Soft Starter Issue

Find the metal object that is stuck in the pump and fish it out of there.

Some of your numbers don't seem consistent.

test the motor without a load....

Do you have any idea how much torque you need to start moving the pump shaft?

1803 Nm, moment of inertia 1.83kg m2

Dear JRaef

Thanks for the response.

The starting parameters mentioned in the motor performance data sheet is as follows

Starting current 914 A

starting torque 1803 Nm

Moment of inertia 1.83kg m2

Rated power 55kW

Nominal speed 990rpm

Number of poles 6

Rated voltage 415V

Pump Datas

Density 1kg/dm3

viscosity 1.57 mm2/s

rated speed 990rpm

flow 96.3l/s

head 26.7

rated power 34.8kW

frequency 50 Hz

If the starting torque of this motor and the actual required torque to get the pump to start turning are as you stated both 1803 Nm then this is your problem. Your motor is under sized for this mechanical load. The slight bit of over design by the motor manufacturer is the only thing that is getting this to move at all. You are getting precisely the results of an undersized motor for the mechanical load. I suspect that somebody mistook the pump's starting load for a motor recommendation specification, or that you are quoting the same number in this and your earlier reply to me.

I would prefer your motor starting torque to be at least twice the starting torque of the load (pump). I've seen smaller margins when static friction was anticipated to be significantly greater than rolling friction. I do not expect static friction with a pump of any type.

I think those are the motor capability values, not the pump torque requirements. It's hard to see those charts, but this appears to be a true centrifugal pump that will have a very low starting torque requirement. It requires 34.7kW and is connected to a 55kW motor, driven by a soft starter at 425% current limit. That motor SHOULD have accelerated with no problem whatsoever. So this is back to something blocking the pump impeller or a defective bearing, in other words a mechanical issue.

The only electrical possibities are a defective soft starter, for example a shorted SCR, or you connected the motor in Delta when it was supposed to be connected in Star. But if it were a shorted SCR, I would be surprised that the soft starter did not indicate that on the display. If you misconnected the motor, the torque capability is reduced by 2/3, so when you also reduce it via the soft starter, it immediately stalls and because the soft starter is artificially limiting current, it stays that way until the fuse blows.

Agreed. My bet is on a bearing failure, with the supposition that this did at one time work properly. My second bet is that the impeller of this sewage pump is clogged with some semi-solid muck. A torque measurement or at least a wrench assisted hand feeling of the pump shaft load will help to identify this. Come to think of it, there is also the possibility that a down stream obstruction (collapsed valve, clogged screen) could be happening. A pressure reading would help sort out this possible problem.

Good point by Redfred. If the pump WAS working and no longer accelerates, then it is surely a mechanical issue such as a blockage or bearing. Only if this is the INITIAL commisioning start up can my other possibilities can enter into it.

data sheets

Before supplying the power for pump, did you remove the coupling of the pump? We always should run no load before power for the device. Or you can use a small motor for testing all function of the softstarter. After that you use it with your motor.

I have all soft-Starters for compressor applications i will tell you settings just do them and you will get started motor easily:-

Set current limiting value at 250% and use fuse rating of 150A then you will start that motor very easily

Do remember that the fuses are there to protect the wiring, not the motor. The motor should be protected with an overload trip device in the starter circuit.

check your motor there should be some restriction of some kind and make sure your starter can stand full load. i had a similar problem, but in my case it was the drives and they where blowing. lost six of them.

Hi All

thanks for the interest in my query and the replies. The problem is now solved and the pump is running. we are still not clear about the trouble. we assume that it could be a mis-programming or setting in the soft starter. the panel manufacturer has yet to confirm this. just to get themselves saved from the blame the panel builders allegation was that the motor winding was not getting the full voltage and it is due to the bad quality of terminal lug used for terminating the cable(By the way we are not accepting this allegation, and love to believe that it is a soft starter setting issue)

I can imagine that if the "bad lug" was exhibiting so much resistance so as to cause the current to increase to 400A on something designed for around 100A, and stay there until the fuse blew, the terminal would have melted.