Soft Starter Design

[Besoeker]

Yes, that's correct.

In a soft starter, you have to determine WHEN to fire the SCRs in order to affect the Phase Angle Firing that reduces the RMS voltage getting to the motor. That "when" is based on accurately knowing the "zero cross" point of the sine wave of each individual phase,

We had problems with that on one manufacturer's drives.The problem was because of a rather dirty supply. Not altogether uncommon in industrial environments

 

[ejon1801]

Thanks Jraef

You seem very experienced with soft starters. I called around to some of those companies to see if there was any soft starters that were compatible with the corner grounded delta systems. Some did not know, one company said yes, however there was no documentation to support the statement. I wish I could find a company that, part of their manual, shows how to connect to a corner grounded delta:?. But its been quite difficult, I have to know for sure , and connect it appropriately.

 

[Jraef]

Thanks Jraef

You seem very experienced with soft starters. I called around to some of those companies to see if there was any soft starters that were compatible with the corner grounded delta systems. Some did not know, one company said yes, however there was no documentation to support the statement. I wish I could find a company that, part of their manual, shows how to connect to a corner grounded delta:?. But its been quite difficult, I have to know for sure , and connect it appropriately.

It's not that it is "corner grounded" Delta, it's just that it is Delta. As I said, Allen Bradley, Mototronics and Benshaw are the only ones left that I know for sure do not care if the voltage is Wye or Delta. Any of them that use MOVs on the front-end to protect marginally rated SCRs will not like Delta, because the MOVs will be connected in a Wye and referenced to ground, so they attempt to become the Wye point for your entire system and blow up the first time there is a fault ANYWHERE. But some, like A-B, offer the MOV unit as an accessory, so you just don't order that. Most of the low cost IEC designed units will have the MOVs on the front-end built-in.

Another issue is the newer trend to use "2 phase" power designs. It's still for use in a 3 phase system, but to save money they only put SCRs on two of the phases; the third (usually the center) phase is just a piece of bus bar, it's hot all of the time. Personally I think this is a poor design and should be avoided because it is bad for the motor if the acceleration takes too long and risky no matter what because if one SCR shorts, you have no way to stop the motor current and you burn it up. I'm not sure how that might be affected by being fed with a corner grounded system though. It might be OK as long as the corner grounded phase is connected to that bus bar phase, but if connected to either of the other two, the leakage might look like a ground fault.

I'd say if people don't know, don't trust them because it says something about them just in the fact that they don't know and apparently can't find out, or don't care enough to find out.

 

[ejon1801]

2 phase SS power designs

2 phase SS power designs

Thanks again Jraef, I did not know there were recent soft starters with SCR's on only two phases. I really need to have a look at those! To protect the motor probably a isolation relay (with a 120v coil) needs to be connected to the soft starters main contactor output (in the control circuit), with isolations contacts (for that relay) being put in line to protect the motor in case of shorted SCR's..what do you think? I will try and find SS with two phases of SCRs, because our system is delta B phase grounded, and that seems to be a suitable fit for my situation. They may have made it like that for grounded delta systems. Anyway thanks again..not meaning to bug you, but it is a rarity to find someone who have years of experience working with soft starters. And I have to be 100% right on this design

 

[Jraef]

Thanks again Jraef, I did not know there were recent soft starters with SCR's on only two phases. I really need to have a look at those! To protect the motor probably a isolation relay (with a 120v coil) needs to be connected to the soft starters main contactor output (in the control circuit), with isolations contacts (for that relay) being put in line to protect the motor in case of shorted SCR's..what do you think? I will try and find SS with two phases of SCRs, because our system is delta B phase grounded, and that seems to be a suitable fit for my situation. They may have made it like that for grounded delta systems. Anyway thanks again..not meaning to bug you, but it is a rarity to find someone who have years of experience working with soft starters. And I have to be 100% right on this design

An isolation contactor works, but adds to the cost, MORE than just buying a soft starter with 6 SCRs! And that doesn't help with the fact that by ramping two of the phases and the 3rd being full voltage, the current, while ramping, is severely unbalanced and the motor experiences a lot of negative sequence current, which "fights" the other two phases to accelerate the load. So this type of device requires MORE current per unit of motor shaft torque than a 6 SCR design. Basically the "two-phase" concept is a way of capturing the market for people too cheap to care about the consequences. They are only useful primarily for small centrifugal pumps and fans where the accel time is short and the load is low until it is almost at full speed. Unfortunately that's the same place where a small VFD plays well, AND can save energy, so that market has been lost to soft starters. The problem is, some of the soft start mfrs take it too far and use this concept up to 250HP!

Here's an example of one, you can see the diagram on the bottom left corner showing you that there are only SCRs on 2 of the phases. At least they are honest about it, not all of them are.

 

[ejon1801]

Thanks for showing me the SS with the two phases. Almost all vendors that I've spoken to either do not know if their starter is compatible with grounded delta, or cannot explain why it is not. Its been very frustrating. All I wanted to know was what criteria made their starters compatible with the grounded delta systems and most do not know. I wish the power system wasnt so old. However, I appreciate all of your responses.

 

[Johnnybob]

Thanks for showing me the SS with the two phases. Almost all vendors that I've spoken to either do not know if their starter is compatible with grounded delta, or cannot explain why it is not. Its been very frustrating. All I wanted to know was what criteria made their starters compatible with the grounded delta systems and most do not know. I wish the power system wasnt so old. However, I appreciate all of your responses.

Alot of supply houses will have a factory rep or specialist on staff. You might try getting ahold of one. One of our local suppliers has 2 A-B (or Rockwell) factory reps, one for exclusively (mostly) A-B PLCs and one for Drives and motion contrtols. Just call around till you find one. You can also Google a drive brand and model # and usually come up with a free PDF users manual.