Soft starter problem 3

[pbcd]

We have an Air Conditioning unit with a Benshaw soft starter. Last week when I turned on the breaker (not the start control) the SCR bypass had a huge arc, which melted the bypass contacts. I had the manufacturer rep. come down to do the troubleshooting and repairs. He megged the motor, ohmed the SCRs and gates and replaced the one bypass that arced. Upon turning on the breaker (not the start control), the soft starter blew again. This time burning out the pulse transformers (all six) on the power circuit board (trigger board). Any info., as to what may be the cause of this problem is greatly appreciated.

 

[jraef]

I would say that was not a very thorough check if that is all he did. The Up-to-Speed circuit that controlled the Bypass contactor(s) may have been welded, the bypass contactor(s) may have been jammed, any number of other relatively minor things could have caused the original problem. Then if is was a digital starter there may have been a corruption of the microprocessor, so a run-through of all of the parameters would have been in order. Even if it was an older analog starter (pots, no keypad), he should have disabeled the bypass and done a startup with the soft starter only, then added the bypass if everything else checked out.

Being that you mentioned that "he replaced the one bypass that arced", I assume it was one of their versions that has separate bypass contactors for each phase. In Benshaw's older version of this, it had a completely separate control board just to control the 3 separate contactor coils. Did he run a diagnostic on that board? If it was their newer digital one, again there could have been some sort of corruption of the microP that could have been easilly fixed before the catastrophic failure.

Since the problem was obviously not fixed the first time, the second failure was probably a result of the continued stress.

Fixing one thing and then trying a "smoke test" is a bad way of finding problems.

"Venditori de oleum-vipera non vigere excordis populi"

 

[pbcd]

Thank you for your response. Yes there are 3 bypass contactors with a bypass control board. My original mindset was that there was a short that caused the arcing. I am new at this so I am trying to get a better understanding of how things work and what may have went wrong. Hope you don’t mind, but I have several questions. Based upon your response, are these deductions correct?

- The arcing is from the high Lock Rotor Amps going through the bypass contacts that are designed for Full Load Amps.
- The problem is due to the bypass contacts being closed during the initial start up.
- The contacts could have been welded closed, stuck closed or given a wrong signal to close.
- If we disabled the bypass the second time, we would have been able to provide power to the starter to perform a diagnostic test.

If the bypass contacts was open and the motor was shorted between windings or grounded OR if the trigger board missed fired, I assume the bypass would not have arced, but the SCRs would have been damaged (no arcing)?

To disable the bypass for testing should I be disconnecting the motor leads or should I disconnect L1, L2 & L3 to the SCRs?

I appreciate your help. Thank you!

 

[jraef]

"- The arcing is from the high Lock Rotor Amps going through the bypass contacts that are designed for Full Load Amps."

Not necessarilly. Assuming it was an RS6B starter, those contacts were designed to handle 720A. They could theoretically start a 500HP motor X-Line, but were not specifically rated to do so. If you had a 600HP motor that would have been a problem.

"- The problem is due to the bypass contacts being closed during the initial start up."

I don't think you know enough to determine what happened the 1st time, but being that they closed as soon as you closed the breaker, I'd say there was some sort of problem along those lines. If all 3 were closed prior to closing the breaker, I doubt that you would have seen a flash, but if they were chatterring you definitely would. Why they were chattering is undetermined.

"- The contacts could have been welded closed, stuck closed or given a wrong signal to close."

Most likely the last one, wrong signal. But if only 1 or 2 of the 3 were welded or stuck, it may have affected the bypass control circuit and caused the others to chatter and flash, I don't know.

"- If we disabled the bypass the second time, we would have been able to provide power to the starter to perform a diagnostic test."

Yes. That would have been prudent, especially given that you had just had a failure in one unit.

"If the bypass contacts was open and the motor was shorted between windings or grounded OR if the trigger board missed fired, I assume the bypass would not have arced, but the SCRs would have been damaged (no arcing)?"

Too many if's in there. If the bypass' were open, a short in the motor or leads would have had no effect on it until they were commanded to close. Again, they were likely NOT all closed upon your closing the main CB, but closed shortly thereafter (for some reason), causing a flash.

