Star / Delta loading - opinions required !! 1

[Pocko]

Hello,

I hope things are well in your part of the world; just a long winded post to verify a few things...

We have two mill motors (on a multishaft disperser) currently being started in a star/delta arrangement. The production bosses want to put a larger diameter "blade" on the end of each shaft (going from 10" to 12")in order to get quicker grind times. They vaguely tell me this been tried once in the past and the motor's kept tripping out on overload, so they reverted. I've been asked to look over the situation.

Each is a 45kW 3Ø motor with a F.L.C rating of 80.9A, 415v, S.F 1.15, 1445rpm.

The overload is situated correctly in the circuit (bottom of the Line contactor), and is currently set to it's max. range of 40A. It seems to be a single range overload.

The cores to the star/delta windings are rated at around 40A/Ø odd under our code.

My query is in regard to the overload setting, by my calc's (80.9 x 0.58) 46.922A should be the correct setting for this overload. So i believe they can not get the full work out of these motors because of the overload's range limitatons.

I can't really allow the extra 15% to the total in this case because the motor's located in a hazadous area, but that extra 7 amps may be enough to tip the scales in our favour. I don't know if the last sparky tried to upsize the overload to 48A for his trial, but i assume he didn't. I'd like to see the cable's upsized too.

I'm going to tong the supply to the line contactor tomorrow during a batch if i get time, to see how hard they are working it at the moment.

If we upsized the overload, and threw the bigger blade on only to find the motor was overloading, this would be a nasty situation for us as it would be a BIG job to empty this vessel with raw product in it (when a batch starts run time is usually around 8 hours). Success would equal batch times of around 4-5 hours and would make many people happy!

Do you think this modification is viable? I'll post any extra information i find out.

Thank you for your time and any thoughts on the situation would be welcomed, Regards - Chris.

 

[Skogsgurra]

A very good thing to do would be to measure how hot the motor windings get during one batch.

If you have a possibility to measure the motor winding resistance cold (ambient) and then after a batch has been run, you will get a very good picture as to how heavily loaded the motor is. If winding temperature stays well below maximum allowed temperature, then a test with higher load and subsequent measurement will put you in a much better situation for a decision how to proceed. You will probably need to use a four-terminal measurement to get good resistance readings.

I think, personally, that a 20 percent increase in blade diameter will cause at least a 40 percent increase in load. Probably a lot more since the area covered will go up 44 percent and the speed that the extra blade tips work with does also increase.

Why not start with a set of 11" blades? Success is more likely and the gain will still probably be in the 20 to 30 percent range. And you will all be a little happier

 

[DickDV]

Pocko, maybe I'm missing something here but, why are the overloads set at 40 amps when the motor nameplate says the motor FLA is 80.9 amps?

Or, to ask the same question another way, why are you dividing the FLA by the square root of 3 to get overload current settings.

Normally, the overload heaters would be chosen based on FLA.

Maybe you are dealing with undersize conductors but, to protect them, the fusing or circuit breaker is there, not the overload block.

 

[Pocko]

Hi Guys,

Thanks for the quick reply and for your input so far.

skogsgurra : I actually took the cold winding resistances this morning, but we had no production today. I'll try to obtain a "warm" reading tomorrow. I can easily do a surface temperature test, but a winding temp. test may be awkward. Any suggestions on methods there?


DickDV : First off for clarity, these are bi-metallic o'load's. I've always been told when using Star/Delta starting, the overload calc's are based on : Motor F.L.A / sqrt3 = the Phase value of the current, as the overload's thermal elements are inside the Delta loop. This provides both Star and Delta protection.

I'm more than happy to be proved wrong though as this would instantly solve the problem (...and it's been a while since i had to think about this stuff so i'm a bit foggy!).

 

[motorsdirect]

I would say that you are certainly using the wrong o/l value....your o/l should be set at the rated full load current of 80.9A..

regards
Jeff

http://www.motors-direct.co.uk

 

[jraef]

No, he is correct if the OL relay is only on the delta contactor section of 3 of the 6 leads to the motor. That is the standard way of doing it.

