question

Nunow

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Jun 2005
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I have a question about upgrading a relay-based control system of a plant to a PLC-based system.A friend told me that star-to-delta starting of a motor is preferably left to be hard-wired controlled instead of a PLC program but he did not give me any convincing reason.I need this information because I am doing a mini-project about the Advantages and disadvantages of a relay-based systems.
Could anyone give me reason(s)for this.
 
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.
 
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PhilipW said:
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.

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.
 
Motor Starter

Seen it both ways. Since the starters are usually prewired, new installs just get the PLC to Start, and not mess with the guts. On upgrades it is common to remove the relays and timers and use a PLC. Some people are uncomfortable with this. Others swear by it.Take care Bruce.
 
PhilipW It is the same PLC that controls all the Process side of things.. That being said It is a Specialty application. The motor is on the start winding for 20 mins before kicking over to Delta. Heavy load directly coupled to the motor. (Milk sperator)
 
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.
 
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BobB said:
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.

I agree with you Bob

All our systems with plcs have direct plc to star delta controls, only thing that is always common is a mechanical interlock along with the NO NC aux's on the star and delta contactors being used on both the plc outputs and for feedback on the inputs.

I guess at the end of the day it is personal preference and either works.

I have been called to plenty of failed hard wired star delta circuits but not (yet) any plc controlled ones, the faults are normally always a timer fault.
 
darrenj said:
PhilipW It is the same PLC that controls all the Process side of things.. That being said It is a Specialty application. The motor is on the start winding for 20 mins before kicking over to Delta. Heavy load directly coupled to the motor. (Milk sperator)
About 20 years ago I did a PLC3 system in a large dairy factory here with about 10 separators (Alfa-Laval and Westphalia). All were star-delta controlled by the PLC. From memory, they were in star for about 20-30 seconds only.
 
Timing depends on the starting load. We usually set the switch over time to however long it takes for the Wye current to stabelize.

However 90% of the time a soft starter cost about the same as a Wye-Delta setup; if that the case, go soft start.
 
Ah well it looks like I was dead wrong and everyone controls their Y-Delta starter from the PLC judging by all the examples given.

And Mike is right, a Softstarter is the only real choice these days. But now I await all those impassioned defenders of the electromechanical Y-Delta starters to burst into print!
 
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But now I await all those impassioned defenders of the electromechanical Y-Delta starters to burst into print!
Philip
I'm with you 100%. Not only do I use a separate timer, I use a dedicated S-D timer from the same vendor as the contactors. The reason is, the transition time is also important. It has to be long enough to extinguish the arc on the star contactor, but short enough that the motor speed doesn't drop too much. I'd never heard of using a PLC to do this until I found this forum, but then I'm old, and I still remember using pneumatic timer blocks on the contactor
 
The reason is, the transition time is also important. It has to be long enough to extinguish the arc on the star contactor, but short enough that the motor speed doesn't drop too much
So? PLC can do exactly the same thing.

but then I'm old, and I still remember using pneumatic timer blocks on the contactor

So am I and you can still buy the horrible things. They are cheap though.
 

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