Three Phase Induction Motor

Transition spikes. When the motor starter switches from Star(Y) to Delta, all of the contactors must be open for a brief moment (open-transition). During this time, the motor will slow down ever so slightly and when re-connected in Delta, it may have dropped down below 90% speed, in which case there will still be an inrush spike of up to 600% FLA anyway, the very thing you were trying to avoid. There will also be a spike of torque at that time which can sometimes be damaging. In addition, there is a phase shift that occurs between Y and Delta, so at the moment of transition, depending on where the voltage phase angle was when you do it, the reconnection may result in even higher torque and current spikes. lastly, if the transition is too short, residual magnetism in the motor may have it still acting as a generator when reconnected in Delta, but it will be a generator out of phase with the line, so a massive voltage spike can occur that exacerbates the other issues. Current transition spikes of up to 2000% of FLA have been recorded on Y-Delta starts, and the resultant torque stresses have been known to shear off motor shafts. In any case, there is always a current spike during transition and that can and will damage other nearby equipment.

This issue can be mitigated by using a Closed Transition Y-Delta starter, but it involves using a 4th contactor that places resistors in the circuit during transition. The problem is, this makes the starter more expensive than an Electronic Soft Starter, so there is no advantage in using it.