Starting Reactors for Soft Starters due to cable length. 1

[gokulkrish2]

Hi all,

We are in the process of purchasing soft starters for our deep well pump motors. They are going to be from EATON Cutler hammer and are for 1000HP motors. (Actually they are 2 x 500HP motors working in tandem). EATON has recommended that we have to have starting reactors because of the capacitance issues due to the long length of the cable.

I want inputs from you guys on when reactors are needed. Here is the some basic information on the system

Motor Power: 1000 HP
Motor Voltage: 4160V
Cable length: not more than 1500ft
Application: deep well pumps. We want to soft start to reduce mechanical stress on the motor and pipings. Also, we have to ramp up time constraints that the motor must be started by 2 secs. So, taking a quick guess, i assume we will have to have around 400% of starting current atleast to ramp the motor up quickly.

Taking all this in to consideration, are starting reactors required? Is Soft starters still a feasible solution or should we use VFD's??

Any other things we are ignoring on this issue??

Thanks

gokul

 

[waross]

variable. The capacitive constant of water is high. Consider the capacitance of the wetted length of down hole cable, if pumping water.
With VFDs you can match pumping rate to usage rate. Much less need to start. The pumps run at reduced speed/capacity continually.
Wait for a few more answers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Either VFD or softstarter has to achieve the same thing - start in 2 seconds. So, they both need to produce the same torque at the motor to get that acceleration which translates into the same torque in the pump and piping etc.

The VFD is capable of doing it with less motor current and it is also capable of changing the running speed to control flow.

At 1500ft of motor cable the soft-starters will likely need line reactors.

 

[jraef]

The load reactors are a legitimate concern, typically recommended on MV applications over 1000ft for the reasons they proscribed (the distance isn't a hard and fast rule, just a reasonable point to start). Cable capacitance in long MV conductors can cause a voltage spike to come back on you and take out the SCRs in the soft starter. You can sometimes deal with that in the cable selection on above-ground installations, but the cable for submersible pumps is usually not optional.

Your issues are covered pretty well, although as Waross says, a VFD can match flow requirements if that is necessary. The only other reason to consider VFDs is if you have a severe power limitation and cannot live with the 400% current you are likely to need with the soft starter.

My only concern is the "two 500HP in tandem" issue. For years this was done by having them electrically in series with each other, but lately that has begun to change because of the advent of VFD applications. Do you have separate cables going out to the two sets of windings? If so, you need to have separate OL protection on each set. If not then they are in series. That would make it difficult to use an "off the shelf" VFD. I know there isn't such a thing in MV VFDs anyway, but for the series motors like that the VFD and motor need to be engineered together and usually the only ones that will do that are the pump mfrs. You can do it on your own of course, but they will not likely stand behind the pump unless they do it.


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[gokulkrish2]

Thank you all for a quick and apt response.

The application is just to start and not to control flow. That is why in the first place we decided to go with the soft starters. But we didnt reliaze the capacitance issue before. So i guess we have to have starting reactors and that will resolve the problem. We dont have any power issues so i think we will be good to go with the soft starter.

Jreaf,

Our motor is a connected in series as u mentioned. I understood your concern for a VFD. Is there a concern like that in case of a soft starter? We are dealing with centrilift on this issue and already they are not so cooperative. This motor we are discussing failed 3 times and they blame it on us saying DOL starting was the problem. Moreover they allow only 2 starts a day. I know it sounds ridiculous but thats what they recommend. Any comments on this??

gokul

 

[jraef]

The series windings issue has always been a risky business in my opinion. They are essentially two separate motors but with having only one feed, the protection device cannot differentiate between the winding sets. What they essentially rely upon is the fact that if one winding goes bad, the extreme increase in load in the remaining one will cause the OL to trip very quickly. My suggestion is to set the soft starter's OL curve to Class 5 if you can, no higher than Class 10. Typically you can get a little more from an ESP because of it being in water that is pumping the heat out with it, but on these series motor arrangements you essentially consume all of the "fudge factor" and then some. That is just my opinion though, ESP people tend to defend this arrangement to the death, probably because it is their only solution to a complex problem. Can you ask Centrilift to put thermistors in the motor windings? I know Reda / Schlumberger used to offer that at one time, I always thought that was a good idea.

As a side issue, the Eaton MV-IT soft starter may not have had enough protection features built-in to it do adequately take these precautions, so if that's what you have then I would use a Motor Protection Relay, either the Eaton one or a Mulitlin 369 or 469 (I personally would use the 469). If, as I recently heard, Eaton has jettisoned their own in-house MV starter in favor of brand-labeling the Motortronics MVC Plus Series, the front-end of that starter will have everything you need to protect this motor. I still like the 469 better, but that's just because that unit has saved my bacon on more than one occasion and I tend to stick with things I know work.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[gokulkrish2]

good to know ur recommendation for 469's and thats exactly what we use in our plant. We were using it before too but didnt help with this particular deep well motor for some reason.

I will remember to instruct EATON guys to use class 5 as they are doing the configuration. Also i dont know wat an ESP mean. I dont knw if centrilift will provide thermistors but will ask them if they can. Again to clear my doubt are thermistor and RTD the same or different?

gokul

 

[jraef]

ESP = Electric Submersible Pump, what you have.

RTDs are better than thermistors, just more expensive. But if they are there or can still be added, the 469 will be able to read them already (up to 12) so that's great.


"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