troubleshooting today

Had an interesting morning troubleshooting our scenery elevator today. It's been down for a few days, and I was asked to take a look.

Two days ago, we had a "partial" power failure. Our lovely city-owned electric utility lost a phase out of the substation for the south side of town. Since then, the elevator hasn't worked. Every time someone tried to operate the lift, its 120vac control power circuit breaker would trip.

Turns out the elevator derives its 120vac control voltage from a 480v-120v transformer. I'm guessing the transformer is tied to one of the phases that stayed on, and the one that died (not sure which phase we lost). The transformer was putting out something much less than 120v, but apparently enough to keep the switch-mode power supply happy enough to power the control PLC and thus everything 24vdc.

The main contactor is a 100A unit, with a 120vac coil; it switches power to the VFD, based on E-stop condition, etc... Near as I can figure, the control voltage was high enough to power the SMPS, but low enough to not pull in the contactor fully. The coil melted and eventually burned off enough enamel to cause it to become a dead short, thus tripping the control voltage circuit breaker on the transformer secondary. I never knew a coil could extrude so much molten, stinky plastic! Props to Allen Bradley for selling the coil as a separate part, even if it's too expensive (70% of the cost of a new contactor).

A new main contactor coil is arriving tomorrow, and a phase-loss monitor relay is showing up early next week. I plan on wiring that to switch 120v control voltage off if we have another phase-loss event with the utility. If the contactor is the only thing damaged, our elevator will hopefully work tomorrow.

It was an interesting morning to say the least. Time will tell if there's anything else the power failure cooked.

I have to remember to send the city electrical utility a thank you card once this is all done I'd have rather they kept everything off until their fault was fixed, instead of giving us partial power and damaging our gear. I haven't worked here too long, but this dropping-a-phase thing is apparently not uncommon.


-rpoet

I know this sounds different but think about false trips.
just concerned about a random momentry spike stopping the lift mid floor.
so check the relay before the lift commences its next journey

Thanks for the caution. The relay I've ordered is self-resetting, and has an adjustable window, up to 5 seconds, before the relay trips. I think 5 seconds should take care of ignoring momentary spikes, without risking damage to anything.

Also, if the relay is tripped, the lift won't run at all since all control power will be locked out.


-rpoet

Another causality from the (partial) power outage - I just had to replace the power supply in the digital readout for our machine shop's mill. Our utility should really sort themselves out.


-rpoet

Equipment Damage from Lack of Animal Guards

It is common for local electric utilities to use step-down 3-phase sustation transformers to receive power from the generator. Each of the 3 phases may feed a single-phase 2300 volt or 7200 volt line that runs to different parts of the city. This time of year, squirrels and other animals use the lines as highways to their nut trees. 1 squirrel + 1 nut = 1 phase out and major damage to many users.

As I sat watching the dozens of squirrels pass by on the wires to my neighbor's pecan tree, one of the squirrels climbed on top of the pole transformer feeding my house. When his tail hit the hot wire, there was a flash of fire, a loud boom, a dead squirrel falling, and a power outage on that phase of the local substation.

I had an idea for a pole transformer protector, a non-conductive plastic cone that fits over the transformer, with a hole and a flange for an attachment mechanism (plastic tie that tightens around the insulator once the cone is in place). It needs to be cheap, durable, and easily installed on existing pole transformers. This simple device could save millions of dollars if used on every transformer. If anyone wants to develop this idea, then I am willing to share the credits. I know, there are plenty of squirrel guards out there, but most try to protect the insulator, not the top of the transformer. Some go around the pole or the lines, a waste of time because squirrels easily jump from tree limbs to the power lines.

Never mind, I see someone has already patented the idea. Now we just have to convince utilities to use it. A few class-action lawsuits for equipment damage should do the job.
http://www.patentstorm.us/patents/5650594/description.html

in my area we are 90% underground - to 22kv/415 Kiosks out to the houses
bit easier.
At my old job there ws this Crow that tested the Phase to ground of a 66KV supply potential across it's wings. no guts left