Electrical Engineering (EE) > Short Notes for Electrical Engineering > Short Notes: DC Machines

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Page 1 Induced emf equation a NZ P E = 60 A ? ?? ?? ?? ? ? = flux per pole wb ? ? ? N = speed of machine rpm P = number of poles A = number of paralled path Z = number of conductors A = 2 for wave winding A = P for lap winding If speed is given in rad/sec a ZP E = 2A ?? ?? ?? ? ?? where ? = speed (rad/s) ? ? m PZ = K 2A ?? ?? ?? ? ? ? ? ? ? m PZ K = = machine constant 2A ? Developed Torque ma T = K I ? m PZ K = = machine constant 2A ? = flux per pole ? a I = armature current Page 2 Induced emf equation a NZ P E = 60 A ? ?? ?? ?? ? ? = flux per pole wb ? ? ? N = speed of machine rpm P = number of poles A = number of paralled path Z = number of conductors A = 2 for wave winding A = P for lap winding If speed is given in rad/sec a ZP E = 2A ?? ?? ?? ? ?? where ? = speed (rad/s) ? ? m PZ = K 2A ?? ?? ?? ? ? ? ? ? ? m PZ K = = machine constant 2A ? Developed Torque ma T = K I ? m PZ K = = machine constant 2A ? = flux per pole ? a I = armature current Classification of DC Machine (i) Separately excited (ii) Shunt excited (iii) Series excited (iv) Compound Excited Page 3 Induced emf equation a NZ P E = 60 A ? ?? ?? ?? ? ? = flux per pole wb ? ? ? N = speed of machine rpm P = number of poles A = number of paralled path Z = number of conductors A = 2 for wave winding A = P for lap winding If speed is given in rad/sec a ZP E = 2A ?? ?? ?? ? ?? where ? = speed (rad/s) ? ? m PZ = K 2A ?? ?? ?? ? ? ? ? ? ? m PZ K = = machine constant 2A ? Developed Torque ma T = K I ? m PZ K = = machine constant 2A ? = flux per pole ? a I = armature current Classification of DC Machine (i) Separately excited (ii) Shunt excited (iii) Series excited (iv) Compound Excited Page 4 Induced emf equation a NZ P E = 60 A ? ?? ?? ?? ? ? = flux per pole wb ? ? ? N = speed of machine rpm P = number of poles A = number of paralled path Z = number of conductors A = 2 for wave winding A = P for lap winding If speed is given in rad/sec a ZP E = 2A ?? ?? ?? ? ?? where ? = speed (rad/s) ? ? m PZ = K 2A ?? ?? ?? ? ? ? ? ? ? m PZ K = = machine constant 2A ? Developed Torque ma T = K I ? m PZ K = = machine constant 2A ? = flux per pole ? a I = armature current Classification of DC Machine (i) Separately excited (ii) Shunt excited (iii) Series excited (iv) Compound Excited Page 5 Induced emf equation a NZ P E = 60 A ? ?? ?? ?? ? ? = flux per pole wb ? ? ? N = speed of machine rpm P = number of poles A = number of paralled path Z = number of conductors A = 2 for wave winding A = P for lap winding If speed is given in rad/sec a ZP E = 2A ?? ?? ?? ? ?? where ? = speed (rad/s) ? ? m PZ = K 2A ?? ?? ?? ? ? ? ? ? ? m PZ K = = machine constant 2A ? Developed Torque ma T = K I ? m PZ K = = machine constant 2A ? = flux per pole ? a I = armature current Classification of DC Machine (i) Separately excited (ii) Shunt excited (iii) Series excited (iv) Compound Excited Characteristics of DC Generator External characteristics If no-load voltage is same for all types of generators: There are two categories of compound generators/motors 1. Cumulative Compound => If series field flux aids the shunt fields flux. 2. Differentially Compound => If ser es field flux opposes the shunt field flux. If full – load voltage of all generators is kept same 1 ? series excited 5 ? separately excited 2 ? over compound 6 ? shunt excited 3 ? level compound 7 ? differentially compound 4 ? under compound Conditions for voltage build-up in Shunt Generator 1) There must be residual flux. 2) Correct polarity of field winding with respect to armature winding so that field flux aids residual flux for a given direction of rotation. 3) Field Resistance must be less than critical value ? ? f f cr R < R Critical resistance is equal to the slop of air-gap line. 4) Speed of rotation should be more than critical value for a given field resistance f R . cr N > N E yRead More

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