The advantage of slipring induction motor over the squirrel cage induction motor is

slip ring motor have high starting torque compare to squirrel cage motor and have good speed control
also have high over loading capacity, smooth acceleration under heavy loads.

A 3 phase, 6 pole induction motor operates on 440 V, 50 Hz, supply. If the actual speed of the motor is 960 rpm the slip will be

Ns = 120f/P
Slip=(Ns-Nr)/Ns
Ns = 1000 and Nr = 960
from equation of slip, S = (1000-960)/1000
S = 0.04 or 4%

The mutual inductance between two unity coupled coils of 9 H and 4 H is

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Fot fast load flow calculations, the best method is

An over excited synchronous motor draws current at

Over excitation : leading current
Under excitation : lagging current
Normal excitation : current with unity PF (in phase current)