Across- the- line vs inside the delta

[Jett]

Is there a significant issue, for or against, across the line vs inside the delta 6 lead motor connections with solid state soft starter applications? I have a 300 HP and a 800HP 480V 6 lead motors on, ammonia compressors, that are currently connected across the line. We are wanting to replace the outdated existing across the line soft starters with new inside the delta starters. Is there a reason we should go back with across the line starters and not convert to inside the delta connections?

 

[Jraef]

Is there a significant issue, for or against, across the line vs inside the delta 6 lead motor connections with solid state soft starter applications? I have a 300 HP and a 800HP 480V 6 lead motors on, ammonia compressors, that are currently connected across the line. We are wanting to replace the outdated existing across the line soft starters with new inside the delta starters. Is there a reason we should go back with across the line starters and not convert to inside the delta connections?

Several reasons not to based on it being unnecessarily risky. Only one reason to do it; it’s cheap (so long as you ignore the risks). It is not better is any technical way to go with the inside the delta option. Avoid it if you can.

 

[drktmplr12]

Is there a significant issue, for or against, across the line vs inside the delta 6 lead motor connections with solid state soft starter applications? I have a 300 HP and a 800HP 480V 6 lead motors on, ammonia compressors, that are currently connected across the line. We are wanting to replace the outdated existing across the line soft starters with new inside the delta starters. Is there a reason we should go back with across the line starters and not convert to inside the delta connections?

it's easier for someone to get electrocuted because they didn't realize exactly what inside delta meant. of course signage and appropriate disconnecting means or isolation contactor will help mitigate the risk. motor circuit conductors may also need adjusting. at this point might as well have gone ATL.

how much will you really save doing inside delta and making it safe? the mfr of the starter will probably not highlight the additional costs.

 

[Jraef]

...

how much will you really save doing inside delta and making it safe? the mfr of the starter will probably not highlight the additional costs.

That's exactly right. Yes, you can mitigate the dangers by adding an isolation contactor, but the amount you save by using inside the delta vs the cost of adding the isolation contactor makes it all an exercise in futility. So as I said, it's ONLY cheaper if you ignore the risks.

I was on my cell phone earlier and posting the risk issues was too tedious. Now I have a keyboard.

1) The SIGNIFICANT risk of hooking it up incorrectly. Yes, they provide instructions and when you first install it, if you follow the instructions CAREFULLY it works fine. But the NEXT guy to come along, who has to swap out the motor on a Saturday at 3:00AM and can't find those instructions, is going to make a mistake and blow out the SCRs on the soft starter, meaning it is down even longer.

2) With all soft starters, there is a slight risk of a severe line transient causing an SCR to short out, meaning it becomes a full time conductor. Soft starters usually have a method of detecting this and will disable any subsequent start so that the motor is not started with a sever imbalance. But when just ONE of the 6 SCRs shorts out on a standard (in-line) soft starter, there is no inherent added risk of damage to the motor, because there is no path for current to flow through the motor unless another SCR is turned on (or shorted), and having TWO SCRs short out is extremely unlikely. But in the inside the delta arrangement, each motor winding is permanently connected to the line source on one side, the SCRs are only on the other side of each winding. This arrangement is why the inside the delta version is cheaper, the SCRs are only going to see 58% of the line current, so they can be smaller. But because of this arrangement, if only ONE of the SCRs shorts in any phase, there is current flowing unrestricted to one winding of the motor and no way to stop it automatically unless you use a Line Isolation Contactor ahead of the Soft Starter, tied to that Shorted SCR detection circuit.

So as mentioned, adding that isolation contact defeats the cost savings aspect of using the 58% rating of the soft starter. Most people who want to save that money are OEMs who are not concerned with the long range operating costs of a machine, they just want to save the initial $$ and pass the risk onto the end users, who usually don't know that there are risks. So the OEMs just provide the inside the delta version to save their $$ and don't use the line isolation contactor, exposing the end user to the risks.

 

[Jett]

I understand the issue with the isolation contactor. My question is more about if there are any issues with motor efficiency, life, temperature rise or other motor ratings parameters.

 

[Jraef]

I understand the issue with the isolation contactor. My question is more about if there are any issues with motor efficiency, life, temperature rise or other motor ratings parameters.

No. Makes no difference whatsoever, as long as nobody makes any mistakes connecting it.