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Soft Start - Starting Current for Centrifugal Pumps 1

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X49

Electrical
Apr 30, 2009
106
I'd like to know how low I can set the current limit on a solid state soft start when starting a centrifugal water pump. The best way would be to test it, but in this case it's not practicle. What information would I need to estimate the minimum starting current?

The motor is 40hp. When the pump starts, it will be pumping into a pressurized water line. The total head is in the 10m to 30m range.

Schneider soft start literature suggests 300% FLA for centrifugal pumps. According to my genset sizing software that will put starting voltage dip of the genset at around 34% when all loads start simultaneously, which may be too high. The genset is existing and I'm trying to add some loads without having to replace it, so I'd prefer to set the soft start at less than 300% FLA.

Thoughts?
 
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Generators have a tendency not to like power electronics (actually it is the AVR that doesn't like the power electronics) and you might not be able to use the soft starter at all. Testing would probably be quite useful.
 
I should clarify. This pump and soft start are existing. I am adding a couple of other smaller motors that may start simultaneously, and as a result the voltage dip may be too high. The soft start has a bypass contactor so harmonics won't be an issue.
 
Simple, don't let multiple motors start simultaneously.
 
X49, you are mixing the terms "current limit" with "starting current". They are two distinctly different things.

Starting current or break-away current needs to be set at the lowest value that will just crack loose the pump shaft. Setting it higher than that simply causes more inrush current than is necessary.

A value of 300% is reasonable because it is about half the motor's across-the-line inrush. At that level the motor shaft torque is one-half squared or one-quarter of the nameplate value. Setting starting current lower than 300% results in such low torque that the motor is barely able to start itself, much less any load.

Current limit is the maximum available current available from the softstarter and would have to be at least as high as the maximum load current required at full speed. It does not affect starting torque: thus two different settings.
 
What davidbeach said. Inhibit starting of the new motors while the pump is starting.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
X49,
Predicting exact starting current settings for a soft starter is not an exact science. If you had every detail of the motor, pump, load torque requirements curve, power system etc, you may be able to plug all of that into a very expensive TMS program like SKM or ETAP and get a more "educated guess", but it's hardly worth it for a 40HP pump. The widely accepted method is to pick a starting point, i.e. the 300% suggested by the mfr., and do a little trial and adjustment.

The above comments regarding staggered restarting is the typical solution to problems such as you are describing. Generators have a certain amount of reserve capacity, and by staggering a restart you can utilize that reserve in smaller increments to facilitate a complete start sequence without over burdening it.



DickDV, I think you are mixing current limit from VFD world with current limit in soft start world. THEY are different things. A soft starter does (can) in fact limit the starting current, either immediately or after a ramp into it.


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X49,

I would do like this.
Try to start pump with some long starting time exmaple 100sec. Put some big current limit maybe 5*In. Then measure starting current (or look in display of softstarter if it some better softstarter). Then set current limit on some value about 30% higher then maximum current that you measured on this test start. Decraese starting time on some value like 20sec. If current reach limit with this settings increase starting time.

One important thing more. Do you have on this network anything esle except motors (for example some coils of contactors). If you have some coils pay-attention because you can burn contactors if voltage is too low (I do it one time on some big pumps). If you have only motors than this voltage drop cant produce any damage (maybe only motors will not run to full speed but it should).
 
jraef, I was assuming that the softstarter was a reduced voltage starter. You are correct that there are softstarters out there that operate via a starting ramp current limit which could be what X49 was referring to.

I would think that a current limit starting ramp would be problematically on a centrifugal pump since the running current due to the actual load becomes probably higher than the acceleration current as the pump approaches full speed.

Since I am not familiar with current limiting softstarters, I'd like to know how they handle the higher load currents at full speed.
 
The new loads are located in a facility a few hundred metres from the genset and I was not planning on having a controls/communications link between the buildings. Also, the existing pump controls are pretty basic (no PLC) so ensuring staggered starts would require some work.

DickDV, the soft start is solid state and is programmable for constant current mode or current ramp mode. Constant current mode is recommended for low starting torque loads like centrifugal pumps.

I think I will either upgrade the existing genset with a PMG alternator with higher starting capacity or install another genset at the new facility.

Thanks for all your responses.
 
Do do the staggered restart, all you need is a restart timer in each controller enclosure, all set to different times. Some soft starters, and the Schneider may be one of them, even offer this as a standard programmable feature built-in to the soft starter controls. You might want to check.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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A current limit/start current setting of 300 - 350% motor FLC would normally enable the motor to develop enough torque to accelerate the pump to rated speed.

Most soft starters these days will operate correctly within a specified frequency range. Once outside this, the starter will trip in order to protect itself. This is where soft starters fed from gen-set supplies can be problematic.

When load is added to the gen-set the prime mover slows down. The alternator is directly coupled to the prime mover, therefore the output frequency reduces.

It is the function of the governor to deliver more fuel in order to assist the prime mover to maintain speed. If the load can be applied gradually, the governor has more time to respond and is able to maintain speed more closely.

Soft starters that offer a current ramp facility (initial current, max current and ramp time) and those with wider frequency tolerances and frequency out of range inhibit times are best suited to gen-set applications.

Not sure that the above helps your specific case but might be worth noting for the future.

Regards,
GGOSS
 
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