If the rated voltage for the delta connection is applied to a star-connected motor, the resulting torque and current are about one third of the characteristic torque and current for the delta connection. As a result, at the transition from star to delta, the change in current and torque is about a three times increase. Therefore, an increase from 40 to 130 amps is within the normal range.
If the current doesn't drop to a normal running current in six to ten seconds after the transition, the motor is not adequate for the load, there is a problem - possibly an unconnected phase in the delta connection, there is a mechanical problem with the load or some other problem.
Star-Delta Starting Characteristic Curves and Explanation
The torque and current vs. speed curves shown below are for a typical IEC Design N or NEMA Design B motor with star-delta starting. The load torque requirement shown increases as the speed increases as it would for a fan or centrifugal pump load. The substantial torque requirement at zero speed would be a requirement of a pump with a substantial static head. Note that the torque available to accelerate the motor and load is the difference between the steady-state load torque requirement and the available motor torque shown by the motor cur torque vs. speed curves.
In this case, there is very little star-connection acceleration torque available between 40% and 70% of full speed. If the load torque was a little more, or the motor torque a little less, the motor capability torque curve would cross the load torque requirement torque curve and the motor would not accelerate the load to a higher speed until the star-delta transition. The load and motor curves are shown as crossing at a little above 90% of rated speed, a little below the star-delta transition speed.
At the star-delta transition, the torque and current increase to the points at which the vertical transition line crosses the delta torque and current curves.