Question

A 3-phase induction draws a starting current of 5 times the full load current when started direct on-line. The full load slip is 4 percent. Determine the starting torque of motor in terms of full load torque when started by means of:

a) Star delta starter
b) An auto-transformer starter with 50% tapings.

Answer 1

The starting torque of a motor can be calculated when started by a star delta starter or an auto-transformer starter with 50% tapings.

Step-by-step explanation:

Starting torque of a motor is directly proportional to the square of the starting current.

a) Star delta starter:

1. When started using star delta starter, the starting current is reduced to one-third of the direct on-line starting current, which means the starting current becomes 5/3 times the full load current.
2. As the starting current is reduced by one-third, the starting torque will be reduced by one-third squared, which is one-ninth of the starting torque when direct on-line starting.

b) Auto-transformer starter with 50% tapings:

1. When started using an auto-transformer starter with 50% tapings, the starting current is reduced to half of the direct on-line starting current, which means the starting current becomes 5/2 times the full load current.
2. As the starting current is reduced by half, the starting torque will be reduced by half squared, which is one-fourth of the starting torque when direct on-line starting.