Question

A three phase source variable frequency is required for an experiment. The frequency changer system is as shown in figure below.

The induction machine is a three-phase 460 volts, 60 Hz supply. The variable frequency output is obtained from the rotor terminals. The frequency is to be controlled over the range 15 -120 Hz. The speeds in rpm of the-system to give 120 Hz is (Given number of poles = 6)

• A. 500 and - 900 rpm
• B. 3600 and 1200 rpm
• C. -1200 and 3600 rpm
• D. 900 and 1500 rpm

Answer 1

The correct option is C -1200 and 3600 rpm

For
$f_2=120Hz$

$s=\pm \frac{120}{60}=\pm 2.0$

$n=(1\pm s)n_s$

$=(1\pm 2.0)1200$

$=-1200and3600rpm$