ET 332a Dc Motors, Generators and Energy Conversion Devices 1 Lesson a.pptx

LEARNING OBJECTIVES Lesson a.pptx 2 After this presentation you will be able to:  Determine the operating point of a dc compound motor  Observe the effect of driving a constant torque load on motor performance  Compute the value of field resistance to produce a given speed  Solve non-linear dc motor problems given a magnetization curve.

Lesson a.pptx 3 Example 20-1: Compound motor with non-linear magnetic circuit. Ten percent of series field used to offset armature reaction. 240 V, 125 HP, 850 rpm, R a = ohms R IP = ohms Series field resistance R s = ohms Shunt field resistance R f = 49.2 ohms Series field turns N s = 4.5 t Shunt field turns N f = 577 t Efficiency at rated load 85.4% Drives a constant torque load Compute a.) I f b.) I a c.) developed torque d.) I a when a series R is added to increase speed to 900 rpm. e.) The field resistance in series with the shunt coil to produce the 900 rpm speed.

Lesson a.pptx    49.2  ????  240 V n 1 = 850 rpmN f = 577 t N s = 4.5 t Magnetization curve given in figure 11-7 in text Find I a from rated output power and efficiency Answer a.)

Lesson a.pptx 5 Answer b.)

Lesson a.pptx 6 Answer c.)

Lesson a.pptx 7 Part d.) I a2 = ??? Use Fig 11-7 to find the total field mmf.

Lesson a.pptx 8 For non-linear operation, torque in proportional to B p ∙I a For constant torque load, T D1 = T D2 Solve for I a2 This relates I a2 to B p2 Need another equation

Lesson a.pptx 9 Use speeds n 1 = 850 rpm n 2 =900 rpm. Speed is inversely proportional to B p. Speed Ratio

Lesson a.pptx 10

Lesson a.pptx 11 Solve the quadratic equation using the quadratic formula

Lesson a.pptx    49.2  ????  240 V n 1 = 850 rpmN f = 577 t N s = 4.5 t Part e.) Find the value of R x to produce 900 rpm with constant T load Use the value of B p2 computed from the previous part. Use the magnetizing curve from the text find the total mmf required to product the computed flux density.

Lesson a.pptx 13 Number of turns in each field winding F net = 4000 A-t Current computed from the previous part.

Lesson a.pptx A-t/pole Answer e.)

ET 332a Dc Motors, Generators and Energy Conversion Devices Lesson a.pptx 15