Motor current during soft stop operation 4

[sibeen]

I have been asked to do a review of an incident at a site that occurred over 6 months ago and where the site conditions have changed considerably since. The information gathered from the site, by others, during the events was sparse.

The site has a 2 pole, 400V, 30 kW pump which was used to pump water into an uphill filling tank. The motor was controlled by a Danfoss MCD 202-030 soft starter, these soft starters have thyristors installed on two out of the three phases (red and blue). The configuration had experienced multiple instances of fuses failing. The original fuses were rated at 160 amps, type unknown. These were eventually upgraded to SIBA aR model 20 282 20.180 (180 amp) fuses, in line with ratings shown in the Danfoss manual.

After the fuse upgrade the soft starter began blowing thyristors on the Blue phase during a ‘soft stop’. Monitoring was placed at the site which indicated that the Blue phase voltage was falling significantly, upstream of the soft starter, whenever a soft stop was initiated. This fault occurred on at least four occasions.

The theory that was presented was that the site would be running at a small load, a soft start would be initiated, and the pump would start up without any issue and the internal bypass contactor within the soft starter would short out the thyristors. The pump would then operate for around eight hours and at that point the pump would be asked to stop. The theory goes that during operation there was a ‘dry joint’ type connection, on the blue phase, upstream which would heat up, and when the soft start was initiated the increase in current of around 400% full load current would cause a voltage dip then causing even more current to flow to the motor. This high current was enough over a few seconds of interval to cause the blue phase thyristor(s) to fail. That’s the theory that I’ve been asked to investigate.

Unfortunately, I’m not a motor person . So, my question is, what does the current into a motor do when a soft stop in called for under normal operation? I can find plenty of sources which will show what happens to the current during soft starting of a motor, but have been unable to locate a graph showing the same for a soft stop. Do I have a 400% increase in the current during a soft stop when the pump would still be under some load? If that is the case would there be some equation I could use to estimate the current maximum during this operation?

 

[itsmoked]

I'd find out if the initial fuses blowing where always the same or only phase. That would help with determining if one supply phase has a problem or conversely if the SS possibly had a control/phase problem.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[sibeen]

Thanks, Keith. From the information that I've been able to gather (not much) there was at least one occasion where the fuse blew on the white phase and on other occasions on the blue phase. I should have mentioned that the soft starter was replaced, after failure, on four occasions, not just repaired on-site.

 

[BlackJackJacques]

I do not know what a dry joint is...a poor connection?

I see many similar failures where the ss controller is not properly matched to the motor --usually when the controller is later replaced, and sometimes the motor.

Sometimes, if a ss is applied, the reduction in motor torque at starting is too low to meet the needed flow/head. Sometimes, someone decides to replace the old motor with a new high efficiency motor, not realizing that the inrush current for the new motor can be 1700% fla, and toss on top of that the torque reduction issues of a ss, then it is easy to see problems. I'd start at the beginning to first id the load, and then map that to what a proper motor/controller should look like, and then contrast that to your existing equipment/failures. In your case where the motor has operated for 8 hours, stopped and asked to start again (soon after stopping?) could be you have a borderline motor/controller sizing condition to begin with, and then when it is asked to restart soon thereafter (without the ability to cool down) and then deal with high inrush, resulting in failure.



Jim

 

[waross]

Apart from your system issues, why are you soft stopping a pump?
Start monitoring the voltage at different points upstream of the motor to find the bad connection.
You can start at the supply transformer to see if the problem is in your plant or on the supply grid.


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

 

[sibeen]

Bill, as far as I know, and that's very little on this job, the soft stop was to reduce water hammer.

OK...I can see now that I've made a few rookie mistakes. Firstly I've posted way too much information and detail in my OP. Engineers are of course going to pile in and diagnose the issue and suggest solutions. My bad.

Secondly, I don't post here often. If anyone clicks on my profile they'll see that I joined back in '03, and unfortunately for me I was on the wrong side of 40 even back then. So perchance I may have some slight inkling of basic fault finding. Hopefully that did't come off too snarky

I will never see this site. It will be a desktop review only.
There won't be any additional testing on the site; things have changes considerably since. The soft starter was changes out for a VFD (good).

I really do just need to know whether the current into a motor will increase during a soft stop application, and if so an equation or two that may assist with giving me a swag at what has gone on.

 

[waross]

I expect that the soft starter has a current limit setting. The current will be what the limit is set for.
Back in the days when we used plugging to stop U-frame motors, the current for a DOL stop was about the same as the DOL starting current, but lasted longer. The current for a soft start will be less than DOL unless the settings wrong.
I haven't seen water hammer issues when stopping. If there are issues, I imagine that stopping faster than a coasting stop would make the issue worse.
I can see the value of a soft stop on a high inertia application such as a large band mill that must be stopped to change saw bands. Without some type of braking the coasting time is too great in a production environment.
I sympathize with your position. It sounds as if the customer may be designing and trouble shooting based on swags rather than facts.

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

 

[Marke]

Hello Sibeen

When you apply a soft starter to an induction motor, the voltage applied to the motor is reduced and consequently the current is also reduced.
The reduction in start current reduces the start torque by the voltage reduction squared, but, the motor must develop sufficient torque to accelerate the motor up to full speed. The minimum start torque requirement is determined by the pump torque speed curve and the start current is determined by the high slip current to torque conversion of the motor.
The modern high efficiency motors have a much higher full voltage start current than the older motors that were around when we first started developing soft starters. The motors of the 1980s typically exhibited a LRC (LRI) of around 600%, but the modern motors are commonly around 850%. To start a modern motor, the minimum start current is much higher and can require a larger starter in order to not overload the SCRs. Many soft starters are rated with a start current of 300% which is too low for modern motors.

