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Sherbius Drive of Electrical Machines
Transcript of Sherbius Drive of Electrical Machines
? - For most of the industrial applications, mechanical loads require to vary the speed within precise limits. COMPONENTS OF SHERBIUS SYSTEM Double range Scherbius Equipment Other Topologies Adjustable Speed Problem Earlier Drives Sherbius System - Drive for Wound Rotor Machines : Instead of voltage drop on variable resistance, variable frequency voltage is applied to the rotor windings. Motor - Generator Sets Components of
Sherbius System REFERENCES - HESLA E. ,BLALOCK T. J. .Electrification of a Major Steel Mill Part 5: Scherbius and Kraemer Drives. 2007 EMRE DURNA - Speed variation should be satisfied by considering the overall efficiency and size of the system. - Rolling Mills, Pump Applications, Mixing Machines - Overall system cost should be as low as possible Wound Rotor Machines - Extra generator is used to drive the motor at required speed. - Earliest solution - Introduces complexity and inefficiency. - Existance of two machines; bulky, costly, inefficient - Rotor side of the machine is wounded instead of short-circuited squirel cage rotor. - Adjustable speed is achieved by connecting rheostat on slip ring connectors. - Inefficient due to rotor resistance and speed depends on load variations. - Simple, Cheap, Easy to Use BUT Inefficient, Dependency on load variations - This voltage is produced by an auxiliary machine called Sherbius Regulating Machine. - First marketed by General Electric Company, in about 1916. - Licensing agreement with the inventor Arthur Sherbius, German electrical engineer 1878 - 1929. - Lately known as the inventor of famous code machine ENIGMA, which is used effectively in II. World War. Regulating Transformer - The operating speed of the main motor is adjusted using this adjustable autotransformer. - Connected to the stator side of the Sherbius Regulating Machine - Fixed position of transformer tap provides the load independent speed regulation. Features of Sherbius System Sherbius Regulating Machine - Stator of the Sherbius Regulating Machine is connected to regulating transformer. Auxiliary Induction Machine - The machine is commutator type but wounded for 3 phase operation. Three sets of commutators are 120 degrees apart from each other so that the stator side should produce 3 phase voltage. - In some applications, Sherbius Machine is mounted on the shaft of main motor. In this case, the power ,which otherwise wasted in resistance, can be returned to the shaft as useful power. (CONSTANT POWER DRIVE) - In some applications (like in this topology), shaft of Sherbius Machine is directly coupled to an auxiliary squirrel-cage induction machine so that the power absorbed by the Sherbius Machine can be returned to the mains. ( CONSTANT TORQUE DRIVE ) - Used as a Generator in Motor - Generator set of the Sherbius System. - Directly coupled to the Sherbius Regulating Machine in order to convert absorbed power to the mains. - For some applications this machine can be replaced by Synchronous Machines. - Can be Wound Rotor type to integrate soft - starting resistances. Features of Sherbius System - Better efficiency than earlier methods. - Complex, Extra Induction Motor - Speed regulation is independent of load conditions. Advantages - Can be controlled manually by operator. Disadvantages - Huge size, Costly - Handling failures require well-trained staff - The given topology (shunt excitation of Sherbius Machine) is only for the speeds lower than synchronous speed. - The speed of the main motor is determined by the rotor frequency applied by Sherbius Machine. The voltage produced by machine is totally depends on the main motor, Sherbius machine should give whatever the required voltage is for corresponding shaft speed. - Sherbius System can not change the speed when the machine is not loaded. (No rotor current when the main machine is at synchronous speed) Adv vs Disadv Other Topologies - LEONHARD W. Control of Electrical Drives, 3rd Ed. 2001 - BAKSHI U. A., BAKSHI M.,V. Electrical Machines - II . 2009 - ADKINS B. , GIBBS W. J. Polyphase Commutator Machines. 1951 - Can not adjust speed at no load conditions