Author Topic: Voltage Monitor Fail  (Read 109 times)

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Offline calzap

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Voltage Monitor Fail
« on: October 25, 2020, 07:56:03 pm »
We use 240 VAC, 1 phase, 2 or 3 HP pumps to move water.  The pump controllers have voltage monitors to protect themselves and the pumps from the vicissitudes of power company voltage excursions.  The monitors are made by ICM Controls, model ICM-491.  The monitor for the pump that cycles the most has to replaced about every 2 years.  Typically, it shows signs of having overheated with a partially melted and cracked case.  See pictures.

The contactor coil power that passes through the relay of the monitor is 24 VAC.  The contactor coil has an inrush of 70 VA and sealed power consumption of 12 VA.  Although the voltage monitor has built-in transient protection, I place a pair of MOVs across its 240 VAC terminals.   The industrial control transformer that supplies the 24 VAC has a couple of RC snubbers across its outputs.  I assumed in the past the cause of failures were voltage transients; turned out to be wrong.  The maximum temperature rating of the monitor is 75C.  The controller cabinet has a fan with a 45C thermostat.

The relay rating of the monitor is 5 A resistive with no inductive specs.  The inrush current of the contactor coil is about 3 A.  If you’re thinking it’s intended only for resistive loads, take a look at the manufacturer’s wiring diagram below.  The controller circuitry is essentially the same but with surge protection added (see below).

I talked to an ICM engineer about the problem.  Best he could come up with was to start using their more expensive model (ICM-492) that has a digital display.  It costs about twice as much, but the contacts are rated at 10A (again resistive!).  However, it wasn’t the relay that failed (read postmortem below).  Once I run out of my inventory of ICM-491s, maybe I’ll give the ICM-492s a try.  It’s obvious when ICM-491s fail, and it’s a quick job to replace them.  Cost is about US$30.   And yeah, I could use another brand of voltage monitor, but the controller layout is configured for ICM491/492s.  At $15 per year it’s not worth redoing the controller.

Postmortem:

Inside, there are two PCBs.  The top one is elevated on nylon standoffs with seven conductors (two pairs and a triplet) going to the lower PCB which is embedded in potting compound.  The ICs on the upper PCB are a LM358 op amp and a 12C671 PIC MCU.

All the external contacts go directly to the lower PCB.   Several components on the lower PCB stick above the muck, and one is the relay which showed no signs of damage.  The hot spot was across the PCB from the relay and was present on both sides of the PCB.  Time to boil the flesh from the bones, so to speak.  Into a pot of boiling water the potted PCB went.  This trick works well to soften and loosen potting compound that has not been overheated, but an overheated spot hardens the compound and makes it more adherent.  After repeated trips to the boiler and chipping away small bits of potting compound, the flaming culprits were revealed … a trio of high wattage, wire-wound resistors. 

Looks like planned obsolescence.  Put in some heaters and let them slowly cook the device.  Why use potting compound here anyway?  It certainly wouldn’t prevent someone from examining the layout, and the device would not normally be subjected to high vibration or dampness.  The potting compound added to the expense and retarded cooling of the power resistors and nearby components.  It would prevent someone from modifying or repairing the device though.  And maybe provide some protection against a HVAC tech being heavy handed with the quick connects.  But c’mon … you spaced one pair of power resistors in a big hole in the PCB … the other sits in a hole too … presumably all done for ventilation … and then you filled the holes with potting compound!

 Mike in California
 

Online nuclearcat

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Re: Voltage Monitor Fail
« Reply #1 on: October 25, 2020, 10:49:26 pm »
IMO you get what you pay for ($30). Quite old (looking) TH design, i wont be surprised if they are selling old stocks, maybe below the price, because device doesn't last long.
 


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