Recent questions tagged synchronous-generator

Description : Discuss how the ventilation and cooling of large high speed alternator is carried out.

Last Answer : For high speed alternator two cooling methods are available and they are conventional cooling and direct cooling. In conventional cooling methods, radial and axial ventilating ducts are provided in the ... oil or hydrogen are circulated in the ducts to remove the heat directly from the conductors.

Description : How does increase in steam input of the Alternator connected to infinite bus-bars affect this operation?

Last Answer : KW output power of alternator increase with increase in steam input keeping excitation constant.

Description : How does increase in excitation of the Alternator connected to infinite bus-bars affect this operation?

Last Answer : Increase in excitation level of the synchronous generator will effectively increase the reactive power supplied by the generator keeping steam input constant.

Description : What steps are to be taken before disconnecting one Alternator from parallel operation?

Last Answer : The following steps are to be taken before disconnecting one Alternator from parallel operation · The prime-mover input of the outgoing generator has to be decreased and that of other generators ... delivered by the outgoing generator is zero, it has to be disconnected from parallel operation.

Description : What happens if speed of prime mover is changed keeping excitation of alternator constant?

Last Answer : If speed of prime mover is changed, the speed of alternator also changes hence frequency also changes. The frequency increase with increase in steam input and hence speed and vise versa.

Description : What happens if excitation of alternator is changed keeping speed of prime mover constant?

Last Answer : The emf induced will be changed and hence terminal voltage will also change. The terminal voltage will increase with increase in excitation and vise versa.

Description : List the factors that affect the load sharing in parallel operating generators?

Last Answer : The total active and reactive power delivered to the load, connected across the common bus-bars, are shared among Synchronous generators, operating in parallel, based on the following three factors ... characteristic/input · Excitation level and · Percentage synchronous impedance and its R/X ratio

Description : Why synchroscope method is more better compared to others?

Last Answer : It is very accurate as it doesnot require personal judgement

Description : How synchronoscope is used for synchronizing Alternators?

Last Answer : Synchronoscope can be used for permanently connected Alternators where the correctness of phase sequence is already checked by other means. The rotation of pointer is proportional to the difference in ... alternator is running slow. If the difference in frequency is large the pointer will oscillate

Description : What is a synchroscope?

Last Answer : Synchroscope is an instrument, which shows the phase relationship of emf of the incoming alternator. It also indicates whether the incoming alternator is running slow or fast.

Description : What are the advantages and disadvantages of three dark lamps method of synchronizing?

Last Answer : Advantages: · The synchronous switch using lamps is inexpensive · Checking for correctness of the phase sequence can be obtained in a simple manner which is essential especially when the Alternator ... The frequency of the incoming Alternator in relation to the bus-bar frequency is not available.

Description : What happens in all dark lamp method ?

Last Answer : If frequency and phase difference of incoming alternator and busbar are not same then lamps connected across incoming alternator will flicker. The flickering decreases as frequency will become near to each other by adjusting the speed by prime mover.

Description : What is synchronizing?

Last Answer : The operation of connecting an alternator in parallel with another alternator or with common bus bars is known as synchronizing.

Description : What are the methods of checking the phase difference?

Last Answer : 1. All Dark lamp method 2. Two bright one dark lamp method 3. Synchroscope

Description : What is meant by infinite bus-bars?

Last Answer : The source or supply lines with non-variable voltage and frequency are called infinite bus-bars. The source lines are said to have zero source impedance and infinite rotational inertia.

Description : State the condition to be satisfied before connecting two alternators in parallel

Last Answer : The following are the three conditions to be satisfied by synchronizing the additional Alternator with the existing one or the common bus-bars. · The terminal voltage magnitude of the incoming ... incoming Alternator voltage must be the same as the bus-bar voltage. . Zero phase difference

Description : What is need of parallel operation of alternators?

