Handbook of Electrical Engineering

110 HANDBOOK OF ELECTRICAL ENGINEERING

0.02208 per-unit. The following ohmic values apply at 415 V for an equivalent star-wound stator:-

R 1 =0.179 X 1 =0.438

R 20 =0.0145 X 20 =0.8230

R 21 =0.253 X 21 =0.333

Rc =115.0 Xm=17.0

Calculate the following:-

• Full-load current from the supply.


• Full-load power factor.


• Full-load efficiency.


• Full-load torque.


• Starting current from the supply.


• Starting power factor.


• Starting torque.

Figure 5.1 is the appropriate equivalent circuit for the calculations.

a) Solution for full-load.

The applied voltage per phaseVpis:-

Vp=

415. 0

√

3

= 239 .6 volts

The rotor resistanceR 2 and reactanceX 2 are:-

R 2 =( 0. 253 − 0. 145 )× 0. 02208 + 0. 145 = 0 .1474 ohms

X 2 =( 0. 333 − 0. 823 )× 0. 02208 + 0. 823 = 0 .8122 ohms

The rotor ‘output power’ resistanceRoutat the given slip is:-

Rout=R 2

(

1 −s

s

)

= 0. 1474

(

1 − 0. 02208

0. 02208

)

= 6 .5273 ohms

The total rotor impedanceZ 22 is:-

Z 22 =R 2 +jX 2 +Rout= 0. 1474 +j 0. 8122 + 6. 5273

= 6. 6747 +j 0. 8122