Question

A 6 pole, 50 Hz, 3-phase induction motor has blocked rotor reactance per phase equal to four times of the rotor resistance per phase. Which of the given statements is not correct?

• A. At maximum torque condition, the value of speed of rotor is 750 rpm.
• B. The value of slip at maximum torque is $\frac{1}{4}$
• C. The value of synchronous speed is 900 rpm.
• D. The value of synchronous speed is dependent on number of poles.

Answer 1

The correct option is C The value of synchronous speed is 900 rpm.

$\text{For maximum torque}\left(T_{max}\right)$

$S_{max,T}=\frac{R_2}{X_2}$

$S_{max,T}=\frac{1}{4}$

$\text{Synchronous speed,}$
$N_s=\frac{120\times 50}{6}=1000rpm$

$\text{Now at}{T}_{max}\text{speed of motor,}$
$N_r=(1-s)N_s$

$=(1-0.25)\times 1000$

$N_r=750rpm$