What is the advantage of having a 12 lead 460V,60Hz WYE-DELTA Motor over 6 lead 460V WYE-DELTA motor ? The motor is for deepwell application & mechanical type reduced voltage starter is used in both the cases.
The advantage of 12 leads is important due to the motor could run at two diferent voltages (230 V- 460 V) and could be started in star-delta for both voltages(if the rated connection is delta), if you will use the motor only for 460 V and will be started with a soft-starter or across the line then use a 3 leads motor.
Most of the time if a submersible motor is wired wye-delta, it is at customer request for starting purposes. Cable is expensive and there are other ways for dual voltage motors to be built that does not require the cable for differing voltages :although dual voltage is not common except on the smaller deepwell submersible motors(we only do up to 30 HP).
They could be using 12 leads to reduced the individual conductor size. This is done because sometimes the cable for a standard 6-lead wye-delta will not fit between the motor/pump and the well casing, or the current demand makes it cost effective. Current and voltage drop are important when sizing the cable, especially since the cable runs can be extremely long (100' is short). We regularly have to upsize cable to get the voltage drop below 5%.
If by "mechanical type" you mean an Autotransformer type starter, then there is no advantage for using a 12 lead motor for a deepwell pump. But 12 leads allows the alternate use of a Part Winding starter, which is popular in that industry, or a Wye-Delta, which is not popular in the US for deepwell pump systems.
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It's not a dual voltage 230V-460V motor. It's a metric rated motor for 460V 60Hz & 380V 50Hz. Its run's in Delta+Delta connection. As Jraef assumed it's should be applicable for part winding start.