Dc_Synchronous_Machines

DC Machines: Stator Main Parts: • • • • A stationary stator : carries field poles or field coils Rotor: carries armature or armature windings Commutator: for ac to dc conversion Brushes : slides on commutator and collect or supply dc current DC MACHINES • The field poles, which produce the needed flux, are mounted on the stator and carry windings called field windings or field coils • The armature core, which carries the armature windings, is generally on the rotor and is made of sheet-steel laminations. • The commutator is made of hard-drawn copper segments insulated from one another by mica. • armature windings are connected to the commutator segments over which the carbon brushes slide and serve as leads for electrical connection. The armature winding is the load-carrying winding. Electromagnetic Energy Conversion: 1. When armature conductors move in a magnetic field (produced by the current in stator field winding), voltage is induced in the armature conductors. When current carrying armature conductors are placed in a magnetic field (produced by the current in stator field winding), the armature conductors experience a mechanical force. 2. These two effects occur simultaneously in a DC machine whenever energy conversion takes place from electrical to mechanical or vice versa. Electromagnetic Force, f f=Bli, where B, f and i are mutually perpendicular. Motional Voltage, e e=Blv, where B, v and e are mutually perpendicular. Wire loop rotating in a magnetic field. 6 Elementary DC Generator with Commutator and Brushes: Brush 7 Force on a current-carrying wire in a magnetic field. 8 Flux compression and resulting force. 9 Commutator and Brushes on DC Machine To keep the torque ( or current) on a DC machine from reversing every time the coil moves through the plane perpendicular to the magnetic field, a split-ring device called a commutator is used to reverse the current at that point. The electrical contacts to the rotating ring are called "brushes” . Commutator Action Commutator Segment1 A + V + V + V + V - - - - AA Commutator Segment 2 Armature conductors: connected to commutator A AA DC Machine Equivalent circuit  The magnetic field produced by the stator poles induces a voltage in the rotor (or armature) coils when the Rotor is rotated.  This induced voltage is represented by a voltage source.   The field circuit is represented by a resistance 17 Equivalent Circuit of a DC Machine If + Ia_mot Ra Vf Field circuit Armature circuit Rf + Ea Vt Ra Rf Ea Ia Shunt DC Machine: Field and armature are in parallel Vt IL + Ia_ gen If + Vf = I f Rf Vt Ea + I a Ra = Vt Ea − I a Ra = (motor) (Generator) Generated emf and Electromagnetic Torque Vf = I f Rf Vt Ea + I a Ra = Vt Ea − I a Ra = (motor) (Generator) Motor: V > Ea Generator: V < Ea Voltage generated in the armature circuit due the flux of the stator field current Ea = K a φ ωm Ka: design constant Electromagnetic torque: Te = K a φ I a Power: = Ea I a Teωm P = MACHINE CLASSIFICATION  DC machines may be classified on the basis of the interconnections between the field and armature windings. CLASSIFICATION OF DC MACHINES There are several possible connections for field and armature circuits - classification of DC machines is determined by the way they are connected 1. Separately excited dc machine: Field and armature are separate and are excited separately. + Vt − + Vf Field _ Armature CLASSIFICATION OF DC MACHINES 2. Shunt DC Machine: Field and armature are connected in parallel. Vf = Vt + + Vf Field _ Vt − Armature CLASSIFICATION OF DC MACHINES 2. Series DC Machine: Field and armature are in series. Ia If If = I a + Vt − Field Armature CLASSIFICATION OF DC MACHINES 3. Compound DC Machine: has both series and shunt field windings. 1) Cummulative Compounded DC Machine: Magnetic flux produced by both windings are additive 2) Differentially Compounded DC Machine: Flux produced by both windings opposes each other (subtractive) + Vt Series Field Shunt Field − Cummulative Compounded DC Machine: Field produced by windings FF and SS are additive DC Machines: Some Facts  The direct current (dc) machine can be used as a motor or as a generator.  DC Machine is most often used for a motor.  The major advantages of dc machines are that speed and torque can be controlled easily and independently.  However, their application is limited to mills, mines and trains. As examples, trolleys and underground subway cars may use dc motors. 26  In the past, automobiles were equipped with dc dynamos to charge their batteries.  Even today the starter is a series dc motor  However, the recent development of power electronics has reduced the use of dc motors and generators.  The electronically controlled ac drives are gradually replacing the dc motor drives in factories. 