Synchronous machine

Synchronous machine

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  Synchronous Machines Mr. V.S. Wadkar Assistant Professor, Department of Electrical Engineering, SITCOE
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  Synchronous Machines • Synchronousgenerators 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
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  Synchronous Motors • Construction –Stator identical to that of a three-phase induction motor – now called the “armature” – Energize from a three-phase supply and develop the rotating magnetic field – Rotor has a DC voltage applied (excitation) – Rotor could be a permanent-magnet type
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  Synchronous Motors • Operation –Magnetic field of the rotor “locks” with the rotating magnetic field – rotor turns at synchronous speed • The rotor acting as a bar magnet will turn to line up with the rotating magnet field. The rotor gets locked to the RMF and rotates unlike induction motor at synchronous speed under all load condition
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  Stator Construction • Statoris identical to the induction motor • Laminated low silicon steel rings joined together • Slots insulated with Mylar • Example of 36 slot stator with 3 coil conductors per slot, 12 slots per phase
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  Stator Construction • Slotinsulator inserted by hand • Coils inserted by hand • Stator is identical to the induction motor • Laminated low silicon steel rings joined together • Slots insulated with Mylar • Example of 36 slot stator with 3 coil conductors per slot, 12 slots per phase Stator frame Stator slots with insulator
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  Stator Construction • Coilscan be placed in single or double layers Stator slot
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  Rotor Construction • SalientPole Two types of rotor • Cylindrical
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  Rotor Construction Salient Pole Differencebetween pole face curvature and stator creates non- linear variation in flux across pole face Non-linear variation in flux across pole face produces sinusoidal change in the induced EMF
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  Rotor Construction Cylindrical Difference incoil spacing creates non- linear variation in flux around the rotor surface Non-linear variation in flux around rotor surface produces sinusoidal change in the induced EMF
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  Rotor Construction Cylindrical Difference incoil spacing creates non- linear variation in flux around the rotor surface Non-linear variation in flux around rotor surface produces sinusoidal change in the induced EMF
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  Calculations Synchronous speed Induced EMF fS= supply frequency required p = pole pairs Φ = flux per pole set by rotor current z = conductor in series per phase Volts per phase RPM
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  Operation as aSynchronous Generator Generated EMF relationship • Rotor speed • Rotor current Relationship between open circuit stator EMF and rotor current is a straight line until the steel begins to saturate when it becomes non-linear. The open circuit EMF generated depends upon saturation Open circuit stator EMF Rotor current linear
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  Operation as aSynchronous Motor The V-curve The rotor current can be adjusted to vary the power factor of the stator Unity power factor is achieved when stator current is at its minimum This machine can be used to correct power factor of induction motors when connected in parallel
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  Application of SynchronousMotors Synchronous motors are usually used in large sizes because in small sizes they are costlier as compared with induction machines. The principal advantages of using synchronous machine are as follows: – Power factor of synchronous machine can be controlled very easily by controlling the field current. – It has very high operating efficiency and constant speed. – For operating speed less than about 500 rpm and for high-power requirements (above 600KW) synchronous motor is cheaper than induction motor. In view of these advantages, synchronous motors are preferred for driving the loads requiring high power at low speed; e.g; reciprocating pumps and compressor, crushers, rolling mills, pulp grinders etc.
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  Concept of theinfinite bus When a synchronous generator is connected to a power system, the power system is often so large that nothing the operator of the generator does will have much of an effect on the power system. An example of this situation is the connection of a single generator to the Canadian power grid. Our Canadian power grid is so large that no reasonable action on the part of one generator can cause an observable change in overall grid frequency. This idea is idealized in the concept of an infinite bus. An infinite bus is a power system so large that its voltage and frequency do not vary regardless of how much real or reactive power is drawn from or supplied to it.
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  Before connecting agenerator 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. Synchronization Load Generator 2 Generator 1 Switch a b c a/ b/ c/
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  Parallel operation ofsynchronous generators There are several major advantages to operate generators in parallel: • Several generators can supply a bigger load than one machine by itself. • Having many generators increases the reliability of the power system. • It allows one or more generators to be removed for shutdown or preventive maintenance.