If the trigger board misfired, it may have affected the bypass control board and caused it to command a bypass closure. Only Benshaw can answer that, but it is true for several other brands that I have seen. Misfiring of SCRs is somewhat rare, but it can happen, usually as a result of condensation on the PC board(s). It is more possible however that the same thing that would cause the trigger board to misfire could also cause the bypass board to freak out as well.

Misfiring does not always result in damaged SCRs. They are usually damaged by voltage spikes, and less often by a current spike or overheating. Misfiring for an extended period can cause the last 2, but not a voltage spike. So I guess you were trying to use the good SCR test as a way of determining if there was a misfire, but that is not valid.

"To disable the bypass for testing should I be disconnecting the motor leads or should I disconnect L1, L2 & L3 to the SCRs?"

If you disconnect either of those you will not be able to test anything! Disable the bypass by disconnecting the control wires going to their coils. That way even if the bypass board is faulty, it should not affect the trigger board. The motor should start and run normally, although Benshaw may have a bypass discrepancy check circuit that would trip you off-line after a few seconds of running at-speed with no bypass. You probably will not be able to run it continuously like that anyway because the heat sinks are not designed for that, but it will tell you if there is a problem with the trigger board or not.


"Venditori de oleum-vipera non vigere excordis populi"

 

[pbcd]

Thank you very much for your informative and detailed response. It is interesting that you mentioned condensation. We did notice that there was a small layer of condensation on the power lines to the breaker within the soft start cabinet. There is also a rust stain running down on the cabinet starting from the bottom of the cooling fan. We have no explanation as to why.

While working in the starter cabinet, can I get zapped from the capacitor?

I assume that the pulse transformers blew up due to high current /voltage via the damaged SCR?

For future reference (when arcing has taken place): Prior to electrically disabling the bypass and attempting a diagnostics test, I assume it would be prudent to remove all bypass contactors to inspect for any physical problems such as binding or welding?

Thanks again for your patience and your time.

Phil

 

[jraef]

"While working in the starter cabinet, can I get zapped from the capacitor?"

That is a concern on VFDs. There should not be a capacitor in there, soft starters do not have them (other than little ones on the snubber, and they dissipate almost immediately through the motor windings). Unless there are Power Factor Correction Capacitors. If so, that is a whole new ball game, even for the previous issues! If you have PFC caps, see if they are connected to the line side or the load side of the starter power circuit. Post your answer and I'll tell you what else to look for.

"I assume that the pulse transformers blew up due to high current /voltage via the damaged SCR?"

As I said earlier, Benshaw would have to answer that because only they know how the pulse transformers on their system are connected. Some designs use a small SCR to switch line voltage from the Phase Locked Loop sensing circuit to go into the pulse transformers, and since Benshaw uses PLL firing, they may use that method. Others use the control power source, i.e. 120V to feed the pulse transformers, in which case anything happening on the power line is dampened by the control power transformer. Either way it is doubtfull that it would be voltage related unless you were struck by lightning, or PFC caps were connected wrong (see above). The current spike could create all kinds of havoc on the other hand. By the way, I thought the SCRs were OK?

Whenever the MS (magic smoke) or BF (blue fireworks) come out of an electrical device, inspection of the switching surfaces is always a good idea, even the ones that did not actually show MS/BF. Unless the current path was to ground, the other part of a short circuit would have to pass through at least one of the other contact sets, if not both.

I guess my point in this was that if I saw a catastrophic failure like that, I would want to know why it failed, not just replace the obvious burned components. It didn't appear to me from your description that the technician was very thorough. The rust for instance would have been a big clue to em and I would have recommended a strip heater.

"Venditori de oleum-vipera non vigere excordis populi"

 

[pbcd]

The model number of the Benshaw starter is UTCIDCBRSM6B-300A-460-1. I noticed RSM6B is part of the model number. The display panel is labeled RediStart Micro. Apparently after the second flash all the SCRs, the power card and the bypass card were damaged. Benshaw is checking for availability and cost of replacement parts verses replacement of the complete starter.

The rep. is concern about moisture and mentioned about changing the enclosure. I don’t understand how changing the enclose will solve anything since the fan is continuously drawing outside air across the interior of the cabinet. You recommended a strip heater, but it would seem to be counter productive with the fan designed to remove heat?

