Showing posts with label Alternator. Show all posts
Showing posts with label Alternator. Show all posts

Voltage Regulation Of Synchronous Machines [Alternator] By Direct Loading Test

Voltage Regulation Of Synchronous Machines [Alternator] By Direct Loading Test

Today in this post we are going to learn what is Voltage regulation of synchronous machine and different methods to calculate Voltage regulation of synchronous machine.

Definition for Voltage regulation of a synchronous machine:

Voltage Regulation of synchronous machine is defined as the difference between terminal voltage at no load and terminal voltage at full load and excitation , speed must remain same.Voltage Regulation of synchronous machine is generally calculated in percentage of full load terminal voltage.

Objectives for calculating Voltage regulation of a synchronous machine:

1. Parallel operation of alternators is affected by the voltage regulation. By calculating voltage regulation of synchronous machine we can adjust the parallel operating machines to be in synchronism.

2. Calculating voltage regulation of a synchronous machine determines the type of automatic voltage control equipment required for resisting the voltage changes.

3.When the load is thrown off voltage rise must be known because with the rise in voltage the insulation must be able to withstand this rise.

So calculation of voltage regulation of synchronous machine has a great importance.

General expression for calculating Voltage regulation of synchronous machine:

Now let us derive general expression for calculating voltage regulation of a synchronous machine

Let E be the terminal voltage of the synchronous machine at no load. Now if the synchronous machine is given full load the terminal voltage will no longer be E because of the losses so let the terminal voltage now be V.

So general expression for Voltage regulation of a synchronous machine is given by

Voltage regulation% = (E - V / V) × 100

Methods for calculating voltage regulation of synchronous machine:

There are two types of methods for calculating voltage regulation of synchronous machine.

1. Direct load test method.

2. Indirect Method.

Indirect method of calculating voltage regulation of synchronous machine can be further classified into 3 types:

3.Zero power factor method or potier method.

Direct load test method for calculating voltage regulation of synchronous machine:

Now let's see how to calculate voltage regulation of synchronous machine by using direct load test method:

Circuit diagram for calculating Voltage regulation of synchronous machine by direct load test:

Circuit connections for calculating voltage regulation of synchronous machine by direct load test:

1.Firstly connections are to be made as given in the circuit diagram:

2. Armature which is star connected is connected to the three phase load with the help of TPST. TPST is a switch and it means triple pole single through. 

3. A rheostat is connected in series with the field winding. 

4. Field winding is excited by using D.C supply and flux is adjusted by adjusting the rheostat. Flux adjustment is nothing but adjust the current flow through field winding.

Procedure for calculating voltage regulation of synchronous machine by direct load test:

 1. Adjust the prime mover such that the alternator rotates at synchronous speed Ns.

 we know Eph α 𝞍 from emf equation

2. Now DC supply is given to the field winding and the current flow through field is adjusted so that the flux is adjusted such that the rated voltage is obtained at its terminals which can be seen on the voltmeter connected across the lines.

3. Now load is connected to alternator with the help of TPST switch.

4.The load is then increased such that the ammeter reads rated current. This is full load condition of alternator. Now as load is connected due to armature reaction there is loss in voltage so let the induced voltage be V. 

5.Now again adjust the rheostat of the field winding to get rated voltage at alternator terminals.

6.Now remove the load by opening TPST switch and the excitation , speed should not be changed it should be same as before removing the load.

7. As there is no load there is no armature reaction the induced emf is equal to terminal voltage which is E.  

Now we can calculate voltage regulation of synchronous machine by 

Voltage regulation% =( E - V / V) × 100 at a specific power factor.

Limitations for calculating voltage regulation of synchronous machine by using direct load method:

This method is applicable only for small capacity machines for larger capacity machines it is not economical because that much load cannot be given directly.

In this way we have calculated the voltage regulation of synchronous machine by direct load test method.

For larger capacity machines voltage regulation can be calculated by Indirect method.

In the next post we can see how to calculate voltage regulation of synchronous machine by Indirect method.

