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DcMotors031406-1

DcMotors031406-1 - Characteristics of DC Motor/Generators...

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Characteristics of DC Motor/Generators M.D.Fox update 5 Oct. 2004; 3/14/06 Introduction The purpose of this lab is to explore the characteristics of DC motors. DC motors are a form of electro- mechanical energy converters. Their efficiency depends on electrical losses due to armature resistance, unrecov- ered energy storage in magnetic fields and core losses including hysteresis and eddy currents, as well as mechani- cal losses due to friction, and vibration. The motors studied are Minertia permanent magnet DC motors with brushes. Since these motors were equipped with an attached rotary encoder it was easy to obtain exact shaft velocity. Two operating conditions were studied, an unloaded solitary motor, and a motor/generator pair in which a motor was shaft coupled to a paired generator, where the load could be varied in controllable fashion. Also examined was motor variability with respect to armature resistance, generator constant, and mechanical losses. The results, which generally follow theory, are presented below. While two of the motors studied were fairly similar, the third motor had significantly different characteristics. We start with some brief theory of permanent magnet DC motors and generators. Then we discuss materials and methods used in making the measuurements. A section is devoted to measurements on individual motors, followed by another section on measurements on motor/generator pairs. Finally we provide discussion and conclusions.
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Theory; Permanent magnet DC Motor. Figure 1 + V A - R A + E A - T M I A D T + w M - Model for permanentmagnet DC motor. T = external torque @ N m D , T M = torque in the air gap, w M = mechanicalangular velocity @ rad ê s D , D = mechanicaldamping factor @ N m s D , V A = terminalvoltage @ J ê C D , E A = induced voltage, @ J ê C D , R A = armatureresistance, A J s ÄÄÄÄÄÄÄÄ C 2 E , I A = input current A C ÄÄÄÄÄ s E . The fundamental relationships for a DC motor include torque/speed relations on the mechanical side and voltage/current relationships on the electrical side as follows [1]: T = T M - w M D V A = E A + I A R A From energy conservation, the electrical power input [after armature resistance losses] in Watt = Joule/- Second must equal the mechanical power output [before frictional losses]. E A I A = T M w M Exercise: confirm that the units on both sides of Eq. 3 reduce to power. Under no external load conditions, @ and neglecting other loss mechanisms D the total power utilized @ dissipated D will be I A 2 R A electrical loss in the armature and w M 2 D in the mechanical section. Typically the back emf E A is related to the angular velocity w M by the generator constant K, i.e. E A = K w M . E A is due to the generator activity of the motor. Assuming small armature resistance, so I A R A is negligible, a constant relationship should be obtained between the input voltage V A , and the angular frequency, w . This is the basis for using the input voltage to control speed in a DC motor. However, note that the speed will sag when a load is applied to the motor.
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