Soft starter rating selection 1

[SA07]

Hi
We have a new water pump & motor to be installed. The motor is 160 kW. Is there a standard, table or theory which explains how to select the rating of the soft starter?

 

[BlackJackJacques]

The main factor is if the soft starter in the reduced voltage mode can meet the starting torque of the motor. You need to first know the type of motor and its starting torque. You can figure starting torque from motor nameplate data.

 

[waross]

The soft starter manufacturers will provide sizing information, often online.

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

 

[jraef]

You buy a soft starter rated for 160kW at the voltage you are working with...

There are issues and pitfalls to watch out for however. One of the big ones now is that many suppliers are trying to "ease the pain" of a perceived higher cost (compared to Star-Delta) by selling what are referred to as "2 phase" soft starters. They are intended to work on 3 phase systems and 3 phase motors, but to cut costs they only SOFT START two of the 3 phases, the third pole is a piece of bus bar going straight through it. Some of them don't even let you know they are doing this, they just ignore the issue. But this is not good for your motor and has an inherent danger of actually destroying your motor if something goes wrong. So always ask if the Soft Starter has 4 SCRs or 6 SCRs, that's the tell-tale design issue. 4 SCRs is what you want to avoid.

The other important aspect is what's referred to as the Overload Capacity of the starter. If you are using this on a typical centrifugal pump, you don't need to worry about that too much, most products will suffice. But if you have any sort of positive displacement pump, then you must be very careful to make sure you get the highest duty rating you can afford. A typical design criteria for a "normal duty" version, meaning one for a centrifugal pump, would be capable 350% current for 30 seconds. If you have a positive displacement pump, you want something good for at least 450% for 30 seconds, longer if possible.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[danby64]

I made the mistake once of buying a soft start that was rated the same HP as my motor. We were running a very high inertial load though, and ran into problems where the soft start would fault on a SCR HI TEMP alarm.

Not sure if you're using Rockwell soft starts, but they have a tool, here, where you can input details about your load and motor, and it'll tell you what % thermal capacity your soft start has reached. If it's higher than 80% you might want to upsize the soft start, otherwise you'll run into the same issue I had. Our fix, instead of replacing the soft start, was to increase the Overload Shunt time setting so it was just under the Ramp Time setting.

 

[sibeen]

jraef, could you please expand on why the two phase version of a soft starter is so problematic.

 

[RRaghunath]

jraef, Two phase only thyrister control - reminds me of the heater panels we have in the process plants. The problem we used to face is the earth fault release in the upstream 415V ACB that feeds the heater panel used to operate whenever the heater is made on. The ELR in the heater panel itself was not affected.
Just wanted to share, though may not be related directly.

Rompicherla Raghunath