PCEL-4302 ELECTRICAL MACHINES-II

1 LECTURE NOTES ON ELECTRICAL MACHINES-II PCEL-4302 ELECTRICAL MACHINES-II 5TH SEMESTER IN ELECTRICAL ENGINEERING LECTURE NOTES ON ELECTRICAL MACHINES-II SYLLABUS LECTURE NOTES ON ELECTRICAL MACHINES-II LECTURE NOTES ON ELECTRICAL MACHINES-II Disclaimer This document does not claim any originality and cannot be used as a substitute for prescribed textbooks. The information presented here is merely a collection by the committee faculty members for their respective teaching assignments as an additional tool for the teaching-learning process.

2 Various sources as mentioned at the reference of the document as well as freely available material from internet were consulted for preparing this document. The ownership of the information lies with the respective authors or institutions. Further, this document is not intended to be used for commercial purpose and the committee faculty members are not accountable for any issues, legal or otherwise, arising out of use of this document.

3 The committee faculty members make no representations or warranties with respect to the accuracy or completeness of the contents of this document and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. LECTURE NOTES ON ELECTRICAL MACHINES-II MODULE-I CHAPTER-3 Salient Pole type Three Phase Synchronous Generators LECTURE NOTES ON ELECTRICAL MACHINES-II Introduction: The simple cylindrical theory of a synchronous generator ignores the effect of the reluctance torque on the generator.

4 Shows a salient-pole rotor with no windings inside a three-phase stator. The stator magnetic field produced in the air gap of the generator induces a magnetic field in the rotor. The flux induced in the rotor will act along the axis of the rotor. Since there is an angle between the stator magnetic field and the rotor magnetic field, a torque will be induced in the rotor which will tend to line up the rotor with the stator field. The magnitude of this torque is proportional to sin (2 ) where is the angle between the two magnetic fields.

5 This torque is known as the reluctance torque . : A salient-pole rotor, illustrating the idea of reluctance torque. : Development of the Equivalent Circuit of a Salient-Pole Synchronous Generator There are four elements in the equivalent circuit of a synchronous generator: 1. The resistance of the stator winding 2. The self-inductance of the stator winding 3. The internal generated voltage of the generator E 4. The armature reaction of the synchronous generator The first three elements as above remain same as in the case of cylindrical rotor theory of synchronous generators, but the 4th term representing the armature-reaction effect must be modified to explain the salient-pole rotor theory.

6 Shows a two-pole salient-pole rotor rotating anti- clockwise within a two-pole stator. The rotor flux density BR points upward. The equation for the induced voltage on a moving conductor in the presence of a magnetic field is The voltage in the conductors in the upper part of the stator will be positive out of the page, and the voltage in the conductors in the lower part of the stator will be into the page. The plane of maximum induced voltage will lie directly under the rotor pole at any given time.

7 LECTURE NOTES ON ELECTRICAL MACHINES-II LECTURE NOTES ON ELECTRICAL MACHINES-II : The effects of armature reaction in a salient-pole synchronous generator. The rotor magnetic field induces a voltage in the stator which becomes maximum in the conductors directly under the pole faces ( ). When a lagging load is connected to the generator, a stator current will flow that becomes maximum at an angle behind EA ( ). This stator current IA produces a stator magneto-motive force (mmf) in the machine ( ).

8 The stator mmf produces a stator flux density BS ( ). The reluctance of the direct-axis flux path is lower than the reluctance of the quadrature-axis flux path; as a result, the direct-axis component of mmf produces more flux per ampere-turn than the quadrature-axis component. The direct- and quadrature-axis stator fluxes produce armature reaction voltages in the stator of the machine ( ). If a lagging load is now connected to the terminals of this generator, then a current will flow whose peak is delayed behind the peak voltage.

9 This current is shown in Fig. The stator current flow produces a magneto motive force that lags 900 behind the plane of peak stator current, as shown in Fig. In the cylindrical theory, this mmf then produces a stator magnetic flux density BS that lines up with the stator mmf. However, it is actually easier to produce a magnetic flux density in the direction of the rotor than it is to produce one in the direction perpendicular to the rotor. Therefore, the stator mmf can be resolved into components parallel to and perpendicular to the axis of the rotor.

10 Each one of these mmf components produces a magnetic field with more flux per ampere-turn being produced along the direct axis as compared to that along the quadrature axis. Ed EA Eq Ed V Eq LECTURE NOTES ON ELECTRICAL MACHINES-II Fig. : The phase voltage of the generator is just the sum of its internal generated voltage and its armature reaction voltages. The resulting stator magnetic field is shown in Fig. , compared to the field predicted by the cylindrical rotor theory. Now, each component of the stator magnetic field produces a voltage of its own in the stator winding by armature reaction.