1 Electrical Machines EELE 4350 Dr Assad AbuJasser

1 Electrical Machines (EELE 4350) Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

2 Assad Abu-Jasser, Ph. D Electric Power Engineering Site. iugaza. edu. ps/ajasser@iugaza. edu. ps Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

3 Chapter 7 Synchronous Generators Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

4 Introduction Synchronous generators are built inaremuch larger referred sizes than Alternating-current (ac) generators commonly to dc as counterparts. Increase inorpower requires thicker conductors to carry high synchronous generators alternators currents & minimize copper losses and stator offers this option and A synchronous machine, whether it istoa hold generator or awinding motor, operates at therefore it is the preferred member armature a synchronous speed, that is, at speed of rotating magnetic field created The output of a N synchronous generator is of alternating type and the by the field coils s=120 f/p (rpm) armature conductors in the stator can be directly connected to the The emf induced in the armature of a dc-generator is of transmission line eliminating the need winding for slip rings alternating type. Therefore, it can be converted to an ac generator by (a) Since mostthe of the heat is produced by the an outer replacing commutator with a set of armature slip rings winding, (b) rotating the stationary can be cooled speed more efficiently than inner rotating armature atmember constants synchronous member Induced voltage is produced when there is a relative motion of conductor Since armature winding of synchronous is morethe involved than with respect to magnetic flux. It does not machine matter whether conductors field winding, it is easier toor construct it on stationary member conductor rotate in a stationary field a rotating field links a stationary Since the induced voltage is it is easier to insulate it when it is The second arrangement is quite morehigh, suitable for synchronous generators. wound in the stationary member Thus, the stationary member (stator) of synchronous generator is the armature and the rotating member (rotor)on carries the is field winding to The placement of low-power field winding the rotor easier via slip provide rings. If flux the field is supplied by permanent magnets, the slip rings can be dispensed with Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

5 Construction of a Synchronous Machine Stator The stator (armature) is made of thin laminations of highly permeable steel These laminations are held together by the stator frame that made of cast iron or mild steel plates & designed to carry flux and provide mechanical support The inside has slots that intended to house thick armature conductors Conductors symmetrically arranged to form balanced polyphase winding The axial length of stator core is short for slow-speed, and large in diameter The axial length can be many times its diameter for high-speed generators these are totally enclosed and require forced-air circulation for cooling Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

6 Stator Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

7 Construction of a Synchronous Machine Rotor There are two types of rotors used: cylindrical rotor and salient-pole rotor The rotor rotates at the synchronous speed by a prime mover such as a steam turbine Rotor has as many poles as the stator and its winding carries dc current to produce constant flux per pole Field winding receives its power from a 115 - or a 230 -V dc generator driven by same prime mover or a separate motor Salient-pole rotor is used in low- and medium-speed generators to reduce windage loss Cylindrical rotors offer quiet operation at high speeds, better balance, and reduces windage loss Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

8 Construction of a Synchronous Machine Armature Winding The stators (armature) of synchronous generators are wound with three distinct and independent windings to generate 3 -phase power. Each winding represents one phase of a three-phase generator The three windings are exactly alike in shape and form but are displaced from each other by exactly 120 o electrical to ensure 3 -phase emfs The three-phase windings may be connected to form star (Y) or a delta (∆) connection. When Y connected the neutral point is also brought out A double-layer winding is often used to wind the armature of a synchronous generator. The number of coils is equal the number of slots The number of coils per phase (or the number of slots per phase per pole for a double-layer winding) is n=S/(Pq), S=slots, P=poles, and q=phases. Where n is the phase group or phase belt 4 -pole, 24 -slot synchronous generator has n=2 and there are 12 phase groups (poles х phases). All coils in a phase group are connected in series Each coil in a phase group can be wound as a full-pitch i. e. spans for 180 o electrical. Fractional pitch is common where the span is less than 180 o Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

9 Construction of a Synchronous Machine Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

10 Operation of Synchronous Generator Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

11 Operation of Synchronous Generator Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

12 Example 7. 1 A three-phase, 4 -pole, synchronous generator has 24 slots. Determine the slot pitch, the coil pitch, and the number of coils in a phase group. Sketch the placement of the coils for one of the phases. Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

