Dnk, Where did you hear of one of them?. My experience tells me either Star-Delta (where the motor starts in Star and runs in Delta). But a Star-Star starter?. The reason behind the Star-Delta start is the lower current upon start-up, hence a lower voltage drop on the supply lines. And Star-Delta requires 3 contactors. Having said that, I can find no advantage in starting a motor in Star and also running it in Star.
There are enough different names for the same concept that I won't claim to know what a Y-Y starter actually _is_. Here is a guess
In a star-delta starter, you don't actually reduce the voltage supplied to the motor at start. Rather you change the way the motor windings are connected together so that the current drawn by the motor is reduced. The voltage 'seen' by each winding _is_ reduced because of the different connection. The supply voltage remains constant, but there is a 1/root(3) change in coil voltage in star versus delta.
As an analogy, consider a lamp 'dimmer' made with switches. You have two lamps; in one switch configuration the lamps are in parallel across the supply, seeing the full supply voltage; in the other the lamps are in series. The supply voltage remains constant, but each lamp now 'sees' half the supply voltage.
Given this, there is no theoretical reason why a motor starter could not incorporate some different switching arrangement than star/delta, one that (say) reconnects a motor between 2 series star and 2 parallel star. This would give you a 1/2 change in coil voltage. With a complex enough switching arrangement, one could step through different connections and 'sneak up' on full voltage.
I've never seen such switching arrangements used. A 'soft start' probably makes more sense
Star-delta starters are the norm here for such things: 240V to each winding during star/wye start-up then 415V across each winding after switching to delta.
Given that North American motors are perhaps more likely to have windings in some sort of 2:1 ratio to provide for 120/240/480V operation, maybe Jon's idea about running series and parallel wye is correct.
Thanks guys, I've seen star-delta starters, and I thought all of them were mechanically interlocked so both contactors can not be energized at the same time? And the Y (star) contactor had jumpers across each phase..
The thing I'm dealing with right now, is a 300 hp motor, 480V 6 lead type. It has two starters, not interlocked together and the second starter is pulled in after the timer times out, just like a y-d..
I assumed they called this a star-star..
FYI, I'm putting in some new soft starter/optimizers.