Siemens Sirius Soft Starter 3RW44 failure 2

[ZaidEvans]

We have a Siemens Sirius Soft Starter 3RW44 supplied as part of a pump system. The 3RW44 soft start is connected to a Siemens 230 kw motor (, 60 Hz, 380 volt, 415A on the plate). The motor is mounted and connected to a floating pump (chain drive).

We have a 750 KVA Doosen generator connected to a Marelli power unit as the power source. This is enough power to operate a 230 kva motor during soft start.

Commissioning of this 3RW44 has failed despite 3 visits from Siemens to calibrate the soft starter 3RW44 with the Siemens motor. We have had this unresolved problem going for 5 months now and the Siemens experts are unable to offer a solution to date. We are a new shrimp farm in Saudi Arabia and with no pump there is no farm.

On the final switch over to full motor (and therefore pump) operation the softstart 3RW44 shuts the whole thing down. We have two identical pumps, motors and soft starters and both shut down in this way. We have tried slow ramp up, fast ramp-up and always at the end of the soft starter cycle, the sofstarter shuts down and stops the motor. The soft start message says it registered high voltage to the soft starter unit. The software settings of the 3RW44 do not include a way to alter or override this message.

On a slow ramp up, you can hear the generator slowly picking up load, but at no time does the generator sound overloaded. Then at the switchover to full pump operation, the soft starter stops the motor.

Siemens has failed to provide a solution. There is a simple solution somehwere here and I hope somebody on this forum has it.

Strangely, the one pump did start perfectly and run once for 30 minutes. However after shutting down and then changing 2 Amp fuses to the softstart 3RW44 neither units could be started again. These 2 Amp fuses kept blowing and Siemens instructed we use 6 Amp fuses later.

I have personally witnessed about 60 or more startup attempts.

The pump is suited to a 230 kva motor, so it is not the pump and it is not the water head either.

I would welcome a solution if possible. We have a case here of technology defeating humans!

 

[bentov]

I believe you're experiencing overvoltage at your control power terminals (A1 & A2) because your generator mains voltage (and corresponding control voltage from a Control Power Transformer) rises at the end of the startup ramp (normal behavior as the generator's excitation system "overshoots" a little with the rapid dropoff in current demand). The Siemens soft starters have a relatively narrow tolerance (plus or minus 10% of nominal, as I recall) and are very unforgiving in this regard.

To check this, verify the spec in your manual & compare to what your system supplies. The spec is probably "115V plus or minus 10%", so acceptable range is 103-127V. If your "at rest" control voltage is around 120-125 or more (very common) it probably rises well above 127V during the generator's "overshoot" moment. We've had a lot of trouble with this, always on the high side, with Siemens soft starters supplied by (fluctuating) utility power. Since it's a brief transient condition it's difficult to detect. Tech support tends to suspect more interesting problems, easy to get distracted with parameter adjustment experiments that have no effect on this. Our current fix is a tacked on CPT with a 120V primary & 12/24V secondary (we use a Square D 9070T50D13), connected as a "buck" autotransformer supplied by the secondary of the existing CPT, so that 12V is always subtracted. When line volts are high, control power is thus 130-12= 118; when line volts are low, control power is 115-12= 103, flirting with the low range of 115x.9= 103.5. It's a compromise for sure, but mostly our problems are on the high side as I expect yours would be.

You're actually in better position to compensate since you control the power source. You might be able to simply adjust your generator's main output voltage downward a bit to solve this problem . . .

 

[ZaidEvans]

Hi Bentov, thankyou for your reply - you have identified exactly the "overshoot" moment that has been noted by the Siemens technicians. We will likely work with the generator supplier to try and implement your two solutions. The manual has 110% ("Control supply voltage is above 110 % of the required nominal voltage for longer than 2 s (voltage peaks, wrong control supply voltage).") as the tolerance. The overshoot is higher than this and naturally the generator was blamed for this.

By the way Siemens sent us a "solution" soon after my posting above. They advised we get a bigger generator, stating that "The generator rated current is only 1140 A. This may not be sufficient to sustain the inrush current."

I will keep you updated on this.

 

[Skogsgurra]

This is exactly the way EngTips should work! PLS for bentov.

A note: To have a soft start that cannot take more than +/-10 % is customer punishment and should never happen. Soft starters are used in exactly these situations and a company that practically started the electrical business should never allow itself to let things like this happen.

Telling the customer to get a bigger generator is adding insult to injury.

A simple recorder would have revealed the problem in a few minutes. Didn't the Siemens support have one available? Or did they only discuss parameters? As always, I am tempted to say.


Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[rasevskii]

Another solution might be to supply the 110V control supply from an independant source such as an inverter supplied by a battery system (kept float charged by a charger). Also a SOLA voltage stabilizer could be used to stabilize the control supply. That is an electromagnetic unit resembling a small transformer, I would imagine that these are still being made.

regards, rasevskii

 

[rasevskii]

As a test, you could connect up a small gen set (like one of those Honda suitcase sized things) to supply the control power separately.