You can download PDF form of voltage regulation of synchronous machines here


EMF Method or Synchronous Impedance Method | Voltage Regulation of Synchronous Generator [Alternator]

Voltage Regulation of Synchronous Generator [Alternator] By EMF Method or Synchronous Impedance Method

EMF method: This method is also known as synchronous impedance method.Here the magnetic circuit is assumed to be unsaturated. In this method the MMFs (fluxes) produced by rotor and stator are replaced by their equivalent emf, and hence called emf method.To predetermine the regulation by this method the following information is to be determined.Armature resistance/phase of the alternator, open circuit and short circuit characteristics of the alternator.

Here we discuss Voltage Regulation of Synchronous Generator [Alternator] by EMF Method or Synchronous Impedance Method.this is better method than direct loading but not best methods to find out voltage regulation.

Synchronous Impedance Method:

To perform  voltage regulation by emf method we need to calculate the following data.
1.Armature Resistance per phase [Ra]
2.Open Circuit characteristics which is a graph between open circuit voltage [Vo.c.] and field current.
3.Short circuit characteristics which is a graph between short circuit current [Is.c.] and field current.
Voltage Regulation Synchronous Generator by Synchronous Impedance Method
In Synchronous Impedance Method we need to calculate OC and SC characteristics to find Synchronous these steps to find out OC & SC test values.
Open Circuit Characteristic (O.C.C.):-

The open-circuit characteristic or magnetization curve is really the B-H curve of the complete magnetic circuit of the alternator. Indeed, in large turboalternators, where the air gap is relatively long, the curve shows a gradual bend. It is determined by inserting resistance in the field circuit and measuring corresponding value of terminal voltage and field current. Two voltmeters are connected across the armature terminals. The machine is run at rated speed and field current is increased gradually to If1 till armature voltage reaches rated value or even 25% more than the rated voltage. Figure illustrates a typical circuit for OC test.The major portion of the exciting ampere-turns is required to force the flux across the air gap, the reluctance of which is assumed to be constant. A straight line called the air gap line can therefore be drawn as shown, dividing the excitation for any voltage into two portions,

(a) that required to force the flux across the air gap, and
(b) that required to force it through the remainder of the magnetic circuit.
The shorter the air gap, the steeper is the air gap line.

Procedure to conduct OC test:
(i) Start the prime mover and adjust the speed to the synchronous speed of the alternator.
(ii) Keep the field circuit rheostat in cut in position and switch on DC supply.
(iii) Keep the TPST switch of the stator circuit in open position.
(iv) Vary the field current from minimum in steps and take the readings of field current and
stator terminal voltage, till the voltage read by the voltmeter reaches up to 110% of rated voltage. Reduce the field current and stop the machine.
(v) Plot of terminal voltage/ phase vs field current gives the OC curve.

Short Circuit Characteristic (S.C.C.):-
The short-circuit characteristic, as its name implies, refers to the behaviour of the alternator when its armature is short-circuited. In a single-phase machine the armature terminals are short-circuited through an ammeter, but in a three phase machine all three phases must be short-circuited. An ammeter is connected in series with each armature terminal, the three remaining ammeter terminals being short-circuited. 

The machine is run at rated speed and field current is increased gradually to If2 till armature current reaches rated value. The armature short-circuit current and the field current are found to be proportional to each other over a wide range, as shown in Figure, so that the short circuit characteristic is a straight line. Under short-circuit conditions the armature current is almost 90° out of phase with the voltage, and the armature mmf has a direct demagnetizing action on the field.The resultant ampere − turns inducing the armature emf are, therefore, very small and is equal to the difference between the field and the armature ampere − turns. 

This results in low mmf in the magnetic circuit, which remains in unsaturated condition and hence the small value of induced emf increases linearly with field current. This small induced armature emf is equal to the voltage drop in the winding itself, since the terminal voltage is zero by assumption. It is the voltage required to circulate the short circuit current through the armature windings. The armature resistance is usually small compared with the reactance.

Short-Circuit Ratio:

The short-circuit ratio is defined as the ratio of the field current required to produce rated volts on open circuit to field current required to circulate full-load current with the armature short-circuited.

