Lesson 20 332 a pptx ET 332 a

Lesson 20 332 a. pptx ET 332 a Dc Motors, Generators and Energy Conversion Devices LESSON 20: NON-LINEAR OPERATION OF COMPOUND MOTORS 1

Lesson 20 332 a. pptx LEARNING OBJECTIVES After this presentation you will be able to: Ø Ø 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. 2

Lesson 20 332 a. pptx NON-LINEAR COMPOUND MOTOR CALCULATIONS 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, Ra = 0. 0172 ohms RIP = 0. 005 ohms Series field resistance Rs = 0. 0023 ohms Shunt field resistance Rf = 49. 2 ohms Series field turns Ns = 4. 5 t Shunt field turns Nf = 577 t Efficiency at rated load 85. 4% Drives a constant torque load Compute a. ) If b. ) Ia c. ) developed torque d. ) Ia 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. 3

Lesson 20 332 a. pptx EXAMPLE 20 -1 SOLUTION (1) Ns = 4. 5 t Magnetization curve given in figure 11 -7 in text 0. 005 W 0. 0023 W ? ? W 0. 0172 W 49. 2 W n 1 = 850 rpm 240 V Nf = 577 t Answer a. ) Find Ia from rated output power and efficiency 4

Lesson 20 332 a. pptx EXAMPLE 20 -1 SOLUTION (2) Answer b. ) 5

Lesson 20 332 a. pptx EXAMPLE 20 -1 SOLUTION (3) Answer c. ) 6

Lesson 20 332 a. pptx EXAMPLE 20 -1 SOLUTION (4) Part d. ) Ia 2 = ? ? ? Use Fig 11 -7 to find the total field mmf. 7

Lesson 20 332 a. pptx EXAMPLE 20 -1 SOLUTION (5) For non-linear operation, torque in proportional to Bp∙Ia For constant torque load, TD 1 = TD 2 Solve for Ia 2 This relates Ia 2 to Bp 2 Need another equation 8

Lesson 20 332 a. pptx EXAMPLE 20 -1 SOLUTION (6) Use speeds n 1= 850 rpm n 2=900 rpm. Speed is inversely proportional to Bp. Speed Ratio 9

Lesson 20 332 a. pptx EXAMPLE 20 -1 SOLUTION (7) 10

Lesson 20 332 a. pptx EXAMPLE 20 -1 SOLUTION (7) Solve the quadratic equation using the quadratic formula 11

Lesson 20 332 a. pptx EXAMPLE 20 -1 SOLUTION (8) Part e. ) Find the value of Rx to produce 900 rpm with constant T load Ns = 4. 5 t Use the value of Bp 2 computed from the previous part. 0. 005 W 0. 0023 W ? ? W 0. 0172 W 49. 2 W n 1 = 850 rpm 240 V Nf = 577 t Use the magnetizing curve from the text find the total mmf required to product the computed flux density. 12

Lesson 20 332 a. pptx EXAMPLE 20 -1 SOLUTION (9) Number of turns in each field winding Current computed from the previous part. Fnet = 4000 A-t 13

Lesson 20 332 a. pptx EXAMPLE 20 -1 SOLUTION (10) 4000 A-t/pole Answer e. ) 14

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