Last Answer : 1. Optimum utilization of alternators as most of the power stations are interconnected by grid. 2. Reduced operating cost: number of alternators operating at a time can be changed depending on ... demands. 3. Easy maintenance: Alternator may be shut down for maintenance 4. Continuity and reliability

Description : Why is the MMF method of estimating the voltage regulation considered as the optimistic method?

Last Answer : In this method the actual value is more than the result obtained.

Description : What are the test data required for predetermining the voltage regulation of an Alternator by MMF method?

Last Answer : Data required for MMF method are : · Effective resistance per phase of the 3-phase winding R · Open circuit characteristic (OCC) at rated speed/frequency · Short circuit characteristic (SCC) at rated speed/frequency

Description : In what way does the ampere-turn method differ from synchronous impedance method?

Last Answer : The ampere-turn /MMF method is the converse of the EMF method in the sense that instead of having the phasor addition of various voltage drops/EMFs, here the phasor addition of MMF required for the voltage drops are carried out. Further the effect of saturation is also taken care of.

Description : Why is the synchronous impedance method of estimating voltage regulation considered as pessimistic method?

Last Answer : Compared to other methods, the value of voltage regulation obtained by the synchronous impedance method is always higher than the actual valuei.e actual value is lower than the results obtained and therefore this method is called the pessimistic method.

Description : What are the advantages and disadvantages of estimating the voltage regulation of an Alternator by EMF method?

Last Answer : Advantages: · Simple no load tests (for obtaining OCC and SCC) are to be conducted · Calculation procedure is much simpler Disadvantages: · The value of voltage regulation obtained by this method is always higher than the actual value

Description : Name the various methods for predetermining the voltage regulation of 3-phase Alternator.

Last Answer : The following are the three methods which are used to predetermine the voltage regulation of smooth cylindrical type Alternators · Synchronous impedance / EMF method · Ampere-turn / MMF method · Potier / ZPF method

Description : What is the necessity for predetermination of voltage regulation?

Last Answer : Most of the Alternators are manufactured with large power rating , hundreds of kW or MW, and also with large voltage rating upto 33kV. For Alternators of such power and voltage ratings ... performance like voltage regulation then can be predetermined at any desired load currents and power factors.

Description : Define the term voltage regulation of Alternator.

Last Answer : The voltage regulation of an Alternator is defined as the change in terminal voltage from no-load to load condition expressed as a fraction or percentage of terminal voltage at load condition ; the speed and excitation conditions remaining same.

Description : An Alternator is found to have its terminal voltage on load condition more than that on no load. What is the nature of the load connected?

Last Answer : The nature of the load is of leading power factor , load consisting of resistance and capacitive reactance.

Description : What is meant by load angle of an Alternator?

Last Answer : The phase angle introduced between the induced emf phasor, E and terminal voltage phasor , U during the load condition of an Alternator is called load angle.

Description : Why synchronous generators are to be constructed with more synchronous reactance and negligible resistance?

Last Answer : The presence of more resistance in the Synchronous generators will resist or oppose their synchronous operation. More reactance in the generators can cause good reaction between the two and help the ... to remain in synchronism in spite of any disturbance occurring in any one of the generators

Description : What is meant by synchronous impedance of an Alternator?

Last Answer : The combination of armature resistance and synchronous reactance is known as synchronous impedance

Description : What is synchronous impedance?

Last Answer : It is the combination of synchronous reactance and armature resistance.

Description : What is synchronous reactance?

Last Answer : It is the combination of leakage reactance and armature reactance

Description : What are the causes of changes in voltage in Alternators when loaded?

Last Answer : When alternator is loaded the armature carries load current. Due to this load current the voltage drops occur in alternator. Hence terminal voltage decreases due to following voltage drops: 1. Voltage ... the leakage reactance of the winding, Xl 3· Voltage drop due to the armature reactance, Xa

Description : Explain armature reaction of an alternator and its effect on main flux at various power factor.

