Motors

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## Motors

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**Motors**Lecture 11.4**Hans Christian Oersted (1777 – 1851)**X 1822 In 1820 he showed that a current produces a magnetic field. Ref: http://chem.ch.huji.ac.il/~eugeniik/history/oersted.htm**André-Marie Ampère (1775 – 1836)**French mathematics professor who only a week after learning of Oersted’s discoveries in Sept. 1820 demonstrated that parallel wires carrying currents attract and repel each other. attract A moving charge of 1 coulomb per second is a current of 1 ampere (amp). repel**What do Maxwell’s Eqs. Predict?**B = magnetic flux density (magnetic induction) m = magnetic permeability Magnetic field lines must be closed loops Force on moving charge q Lorentz force B**Vector Multiplication**Scalar (Dot) Product Vector (Cross) Product A A q q B B Direction of C given by "Right-hand rule" (into the screen)**Force on current in a magnetic field**Force on moving charge q -- Lorentz force Current density, j, is the amount of charge passing per unit area per unit time. N = number of charges, q, per unit volume moving with mean velocity, v. j Force per unit length on a wire is**B**Force out i + - i Force in X Rotating Machine**X**Rotating Machine B Force in i - + i Force out**B**Force out commutator i + - i brushes Force in X Rotating Machine**What do Maxwell’s Eqs. Predict?**Corresponds to Faraday’s law of electromagnetic induction A changing magnetic flux B density induces a curl of E**Back emf**X X B B Force out r i a q l + - b i Force in**B**Force out i + - i Force in X Back emf a b**B**Force out commutator i + - i brushes Force in X Back emf**X**X X X Armature with four coil loops S N**Motor Circuit**Power and Torque