Also on the main gen set, see that the AVR is not set for droop operation, assuming that it is not paralleled with any other generator. On a modern brushless exciter, the voltage change from no-load to full laod (at no droop) should be less than about 2 percent.

On the inverter-battery idea, an ordinary UPS as used for computers (of the correct voltage and capacity, of course)could be used. We got around a bad problem once on an excitation system for a black-start generator in the Emirates, using an ordinary computer UPS.

regards, rasevskii

 

[jraef]

The 3RW44 voltage tolerance was changed to 115% some time ago because of this problem. If your manual is for the units you have, they are likely over 2 years old. If you downloaded the manual from a website, it may just be an old manual. There is a series number that can be read from the 3RW44 display that will indicate whether or not the units you have are the upgraded ones or not. I can no longer remember the series designation, but I think it might be "E" that indicates the higher voltage tolerance.

I posted my response to your question in the "other" forum. you posted it in. I don't believe that genset is too small. My most likely scenario was that the 3RW44 is under sized and at the moment of transfer to bypass, the current exceeds the protection threshold for the device itself. But that was because you didn't say in that other posting that the fault was on Over Voltage as you state here. So ignore that other posting... (this is a better forum anyway)

My less likely scenario was that the Doosan genset you have is old (or cheap) and does not have filtering on the AVR, so right at that moment the voltage swings because the harmonics from the phase-angle firing of the SCRs upsets the AVR. That could be causing a voltage spike even in excess of the 115% that shuts down the soft starter. This was at one time a common problem with soft starters and generators, but went away when genset mfrs realized it and installed filtering on the AVR circuits. If yours predates that change, or Doosan doesn't know that, then it may be a simple fix on that side. There used to be some after-market AVR filters for this, but since most genset mfrs now incorporate that as standard it may be hard to find.


"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

 

[ZaidEvans]

The 3RW44 is the model 3RW4447-6BC34. A manual on this has this as 385 A rated operating current, 150 HP @ 230 v and 300 HP @ 460 v. This is borderline satisfactory for a 230 kw motor? I am waiting for the generator agent to give me the AVR specifications.

Thanks so for for this help. I have made more ground here in 2 days than in the last 4 months!

 

[bentov]

ZaidEvans, if you have already tried this 60 times you certainly have nothing to lose. I would suggest you try again immediately with the separate control supply as described above (with small auxiliary generator) to verify the problem. I have attached a sketch of the autotransformer setup we use, cheap thing to try as a permanent fix if the separate control deal works.

Good luck . . .

 

[LionelHutz]

I don't understand your point about the unit switching from ramping to line power. Does that soft-starter switch in a bypass contactor before it has reached full-voltage and operating current? ie, is it switching a bypass contactor part way through the ramping which is then causing a voltage and current transient?

 

[ZaidEvans]

Engineers provided by the generator company have a similar solution, but with some type of voltage regulator/stabilizer. I will get this whole forum discussion to them before they implement anything. I expect Bentov's solution is far cheaper.

As for the switching, there is no separate bypass contactor - this is apparently built into the soft start unit - I do not know the mechanics of this process.

Siemens is still sticking to their guns that the "inrush current of the first motor" (we have 2 pumps and so 2 setups) is too high for the generator, but say that "A bigger generator is only an option. The main issue to solve is to guarantee a steady voltage in mains no matter if that’s achieved by using a bigger generator, changing regulator parameters or using some other generator with different excitation or regulator systems. Neither Siemens nor pump supplier have explicitly stated to use a bigger generator as the only option available."

 

[LionelHutz]

Well, you mentioned that it fails "On the final switch over to full motor (and therefore pump) operation" and honestly I have no idea what that really means. To me, it sounds like the soft-starter is switching something (like a bypass or it's turning the SCR's full-on) before the motor has reached steady-state.

The point about the generator controls not handling the soft-starter harmonics is a valid point. However, when I have run across this, the issue was either early-on or part way through the ramp and you are indicating the motor gets close to operating before the trip occurs which means this likely isn't the problem. The already described regulator over-shoot as the motor current falls is likely the issue and some tuning might be able to help.

You could also look for a ferro-resonant or constant voltage transformer for the control circuit to help a voltage regulation problem.

Maybe time for another soft-starter manufacturer that will solve your issues? It's sad hearing you have stuck with Siemens after 5 months of nothing but failure...

 

[rasevskii]

It looks like Siemens is trying to sell you a new and bigger generator. Is this such an isolated site that no commercial power is available? That seems to be true of many projects in Saudi, years can go by before the grid ever reaches the customers.

In reality, get out there and connect up a temporary control supply as suggested and see if that works. Then you can prove to the owners what can be made to work and what has to be modified. Do you really have only one main gen set with no backup? If so, that is bad planning to say the least.

In any commercial operation in the West, some heads would have rolled if after 5 months still nothing works.

Just some advice from one who has been in many bad site situations, also in Saudi...

rasevskii

 

[jraef]

Actually, it's sad that your soft starter supplier is sticking to the sizing of that unit. I believe it is under sized for that motor in that application.