Short-circuit ratio = If1/If2

Determination of synchronous impedance Zs:

As the terminals of the stator are short circuited in SC test, the short circuit current is circulated against the impedance of the stator called the synchronous impedance. This impedance can be estimated form the oc and sc characteristics.The ratio of open circuit voltage to the short circuit current at a particular field current, or at a field current responsible for circulating the rated current is called the synchronous impedance.

synchronous impedance Zs = (open circuit voltage per phase)/(short circuit current per phase)
for same If
Hence Zs = (Voc) / (Isc)
for same If
From figure synchronous impedance Zs = V/Isc

Armature resistance Ra of the stator can be measured using Voltmeter Ammeter method. Using synchronous impedance and armature resistance synchronous reactance and hence regulation can be calculated as follows using emf method.

Zs =√(Ra)² + (XS)² and Synchronous reactance Xs =  ( Zs)² - (Ra)²

Hence induced emf per phase can be found as 
Eph = √ [ (V cos  Ø+ IRa)²+ (V sin  Ø ± IXS)²]
V = phase voltage per phase = Vph ,
I = load current per phase
in the above expression in second term + sign is for lagging power factor and
– sign is for leading power factor.

% Regulation = [(Eph – Vph / Vph )] x 100

where Eph = induced emf /phase, Vph = rated terminal voltage/phase.

Synchronous impedance method is easy but it will not give accurate results. This method gives the value of regulation which is greater (poor) than the actual value and hence this method is called pessimistic method. The complete phasor diagram for the emf method is shown in above figure.

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EMF Equation Of Alternator / 3 Phase AC Generator EMF Equation

EMF Equation of an Alternator

We know Synchronous Machines generates E.M.F.the amount of EMF generated can be calculated using below simple derivation.

Consider following
Φ= flux per pole in wb
P = Number of poles
Ns = Synchronous speed in rpm
f = frequency of induced emf in Hz
Z = total number of stator conductors
Zph = conductors per phase connected in series
Tph = Number of turns per phase
Assuming concentrated winding, considering one conductor placed in a slot
According to Faraday's Law electromagnetic induction,
The average value of emf induced per conductor in one revolution 
eavg = dΦ /dt
eavg = Change of Flux in one revolution/ Time taken for one revolution

Change of Flux in one revolution = p x Φ
Time taken for one revolution = 60/Ns seconds.
Hence eavg = (p x  Φ  ) / ( 60/Ns) = p x   Φ x Ns / 60
We know f = PNs /120
hence PNs /60 = 2f
Hence eavg = 2   Φ f volts
Hence average emf per turn = 2 x 2 Φ f volts = 4Φf volts
If there are Tph, number of turns per phase connected in series, then average emf induced in Tph turns is
Eph,avg = Tph x eavg = 4 f   Φ Tph volts

Hence RMS value of emf induced E = 1.11 x Eph, avg
= 1.11 x 4  Φ f Tph volts
= 4.44 f  Φ Tph volts

Eph,avg= 4.44 f  Φ Tph volts

This is the general emf equation for the machine having concentrated and full pitched winding.In practice, alternators will have short pitched winding and hence coil span will not be  180o(degrees), but on or two slots short than the full pitch.

***If we assume effect of 
Kd= Distribution factor
Kc or KP = Cos α/2 . This is pitch factor.

Eph,avg= 4.44Kc  Kd f  Φ Tph volts
This is the actual available voltage equation of an alternator per phase.If alternator or AC Generator is Star Connected as usually the case, then the Line Voltage is √3 times the phase voltage.

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Advantages of Rotating Field Over Rotating Armature

In this article we are going discuss about why rotating field is used over rotating armature in alternators or synchronous machine.In the alternator construction we said in large AC generators we use stationary armature and rotating field.There are some advantages are with stationary armature arrangement want to know what are they? here we go....advantages of stationary armature in synchronous machines.

Advantages of rotating field over rotating armature 

1) We use AC power in our daily life which has generation level around 11 kV to 33 kV,it is difficult to get induced emf in armature because we can't mount as many as conductors as required on rotating armature hence we prefer stationary armature.

2) As said in above point large conductors posses high centrifugal forces while rotating.So there will be chance of conductors slipping out from slots.So by using rotating field over rotating armature we can reduce mechanical and electrical stresses.

3) The problem of sparking at the slip rings can be avoided by keeping field rotating which is low voltage circuit and high voltage armature as stationary.

4) It is not that much easy to collect larger currents at very high voltages from rotating armature.

