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ECE 441 1

Induction-Motor Starting

Connect directly across full-voltage using

a contactor for each phase.

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ECE 441 2

Full-Voltage Starting

Major disadvantage is the high in-rush current.

Large voltage drops occur, dimming lights and

forcing computers off-line.

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ECE 441 3

Reduced-Voltage Starting

Autotransformer starting

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ECE 441 4

S, or start contactor close to start the motor at a

reduced voltage the autotransformers are set to 50, 60,

or 80 percent of full-voltage.

At rated speed, open the S contactors and close the R,

or run contactors apply full voltage.

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ECE 441 5

After coming up to speed, Running

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ECE 441 6

Example 5.18

A three-phase, 15-hp, 460-V, 150-A, 60-

Hz, six-pole, 1141 r/min, design B motor

with a code letter H is to be started at

reduced voltage using an autotransformerwith a 65-% tap. Determine

a) locked-rotor torque and expected

average in-rush current to the stator if themotor is started at rated voltage

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ECE 441 7

Example 5.18 continued

b) repeat part (a) assuming the motor is

started at reduced voltage using an

autotransformer with a 65% tap

c) the in-rush current when starting at

reduced voltage.

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ECE 441 8

Example 5.18 Solution

(1141)1255252 5252

575.37rated

Tn TP

T lb ft

Check Table 5.1 for the minimum locked-

rotor torque for a 125-hp, design B, six-

pole motor

,460

,460

125%( )

1.25(575.37) 719.2

lr rated

lr

T T

T lb ft

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ECE 441 9

Example 5.18 Solution continued

From Table 5.9, the average locked-rotor

kVA/hp for a code H motor is

6.3 7.16.70 /

2kVA hp

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ECE 441 10

Example 5.18 Solution continued

The expected average in-rush current to

the stator is

,460

/ 1000 3

6.7 1000 125 3 460

1051

line line

line

line

kVA hp hp V I

I

I A

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ECE 441 11

Example 5.18 Solution continued

With the 65% tap, the voltage across the

stator at locked-rotor is

2 0.65 460 299V V

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ECE 441 12

Example 5.18 Solution continued

The average in-rush current to the stator will

be proportional to the voltage applied to

the stator since the locked-rotor input

impedance is constant.

1051 0.65 683I A

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ECE 441 13

Example 5.18 Solution continued

The locked-rotor torque is proportional to

the square of the voltage.

2 2

22 1

1

0.65 460719.2 303.9

460

VT T lb ft

V

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ECE 441 14

Current Curves

Transition from low to rated

voltage is at slip = 0.125.

shaft speed = ns(1-s)

shaft speed = 1200(1-0.125)

shaft speed = 1050 r/min

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ECE 441 15

Locked-Rotor Torque Curves

Transition from low to ratedvoltage is at slip = 0.125.

shaft speed = ns(1-s)

shaft speed = 1200(1-0.125)

shaft speed = 1050 r/min

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ECE 441 16

The in-rush line current

, ,

, ,

1

0.65 0.65

68.3 0.65 444

HS line HS line

LS line HS line

LSHS

V Va

V V

II A

a