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Engr. Abbas Abbasi Electrical Machines Basic Principles

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Course Info Text Book: Electric Machinery Fundamentals By Stephen J. Chapman Course material available at: http://groups.yahoo.com/group/controlsystemsabbasi

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What is electrical machine? An electrical machine is a device that can convert either mechanical energy to electrical energy or electrical energy to mechanical energy The device that converts mechanical energy to electrical energy is calledgenerator The device that converts electrical energy to mechanical energy is calledmotor The energy conversion in generators and motors takes place under the action of Magnetic Field The device that converts ac electrical energy at one voltage level to ac energy at another voltage level, is called transformer

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So where do we use these electrical devices? Refrigerators Freezers Vacuum cleaners Blenders Washing machines Fans Air conditioners Juicers Tape recorders CD players Grinders Power distribution Rectification Industries Etc.

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Lets have a look on electrical and magnetic quantities QuantitySymbolUnitUnit symbol ResistanceROhm Ω CapacitanceCFaradF InductanceLHenryH CurrentIAmpereA VoltageVVoltV ChargeQCoulombC Magnetic Flux φ WeberWb Magnetic flux densityBTeslaT Magnetic field intensityHAmpere turns/meterA/m PowerPWattW EnergyEJouleJ

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Translational and rotational mechanical quantities Translational and rotational mechanical quantities TranslationalUnitRotationalUnit Force (F)Newton (N) Torque ( τ ) N.m Distance (x)Meter (m) Angle ( θ ) Radians (rad) Velocity (v)m/s Angular velocity ( ω ) Rad/s Acceleration (a)m/s 2 Angular accel. ( α ) Rad/s 2 Mass (M)KgInertia (J)Kg. m 2 Work (W)Joule (J)Work (W)Joule (J)

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Velocity TranslationalRotationalRPSRPM

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Torque Torque is known as a rotational force applied to a rotating body giving angular acceleration. It can be loosely defined as twisting force Product of force applied to the object and the smallest distance between the line of action of the force and the objects axis of rotation

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Newtons law of rotation Newtons law for objects moving along a straight line is: Where F is applied force, m is the mass of the body and a is the acceleration produced Similarly Newtons law for objects rotating in a circular fashion is: Where τ is applied torque, J is inertia and α is angular acceleration

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Work and Power Work (Variable Force) Work (Constant Force) Power Translational Rotational

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Magnetic field principles 1. A current carrying wire produces a magnetic field in the area around it 2. A variable magnetic field induces a voltage in a coil of wire if it passes through that coil 3. A current carrying wire in the presence of a magnetic field has a force induced on it 4. A moving wire in the presence of a magnetic field has a voltage induced on it

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Amperes Law The path of integration in figure above is l c and coil cuts this path N times The units of H are Ampere-turns/ meter Where H Magnetic field intensity dl differential length along the path of integration I net net current in the coil

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Example The coil in the figure below has 20 turns and a current of 2 amperes passes through the coil. Find magnetic field intensity if the mean length of the core is 80 cm. also calculate the new value of H if Turns are doubled Current is doubled Length is doubled

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Magnetic flux (φ) and Magnetic flux density (B) Lines of magnetic field are called flux lines. It is measured in Weber (Wb) Magnetic flux lines passing through unit area is known as Magnetic flux density (B). It is measured in Weber/m 2 or Tesla (T).

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Permeability (µ) The strength of the magnetic field flux produced in the core also depends on the material of the core. Thus, B = magnetic flux density (webers per square meter, Tesla (T)) μ= magnetic permeability of material (Henrys per meter) H = magnetic field intensity (ampere-turns per meter) H represents the effort exerted by the current to establish magnetic field μ represents the relative ease of establishing a magnetic field in a given material

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Relative permeability (µ r ) The permeability of any other material compared to the permeability of the free space is called relative permeability The steels used in modern machines have relative permeability of 2000 to 6000. It means for a given amount of current, 2000 to 6000 times more flux is produced in steel than air for the same value of area

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Magnetic flux expression

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Summary

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