27 Armature Reaction • If a load is connected to the terminals of the dc machine, a current will flow in its armature windings. • • This current flow will produce a magnetic field of its own, which will distort the original magnetic field from the machine’s field poles. • This distortion of the magnetic flux in a machine as the load is increased is called the armature reaction. • Compensating winding is used to prevent this distortion. SYNCHRONOUS MACHINES • Synchronous generators or alternators are used to convert mechanical power derived from steam, gas, or hydraulic-turbine to ac electric power • Synchronous generators are the primary source of electrical energy we consume today • Large ac power networks rely almost exclusively on synchronous generators • Synchronous motors are built in large units compare to induction motors (Induction motors are cheaper for smaller ratings) and used for constant speed industrial drives CONSTRUCTION  Basic parts of a synchronous generator: • Rotor - has field winding supplied by dc power (similar to dc field windings, but on rotor) Stator - similar to the stator winding of 3-phase induction machine • Various Types   Salient-pole synchronous machine Cylindrical or round-rotor synchronous machine Salient-Pole Synchronous Generator 1. Most hydraulic turbines have to turn at low speeds (between 50 and 300 rpm) 2. A large number of poles are required on the rotor d-axis N Non-uniform air-gap D ≈ 10 m q-axis S S Turbine Hydro (water) N Hydrogenerator Salient-Pole Synchronous Generator Stator Cylindrical-Rotor Synchronous Generator Turbine D≈1m L ≈ 10 m Steam d-axis Stator winding  High speed  3600 rpm ⇒ 2-pole  1800 rpm ⇒ 4-pole  Direct-conductor cooling (using hydrogen or water as coolant)  Rating up to 2000 MVA q-axis N Uniform air-gap Stator Rotor winding Rotor S Turbogenerator Cylindrical-Rotor Synchronous Generator Stator Cylindrical rotor Operation Principle The rotor of the generator is driven by a prime-mover A dc current is flowing in the rotor winding which produces a rotating magnetic field within the machine The rotating magnetic field induces a three-phase voltage in the stator winding of the generator Electrical Frequency Electrical frequency produced is locked or synchronized to the mechanical speed of rotation of a synchronous generator: nP f = 120 where f = electrical frequency in Hz P = number of poles n= mechanical speed of the rotor, in r/min Generated Voltage The generated voltage of a synchronous generator is given by E = Kc φ f where φ = flux in the machine (function of field current If) f = electrical frequency Kc= synchronous machine constant E If Magnetisation curve of a synchronous generator. Voltage Regulation A convenient way to compare the voltage behaviour of two generators is by their voltage regulation (VR). The VR of a synchronous generator at a given load, power factor, and at rated speed is defined as Voltage Regulation = Enl − V fl V fl ×100% Where Vfl is the full-load terminal voltage, and Enl (equal to Ef) is the no-load terminal voltage (internal voltage) at rated speed when the load is removed without changing the field current. For lagging power factor (PF), VR is fairly positive, for unity PF, VR is small positive and for leading PF, VR is negative. Equivalent Circuit_1 o o o – – – The internal voltage Ef produced in a machine is not usually the voltage that appears at the terminals of the generator. The only time Ef is same as the output voltage of a phase is when there is no armature current flowing in the machine. There are a number of factors that cause the difference between Ef and Vt: The distortion of the air-gap magnetic field by the current flowing in the stator, called the armature reaction The self-inductance of the armature coils. The resistance of the armature coils. Three-phase equivalent circuit of a cylindrical-rotor synchronous machine The voltages and currents of the three phases are 120o apart in angle, but otherwise the three phases are identical. + Vt Ef1 + jXs Ra Ia1 VL-L VL-L =3Vt Equivalent Circuit_2 Ia + + Ef generator Ia Vt Motor operation jXs Rs + Equivalent circuit of a cylindrical-rotor synchronous machine Phasor Diagram Phasor diagram of a cylindrical-rotor synchronous generator, for the case of lagging power factor Lagging PF: |Vt||Ef| for underexcited condition Synchronization Before connecting a generator in parallel with another generator, it must be synchronized. A generator is said to be synchronized when it meets all the following conditions: • • • • The rms line voltages of the two generators must be equal. The two generators must have the same phase sequence. The phase angles of the two a phases must be equal. The oncoming generator frequency is equal to the running system frequency. a Generator 1 b c Switch Load a/ Generator 2 b/ c/ Power-angle or torque-angle characteristic Real power or torque Pull-out torque as a generator generator −δ −π −π/2 0 motor +π/2 +π +δ Pull-out torque as a motor