Softstarter ramping Down

[jrwb4gbm]

For anyone interested, here is a link to the manual for the make and model softstart 3RW44 Siemens. See Figure 1-5 and page 1-6.
Link:
http://www.industry.usa.siemens.com...rters/Documents/3RW44 System Manual_12.08.pdf

 

[DickDV]

First, an electronic softstarter is NEVER a variable frequency device. It is true that sometimes VFD's in the smaller sizes are used to do only softstarting (and soft stopping) but that is a VFD, not a softstarter.

Modern electronic softstarters effectively vary the voltage to the motor thru SCR chopping of the AC wave. Some softstarters control this voltage ramp only during starting and stopping while some control the resulting motor current during the ramps. Some are selectable in the programming.

Even on the easiest starting loads, you cannot soften the start enough to get below about 280% of FLA. That's because the motor torque comes down by the square of the current reduction so, below 280% inrush, there is not enough torque to start the motor much less a load on the shaft.

Regardless of what Siemens told you, on both accel and decel ramps, 300% FLA is entirely normal and even optimum since most other conditions would result in higher currents.

If you were expecting to accel and decel your pump and stay close to the motor's FLA, a softstarter is the wrong choice. You will need a VFD to acheive that.

Sorry!

 

[Saffa]

Tell Siemens & the engineer to RTFM (Read The F**king Manual). See Section 6.3 on Stopping, page 6-11.

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Note
If guided stopping is selected as stopping mode (soft stop, pump stop or braking), it may be necessary to select a larger dimensioned branch (soft starter, wiring, branch protection elements and motor), since the current exceeds the motor rated current while the motor is being stopped.
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Case closed if you ask me!

 

[Jeff23spl]

Tell Siemens & the engineer to RTFM (Read The F**king Manual). See Section 6.3 on Stopping, page 6-11.

---------------------------------------------------------------
Note
If guided stopping is selected as stopping mode (soft stop, pump stop or braking), it may be necessary to select a larger dimensioned branch (soft starter, wiring, branch protection elements and motor), since the current exceeds the motor rated current while the motor is being stopped.
-----------------------------------------------------------

Case closed if you ask me!

Perfect! Just to clarify: Siemens never told me anything wrong..., the engeneer that i was talkng about are the ones working for the contractor doing the job...Just to avoid any confusion if sometimes my syntax isn't 100% clear

 

[JesperMP]

Jesper Its a 30kw pump, running on a Class 10 56kw 3RW44 Siemens softstart. I had to raise the class to 30 to overcome the thermal model overload obtained by the ramping down with class 15
It's a torque control pump stop algorythm used so not really fast DC braking...But something as to retain the water and i don't understand exactly in fine details how it does that and why the current raise so much. The raise doesn't appear on a standard softstop but the hammering is there.

Do I understand correctly, that if you set it to "normal stop" (coast to stop ?) you dont get thermal overload, but you do get the water hammering ?
And if you set it to "pump stop", you get the thermal overload but not the water hammer ?

I am not that experienced with pumps, but I can imagine that it is not just a matter of how big the pump is, but also what diameter of the piping and the pipe distances before and/or after the pump. There is a difference in how much mass of water that has to be decellerated. It may be a case of that the softstarter is in principle big enough for the pump motor, but for decellerating the water in your particular case it would have had to be overdimensioned.

edit: By the way, I think you definitely need to monitor the winding temperature. The 3RW44 has an integral thermistor monitoring function on the terminals T1-T2.

 

[Jeff23spl]

You can often reduce the current during this phase by increasing the stopping time.

As this algoryth extend the mecanical time to stop (retaining the load), to reduce the heat generated we would need to reduce the time (not increasing it) but the process call for a minimum time longer than usual