HVAC compressor frying contactors

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RustyShackleford

Minister of Fire
Jan 6, 2009
1,331
NC
Being a handy EE type, I'm comfortable replacing contactor (and capacitor) in my compressor. Unfortunately, my compressor has developed a habit of going through contactors pretty quickly. Last summer, I replaced it and had to do so again during the same summer. That one just died yesterday. In a peculiar way where it was stuck CLOSED. But now it won't make the connection.

Googling around, sounds like a solid-state contactor might be the solution. But I'm having trouble locating one that has a 24vac input. And not sure that would solve the problem. There's also an expensive mechanical one like this, that sounds like it might solve the problem: https://www.supplyhouse.com/White-Rodgers-49M11-843-SureSwitch-Multi-Volt-Switch-Relay

But I'm wondering if there's something fundamental wrong with the compressor (25 years old) that's causing this.

Thoughts ?
 
I'd also be suspicious of the compressor. 25 years of wear may have resulted in excessively high start-up currents.
If you have a clamp-on digital amp meter you may be able to check this out. Of course, you'd have to know what the start-up amperage is supposed to be as a diagnostic.
 
I'd also be suspicious of the compressor. 25 years of wear may have resulted in excessively high start-up currents.
If you have a clamp-on digital amp meter you may be able to check this out. Of course, you'd have to know what the start-up amperage is supposed to be as a diagnostic.
I do have a clamp-on. What should startup current be ? (MCA = 21.3, RLA = 16, LRA = 88). Am I likely to be able to measure it, with a plug-in clamp-on for a digital Fluke ?
 
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1. Make sure your motor startup winding is disengaging when reaching synchronous speed. Mechanical start winding engagement switches are by far the most common issue with single-phase motors, esp. in any dusty environment. If the start winding is engaged by means of inrush current relay or potential relay, be sure no prior repair/mod defeated it, which is common (people don't understand how they work).
2. Check start and run winding resistances, as well as insulation between them.
3. Consider adding a surge suppressor to your motor. Voltage spikes as contactor opens is a common cause of contactor failure. Many companies make these, some of which are designed to bolt right into the contactor. Murrelektronik makes ones that can go onto the motor, usually using a spare 1/2" NPT cord knockout: https://shop.murrelektronik.com/ind...lang=1&cl=search&searchparam=motor+suppressor
 
Thanks, @Ashful . I'm punting to my HVAC guy (also a buddy). He's a moonlighting cryogenic tech, so he knows his stuff. Here's what he said: We should put a start capacitor and relay device to start the compressor fast and reduce the arc on the contactor. But I will discuss your suggestions with him too.

Also posted on an HVAC forum, and they my Nest thermostat is a likely culprit. "Google Nest thermostats are known for erratic intermittent operation and it's likely chattering or short cycling the contactor probably when you're not there to catch it. Those are the worst thermostats on the market."

I think I know the answer to this, but ... pre electronic ignition cars had a capacitor (it was called a condenser) in parallel with the "points" to prevent arc'ing damage. Why can't the HVAC do that ? I guess because it's AC, not DC.
 
Hey Rusty,

It's funny that when I read your original post, I didn't see HVAC mentioned anywhere, and I was thinking of a shop air compressor. Now I see the thread title includes HVAC.

So, these compressors are a bit different than a rotary capacitor start induction run motor, which might have a centrifugal clutch to disengage the start windings. HVAC compressor pumps usually have two capacitors, one for start and another for run. These fail pretty routinely (5-10 years), and sometimes get replaced with the wrong value, if the tech doesn't have the right value on his truck. There should be a schematic on the inside panel of your evaporator unit that shows the correct values, or just look up the literature online. I would bet you have the wrong cap value.

As to them disengaging during operation, I'm not really sure what mechanism HVAC compressors use. Definitely not a mechanical centrifugal clutch. It's possible they just stay engaged, or possibly use an inrush current relay or potential relay to perform this operation. If you have a buddy who's an HVAC tech, they would already know these specifics, I'm just coming at it from a general EE background.

You probably already know this, but the run windings of a single-phase motor is like a unicycle or direct-drive bicycle with one pedal. Maybe a treadle lathe or sewing machine is the better analogy. Either way, you can keep the thing going by inertia, if you can just get a push to get started. The start windings are would with smaller wire to create higher resistance. The run winding is nearly purely inductive, whereas the start winding is R + jwL, and thus has a little phase shift from pure jwL. The trouble is, the small amount of resistance created by the smaller run winding does not create enough phase shift to really provide good start-up torque, so they add a start capacitor to pull that phase a full 90 degrees from the run winding and give good start-up torque.

A typical "capacitor start induction run" motor will have the R + jwL + 1/jwC of the start winding optimized near 90° from the run winding near stall, to give max starting torque from a dead stop. Because the inductances change as the motor comes up to speed, the start winding can actually begin to fight the run winding and cause overcurrent (blown breaker) if they fail to disengage. But there are also motors designed to keep both windings engaged all the time, and these would be more optimized toward max torque near synchronous speed. I suspect HVAC compressor motors may fall into this class.
 
Wow, you've probably forgotten more than I ever knew about motors. Very interesting. I am EE, but more on the digital side, and I just don't know much about this stuff.
 
Hell, I've forgotten more than I know, too! Most of my motor knowledge comes from my old hobby of rebuilding commercial equipment, before this whole wood stove thing.