Pocko,
You are correct in thinking that you are not getting full capacity out of this motor in terms of the OL protection. However that does not automatically mean that you can get away with what you are thinking of doing. Just from a purely cautionary stance, it is unsafe to assume that someone before you did not know what they were doing, it is better to assume they did it for some reason you are unaware of. If the original design criteria are no longer available, some detailed investigation would be prudent along the lines of what skogsgurra suggested and even more. For instance, is the exp. proof rating of the motor dependant upon no more than 80% loading? Is it a special design that has a severe rotor thermal damage curve? Was the lower OL setting a crude way of attaining an electronic shear pin?

Lots of questions here.


"Venditori de oleum-vipera non vigere excordis populi"

 

[motorsdirect]

Sorry Pocko...

just goes to show..you should always read the full thread before answering!!

whoops

Jeff

http://www.motors-direct.co.uk

 

[itsmoked]

jraef: Good stuff there. Excellent points! I felt a train wreck coming.

 

[jraef]

Me too. Sometimes I feel it is OK to try and err rather than spend a lot of time theorizing. Practical technicians tend to get results faster than engineers, but with considerably more failures along the way. In this case, especially when he said it was a hazardous location and downtime was so expensive, I see a high risk potential for disaster. Not a good time for trial and error.

 

[itsmoked]

Pocko; I want to commend you on a clear and detailed question too!


As jaraef was mentioning the overloads might be sized entirely for the hazardous area. One part of the hazardous area requirement is external surface temperature. It could be that the 60hp motor can only be exercised to an (example)45hp level, to prevent its exterior from rising to a temp that violates the area requirement.

 

[DickDV]

Inside the delta loop. OK! You are correct. Divide the FLA by the square root of 3.

 

[Marke]

Hello Pocko

The overload is inside the delta loop so it will see 58% of the current. You are correct with you "new" setting.

When did the overload trip before?
There are two possibilities.
The overload couls trip due to the extended start time with the larger blades, or due to the overload with the additional work being done.

If you expect that the production time is going to halve with the larger blades, that would suggest that your work rate is going to double and therefore your power rating may also double. - You may need to increase the size of the motor by quite a margin!!

I would check how much load the motor is currently drawing, and if it is relatively close to the 40 odd amps that the overload is set for (inside delta loop) then I would say that you will need to upsize the motor with the blades. If the current is significantly less than the trip setting, you may get away with it.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Pocko]

Hello again,

Thanks to all respondants so far. An update...The plot has thickened a bit! I've managed to take some current readings today during a "moderate" batch load, and here's what i've found -

1/ The incoming supply lines are reading = 38A avg.
2/ The bottom side of the Line contactor = 22A avg.
3/ The bottom side of the Delta contactor = 22A avg.
4/ The top side of the overload = 22A avg.

I think i'm going to have to physically check the wiring position of the contactor again (it's externally wired - not direct into the contactor). I can't do this today while the things live though!

To me all the readings indicate it's very underloaded. Is it possible the overload is in the line side and has been set at 58%. If so we would have alot of room to move...

Have a great day, Regards -Chris.

 

[mc5w]

The correct position for motor overload relays for a delta connected motor is inside of the delta loop, even if the motor is started across the line. The protects against an open motor winding.

If you want to double production you would probably be better off buying or building more machines. You have a design that works and trying to cur production time by speeding up the "blender" may actually backfire. I really doubt that it takes 8 hours to blend the ingredients and that the 8 hours is a chemical reaction rate limitation, not how fast the ingredients are being blended.

What you need to do is to put some smaller test batches in a tabletop blender and then blend for 1, 2, 4, and 8 hours and then chemically analyze the product. You probably will find that you really need the 8 hours.

 

[aolalde]

Pocko:

I think you are in the right track. the overload should be in the inner delta loop but may be it was wrongly wired in the line.
Your current lectures show 38 amps in the line, very close to the 40 overload setup..

Check first the oveload heaters location.