Question

Show that in star delta starting of squirrel- cage Asynchronous motor the starting current and torque are reduced by a factor of 1/3 compared to DOL starting. A 3-phase, 25 kW,400 V,50 Hz, 8-pole Asynchronous motor has rotor resistance of 0.08Ω and standstill reactance of 0.4Ω. The effective stator/rotor turn ratio is 2.5/1. The motor is to drive a constant-torque load of 250Nm. Neglect stator impedance.

Calculate the minimum resistance to be added in rotor circuit for the motor to start up on load.

Answer 1

In star-delta starting of a squirrel-cage asynchronous motor, the starting current and torque are reduced by a factor of 1/3 compared to DOL starting. The slip of the motor can be used to calculate the minimum resistance to be added in the rotor circuit for the motor to start up on load.

Step-by-step explanation:

In star-delta starting of a squirrel-cage asynchronous motor, the starting current and torque are reduced by a factor of 1/3 compared to direct-on-line (DOL) starting. This reduction is achieved by connecting the motor in a star configuration during starting and then switching to a delta configuration once the motor has reached a sufficient speed.

When the motor is connected in star, the phase voltage is reduced by a factor of 1/√3 compared to the line voltage, resulting in a reduction in starting current and torque.

To calculate the minimum resistance to be added in the rotor circuit for the motor to start up on load, we need to use the equation:

R_2 = (s^2 * R_2') / (1 - s^2)

where R_2 is the minimum resistance to be added in the rotor circuit, R_2' is the standstill resistance of the rotor circuit (0.08Ω in this case), and s is the slip of the motor.