1. ET 332a Dc Motors, Generators and Energy Conversion Devices 1 Lesson 20 332a.pptx

2. LEARNING OBJECTIVES Lesson 20 332a.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.

3. Lesson 20 332a.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 = 0.0172 ohms R IP = 0.005 ohms Series field resistance R s = 0.0023 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.

4. Lesson 20 332a.pptx 4 0.0172  0.005  0.0023  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.)

5. Lesson 20 332a.pptx 5 Answer b.)

6. Lesson 20 332a.pptx 6 Answer c.)

7. Lesson 20 332a.pptx 7 Part d.) I a2 = ??? Use Fig 11-7 to find the total field mmf.

8. Lesson 20 332a.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

9. Lesson 20 332a.pptx 9 Use speeds n 1 = 850 rpm n 2 =900 rpm. Speed is inversely proportional to B p. Speed Ratio

10. Lesson 20 332a.pptx 10

11. Lesson 20 332a.pptx 11 Solve the quadratic equation using the quadratic formula

12. Lesson 20 332a.pptx 12 0.0172  0.005  0.0023  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.

13. Lesson 20 332a.pptx 13 Number of turns in each field winding F net = 4000 A-t Current computed from the previous part.

14. Lesson 20 332a.pptx 14 4000 A-t/pole Answer e.)

15. ET 332a Dc Motors, Generators and Energy Conversion Devices Lesson 20 332a.pptx 15