Closed Transition Starting
The control circuit discussed so far uses open transition starting. This means that the motor is disconnected from the power line during the transition from wye to delta. This may be objectionable in some applications if the transition causes spikes on the power line when the motor changes from wye to delta. Another method that does not disconnect the motor from the power line is called closed transition starting. Closed transition starting is accomplished by adding another three pole contactor and resistors to the circuit (Figure 39 – 14). The added contactor, designated as 1A, energizes momentarily to connect resistors between the power line and motor when the transition is made from wye to delta. Also note that an on-delay timer (TR2) with a
delay of 1 second has been added to the control circuit. The purpose of this timer is to prevent a contact race between contactors S and 2M when power is first applied to the circuit. Without timer TR2, it would be possible for contactor 2M to energize before contactor S. This would prevent the motor from being connected in wye. The motor would start with the stator windings connected in delta.
