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Starting Torque of Star Delta vs Delta Delta

Starting Torque of Star Delta vs Delta Delta

Starting Torque of Star Delta vs Delta Delta

(OP)
Hi there,

This question pertains to a 400HP 575Volt 1200 RPM Motor that uses a RVAT starting system. If we were to take this motor that was orignally designed and wound as a Delta Delta and have it rewound as a Star Delta winding, what would the ramifications be? Assuming that we continue to use the RVAT would the starting torque be compromised. Any other considerations need to be made? Thanks in advance

RE: Starting Torque of Star Delta vs Delta Delta

If you're proposing to energise both windings simultaneously then magnetically it would be in a bit of a mess. I can't convince myself quite what it would do, but the magnetic fields of the delta winding group would be displaced in phase by 30&Deg; relative to the star winding group.
  

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If we learn from our mistakes I'm getting a great education!
 

RE: Starting Torque of Star Delta vs Delta Delta

30&Deg; = 30° - defeated by TGML again!
 
  

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If we learn from our mistakes I'm getting a great education!
 

RE: Starting Torque of Star Delta vs Delta Delta

(OP)
I received some wrong information on this. Thanks Scotty for tipping me off. I did a little more investigating and it turns out the motor will go back as a delta delta winding. The only difference now will be whether or not to stick with the RVAT or go to a newer solid state soft start. What would the pros and cons be to doing this.

RE: Starting Torque of Star Delta vs Delta Delta

RVAT Pros;
1) Lowest line current draw during start because of the transformer action, which makes it slightly better with limited supplies. Line current draw = motor current draw * transformer ratio %.
2) Joe Bubba 'Lectrician can watch the big parts bang on and off, so he can sort of understand how it works (Hmmm... not as valid no with Arc Flash safety eh?)

RVAT Cons:
1) Biggest footprint and very heavy when considering floor loading
2) More expensive than RVSS, in spite of common misconceptions of electronics being expensive.
3) Very little flexibility with regards to adjusting start parameters.
4) Duty cycle is usually more restricted than the motor it is controlling.
5) Joe Bubba 'Lectrician can use his screwdriver to "bump the motor", which leads to premature failure.
6) Parts wear out, replacement contacts often cost as much as an RVSS.
7) Advanced motor protection is a separate device and adds cost, complexity and another point of responsibility.

RVSS Pros:
1) Smaller footprint than RVAT, less weight, easier to install or remove later.
2) Less expensive than RVAT technology.
3) Infinitely adjustable to match conditions. If conditions change, all it takes is a change in programming.
4) Duty cycle is typically the same or better than the motor it is controlling, it will not present an artificial limitation on use.
5) Start sequence and testing is controllable by the starter design; difficult to override.
6) No moving parts that will wear out. A Bypass Contactor, if used, will not see starting or stopping duty so typically the contacts never need replacing.
7) Advanced motor protection is easily available as an integral feature (by virtue of product selection).
8) Soft Stopping of pumps is possible (not possible with RVAT), which lowers the physical stresses on piping systems from water hammer.
9) Electronic braking is possible as an inherent feature (RVAT would need a separate stand-alone brake).

RVSS Cons:
1) Subject to damage from severe voltage spikes, i.e. lightning (can be protected as anything else can).
2) Joe Bubba 'Lectrician can't see anything move so he has no idea how the big magic box works and is often intimidated (needlessly I might add).
3) Line current reduction is not as great as with RVSS; line current = motor current during startup. It's still much less than DOL, it's just that RVAT has a slight advantage here.


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RE: Starting Torque of Star Delta vs Delta Delta

I'm new to this forum. I was just wondering what would be the purpose of rewinding this motor back Part-winding start, if a soft start was intended on being used? What are the differences between a part-winding start vs. star-delta as far as starting torque?

RE: Starting Torque of Star Delta vs Delta Delta

No point in part winding if you have a soft starter. You would just connect all of the windings anyway.

Torque in an electro-mechanical RV start is fixed in steps. Step 1 is reduced, step 2 is DOL. No adjustments other than transition time. Y-Delta is 33% in step one, Part Winding is dependent upon the windings, but is usually 42%. Solid state is infinitely adjustable to fit wjat you need and there is no transition.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Starting Torque of Star Delta vs Delta Delta

I'm not particularly familiar with soft starting methods (they aren't needed at a power plant).  I have probably misunderstood something here.  Maybe someone can set me stright:

I didn't see anything in the original poster that mentioend part winding start in the before or after configuration.  Why are we talking about part winding start?

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RE: Starting Torque of Star Delta vs Delta Delta

flandrax hijacked the thread. I probably shouldn't have responded to him but I was giving him a newbie pass thumbsup2


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Starting Torque of Star Delta vs Delta Delta

Thanks jraef.  I wasn't complaining about the detour... just didn't know if it was connected to the discussion. Now it makes sense.  

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RE: Starting Torque of Star Delta vs Delta Delta

sorry for causing any mass confusion. From the understanding that i gathered from the question, the "poster" wanted to know about rewinding the "delta-delta" motor to a "star-delta" and starting characteristics from one to the other. The picture of the connection plate told me that it was a part-start motor(1,2,3,7,8,9).

I never quite understood the rewinding redesign from "delta-delta" to "star-delta", but that's another subjuct. To me, I thought my question was relevant.

RE: Starting Torque of Star Delta vs Delta Delta

Yes, that's a strange looking nameplate to me.  I agree seems maybe that is suggesting it's a part winding start.  (I have never seen a part winding start nameplate).  

I also see jraef's point - why would we combine RVAT start with part winding start...coudln't they just adjust taps on RVAT.   Unless maybe the motor was installed as part winding motor to begin with and then they figured out they needed to reduce starting current further.  

Beats me.

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