Your friend is right. A star-delta starter must have it's own internal control and interlocking done hardwired with relays and timers. This is an essential safety matter because if the control of a start-delta starter fails, there is the big possiblity of creating a major fault across the incoming mains.
For this reason EVERY start-delta starter in the real world uses a specific and exact relay/interlock design. Of course it is very common for a PLC to be in control of the external Stop/Start command for the star-delta starter, but NEVER to be involved with its internal workings.
Definately do not agree! I do star delta control in a PLC all the time. The only things I will not compromise are status off all contactors into the PLC and a mechanical interlock. They are absolutely essential. Probably have a hundred or more out there controlled by PLC's. Never had a problem.
With hard wired YD I have seen timer failures etc that have caused problems such as the motor being left in star due to the faulty timer.
Not disagreeing with you but we have 2 Y-delta motors that are in complete control by the PLC. (The only thing not controlled is the mechanical interlock between the two contactors) Never had a problem..In fact the only problem we had was erly on when the plc didnt control them..After a power "glitch" the things always wanted to go back on the start winding..there was no need couse they were already up to speed..The PLC now looks at the current and desides.
Quite an expensive system monitoring current but very good. I wish my clients would pay for that sort of control sometimes. It is always "cheep - cheep". They call it value for money.
Do not use YD anymore if given a choice. Always use soft start/stop. I use the Sprechur + Schuch PCS starters. S & S are owned by AB and I am sure AB have them somewhere in their arsenal. These things go to 45kW, usually enough for water pumps, and also have an instantaneous output to control the isolation contactor. Just tell it to go and it does.
The motor is on the start winding for 20 mins before kicking over to Delta. Heavy load directly coupled to the motor. (Milk sperator)
I would suggest to you that probably the wrong type of starter is being used. Common practice is to keep a motor on star for absolutely no more that 45-60 seconds. Many believe that is too long. You are placing enormous stresses on the motor. I would suggest you look at soft start with an adjustable ramp rate or the old faithfull method of using an auto tranny starter.
Having said that, as an apprentice electrical fitter I wound thousands of motors. Wound a motor from a power station - 2500HP - 11kV. An old monster. It was huge. Had to wind stator and rotor. When it came to supporting the stator winding and "rolling" down the rotor winding with gal wire - the gal wire was then flow soldered with an oxy torch - the boss came over and told me how he wanted both supported. I could not believe the strength of support he required. He then said to me "remember, this is a power station. They have all the power in the world. They start the motor DOL!!!" All of a sudden the reason for the strength of support required for the windings became apparent. I was at the power station for the testing of the motor on load driving a huge cooling water pump. When they turned the 11kV on and the motor sat there and "growled", all I could think of was the tremendous magnetic forces being applied to the winding to get the thing to even turn. It survived for many years until the power station was pulled down.
Years later when I was working for GEC, I saw photos of massive motors being transported through London on low loaders. They had to take down all the overhead cables etc to get the motors through the streets. There were 2 prime movers at the front of the low loaders and 2 at the rear. The motors were going to the docks to go on a ship for Russia. They were to drive cooling water pumps for a Russian power station. 10,000 HP, 22kV DOL!!! Made my little project seem insignificant.