1. EEE223 Energy Conversion II Md. Asif Uddin Khan Lecturer, EEE BRAC University

2. Basics of Synchronous Machines Materials taken from: Stephen J. Chapman: Electric Machinery Fundamentals, McGraw-Hill, 5 Th Edition Chapters: 3 and 4

3. Rotor: The part which is Movable. The term is used for AC machines. This contains the Field for DC Machines. Stator: The part which is stationary. The term is used for AC machines. This contains the armature windings for DC machines.

4. Basics of ‘Electromagnetic’ Energy Conversion

5. Similarities Between Synchronous and Induction machine

6. Similarities Between Synchronous and Induction machine (Cntd) 3.Both have same power flow diagram and losses

7. Dissimilarities between Synchronous and Induction machine Induction Machine 1.Field current is supplied by magnetic induction of the stator current into the field winding. This is the reason the air gap between stator and rotor is smaller than that in synchronous machines. 2.Run at a speed less than the synchronous speed and speed can be varied. 3.Rotors in induction machines are squirrel cage type or wound rotor type having a complete set of Y connected 3 phase windings that can be connected to external resistance.

8. Construction of Synchronous machine:

9. Construction of Synchronous machine Stator(armature):

13. Construction of Synchronous machine Rotor (Field): The rotor is different from that in an induction machine. The Rotor of the generator is turned by a prime mover producing a magnetic field within the machine. The rotating magnetic field induces a voltage within the stator windings of the generator. In a synchronous machine the number of poles as designed through the mode of stator winding is also physically reflected in the construction of rotors in two ways Salient Pole form (a magnetic pole protruding out from the surface of the rotor and is usually provided in the generators driven by low RPM hydraulic turbine or in motors with more than 4 poles) Non-Salient Pole form (a magnetic pole constructed flush with the rotor surface and is normally used for 2 or 4 poles high RPM machine). NSP rotors are also termed cylindrical rotors because of their shapes.

14. Rotors are constructed of thin lamination to reduce eddy current losses arising electromagnetic fields which a rotor subjected to.

18. Excitation System of Synchronous machines A DC current must be supplied in a controllable way to the field circuit on the rotor. Since the rotor is rotating a special arrangement is required to obtain the DC power for field windings. There are 2 common approaches for this: 1.Slip Ring and Brush Method 2.Brush less DC (BLDC) exciter a)BLDC without pilot exciter b)BLDC with a pilot exciter

19. 1. Slip Ring and Brush Method:

20. The synchronous machine’s rotor winding is directly supplied from an external DC source through brushes that ride on slip rings. Slip rings are metallic rings completely encircling the rotor shaft but insulated from it. Brushes are a block of graphite like carbon compound and remain stationary while slip rings rotate. Cost effective for smaller machine

21. Problems: I.Regular check for wear needed. II.Brush voltage drop can be large when the machine field current is high. 2. BLDC exciter: This is a separate small three phase synchronous generator whose field is mounted on its stator while armature is mounted on the main machine’s shaft so that the output AC can be rectified into DC using a 3 phase rotating rectifier and will be fed to main machine’s rotor winding.

22. A. BLDC without pilot exciter:

23. Advantage: Since no mechanical contacts occur between the rotor and stator, a brushless exciter requires less maintenance. Problem: The excitation is not completely independent of external power sources. Solution: To make the excitation of a generator completely independent of any external power source, a small pilot exciter can be used

24. B. BLDC with a pilot exciter:

25. Pilot Exciter: A pilot exciter is a small ac generator with permanent magnets mounted on the rotor shaft and a 3 phase windings on the stator. It produces the power for the field circuit of the exciter, which in turn controls the field circuit of the main machine. If a pilot exciter is included on the generator shaft, then no external electric power is required. Emergency/Necessity: Even though machines with brushless exciters do not need slip rings and brushes, they still include the slip rings and brushes so that an auxiliary source of DC field current is available in emergencies.