ET 332a Dc Motors, Generators and Energy Conversion Devices · p1 p2 2 1 n n Find the value of p2...

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ET 332a

Dc Motors, Generators and Energy Conversion Devices

1 Lesson 13 332a.pptx

Lesson 13 332a.pptx 2

Explain how changing field excitation of a shunt motor affects its performance

Explain how the internal feedback inherent in the shunt motor maintains a nearly constant shaft speed.

Use shunt motor equations and circuit model to compute motor operating conditions.

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TD = KT(Ia)

Time

TD

Ia

n

TL

Ea

Ea = Ke(n)

1.) Increasing TL causes motor to slow.

2.) Reduced armature

speed decreases Ea. 3. ) More Ia flows in

armature circuit. More Ia means more

TD.

4.) When TD matches TL

system stabilizes at new operating point. Ia higher:

5.) Ea and n return to

almost the same values

e

a

K

En

acir

aTa

R

EVI


Just as in other machines studied up to now, the motor speed, developed torque and generated emf are all proportional. If an operating point and a percent increase/decease is known, the new operating point can be found using proportions.

Ea is proportional to speed

2

1

2a

1a

n

n

E

E

TD is proportional to armature Current

2a

1a

2D

1D

I

I

T

T

1p

2p

2a

1a

2

1

E

E

n

n

Speed is directly proportional to Ea and inversely proportional to field flux

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A 10 HP 240 volt 1200 rpm motor is operating at rated conditions. Determine the percent change in shunt field flux required to lower the speed to 900 rpm. Assume that armature current remains the same.


n1 = 1200 rpm n2 = 900 rpm

Define speeds for two cases

Set up proportions

G1p

acir1aT1

k

RIVn

G2p

acir2aT2

k

RIVn

acir2aT

G2p

G1p

acir1aT

G2p

acir2aT

G1p

acir1aT

2

1

RIV

k

k

RIV

k

RIV

k

RIV

n

n Since Ia1=Ia2 this simplifies

1p

2p

2

1

n

nFind the value of

p2 assuming p1 = 1

33.11rpm 900

rpm 1200 2p

%33%1001

133.1%100

1p

1p2p Answer

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A 200 volt shunt motor is rated a 5 HP and 1000 rpm. At rated output it draws 25 A of line current. The total armature circuit resistance is 0.5 ohms. The field resistance is 100 ohms a.) find the total rotational losses of the motor at rated

conditions b.) Find the no load speed of the motor at 200 volts


200 V

IT = 25 A

Racir Rf

5 HP 1000 rpm

If

Ia

Find Ia

Rotational losses are the difference between Pem and Pshaft

Find Ea

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Answer


b.) At no-load the only power absorbed is what is required to supply Prot.

Power balance on electric side of motor. Neglecting brushes

Put into standard form

Solve quadratic for Ia

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Torques is proportional to armature current Ia. Lower power requires lower torque and armature current

Speed is proportional to Ea


Solve the previous equation for n2

Answer

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Weakening the field increases speed but reduces torque

Example 13-3: A 500 volt 125 HP 1150 rpm shunt motor operates at rated conditions, driving a constant-torque load. The line current at rated conditions is 204.3 amps. The total armature resistance is 0.0343 ohms the field resistance is 96 ohms. a) Determine the steady-state armature current if a 0.052 ohm

resistor is connected in series with the armature and the field is weakened by 10% from its rated value.

b) b.) Determine the steady-state speed for conditions in part a.


VT=500 V

Racir

0.0343

R1=0.052

Rf=96

IT = 204.3 A

If

Find Ea at rated conditions

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Switch in additional resistance, R1, and weaken field

TD1 = TD2 Constant torque load


Field weakened by 10% reduces the value of KT by 10% assuming no magnetic saturation

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Answer Part a

b.) Find speed under the above conditions


Answer Part b

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ET 332a

Dc Motors, Generators and Energy Conversion Devices


ET 332a Dc Motors, Generators and Energy Conversion Devices · p1 p2 2 1 n n Find the value of p2...

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