Hello.How do I calculate the stator emf at standstill when an 3 phase ac induction motor is starting up?

 

The stator emf will be the applied voltage minus any volts drop in the mains due to starting current

 

The stator emf will be the applied voltage minus any volts drop in the mains due to starting current

What are the mains? Is it the rotor?
So if its a 240 Volt AC its 240 volts minus rotor emf?
Thanks for the reply you gave me.

 

An induction motor at the point of switch-on is a transformer with shorted turn secondaries, causing a short period of saturation.
Max.

 

Mains?. That is the 3 phase feed to the motor. The voltage used here is a nominal 415v phase to phase volts. So there will be three wires feeding the 3 phase supply to the stator.
As Max stated, at switch on the motor appears like a transformer with shorted secondary(see construction of induction motor details) so instantaneous current is only limited by the stator impedance and cabling resistance and source(whatever supply you have) impedance.
If you want to be pedantic you can also add in the impedance of the rotor(reflected back into the stator) to calculate to total impedance the stator presents to the supply on startup. Then you can determine how much supply volts will be at the stator on startup.
Note that induction motors have a LARGE starting current at standstill.