Xd and Xq of Alternator | Slip Test

X_d AND X_q OF A SALIENT POLE SYNCHRONOUS MACHINE.

AIM: To find X_d and X_q of a salient pole synchronous machine by slip test.

APPARATUS: 3 F Auto transformer 1 No.

                                    Ammeter (0-10A) MI. 1 No.

                                    Voltmeter (0-300V) MI. 1No.

                                    rheostats (296 ohm/ 2.8A), (45 ohm/ 5A)

PROCEDURE:

1. Connections are made as per Circuit diagram.
2. Switch on DC supply and run the DC motor at a speed below t he synchronous speed of alternator.
3. Switch on AC supply and increase auto transformer out put to a suitable value note the variation in voltmeter and ammeter.
4. For another speed take the similar readings.
5. Adjust speed to get maximum oscillations in the ammeter and voltmeter.
6. Note maximum and minimum readings of voltmeter and ammeter.
7. Adjust the auto transformer output for a different value and take readings.
8. Vary the speed to a value greater than synchronous speed and repeat the Experiment.

Observations:

N(rpm)	Imin(amps	Imax(amps)	Vmax(volts)	Vmin(volts)	Xd(ohms)	Xq(ohms)	Xd/Xq

Precautions: Slip should be always be +/- 5%

Result:

THEORY (Determination of Xd & Xq – slip test)

            Direct  and quadrature axis reactances of a silent pole synchronous machine can be estimated by means of the test. The machine armature is connected to a 3-phase supply whose voltage is much less than the rated voltage of the machine. While the rotor is run at spee close to synchronous with the field winding fell open circuited. Since the excitation emf is zero, heavy current would be drawn by the armature, if connected to the r ated voltage supply.

            The currents drawn by armature set up an mmf wave rotating an synchronous speed. Since the rotor being run at a speed close to synchronous, the stator mmf moves slowly past the field poles at slip speed (Ns – N). When the stator mmf is alighned with the d-axis (field poles), flux f_j / pole is setup. So that effective reactance offered by the machine is Xd. Similarly when the stator mmf aligns with the q=axis, the flux set up is f_q/pole and the machine reactance is Xq. The current drawn by the armature therefore, varles cyclically at twice the slip frequency. The rmscurrent is minimum when machine reactance is Xd and is maximum when it is Xq. Because of cycle current variations and consequent voltage drop in the impedance of supply lines, the voltge at machine terminds also varies cyclically and has a mimimum value at maximum current and maximum value at mimimum current. The machine reactance can be found as

            Xd = (VE ( a1 Ia min) / (Ö3 Ia(min)

            Xq = (VE (at Ia max) / (Ö3 Ia (max)

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The Armature reaction of Alternator at different power factors

The Armature reaction of  

Alternator  at different power factors 

Armature reaction is meant that the effect of magnetic field set up by armature current on the distribution of flux under  the main poles of a generator.

•As in d.c generator armature reaction is the effect of armature flux on the main flux.

•In case of alternators the p.f. of load has considerable effect on armature reaction.

When load power factor is unity.

•When load power factor is zero lagging.

•When load power factor is zero leading.

Synchronous reactance: For the same field excitation terminal voltage decreased from its no load value E0 to V (for a lagging p.f )

     This is due to

Drop due to armature resistance, IRa            

Drop due leakage reactance, IXL                    

Drop due to armature reactance , IXa

The leakage reactance XL and the armature reactance Xa may combined to give synchronous reactance Xs,

           hence Xs = XL + Xa

Xa varies with p.f of the load, because armature reaction depends on load p.f

•Total voltage drop in an alternator under load  = IRa + JIXs

                  I ( Ra + JXs ) = IZs

      where “Zs” is known as synchronous impedance.

•    When the synchronous reactance is combined with the armature reactance Xs and armature resistance Ra, the quantity obtained is called synchronous Impedance.

•        Zs = Ra + JXs Ω/ phase

•       Ra is usually very small compare to Xs.

      Therefore  Zs may assumed equal to the Xs           for many purposes.

Voltage variation on load

As the load on alternator is varied, its terminal voltage "V" also varies due to the following reasons

1)  Voltage drop due to armature resistance Ra

2)  Voltage drop due to armature reactance  XL 

3)  Voltage drop due to armature reaction.

•We know that terminal voltage decreases on load.

