We had an incident Monday that I thought might be worthy to post. One of our customers owns a large retail store. This past Sunday evening the local fire department began receiving calls overnight Sunday to Monday morning, reporting the smell of something burning in the area around the store. The fire department responded a number of times but could not locate the source of the smell. Around 8:30 AM Monday morning more calls were received and during the search the fire department went to the store's basement motor room which contained 3 hydraulic elevators. When they opened the door they were hit with a blast of heat, and immediately shutdown the elevator circuit breakers. One of the elevator tanks containing 150 gallons of oil was literally boiling. 3 hours later when our technician lifted the lid on the tank he had the use a piece of wood because the tank was still so hot you could not touch it.(See attached pics, at this point 3 hours after the unit was shut down the oil was still boiling, which you can see if you look closely) It had gotten so hot in the tank that the gauge on the valve had melted and the wires to the valve were stiff and brittle. We determined the cause of the problem to be a fused RU contactor, and while the computer generated a fault and took the system off-line, the contractor remained fused closed which caused the unit to continually run, we believe for 10 to 12 hours. The reason the fire department did not find the source of the smell was that there was virtually no smoke and the machine room vent, which is very large, is located down an inaccessible gated alleyway between 2 large buildings. There are working smoke and heat detectors along with a sprinkler system in the motor room yet none of these activated. We are still working through our initial investigation but I was wondering if anyone knew of a system that would shunt power based on oil temperature? This system is relatively new (less than 10 years) yet this still occurred.
Hasn't this unit have a run timer within the control? I would assume there would a MC contactor ( as the first 3 phase input), then the star/delta change over to run the pump. I also know of a stuck contactor function used as well. I would have thought in the event of this stuck RC, the system would pick up this, and trip the run timer, which would in turn trip a MC contactor. Also you asked about a temp control, there was also a over temp function and was a pain when hydro's were driven past their duty cycles and shut down.
Problem is that both the STR and RU contactors are fed directly from the main line. When the contactor fused the only way to shut it down was shunting the main line.
I had this happen once too from welded starter contacts. I can think of very few manufacturers that used a line contactor excluding Montgomery's P contactor. Seems to me a softstart would be the simplest solution. I think these incidences are a strong selling point. Unless anyone has heard of a Siemens softstart failing with more than 1 leg shorted?
I was wondering if a Soft Start could prevent this from happening again, small cost vs what they are going to pay to replace the entire power unit. I'm going to check with our supply house about this tomorrow.
I would second the soft start. No contacts to change, won't mechanically hang up, more energy efficient and relatively cheap insurance for the customers motor. I prefer the Sprecher Schu units my self(more reliable then then Siemens)
With that being said I had a tac20 once with a Siemens soft start that got hit by lighting and found it with the screen blank but it still sent power to the motor non-stop. The oil got so hot the plastic adjustment cover on the I2 valve melted off and toasted the o-rings in the valve, along with burning the oil real bad. Kind of a freak accident, but you never know would will happen when lighting damage is involved.
Certainly could have been worse. Good for everyone involved that no one was injured. In my opinion, now would certainly be the best time to introduce the customer to the electronic starter idea. For a number of reasons we typically site for switching mechanical starters out for an S.S. unit, sealed in contactors as such in your story is not one of the things I typically reference. However, I think it covers to the extreme what could " possibly happen" with the old mechanical type starters. I'd bet your customer seriously considers the idea. Over temp contacts on the motor typically catch this to shut the run contactor off, but as it has been said,with a sealed in contactor, ain't nothing gonna stop it short of 3phase power removal. I'd be curious why (not that it would have mattered) the smokes didn't trip? And the shunt trip device in the machine room? Maybe it didn't get hot enough? Or...nothing works in the equipment room regarding fire code. Now's the time to push that issue too.
Problem is that both the STR and RU contactors are fed directly from the main line. When the contactor fused the only way to shut it down was shunting the main line.
I am unfamiliar with this unit, but are you saying the 3 ph goes thru STR and then RU? The reason why I ask this is the whole purpose of have 2 or 3 contactors is so if 1 does get stuck, is then the run timer trips, stops the control run up, which then would trip 'all' driving relays ie STR and UP. Unless STR is star and UP is delta. If that is the case, then this is a poor design of the 3 phase control. Also over temp thermistor should do the same thing. Every hydro that I have worked on has a MC contactor, then the star/delta changeover system which can protect for this problem.
Also having a soft starter also needs to have an input to say the run timer and over temp has tripped or else the same situation will happen. The best advantage of a S/S is for motor life without the in rush current breaking down motor insulation.
Many years ago Otis had a problem with the 6754 switches in escalator controllers. They were Silver moveable to carbon stationary. Some one decided to make the stationary silver also for longevity. Silver to silver welds on AC. They had to go back to a carbon stationary. Safety circuits could open all day long and motor kept running. When you pulled the main line the relay contacts were welded together and relay was locked in. Always make sure the contactor, if replaced, is rated for AC and not just DC. May end up with some silver soldering and relay locked in.
I thought across the line starters had 2 MCs just for this very reason? If they won't go to a soft start, at least get the second coil in there. Chances of both welding shut are the other side of remote.
Chances are with proper ventilation if there is no fire, nothing will go off for fire or shunt trip. Remember our ambient temp for oil is around 100 degrees, I think TKE usually has their viscosity shut off at around 150. Wonder what the boiling point was?
Line feeds both STR and RU simultaneously, they switch the motor from wye to delta. The good news is all the cars in the store (4)are now getting soft starts. As for the fire system, the only reason that I can figure that the system didn't trip was that this particular unit was directly under the outside vent.(which is very large due to multiple power units in the room)I have been a volunteer firefighter for 20 years and this one is sure strange. I think the lid on the tank probably stopped the oil from catching fire by excluding the oxygen, I'm sure it had reached its ignition temp!
Line feeds both STR and RU simultaneously, they switch the motor from wye to delta. The good news is all the cars in the store (4)are now getting soft starts. As for the fire system, the only reason that I can figure that the system didn't trip was that this particular unit was directly under the outside vent.(which is very large due to multiple power units in the room)I have been a volunteer firefighter for 20 years and this one is sure strange. I think the lid on the tank probably stopped the oil from catching fire by excluding the oxygen, I'm sure it had reached its ignition temp!
Yep genuinely poor design without another contactor in series for protection.
Had an OT call on Westinghouse hydro (circa 1979 relay)weld the starter contacts that got so hot it melted the site tube out and sent 120 gallons of oil across the floor. This happened right after the hospital maintenance man pulled the mainline. When the oil stopped circulating it got hot enough to blow the tube out as the guy cleared the door!!! It melted the nylon track liners that stood in the corner about 2 feet 1 inch at a time. Oil in the hall way and down the shaft!!! What a mess and every fireman from 3 districts beat me there!!!
This is possible with every single Y Delta or across the line elevator out there. Once the contacts weld together, there is no possible way for the control circuit to prevent this. You would have to initiate shunt trip with a thermistor in the oil, or better yet, softstarter as mentioned earlier
