sirius softstarters blowing up fuses 1

[vilhal]

Hello

We have a small pumping station with 2 37kW pumps.
We added two sirius softstarters (2 phases controlled) with line contactors.
Control voltage and lighting are connected to 2nd phase with their own fuses.
First time we tried to start softstarter, plc and kwh meter burned.
After that every time we try to start softstarter lighting fuse blows.
Lights dont flicker or anything, just after a few seconds fuse blows.
Very weird since controlvoltage and lighting have their own fuses.
Both are from 2nd phase which is the same phase that directly goes through the softstarter.
Without softstarter, controlled only with contactor the pumps run fine and there's no problems with fuses.
Also when trying to start with softstarters there's a constant sound. Like it's not even trying to up the voltage, only the 2nd phase going through.
Softstarter leds show phase failure / missing load.

Hopefully someone has some solution to this before we throw away the softstarters and buy frequency converters just to find out it doesnt fix the problem.

Thank You.

 

[jraef]

Control voltage and lighting are connected to 2nd phase with their own fuses.
First time we tried to start softstarter, plc and kwh meter burned.
2 phase soft starters are nasty, they are only done that way to be cheap, they are NEVER good for the motor or the power system. I never allow them.

Even so, what you are describing is unusual. The soft starter should not be affecting your other equipment in making it BURN! But if what is really happening is that the FUSES are clearing UP stream of the soft starter, then that is easy. You have a shorted SCR in the soft starter.

In a REAL soft starter, one shorted SR is not an immediate problem, because there is no valid path for current to flow as long as the other 5 SCRs are still good. But in the cheap 2 phase design, the other half of the current path, the center phase, is ALWAYS present. So as soon as ONE of the remaining 4 SCRs shorts out, you IMMEDIATELY have a current path through the motor winding. The motor will not create torque with only one energized winding, so no rotation, therefor almost no inductance and the current flow is in effect, a short circuit, restricted only by the winding resistance. That will clear your fuses, at least HOPEFULLY, otherwise it damages the motor winding. With a Line Contactor, you will not know this is the case until AFTER the line contactor closes. Generally, you are supposed to have the Fault Contact of the soft starter wired into the Line Contactor coil circuit so that you cannot re-close that contactor after the first fault. You must have bypassed that requirement.




"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[vilhal]

The linecontactor is connected to RUN/ON contact of softstarter. Also the softstarter has indication for being "DEFECTIVE". I would think that this self diagnostics which it has would recognize burnt SCRs? For what i understand if it would be DEFECTIVE, resetting the fault wouldnt even be possible in the first place. So no, linecontactor isnt connected into FAULT-contact, but it is connected to RUN/ON contact which doesnt go on if there is a fault (=FAULT contact is on).
Also if there is a shortcircuit in the SCRs, wouldnt it blow the main motor fuses instead of lighting fuse through which the motor current even doesnt travel?

Actually the only answer we got from talking to lot of experts was that the softstarter has some cheap way to adjust the voltage compared to frequency converter which causes the current to rise. Usually this excess current is spread around the system, but since we only have these 2 softstarters and lights in the system, it has nowhere to go and this causes the problems.

 

[vilhal]

Circuit diagram attached.
In addition to the circuit attached, in parallel to control circuit and F1 we have F2 which has lighting (lamps) connected to it.
Our L is taken from 2nd phase (L2) and it is taken after the main fuses before the individual motor fuses.
So can a damaged SCR cause F2 (or F1) to blow?

 

[LionelHutz]

Possibly a short to ground in the motor circuit causes your phase to ground voltages to shift.

 

[vilhal]

Motor cabling and internal cabinet cabling has been measured and ok.
Thats partly whats troubling here.
Im not an electrician, but if everything is measured and should be ok then how can half of the components in the cabinet burn?

We replaced one of the softstarters with frequency converter and it works like a charm.
One of the problems were still facing is that we cant actually see the nameplates of the motors. So we just have to go with factory defaults and measured currents.
Another mystery thing, our guys measured the current of a pump when it was running directly with a contactor, was 46 amps, now with frequency converter its 55 amps. Other one was measured 64A and now its mysterically showing closer to 80amps.

Another day another mystery.

 

[jraef]

Where are you measuring current on the VFD?


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[vilhal]

Actually from 3 places. We have a fixed meter that measures power, voltage and current of individual phases for the whole cabinet. Then I checked output current from frequency converter and in addition to these two we also used clampmeter (or what it's called?) to measure current of a phase between inverter and motor. These all were in within few amps.
The whole story of this is that we replaced the whole powerdistribution/motor control cabinet. The old one was from the 60s. The pumps themselves are newer but they are under water, thus unreachable to us, and we have no additional info on them apart from the currents we have measured. The original system used star-delta starters. So replacing them with softstarters seemed to be a good solution. But since for the mysterious reasons we cant get them to work, we will just have to take the losses and use frequency converters.
But we need to know the cause for these weird things. Like how the measurements were 10Amps off in the first place? Where is that 20Amps going now? Surely not to the river or the metal structures...
Maybe its the capacitors connected to compensate for the motors that have caused some false readings.