Behind the starter enclosure is another enclosure (I’m guessing about 8” x 8” x 16”). The nameplate data states capacitor (I’ll have to recheck the exact wording). I believe it is connected to the lineside of the overload breaker with a contactor. This what I was concerned about getting zapped.

If just one of the bypass contacts were closed for whatever reasons and there was no ground, then I shouldn’t have a BF situation?

Thank you

 

[pbcd]

jraef

I read your “What is a Soft Start” FAQ. Does a SCR continue to “reject approx. 1.5 watts of heat per running load amp” when the SCR is being gated at the zero-cross point and the motor is getting full line voltage (applicable to a soft starter with no bypass)? Or is most of the heat generated only during the ramp up period?


Thanks,
Phil

 

[jraef]

The heat would be rejected continuously, however in your starter that is not an issue because you have a bypass contactor. That is the likely reason why your rep discussed a different enclosure. Often times OEM equipment suppliers will buy a panel mount starter and use the cheapest or most convenient box they can find, even though it might not be the right rating for the application. By having a bypass contactor, there is probably no need to have ventilation of the enclosure. If your ambient environment is subject to low temperatures however, ventilation will in fact make it worse. The fans on the heat sinks themselves are meant to just dissipate the heat from startup more evenly accross the aluminum. I said "probably" earlier because IF the enclosure is extremely small AND your duty cycle is high, they may have needed the fans to dissipate the extra heat created during ramp-up (unlikely, but possible). If that is not an issue, I too would suggest a sealed enclosure (NEMA 12 or NEMA 4) and a strip heater with a thermostat set to 40F.

If your PFC capacitors are in fact controlled by a contactor, that is the correct way to do it. They should have a ground connection so there should not be a significant risk of contact with them within a few seconds of de-energizing. Check that contactor and control circuit however. If the caps were stuck on when the soft starter was ramping, it may have blown the caps and that could have created a severe voltage spike. Caps and SCRs cannot coexist in a circuit, one or the other will be damaged and it is a race usually lost by the caps.

If only one bypass contact were welded or stuck, and there was no ground foault, there would be no path for a short and no damage. There would be a severe imbalance on startup because 2 of the phases would be ramping while the 3rd would be full voltage, but that would not necessarilly cause damage unless to the motor over a long period of time (overheating caused by the imbablanced current).

"Venditori de oleum-vipera non vigere excordis populi"

 

[jraef]

As I think about this more, I realized that the particular Benshaw starter you described has their "high end" microprocessor control system. It would have (should have?) picked up a welded or stuck bypass contactor as if it were a shorted SCR and disabled the starting of the rest of the system. That leads more towards a control glitch, and again, moisture becomes a prime suspect.

"Venditori de oleum-vipera non vigere excordis populi"

 

[pbcd]

I live in Hawaii so low ambient is not an issue. The existing enclosure is in fact quite large. There is one fan on the side of the enclosure which draws air across the heat sinks. The chiller is on 24/7, so ramp-up is not an issue for our normal use. The chiller is in a machinery space that at times will have a wet floor. Would a seal enclosure with no strip heater be recommended?

There is a 40 KVAR, self-healing dry capacitor. It is connected via a contactor. I will check for the ground connection. “If the caps were stuck on when the soft starter was ramping, it may have blown the caps and that could have created a severe voltage spike.” Did you mean “If the bypass contactors were stuck ……”? Do you know how I can check a cap of that size? “Caps and SCRs cannot coexist in a circuit”, could you please elaborate a little more on this?

The contractor quoted $11K to repair and replace damaged components with no guarantee (parts changer). The other option is $21K for a complete soft start unit replacement. Being that you have an idea of what I have and what is currently available on today’s market, could I get your recommendation?

I appreciate your time.

Phil

 

[pbcd]

jraef

Just thinking ……. If the high end microprocessor can pick up a stuck or welded contactor then wouldn’t it be able to pick up the contacts being closed by a bypass card glitch? If there was a glitch from the trigger board causing a misfire wouldn’t that not result in BF?

 

[jraef]

" Did you mean “If the bypass contactors were stuck ……”? “Caps and SCRs cannot coexist in a circuit”, could you please elaborate a little more on this?"