Be that as it may, the normal bypass engagement in that starter is a result of two conditions: 1) the ramp time setting is complete and/or 2) the motor current must have climbed above 50% of the setting for 2 seconds and folded back to below 125% for at least 2 seconds. In other words there is a system to determine that the motor has reached pull-in speed (approx. 90%) before the contacts engage. But right at that point as the current is falling rapidly is when the AVR of the generator can get "squirrely", especially if not filtered to avoid being affected by the remaining harmonics coming from the SCR firing that is still taking place.

FYI, bypass in the 3RW44 is accomplished with integral contacts but they are not rated for motor starting duty, they are strictly shunt duty rated. That's why the unit must detect that the motor has gone past the pull-in point before engaging and the SCRs are not shut off immediately.


"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

 

[bentov]

Rant part (composed but withheld initially as not pertinent to ZaidEvans’ problem):

I've definitely struggled with control power overvoltage on the 3RW40 & 44 soft starters. In our neighborhood, the utility tends to jack up voltage in anticipation of heavy irrigation pump loads, not unusual to see mains pushing 515 when pumps are off. Once they’re running it drops below 500 again. A powered up 3RW with control voltage above 125 is likely to have a red “DEVICE FAILURE” light on, won’t start. The quick fix is to kill power, re-energize & start the motor before the 2 second trip delay times out – then usually the motor load brings the voltage down enough to keep the thing running. Some customers don’t find that real convenient, also not possible with auto start setups (pressure or level control systems). Another scheme is to rig the start circuit with a relay so the soft start control power supply arrives simultaneously with the run signal, again to apply motor load dampening of the mains voltage within the trip delay period – more work, more stuff to go wrong, interferes with programming & troubleshooting. Siemens makes multi-tap CPT’s but they’re pricey, usually not in stock, so I just started using the buck transformer alternative. Seems to be working so far, though of course now we’re vulnerable on the low side, bunch of pumps might shut down when peak load periods drag down utility mains. I vaguely remember looking into constant voltage power supplies, seems like they were expensive, seemed to me like I shouldn’t have to go through all that “customer punishment” Skogsgurra mentioned . . .

Our first 3RW44 startup (400hp) was a real headache, finally turned out to have inadequate control power VA available (undersized CPT supplied by others, took a while to start looking in that direction). That unit would ramp up, reach speed, then have a quiet moment (no sound from the starter), followed by thumping closure of the bypass contactor and a corresponding current spike that tripped the main circuit breaker. Siemens support was stumped (remotely though, they didn't actually visit). They first believed the unit would behave that way if it "didn't know" that it wasn't big enough(!), so they sent us a larger one (no change). Then they set me up with communications software to monitor the unit, plus I hooked up my own datalogger, etc., etc. - all of which only confirmed there was indeed a cessation of current flow when things got quiet, followed by a big spike with the across the line bypass contactor closure. We finally determined (by reasoning, meters weren't quick enough to catch it) that control power sagged at the point of bypass - due to the contactor coil load (far larger than the electronics demand during ramp up) overwhelming the CPT capacity. So, ramp up finishes, bypass is called for, coil load causes voltage sag so contactor armature fails to travel home, SCR’s open since done with their cycle (the fatal flaw in my view, must be on a timer instead of interlocked to the actual contactor as you would expect) . . . current drops to zero, mains (& CPT output) voltage rises, still energized bypass contactor suddenly has just enough suds to pull in so executes an across the line start with corresponding LRA and torque impacts . . .

Upshot, control power is a very big deal on these, hard to detect and easy to overlook (VA requirements are buried deep in the manual). Certainly we as installers or end users are responsible for supplying specified conditions, but I was very surprised this particular sequence was possible at all. I wrote up my observations & strongly suggested a change in control logic. I got no response; I suppose it’s possible that changes have been quietly made (this happened a couple years ago). Even when everything is installed correctly, that same scenario could arise anytime (adequate power for ramp-up, sag/hiccup at bypass) due to some small control wiring problem, like a proverbial ant in a switch contact somewhere creating a high resistance connection. We were lucky to have a carefully sized (Siemens MCP) circuit breaker on our 400hp startup, that promptly tripped off at each unintended across the line contact closure. What if instead we had snapped a shaft on a brand new 1,000 foot deep well pump, or blown a utility company's transformer? Wonder what those “shunt duty” contacts ended up looking like (pounding dependant on MCP trip curve)?

So, constant parade of "what ifs", no way all will ever be considered & addressed. We still sell these units, also sell and have issues with other brands of soft starts & VFDs, seems like it’s always something . . .

 

[ScottyUK]

From the original post

The 3RW44 soft start is connected to a Siemens 230 kw motor (, 60 Hz, 380 volt, 415A on the plate).

and later on

The 3RW44 is the model 3RW4447-6BC34. A manual on this has this as 385 A rated operating current, 150 HP @ 230 v and 300 HP @ 460 v. This is borderline satisfactory for a 230 kw motor?

The soft starter looks slightly undersized if the motor is heavily laden. Is the switch house air-conditioned, or is the soft starter exposed to Saudi ambient temperatures? High ambient will only make problems worse.


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