 

[jraef]

Ding ding ding! We have a winner!
You have power factor correction capacitors??? Might have helped if you had finished the whole story earlier... Capacitors are a BIG no-no with soft starters! You likely fried the SCRs in the soft starter the instant you energized them, you may have also damaged the capacitors. And capacitors are really bad for VFDs too! There is NO need for the caps now, the VFD will correct the power factor for you. Hopefully you have not yet damaged the output transistors of the VFD!



"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Skogsgurra]

Wow! Capacitors!!! Say no more...

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[vilhal]

geez! ok thank you guys, i will forward this to our guys. it's just money. it comes and goes. :*

 

[vilhal]

and if this solves the problem, we will be calling You Gunnar the next time since obviously we cant find a person in Finland to solve these problems.
This cost us 5500€. 6000$. plus about the same in wages.
So PEOPLE OUT THERE! MAKE SURE YOU KNOW WHAT YOURE DOING!
:*

 

[Skogsgurra]

Why not? I just helped the Fathers of Sirius with a serious problem in a paper mill.

If they hadn't 'reduced workforce' the way they have, a guy nerby could have done it for them. But the beancounters are everywhere now. Beancounters that think that technology equals IT and Telecom. Paint me tired.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

Gunnar, you'll get a kick out of this.

I'm working on a project to retrofit VFDs to some 4,000HP 4160V synchronous blower motors. Your (and my) ex employer came out and quoted the project, no mention of what they will do to protect the existing motor windings from the ravages of VFD operation. I brought it up, the customer queried them, they just said "no problem", no further explanation or discussion offered. I pushed the issue, they said they will have to engage an "expert" on the issue to give more details. That was 6 weeks ago. Apparently they can't find one...


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Skogsgurra]

If they are using the Robicon (re-baptised Si-someting) inverters - no problems. I have done measurements on smoke stack fans to prove that. The Robicon inverters are multi-level and quite nice to the windings. And extremely nice to the bearings.

I have been looking at a Chinese brand lately. They have most sizes and voltages on stock. It seems. And prices are unbeatable. Seems to be the same multilevel techology.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

Thinking a little bit further. The good ol' not-so-sexy KUSA-starter could work very well here. It doesn't create the large current spike that the wye-delta sometimes does. It is quite uncomplicated and does not produce any harmonics. The only drawback is that you need a resistor that needs to be designed.

I got a response from the Chinese MV inverter guy. He confirms what Jeff says about two-phase softstarters. Don't use them.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[sibeen]

Great discussion, gents, and again I learn something new from this site. I'd never heard of a multi level inverter before this thread, not being an MV person, and have been off reading some papers and company documentation et al.

Gunnar, if I'm reading you correctly you won't require any form of output filtering on these style of drives as the dv/dt is low enough not to cause issues. Is that correct?

The topology also lends itself to quite low input THDi.

Has anyone used this topology in the LV enviroment, or will the cost preclude that?

 

[Skogsgurra]

Yes, sibeen, that's right.
Yaskawa has a rudimentary type of multilevel inverter. But they use more of a Neutral Clamp aka Three Level Modulation.
It is the high voltage, typically 3 - 6 kV that necessitates the use of several sub-units to build the necessary voltage.
Each sub-unit is fed from a separate secondary winding on the integrated x kV primary transformer.

The reason that these inverters are so "nice" to the motor is that the motor itself has the necessary x kV insulation, but since each sub-unit works with typical LV (400 - 690 V) the PWM is not a problem for the motor insulation.

The capacitance from stator windings to ground is low (thicker dielectricum) so the stray currents that cause the induced shaft voltage (the main reason for EDM in large machines) are also low and the bearing life is, therefore, hardly influenced at all.

Great technique, you do not need any external transformer. Just put one of these complete units between your 3 or 6 or 11 or whatever kV grid and your motor. Plug in control and off you go. Very simple - everything already tested and verified. I wish I had the need for one...

And, yes, I forgot: since each branch consists of several sub-units that are insulated from each other, it is possible to hot-swap them so you can keep the motor running even if one of the modules fails. And also during repair. How about that for availability?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

Since no-one actually explained this - The capacitors can't be connected between the output of the soft-starter and the motor. They generally don't cause any problems if they are connected on the line side of a soft-starter.

If you like the Robicon and Robiclone drives then you'd really like the new one we're building. It uses 2 terminal power modules in the multi-level inverter. Much simpler on the input side with the same output side advantages.

 

[sibeen]

Lionel, any chance of a sneak preview of the topology of the new units?