E283C9 - EEEB344 Electromechanical Devices Chapter 9...

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EEEB344 Electromechanical Devices Chapter 9 CHAPTER 9 – DC MOTORS Summary: 1. The Equivalent Circuit of a DC Motor 2. The Magnetization Curve of a DC Machine 3. Separately Excited and Shunt DC Motors - The Terminal Characteristics of a Shunt DC Motor - Nonlinear Analysis of a Shunt DC Motor - Speed Control of Shunt DC Motors - The Effect of an Open Field Circuit 4. The Permanent-Magnet DC Motor 5. The Series DC Motor - Induced Torque in a Series DC Motor - The Terminal Characteristic of a Series DC Motor - Speed Control of Series DC Motors. 6. The Compounded DC Motor - The Torque-Speed Characteristic of a Cumulatively Compounded DC Motor - The Torque-Speed Characteristic of a Differentially Compounded DC Motor - The Nonlinear analysis of Compounded DC Motors - Speed Control in the Cumulatively Compounded DC Motor 1
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EEEB344 Electromechanical Devices Chapter 9 1. The Equivalent Circuit of a DC Motor In this figure, the armature circuit is represented by an ideal voltage source E A and a resistor R A . This representation is really the Thevenin equivalent of the entire rotor structure, including rotor coils, interpoles and compensating windings, if present. The brush voltage drop is represented by a small battery V brush opposing the direction of current flow in the machine. The field coils, which produce the magnetic flux in the motor are represented by inductor L F and resistor R F . The separate resistor R adj represents an external variable resistor used to control the amount of current in the field circuit. Some of the few variations and simplifications: i- The brush drop voltage is often only a very tiny fraction of the generated voltage in the machine. Thus, in cases where it is not too critical, the brush drop voltage may be left out or included in the R A . ii- The internal resistance of the field coils is sometimes lumped together with the variable resistor and the total is called R F. iii- Some generators have more than one field coil, all of which appear on the equivalent circuit. The internal generated voltage is given by: E A = K φϖ and the torque induced is τ ind = K φΙ Α 2 (a) The equivalent circuit (b) A simplified equivalent circuit eliminating the brush voltage drop and combining R adj with the field resistance.
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EEEB344 Electromechanical Devices Chapter 9 2. The Magnetization Curve of a DC Machine E A is directly proportional to flux and the speed of rotation of the machine. How is the E A related to the field current in the machine? The field current in a dc machine produces a field mmf given by F=N F I F . This mmf produces a flux in the machine in accordance with its magnetization curve, shown below: Since the field current is directly proportional to the mmf and since E A is directly proportional to flux, it is customary to present the magnetization curve as a plot of E A vs field current for a given speed ϖ ο . NOTE: Most machines are designed to operate near the saturation point on the magnetization curve. This
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This note was uploaded on 03/03/2010 for the course POWER 332 taught by Professor Dr during the Spring '10 term at Ain Shams University.

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E283C9 - EEEB344 Electromechanical Devices Chapter 9...

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