The MX
2
utilizes an intelligent Wye to Delta transition algorithm. During starting, if the measured motor current drops below 85% of
FLA and more than 25% of the Up To Speed timer setting has elapsed, then a Wye to Delta transition occurs. The intelligent transition
algorithm prevents unnecessarily long motor starts which reduces 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
2
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 the resistors in a closed transition starter smooths the transition. A typical closed transition Wye-Delta starter schematic is
shown in Figure 41 on page 160.
The closed transition resistors generally are sized to be in the circuit for a short period of time. To protect the resistors from over
heating, one input should be programmed as a Bypass/2M contact feedback input and the Bypass/2M confirm parameter must be set.
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 on page 112 for
more information).
Based on the typical closed transition schematic shown in Figure 41, 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
2
starter remains in the Wye mode until either:
1. The start command is removed.
2. The Up To Speed Time expires.
3. The measured motor current is less than 85% of FLA and more than 25% of the Up To Speed Timer
setting has elapsed.
4. 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 UTS relay is energized which energizes the 2S contactor.
2. When the 2S contactor pulls in, resistors are inserted in the circuit and the 1S contactor is de-energized.
3. When the 1S contactor drops out the 2M contactor is energized.
4. When the 2M contactor is pulled in, feedback can be sent to the MX
2
control card to confirm that the
transition sequence to Delta is complete.
The starter remains in the Delta or running mode until the start command is removed or a fault occurs.
161
7 - THEORY OF OPERATION
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