$ρ fractional-pitch coil winding factor Pitch, Distribution and Winding Factors kw =Kp. Kd 13$

ρ fractional-pitch coil winding factor Pitch, Distribution and Winding Factors kw =Kp. Kd 13 γ slot-pitch Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

14 Winding Connections Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

15 Induced emf Equation kp pitch factor kd distribution factor kw winding factor Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

16 Example 7. 5 Each coil of a double-layer wound, 16 -pole, 144 -slot, three-phase, Y-connected, synchronous generator has 10 turns. The rotor is driven at a speed of 375 rpm. The flux per pole is 25 m. Wb. Each phase winding is connected in two parallel paths. Determine (a) the frequency of the induced emf, (b) the phase voltage, and (c) the line voltage Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

17 The Equivalent Circuit Armature Power Reaction. Factor (pf=1) Leading Lagging Armature Leakage-Reactance Resistance Voltage Drop If Φp is the flux per pole under no load, then the generated voltage Ea E current Ia in per-phase the armature generated winding voltage produces of a a be the must lag Φp by 90 o The Let The terminal an ac generator depends synchronous flux. A part of voltage the generator flux of called and leakage Ia the flux per-phase links the Since the power factor is unity, upon the load and may be larger or smaller than the current armature supplied winding toonly the and load gives rise to a leakage phase current Ia is in phase with the generated reactance Xa voltage terminal phase voltage Va If Ra is the per-phase resistance of the armature As the phase Ithan passes a X The terminal voltage may actually be current higher winding, leakage then reactance the voltage causes drop a across voltage itdrop is Iaх. R j. I , a a a through armature winding, its mmf the generated the power factor (pf) which leads Ia voltage by 90 o. when produces Φ in phase with Ia and ar The I R voltage drop is in phase with the load a a is leading the effective flux Φe=Φp+Φar current A phasor Ia. Since diagram Ra causes is a graph a power that loss depicts in the Earthe , armature reaction ar produces For unity and factors, terminal generator, relationships it lagging isbetween keptpower asΦ per-phase small as possible generated to emf, lags Φar by 90 o and Ee=Ea+Ear voltage is than theterminal generated voltage minimize Eathe , smaller the copper per-phase losses voltage Va, and The terminal voltage Va is obtained the voltage drops Ia. Ra andfrom j. Ia. XEae=Va+Ia(Ra+j. Xa). Armature reaction reduces effective flux Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

18 Equivalent Circuit and Phasor Diagram Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

19 Example 7. 6 A 9 -k. VA, 208 -V, three-phase, Y-connected, synchronous generator has a winding resistance of 0. 1Ω/phase and a synchronous reactance of 5. 6 Ω/phase. Determine its voltage regulation when the power factor of the load is (a) 80% lagging, (b) unity, and (c) 80% leading. Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

20 Power Relationship Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

21 Approximate Power Relationship Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

22 Example 7. 7 A 9 -k. VA, 208 -V, 1200 -rpm three-phase, 60 -Hz, Y-connected, synchronous generator has a field-winding resistance of 4. 5Ω. The armature-winding impedance is 0. 3+j 5 Ω/phase. When the generator operates at its full load and 0. 8 pf lagging, the field winding current is 5 A. The rotational loss is 500 W. Determine (a) the voltage regulation, (b) the efficiency of the generator, and (c) the torque applied by the prime mover. Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

23 Synchronous Generator Tests OC-Test SC-Test Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)

24 Example 7. 8 A 500 -k. VA, 2300 -V, 1200 -rpm three-phase, Y-connected, synchronous generator is operated at its rated speed to obtain its rated no-load voltage. When a short circuit is established the phase current is 150 A. the average resistance of each phase is 0. 5 Ω. Determine the synchronous reactance per phase. Using the per-unit system, determine the percent voltage regulation when the generator delivers the rated load at its rated voltage and 0. 8 pf lagging. Dr. Assad Abu-Jasser, EE- Department - IUGaza Electrical Machines (EELE 4350)