No, I meant if the capacitor control contactor were stuck on or energized while the starter was ramping. Capacitor charging current looks just like a short circuit to an SCR, meaning that there is too high of a rate of rise in current (known as Di/Dt, short for Delta I (current) over Delta t (time)). Di/Dt can cause the SCRs to "self commutate" or essentially turn themselves on without a gate firing signal. Being that this happens uncontrolled at the wrong time, either the SCRs will short out from rapid current flow that cannot be dissipated into the heat sinks, or the capacitors will burn out (sometimes explosively) from being overcharged. The capacitors usually lose first.

As far as the microP picking up glitches, not necessarilly. Glitches are by definition outside the realm of normal behavior in and of themselves. What I meant is that most high-end soft starters like that have a circuit that detects a LACK of a voltage drop accross the SCRs, that is how we determine if an SCR is shorted. If even 1 bypass contact was stuck closed, it would have the same effect and indicate a shorted SCR, locking out the ramp functions. I cannot answer to Benshaw's ability to pick up auxilliary card failures, but one would hope so!

Ethical rules of this forum keep me from jumping all over this opportunity, but suffice it to say that those prices had better include installation costs performed while a dozen hula dancers keep you entertained!



"Venditori de oleum-vipera non vigere excordis populi"

 

[jraef]

I separated out this portion of my response because the administrators of this forum may delete it for being too specifically commercial, but I didn't want you to miss out on the other technical issues.

It is possible that your contractor is buying that replacement unit from the chiller dealer, who is buying it from the chiller mfgr, who is buying it from Benshaw. If it is out of warranty anyway, I would suggest looking around for a better deal. Just don't go with the cheapest ones out there, some are fairly shabby products. I can recommend calling a good distributor in Honolulu, Famillian NW (they may have changed their name to Ferguson Ent. by now). They sell 4 or 5 good brands that would all be good for you.

"Venditori de oleum-vipera non vigere excordis populi"

 

[pbcd]

There were two previous questions that you did not reply to. Would a seal enclosure with no strip heater be recommended for my application (24/7 operation in Hawaii)? Do you know how I can check a cap of that size?

Thanks again for all the info. You are probably tried of answering all my questions. You may be right about all or some of the mark-ups that may be taking place. I mentioned to management that I could also change parts and start the unit without a guarantee for much less. He mentioned he feels obligated to have the contractor complete the work. It's his call. As far as a complete change-out, it may be a problem going with others because the starter must be able to communicate with the Carrier a/c unit (manual start/stop, auto shut downs and alert / fault displays). But I will keep Famillian in mind. Next time I'll try to do the troubleshooting and repair with the guidance of the manufacturer of the starter. Right now management is deciding to repair or do a complete replacement. I'll keep you posted.

 

[jraef]

Sealed box w/o heater is probably OK, but remember that it only needs to get to 40F for condensation to form. Put a dessicant or corrosion inhibitor in it if there is a lot of humidity present. Most electronic systems are only rated for 95% RH maximum. I know from experience that Hawaii sees 99%- 100% quite a lot. I have been in several big hangars where it rained inside.

Testing capacitors should really be done per the manufacturers suggestions. Some have varying types of protection systems that may interfere with simple resistance tests. The easiest thing to do first is to check the fuses (assuming they provided them) and/or any pop-up indicators that may be included. After that, each mfgr has their own recommended procedures.

A basic test if you get past all of that, is to get an old style analog (needle type) ohmmeter, they are easier to use for this. Set it on its highest resistance setting. Put the test leads across the capacitor terminals (discharged of course). It should make the needle deflect almost all the way to zero and slowly move toward infinate. It will start out fast and slow down as resistance increases. The higher the capacitance, the slower the needle will move, varying with different meters and capacitors. If it immediately reads infinate, it may be a protection device. if it immediately reads zero and doesnt change, it may be shorted.

By the way, I just looked at a manual on some Aerovox caps I recently used, and they indicated they had a discharge resistor in the ground path, so it may take up to 1 minute for them to discharge.


"Venditori de oleum-vipera non vigere excordis populi"

 

[pbcd]

Thanks lot a for the education. I'll keep you posted.