Auto Transformer vs Star Delta

I would only take issue with the use of "smooth" as a descriptor for RVAT (Reduced Voltage Auto Transformer) starting. "Smoother" than Y-Delta, yes, but it is still a 2 step process just like Y-Delta, albeit a much preferred one for other reasons as well*. Like Y-Delta, If the transition timing is off just a little there is still a potential of a torque and current transient that will be as high as LRA. The only true "smooth" acceleration possibility in my opinion is going to come from solid state control, either soft starters or VFDs (the latter being used when speed control is a benefit). Being that in most cases, soft starters are now less expensive than RVAT starters, as well as larger and heavier, I see no reason to use RVAT any longer.

*One of my biggest headaches with open transition Y-Delta is the fact that, given the right set of circumstances that are easy to come by, you can transition at a moment that can cause a massive voltage transient as well as a current and torque transient, and that combination can wreck havoc on the entire connected electrical system. Most people are unaware of this and choose it because it is cheap up front, but the long term cost of ownership is always higher from my experience. People usually just blame the utility for the transients however, so they continue on with using Y-Delta starting. Closed transition Y-Delta, while solving that potential problem, is so much more expensive that once again, solid state soft starters end up a much better solution.

Thanks for all replies.

Have you guys ever seen Auto transformer and star delta contactors work along side each other in the same panel? I have seen one but there are no drawings to comprehend.

Is it possible that while switching from star to delta the transients could be enough to interupt a wifi signal? We have one air compresor with the wye-delta start and it gets booted from the compressor manager at least once per day. The antenna sits on the control cabinet and the data entry point is less than 10 feet away.

Friends,

Guest in post #1 briefly mentioned that ATS only requires 3 cables for the motor. Y-Delta (or star/delta) requires a motor on which both ends of each winding are brought out to the terminal box and then to the starter. With a single-voltage motor, that requires 6 leads, and with a dual-voltage motor, that requires 12 leads. Since many motors are made with only 3 leads (single-voltage) or 9 leads (dual-voltage), those types of motors cannot be used with a Y-delta starter. If the distance from the starter to the motor is significant, the need to run the extra leads can add to a lot of money.

The big advantage ATS have over solid-state methods such as soft-starts or VFD's is the absence of solid-state power devices and their mortality or susceptibility to harm from heat or transients.

So, although ATS are heavier and typically more expensive than Y-delta (open transition) or soft-starts, I see a valid position for them in the market. Because they have a transition, they are not as smooth on the motor or the power line than soft-starts or VFD's. ATS are easy to design and build. Closed-transition Y-delta starters are very hard to design because sizing tables for the transition resistors are very hard to find.

--JMM