More electrical than PLC question

[reefshark]

I'm having a strange problem with an electrified monorail consisting of several trolleys on a 1/4 mile loop.

This monorail runs 24/7, Powered by 480 3phase.

The Problem: Every Monday morning at 6:36, 7:06 or 7:36 the monorail E-stops. The only way to E-stop this monorail is by hitting one of several E-stops, leaving a gate open or opening a door( that is an E-exit).

One other way to cause an E-stop is if the 3 phase drops and the saftey contacters drop out.

I was involved with the building and commissioning of the monorail and know that there is NO way any of the PLC's could be causing this. (even though my customer disagrees)

I have a feeling somewhere at the customers site between 6:36 - 7:36am there is something large turning on and in turn causing the 480vac to dip just long enough to kill the 24vdc power supply (supplied by the 480) in turn dropping the safety relays.

Does this seem possible? Is there some way to capture a voltage drop on the 3 phase?

 

[Tharon]

Is it always at those set times, 6:36, 7:06, or 7:36? Seems odd that it'd be that exact.

They make phase monitors that can latch an output if the voltage dips below a set threshold. All the 480 to 24V 3 phase power supplies I've seen have had quite a big range of input voltages (like, as low as 380-400V) so it would likely be a very large dip in line voltage to cause the 24V power to drop. Easily detectable with a monitor.

 

[jvjr]

I have had similar experiences of what you are describing over the years. However most of them have occurred at foreign job sites.
In Korea all the motor drives on the machines would fault or motor speeds would vary when the high temperature furnaces would fire up.
In India we have a problem right now, before and after shift changes as machines are shutdown then started back up.
I am sure someone makes some type of voltage logging device that would pick this up.

 

[shooter]

A 24 VDC will not dip when a brownout.
However the main contactor running on 110 or even 480 will stop for a moment, and then the power will fail for a moment, causing massive problems.
And yes there is a simple way with three relais and some lamps.
Use the relais on each fase 1 in serial with a lamp and to the Null, not the ground), close the relais by hand and it will shut off when a phase is is missing, or browning. The relais should have a hold contact to keep the relais on.
So if you come in and see a light off it is external.
A phase guard would be nice to put in the system, just to prevent starting up after a brownout.
The customer can have a point, for example when a timer is involved somewhere and this timer is overflowing.
How are the safetyrelais controlled? via a long line al together, maybe a groundloop when something is starting, (the power on the input of a PLC is very small around 10 mA, have the safeties also on a relais, or a lamp so you can see if the input to the PLC is correct,

 

[Mickey]

You can also use a power analyzer to monitor and record what is going on.

They can be rented. See link or your local electrical wholesaler.

http://www.google.com/#hl=en&gs_nf=....,cf.osb&fp=281f5fbf74ffa19b&biw=1275&bih=688

 

[The Plc Kid]

Rent yourself a Dranetz power analyser. It is the best tool on the market and will find your problem in short order.

But you may want to rent because to purchase is about 12-15 K depending on the model and options. I use mine almost daily for machine issues like this.

 

[GlennA]

You said there are several Emergency Stops = e.g twist-to-release ? And there are gates and doors = proxi or sensor and magnet ? Do you have an air pressure or other switch in the Emergency Stop chain ? Are there humans / operators near the monorail ? Is an operator cutting across the track at end of shift or break ? Is there a door or gate that would break the contacts if an operator leaned on it ? Is there a 'human time table' you can match this to ?

 

[GlennA]

Assuming that you are transferring pallets into and out of the mono-rail = is there a transfer to or from a machine or loading dock that happens at that time ?

 

[TConnolly]

I'm going to go along with Glenn and suggest that there is a human factor involved here. It happens on every Monday but does not happen on Tue., Wed., Thur., Fri. It happens only in the morning (at start of first shift) but not in the afternoon (second shift) or night (third shift). Someone making a coffee or doughnut run? Maintenance tech performing a required PM? Operator does something while going through a start up checklist? Someone taking a shortcut out of the area? Weekend third shift operator retrieving lunch box from behind a door on his way out?

 

[Mickey]

Human, at exactly 6mins after? My vote is a timer/clock somewhere.

 

[leitmotif]

The half hour interval suggests to me someone is staggering the starting of very large loads to keep demand charges down. Check with local utility and see what the time base for their demand charge is. 15 minute is common. First thing on Monday makes some sense for this argument also = plant shuts down on Friday and restart on Monday.

If this is only the conveyer then I would tend to restrict the search to something on the same feeder and not a plant wide search.

If you are really sure of the times you do not need much more than a light bulb you should see it flicker. You could put a video camera on it to allow for instant replay to management or more likely (especially if it was me) to convince yourself you were not hallucinating.

Dan Bentler