When the motor is used to soft stop the pump, the voltage is ramped down. As the voltage is ramped down, the maximum torque available from the motor reduces until the motor can not produce enough torque to keep the motor running. At this point, the motor begins to stall and slows down to the point where the torque produced by the motor is equal to the torque required by the pump. If the torque curve of the motor is very peaky, this can result in quite a significant drop in speed. As the motor slows, its impedance drops, (look at the full voltage start current against slip) and the current will rapidly rise significantly.

The current during stop will be a little less than the current during start, but can be present for significantly longer, depending on the soft stop time selected.
If the SCRs are undersized due to the high locked rotor current of the motor, then they will be overloaded during start, and potentially significantly overloaded during stop. This can contribute to premature failure of the SCRs.
I would have a look at the rating of the starter relative to the motor rating and check if any derating is required for the actual start current.
For a standard pump on a high efficiency motor today, I would size the soft starter based on a start current of 450%, or even higher, not 300% as is commonly the case.

There is also another potential issue that can cause semiconductor fuse and/or SCR failure to occur if the starter is marginally rated.
Many modern soft starters employ trigger opto SCRs to trigger the SCRs from the Anode Gate voltage of the main SCRs. If there is a bypass contactor closed across the SCRs, then there is no trigger voltage applied to the gate while the contactor is closed. Gate current can only be applied to the SCRs a period of time after the contactor opens. - the anode voltage must go high in order to develop the gate trigger voltage. This can mean that there is a disconnect period followed by a reconnect on an induction motor and can cause a significant current transient that exceeds the di/dt of the SCR.
I would go up in soft starter rating.

Best regards,

Mark Empson
Advanced Motor Control Ltd

 

[sibeen]

Mark, thanks for the reply and I have to do a ROFL. On Friday I was searching around for papers to read and came across some from AuCom which mentioned your name. The name immediately set off some bells in my head but I couldn't work out from where. I hadn't posted over here for three or four years. I thought I may have met you years ago when I used to do some work with Vectek (sp) in NZ and actually rang Jeremy from ABB in Auckland to check out my theory. He told me he knew you but I wouldn't have through Vectek. I think it was yesterday that it clicked about where I knew the name from and I wasn't sure whether you still posted on this site. Nice to see that you still do.

I've spent the last two days reading yours, and other sites, and think I'm finally getting my head around it all. Thank you again

 

[Marke]

Hi Sibeen

It is a small world!!
I have not been very active on here either, just too much happening at present.
I hope that you sort the problem. Are you still in NZ??

Mark Empson
Advanced Motor Control Ltd

 

[sibeen]

Mark, I'm based in Melbourne. I used to help out Vectek (I hope I have that name right) with UPS issues many years ago and would fly over on occasions to give assistance. I still know a few of the olds and bolds from those days and keep in touch once in a blue moon.

As to the problem, one engineer was claiming that there wouldn't be a current increase to the motor during a soft start. I think I've now enough ammunition to be able to shoot that part down. The problem has been resolved, or at least there is no apparent site issue, as the soft starter was replaced with a VFD about 5 months ago. I've just been asked to get in between two warring parties. Not usually fun

 

[jraef]

When you "soft stop" a motor with a Reduced Voltage Solid State starter, the current profile during soft stop is basically a mirror image of the starting profile IF you had used a purely voltage ramp profile, i.e. no Current Limit. Even RVSS' that offer Current Limit will usually not enable it during Soft Stop, so it may actually look almost the same as the current profile when starting. So in theory depending on how aggressive of a decel profile you use, the current could get very high. That however SHOULDN'T damage the SCRs. But having said that, the problem is that there are no real standards to which soft starters are made, so there are a LOT of products sold, by otherwise reputable manufacturers, which are questionable. One way I can spot them is if they are selling the "2 phase" versions, meaning they only have SCRs on 2 of the 3 phases, the 3rd one (usually the center) is just a piece of bus bar going straight through the unit, line to load. That's what you have, so right there, I'm already suspicious of it.

One bad thing about that cheaper design is that it is INHERENTLY causing a severe imbalance in the motor current as you phase back only 2 of the lines, leaving the 3rd at full conduction. This means your motor is heating up very disproportionately to the expected load and it can cause loss of motor life. Then when implementing soft stop in this type of controller, you have very little control of the motor current and they typically recommend no more than a 10 second decel. That however often conflicts with the stated purpose of mitigating water hammer. So they provide you with a limited decel time setting that may not allow the water hammer mitigation you were seeking!

What I have seen happen then is that the 10 seconds was not enough time to fully mitigate the water hammer and at the end of that 10 seconds, the Soft Starter just turns off. But the valve is not yet closed, so the water back flows into the pump briefly, making it back spin and residual magnetism in the motor winding allows it to become a generator, sending a brief but spectacular voltage spike back onto the SCRs in the Soft Starter. Doing so can cause them to "self commutate", meaning they turn themselves on without being commanded to and since that center phase is always live, there is a valid current path. That then becomes a high current spike (like connecting a generator that is out of phase with the line), blowing the fuses and stressing the SCRs. Repeat to failure.




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

 

[itsmoked]

Well Jeff. I'm really seeing what you mean about the cheap-ass two channel SSs. That made your dissing them considerably more clear. Thanks.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[sibeen]

jraef, thanks for your post and the information, especially the interesting fault scenario at the end. Something else that I'm going to have to throw into the mix.