Technical information

8 – THEORY OF OPERATION
Wye Delta Operation
129
8.5 Wye-Delta Operation
When the Starter Type parameter is set to Wye-Delta, the MX is configured to operate an electromechanical Wye-Delta (Star-Delta)
starter. When in Wye-Delta mode, all MX motor and starter protective functions except bad SCR detection and power stack
overload, are available to provide full motor and starter protection.
The MX utilizes an intelligent Wye to Delta transition algorithm. If during starting, the measured motor current drops below 85% of
FLA and more than 25% of the Up To Speed Time has elapsed, then a Wye to Delta transition occurs. The intelligent transition
algorithm prevents unnecessarily prolonged motor starts thereby reducing motor heating. If a Wye to Delta transition has not already
occurred, a transition always occurs when the complete Up To Speed Time expires.
The MX can operate two configurations of Wye-Delta starters, open transition and closed transition. An open transition starter
momentarily disconnects the motor from the input line during the transition from Wye to Delta operating mode. A closed transition
starter uses resistors that are inserted during the transition so that the motor is never completely disconnected from the input line.
The presence of these resistors in a closed transition starter smoothes the transition from Wye to Delta operating mode. A typical
closed transition Wye-Delta starter schematic is shown in Figure 12 on page 92.
For the Wye-Delta starter mode to operate properly, one output relay needs to be programmed to the RUN output function and
another output relay needs to be programmed to the UTS output function. (Refer to the Relay Output Configuration parameters for
more information)
Based on the typical closed transition schematic shown in Figure 12, when a start command is given the starter enters the Wye
starting mode by energizing the relay programmed as RUN.
The transition to Wye (Starting) mode occurs as follows:
1. Start command is given to the starter.
2. The RUN relay is energized which energizes the 1S contactor.
3. When the 1S contactor pulls in, the 1M contactor is energized.
The MX starter remains in the Wye mode until either:
1. The start command is removed.
2. The Up To Speed Time expires
or
The measured motor current is less than 85% of FLA and at least 25% of the Up To Speed Time has elapsed.
3. A fault occurs.
When the Up To Speed Time expires, the starter changes from Wye starting mode to the Delta or normal running mode by energizing
the relay programmed as UTS. In Delta mode, the RUN and UTS relays are both energized and the motor is connected in the normal
running Delta configuration.
The transition to Delta (Run) mode occurs as follows:
1. The Up To Speed Time expires
or
The measured motor current is less than 85% of FLA and at least 25% of the Up To Speed Time has elapsed.
2. The UTS relay is energized which energizes the 2S contactor.