Last Answer : 1. zero lagging pf: armature flux lags main flux by 180 . Hence effect is demagnetizing i.e the main flux is reduced 2. lagging pf: it has 2 components demagnetizing and cross magnetizing ... 5. zero leading pf: Armature flux is in phase with main flux. Effect is completely magnetizing.

Description : What is meant by armature reaction in Alternators?

Last Answer : The effect of armature flux on main flux

Description : Why alternators are rated in KVA?

Last Answer : The KVA rating of ac machine depends on the power factor of the load. The power factor in turn depends on the operating conditions. The operating conditions differ from place to place. Therefore the KVA rating is specified for all ac machines.

Description : Why are Alternators rated in kVA and not in kW?

Last Answer : The continuous power rating of any machine is generally defined as the power the machine or apparatus can deliver for a continuous period so that the losses incurred in the machine gives rise to a ... generator , the Alternators have only their apparent power in VA/kVA/MVA as their power rating.

Description : Which type of winding is preferred for armature windings of alternator? Why?

Last Answer : Double layer, short pitched and distributed type of armature winding. It eliminates high frequency harmonics hence it makes induced emf more sinusoidal.

Description : Write down the formula for distribution factor

Last Answer : Kd = sin (mβ/2) / msin(β/2) where m - number of slots/pole/phase Slot angle β - angle between adjacent slots in electrical degree =180/(slots/pole)

Description : what is slot angle β?

Last Answer : The phase difference contributed by 1 slot in electrical degree is called slot angle. Slot angle β - angle between adjacent slots in electrical degree =180/n =180/(slots/pole)

Description : Define distribution factor or winding factor or breadth factor or spread factor

Last Answer : The distribution factor is defined as the ratio of vector sum to arithmetic sum of emf induced in the conductor of one phase spread. OR It is ratio of resultant emf induced when coils are distributed to the resultant emf induced when coils are concentrated.

Description : What is concentrated winding?

Last Answer : If all the conductors or coils belonging to a phase are placed in one slot under every pole then winding is called concentrated winding.

Description : What is distributed winding?

Last Answer : When coil-sides belonging to each phase are housed or distributed in more than one slot under each pole region then the winding is called distributed winding

Description : Why is short pitch winding preferred over full-pitch winding ?

Last Answer : Advantages · Sinusoidal waveform: Waveform of the emf can be approximately made to a sine wave and distorting harmonics can be reduced or totally eliminated. · Economical :Conductor ... harmonics are eliminated, the eddy current and hysteresis losses are reduced which increases overall efficiency

Description : Define pitch factor or coil span factor.

Last Answer : The pitch factor is defined as the ratio of vector sum of emf induced in a coil to arithmetic sum of emf induced in the coil OR It is the ratio of resultant emf induced when coil is short pitch to the resultant emf induced when the coil is full pitched.

Description : What is full pitch winding?

Last Answer : If coil pitch is equal to pole pitch it is called full pitch winding

Description : What is short pitch winding?

Last Answer : If coil pitch is equal to less than pole pitch it is called short pitch winding

Description : Define pole pitch.

Last Answer : It is centre to centre distance between two adjacent poles. Two poles are responsible for 360 degree electrical of induced emf so one pole is responsible for 180 degree electrical of induced emf. Hence 180 degree electrical is called 1 pole pitch.

Description : What is the relation between electrical degree and mechanical degree?

Last Answer : Electrical angle = (P/2) x Mechanical angle Where P is number of poles

Description : How does electrical degree differ from mechanical degree?

Last Answer : Mechanical degree is the unit for accounting the angle between two points based on their mechanical or physical placement. Electrical degree is used to account the angle between two points in rotating ... field. 180 electrical degree is accounted as the angle between adjacent North and South poles.

Description : Why is the stator core of Alternator laminated?

Last Answer : The stator core of Alternator is laminated to reduce eddy current loss.