LESSON 20: NON-LINEAR OPERATION OF COMPOUND MOTORS

ET 332aDc Motors, Generators and Energy Conversion Devices

1

Lesson 20 332a.pptx

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.

NON-LINEAR COMPOUND MOTOR CALCULATIONS

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, Ra = 0.0172 ohms RIP = 0.005 ohms Series field resistance Rs = 0.0023 ohmsShunt field resistance Rf = 49.2 ohmsSeries field turns Ns = 4.5 tShunt field turns Nf = 577 t Efficiency at rated load 85.4%Drives a constant torque loadCompute 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.

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

4

LoadT= constant

RIP

Interpole

Rf

Ra

Rs

Series Field

Rx

Shunt Field

EaVT

0.0172 W

0.005 W0.0023 W

49.2 W

???? W

240 V

n1 = 850 rpm Nf = 577 t

Ns = 4.5 t

Magnetizationcurve given infigure 11-7 in text

Find Ia from rated output power and efficiency

Answer a.)

Lesson 20 332a.pptx

5

EXAMPLE 20-1 SOLUTION (2)

Answer b.)

EXAMPLE 20-1 SOLUTION (3)

Lesson 20 332a.pptx

6

Answer c.)

Lesson 20 332a.pptx

7

EXAMPLE 20-1 SOLUTION (4)Part d.) Ia2 = ??? Use Fig 11-7 to find the total field mmf.

0 1 2 3 4 5 6 70

0.2

0.4

0.6

0.8

1

1.2Motor Magnetizing Curve

mmf (in 1000 A-t/pole)

Flu

x D

ensi

ty (

T)

.88fnet

1000

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

8

For non-linear operation, torque in proportional to Bp∙Ia

For constant torque load, TD1 = TD2

Solve for Ia2

This relates Ia2 to Bp2

Need another equation

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

9

Use speeds n1= 850 rpm n2=900 rpm. Speed is inversely proportional to Bp.

SpeedRatio

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

10

Lesson 20 332a.pptx

11

EXAMPLE 20-1 SOLUTION (7)Solve the quadratic equation using the quadratic formula

EXAMPLE 20-1 SOLUTION (8)Lesson 20 332a.pptx

12

LoadT= constant

RIP

Interpole

Rf

Ra

Rs

Series Field

Rx

Shunt Field

EaVT

0.0172 W

0.005 W0.0023 W

49.2 W

???? W

240 V

n1 = 850 rpm Nf = 577 t

Ns = 4.5 t

Part e.) Find the value of Rx to produce 900 rpm with constant T load

Use the value of Bp2 computed from the previous part.

Use the magnetizing curve from the text find the total mmf required to product the computed flux density.

Bp2 0.83 T

Rf 49.2

EXAMPLE 20-1 SOLUTION (9)Lesson 20 332a.pptx

13

0 1 2 3 4 5 6 70

0.2

0.4

0.6

0.8

1

1.2Motor Magnetizing Curve

mmf (in 1000 A-t/pole)

Flu

x D

ensi

ty (

T)

.83

fnet

1000

Number of turns in each field winding

Nf 577 t

Shunt

Ns 4.5 t

Series

Fnet = 4000 A-t

Ia21 477.2 A

Current computed from the previous part.

EXAMPLE 20-1 SOLUTION (10)

Lesson 20 332a.pptx

14

4000 A-t/pole

Answer e.)

ET 332aDc Motors, Generators and Energy Conversion Devices

Lesson 20 332a.pptx

15

END LESSON 20