4) It is easier to collect larger currents at very high voltages from a stationary member than from the slip ring and brush assembly.The voltage required to be supplied to the field is very low (110 V to 220 V dc.) and hence can be easily supplied with the help of slip ring and brush assembly by keeping it rotating.

5) The ventilation arrangement for high voltage side can be improved if it is kept stationary.

6) It is better to rotate low inertia system than high inertia system so we use low voltage on rotor side so that inertia can be reduced along with insulation on the rotor side which also reduces cost of the system

7) As we collect power from armature a rotating field type makes it easier to collect power from the armature,and greater output can be collected at low losses compared to that of rotating armature.

8) If field is rotating, to excite it by an external d.c. supply two slip rings are enough.One each for positive and negative terminals. As against this, in three phase rotating armature the minimum number of slip rings required are three and can not be easily insulated due to high voltage levels.

By considering the all above reasons we can say for very high voltages, rotating field type of arrangement in alternators is better than stationary field type. For small voltage rating alternators rotating armature arrangement may be used.

Construction of Alternator or Synchronous Machine

3 phase synchronous generator or alternator or synchronous machine construction 

Alternator is the machines which generates the power which we are using today.Now let us discuss about construction of alternator or synchronous machine.The most important parts in alternator design  are stator & rotor.Modern alternators prefer rotating field type of construction.Unlike d.c. generators in case of alternators construction the winding terminology is different. In alternators the stationary winding is called 'Stator' while the rotating winding is called 'Rotor’.

Note: Most of alternator have stator as armature and rotor as field, in general.

Constructional Details of Rotating Field Type of Alternator[AC generator]

Stator of Alternator or Synchronous Machine 

The stator of synchronous machine is a stationary armature.It consists of a core and the slots to hold the armature winding like the armature of a dc. generator. The stator core of alternator is laminated to reduce eddy current losses.Stator is built up of special steel stampings insulated from each other with varnish or paper.Silicon steel is used as armature core to minimize hysteresis losses. The entire core is fabricated in a frame made of steel plates. The core has slots on its periphery for housing the armature conductors. Frame does not carry any flux and serves as the support to the core.Holes are made in the frame to maintain ventilation.The section of an alternator stator is shown in the Fig.
Rotor of Alternator or Synchronous Machine

There are two types of rotors used in alternators.
1) Salient pole type
2) Smooth cylindrical type.

Salient Pole Type Rotor[Construction of Alternator]

Salient pole type of rotor is also called projected pole type as all the poles are projected out from the surface of the rotor.The poles are made up of thick steel and they are laminated. The poles are mounted on the rotor with the help of bolts [shown in the fig].
The pole face has been given a specific shape. The field winding is provided on the pole shoe. Salient pole type rotors have large diameters and small axial lengths. The limiting factor for the size of the rotor is the centrifugal force acting on the rotating member of the machine. As mechanical strength of salient pole type is less, this is preferred for low speed alternators ranging from 125 r.p.m. to 500 r.p.m. The prime movers used to drive such rotor are generally water turbines and IC. engines.

Features of Salient Pole Type Rotor

1.They have a large horizontal diameter compared to a shorter axial length.
2.The pole shoes covers only about 2/3rd of pole pitch.
3.Poles are laminated to reduce eddy  current  loss.

Smooth Cylindrical Type Rotor[Construction of Alternator]

Smooth Cylindrical Type Rotor is also called non-salient type or non-projected pole type or round rotor construction.The rotor consists of smooth solid steel cylinder, having number of slots to accommodate the field coil. The slots are covered at the top with the help of steel or manganese wedges. The un-slotted portions of the cylinder itself act as the poles. The poles are not projecting out and the surface of the rotor is smooth which maintains uniform air gap between stator and the rotor. These rotors have small diameters and large axial lengths.
These rotors have small diameters and large axial lengths.This is to keep peripheral speed within limits.The main advantage of this type is that these are machanically very strong and thus preferred for high speed alternators ranging between 1500 to 3000r.p.m.Such high speed alternators are called 'turbo-alternators'.The prime movers used to drive such type of rotors are generally steam turbines,electric motors.

The cylindrical rotor alternators are generally designed for 2-pole type giving very high speed of Ns = (120 × f)/P = (120 × 50) / 2 = 3000 rpm.

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