•Eo = no load emf , that is also equal to maximum value of induced emf.

•E = load induced emf,

•E is vectorially less than Eo / IXa

•V = terminal voltage, it is vector ally less than Eo / IXs

•Z2 =  Ra2 + XL2

 

•I = Armature current per phase

Ф = load power factor angle

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Previous year Questions -ELECTRICAL MACHINES -II

Previous year Questions

Electrical Machines –II

JNTUH R18 B.Tech II  EEE

Short answer Questions :

• List any two differences between cage rotor and wound rotor. [2]
• Explain the concept of RMF. [3]
• Draw torque – slip characteristic of three phase IM. [2]
• What is cogging? Explain? [3]
• Why two reactances are required for the analysis of Salient pole alternator? [2]
• List the differences between distributed and concentrated windings. [3]
• What is synchronizing torque? [2]
• What is Synchronous induction motor? Explain? [3]
• Why single phase IM is not self starting? [2]
• List Applications of shaded pole motor.
• What is rotating magnetic field of a three phase induction motor. [2]
• Define slip of an induction motor. Write the equation for the same. [3]
•  What is cogging of an induction motor? [2]
•  List three different speed control methods of a three phase induction motor. [3]
•  List any two advantages of rotating magnetic field in Synchronous Generators. [2]
•  List any three differences between smooth Cylindrical and Salient pole types of Alternator. [3]
• Why a 3-phase synchronous motor always runs at synchronous speed? [2]
•  List any three conditions to be satisfied for parallel operation of alternators. [3]
• Classify different types of single phase induction motors. [2]
•  Explain Double revolving field theory of Single phase Induction motor. [3]
•  What is slip and slip speed of an Induction motor? [2]
•  Give the applications of 3-phase induction motor. [3]
•  What is crawling in Three-phase Induction motor? [2]
•  What is the principle of V/f speed control in Induction motor? [3]
•  What is pitch factor? [2]
•  Draw the load characteristics of an alternator. [3]
• What is Synchronous reactance of an alternator? [2]
•  Write short note on Synchronous induction motor. [3]
• State the disadvantages of single phase Induction motor. [2]
• Give the applications of shaded pole induction motor. [3]
•  Write the merits and demerits of slip-ring induction motor. [2]
• Write the effects of crawling and cogging on the performance of induction motor. [3]
• What is the procedure to conduct the blocked rotor test on 3-ϕ induction motor? [2]
• Why starting methods are needed for 3-ϕ induction motor
• What is the effect of skewing the rotor slots of an induction motor? [2]
• Draw the speed-torque characteristics of a deep-bar double cage induction motor. [3]
• What are the limitations of speed control by conventional methods? [2]
• What will be the effect on torque developed by an induction motor if applied voltage is reduced to half with frequency unchanged?
• Define slip. And explain necessity of slip in an induction motor. [2]
• Define frequency of rotor current and its condition during starting and synchronous speed. [3]
• What information can be obtained from circle diagram of 3-ϕ Induction Motor? [2]
• What is the necessity of speed control of a three phase induction motor?
• Derive the condition for maximum torque at the time of starting in a 3-ϕ I.M. [2]
•  Derive the condition for maximum torque under running conditions in a 3-ϕ I.M. [3]
• Explain the working principle of Induction generator. [2]
• Explain the speed control of 3-ɸ IM using cascade connection

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Parallel operation of Alternators

Parallel operation of Alternators

Necessity of Operating the alternators in parallel

Continuity of service

•If power plant consists  only single unit

•In the event of breakdown of the prime mover or the

   Alternator

•Entire station will be

   shut down

•If several small units

   run in parallel

•If one unit fails the

   power is supplied by

   remaining units

To improve Efficiency

•Alternators operate most efficiently when delivering full load.

•Hence when load is light, single unit is used.

•When load increases, another unit can be run in

    parallel to share the load

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Current Affairs  

 Key points  

Railway

Tejas Express:

 After the successful run of Lucknow-Delhi Tejas Express, the second premium Tejas train is ready to run between Ahmedabad and Mumbai. 
 This will be yet another step of the Ministry of Railways in its endeavour to improve the overall traveling experience to rail passengers by providing state of the art facilities. 

 The train is fully air-conditioned and will have two executive class chair cars with 56 seats each and eight chair cars 78 seats each. The train will totally carry capacity of 736 passengers. 

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