in the connecting circuits.

- For the exciters of the same capacity and speed, the 250-volt machine would require a commutator of only one-half the length, which is of exceeding importance in the case of high peripheral velocities.

SYSTEM OF EXCITATION

The most common and simplest system of excitation used in general practice involves a self-excited direct-current shunt generator.The alternator field current is controlled by varying the exciter voltage by means of a regulating rheostat in the shunt field of the exciter

For the larger machines it is desirable to use a separately-excited direct-current generator as the source of alternator excitation, the power supply for the field of the main exciter being supplied by a pilot exciter.

As before, the alternator field current is varied by rheostatic control of the main exciter

EXCITER CHARACTERISTICS

A. PILOT EXCITERS

- normally designed to operate at constant voltage independent of load, they are compound-wound direct-current machines with practically flat load voltage characteristics.

B. SEPARATELY EXCITED MAIN EXCITERS

The main advantage of a separately excited dc generator, over the standard self-excited type, is the better voltage stability of the former.Any voltage may be obtained over any range,depending on the degree of excitation.It is the feature which makes the separately excited exciter particularly desirable for large alternators.

The field circuit should be design with low inductance in order that the changes in excitation may take place quickly.

METHODS OF DRIVING EXCITERS

1. For high speeds and low speeds it may, therefore, become economical to use separate prime-mover drives for exciters.

2. In the case of steam plant the prime-mover may be a steam turbine.As a general rule, however, a steam turbine has too high a speed in the small sizes that are required for exciter drive;hence it is not used very widely except as a spare which may operate as an emergency.

3. In a hydraulic plant, the hydraulic design is generally too complicated and the cost too high to consider water-wheel-driven exciters; hence in such plants motor drive is generally used except for those plants using the direct-connected system.

AUTOMATIC VOLTAGE REGULATION

Good voltage regulation of power networks is probably the most important element in the normal process of delivering good service to the costumer.

Voltage Regulators may classified as;

1.Vibrating type regulator

- is one of the older types and still found in use, but it no longer holds first place.

- there seems be a definite trend away from it toward the rheostatic type.

2. Rheostatic Type Regulator

-may be classified as “direct acting” or “indirect acting”

Vibrating-type regulatorVibrating-type regulator

-the alternating voltage regulated indirectly by rapidly the alternating voltage regulated indirectly by rapidly opening and closing a shunt circuit across the exciter opening and closing a shunt circuit across the exciter rheostat, thus varying the exciter voltage in order to the rheostat, thus varying the exciter voltage in order to the desired alternating voltage.desired alternating voltage.

2 main parts:2 main parts: 1. Direct-current control system1. Direct-current control system - - simple and having a main control magnet and a simple and having a main control magnet and a relay magnet connected across the exciter mains.relay magnet connected across the exciter mains.

2. Alternating-current control system2. Alternating-current control system --has a magnet of ordinary solenoid type,having a has a magnet of ordinary solenoid type,having a laminated iron core which is attracted upward by the laminated iron core which is attracted upward by the magnetizing force.magnetizing force.

General Electric DiactorGeneral Electric Diactor

Three main parts:Three main parts: 1. Resistance plates – has a silver-button insert 1. Resistance plates – has a silver-button insert passing through the plate near the front end. passing through the plate near the front end. 2. Metal contact plates– separates the 2. Metal contact plates– separates the individual resistance plates at the rear of the individual resistance plates at the rear of the stack.stack. 3. Insulating spacers – located at the center and 3. Insulating spacers – located at the center and act as a fulcrums on which the resistance plates act as a fulcrums on which the resistance plates may be tilted.may be tilted.

INDIRECT RHEOSTATIC-TYPE REGULATORINDIRECT RHEOSTATIC-TYPE REGULATOR

The heart of this regulator is in the main The heart of this regulator is in the main control unit.This control unit is composed of a control unit.This control unit is composed of a three-phase torque motor with the movable three-phase torque motor with the movable contact arm rigidly coupled to its shaft.The contact arm rigidly coupled to its shaft.The torque of this motor is proportional to the torque of this motor is proportional to the average of the three-phase voltage.average of the three-phase voltage.

Air dashpot or magnetic dumping deviceAir dashpot or magnetic dumping device -- device used to prevent the regulator from -- device used to prevent the regulator from acting during a momentary change in voltage acting during a momentary change in voltage not requiring a change in excitation.not requiring a change in excitation.

PARALLEL OPERATIONPARALLEL OPERATION

--- --- The division of load among alternating-current The division of load among alternating-current generators operating in parallel in dependent on the power generators operating in parallel in dependent on the power supply to each generator as controlled by the governor of supply to each generator as controlled by the governor of its prime mover and is practically independent of its prime mover and is practically independent of excitation.excitation.

Two main functions of the regulator to obtain successful Two main functions of the regulator to obtain successful parallel operation of 2 or more generators with individual parallel operation of 2 or more generators with individual voltage-regulator control:voltage-regulator control: 1. they must hold the voltage constant1. they must hold the voltage constant

2. They must provide proper division of reactive kilovolt-2. They must provide proper division of reactive kilovolt-ampere load among the generators.ampere load among the generators.

LINE-DROP COMPENSATORLINE-DROP COMPENSATOR

--- is used to reproduce in miniature, at the --- is used to reproduce in miniature, at the regulator, the resistance and reactance drops to a regulator, the resistance and reactance drops to a predetermined point in line.predetermined point in line.

FUNCTION:FUNCTION: -- is to lower the impressed voltage on the -- is to lower the impressed voltage on the torque motor or solenoid unit as load comes on, torque motor or solenoid unit as load comes on, thereby causing the regulator to maintain normal thereby causing the regulator to maintain normal voltage at some predetermined point on the voltage at some predetermined point on the system regardless of the power factor of the load.system regardless of the power factor of the load.

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