Funda @ 3 Idiots

Remember when Viru Sahashrabudhhi, the director of ICE, asked Raju Rastogi(one of 3 idiots), how does an induction motor starts ? The answer was logically spontaneous and correct - "Vhrummmmmmmmm". This brilliant movie by Raju Hirani didn't even left a single option of turning down of any idea of visiting the cinema hall second time.

But how does an induction motor starts? Today's blog is dedicated to all those Electrical Engineers who wouldn't mind recollecting the age-old funda once more. And also for non electrical guys who might take it as the "BANGALI BABA KA CHURAN" - stupid but fundamental knowledge appetizer. Here's the answer [soulfully what I learnt during my engineering days] :-

Induction motors are of two types- squirrel cage and phase wound motors.Physics of electromagnetism teaches us that a motor at rest when exposed to any potential for due starting, extracts a huge current called in-rush out of the main supply.

In squirrel cage motors as the rotor conductors are shorted by end-rings, its not possible to add additional resistance for limiting the current while starting.But we can limit the current by reducing the supply voltage at starting.
Here the high current inrush is limited by using a star-delta starter. At the instant of starting the stator winding is connected in star there by voltage per phase becomes=V/(1.732). Eventually the voltage is found reduced. After the rotor attains sufficient speed the stator winding is made delta for any further normal operation. In phase wound type motor the rotor condutors are shorted through resistors and the current is limited by applying resistors.

Next time when you switch on your water pump for collecting water at the over head tank, do apply a little interest on what kind of motoring action you just initiated. Could you identify any starter there? or is it a direct-on-line starting?