But I'm also an EE, high power RF/microwave amplifiers, etc. I did my first BSEE on digital (ECE), but never really worked in that field. I loved doing the huge logic problems, Karnaugh mapping, PAL's, all that crap. I guess it spoke to my OCD.
 
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HVAC buddy came today. Primarily to put some stop-leak in, after he recharged from a VERY slow leak that eroded performance over the past few years.

But it looks like the contactor problem may simply have been ants. Lots of ants. Except I'm not sure that explains the stuck-closed/on that I saw. If the thing keeps acting up, he said we can replace the compressor with an R22-compatible one (and fill with R438a) for maybe $500 plus labor. But don't get the performance upgrade of an all-new system.

The thing that's holding me up from an all-new system is trying to decide if these new heat pumps have good enough cold-weather performance to not require any auxiliary heat. Current system uses propane for backup, and my electrical panel is already way too overloaded to do heat-strips. There are new variable-speed heat pumps (kinda like minisplit technology) that have, on paper at least, adequate cold-weather performance to not need backup. But it's un-clear if that's really workable.
 
But it looks like the contactor problem may simply have been ants.
Funny, this was my first thought when I read your original post but then I read on and dropped the idea.
When I lived in Texas this was a common problem with Fire Ants. I think the sweetness from ozone produced by the spark across the points attracts them.
I saw it in my HVAC outdoor unit and even in relays in vehicles.
 
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I have the same problem here, ants in my low-voltage transformers and receptacles used for garden lighting. I found an ant trap in each enclosure solved the problem. Usually something like this, and changed maybe every 3-5 years.

Amazon product ASIN B000VA90FU
Obviously, liquid traps don't work here, since I'm usually tucking them up sideways inside a transformer or receptacle box cover.
 
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I have the same problem here, ants in my low-voltage transformers and receptacles used for garden lighting. I found an ant trap in each enclosure solved the problem. Usually something like this, and changed maybe every 3-5 years.
I have an extensive landscape lighting system (my own fully-custom design, but that's another story) but they haven't messed with it ... yet.
Obviously, liquid traps don't work here, since I'm usually tucking them up sideways inside a transformer or receptacle box cover.
Those look a lot like some we've been using indoors, that have a thick goo, which WILL leak out eventually.
 
Yeah, I couldn't find the exact ones I'm using on Amazon. I had assumed that ones I showed were dry granular or solid powder puck, but I might have been mistaken.
 
btw mca=maximum circuit ampacity. lock rotor is what it starts as or if the compressor is frozen mcb is maximum circuit breaker. if you windup getting a new compressor and the breaker is to large they will void your warranty. if you are thinking of a new heat pump the ecoer brand has a 100% heating all the way down to 15 degrees and has 40 speeds that the compressor can run at. it also has a 10 year warranty and the company and the a.c. guy will know there is a problem before the home owner. i wire these all the time matter of fact the one we installed 2 days ago came up on my phone as overcharged and shut itself down. they have a automatic charge feature and it screwed up and the board was replaced free of charge
 
if you are thinking of a new heat pump the ecoer brand has a 100% heating all the way down to 15 degrees and has 40 speeds that the compressor can run at.
Those look good. I'm trying to figure out if I can, or should, go with a heat pump without backup heat. It would have to be propane (I don't have enough electrical capacity for heat strips). FWIW, I am in central NC.

One thing that concerns me abiut fancy heat pumps (that might be able to not need backup) is that my duct system is pretty badly imbalanced. I worry that when it throttles down (like a minisplit does) that some rooms just won't get any air.
 
Those look good. I'm trying to figure out if I can, or should, go with a heat pump without backup heat. It would have to be propane (I don't have enough electrical capacity for heat strips). FWIW, I am in central NC.

One thing that concerns me abiut fancy heat pumps (that might be able to not need backup) is that my duct system is pretty badly imbalanced. I worry that when it throttles down (like a minisplit does) that some rooms just won't get any air.
Has the capacitor been checked?

The two stage equipment is a good value for temperate climates in NC. What is your design temp?

No heat strips will mean it blows cold air during defrost.

 
I put in an outdoor wood yard in Eastern NC years ago, it had a lot of high horsepower gear including a truck dumper that picked up an entire tractor trailer rig including cab and tilted it upright plus some vibratory screens and some rechippers. It was cheaper to install a local Motor Control Center section in block building out near the gear than to run cables back to the main building. There were fire ants around and every conduit entering the building had to be equipped with special fittings normally used for explosion proof projects. After the wires were pulled something that looks like cotton is stuffed in the conduit and then a pourable liquid cement is poured into the fitting to seal it off. Even with that they expected the ants would eventually find a way in the building and into the gear.

The plant shut down after a couple of years as NC changed their renewable incentives and last thing I knew it was still sitting there mothballed. No doubt the ants have moved in.
 
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Has the capacitor been checked?

The two stage equipment is a good value for temperate climates in NC. What is your design temp?

No heat strips will mean it blows cold air during defrost.

Yeah, capacitor is fine.

Design temp is 20. It does get colder here, teens maybe a couple nights per year. Used to go into single digits maybe one night per year, and below zero one night every few years - but not any longer.
 
Yeah, capacitor is fine.

Design temp is 20. It does get colder here, teens maybe a couple nights per year. Used to go into single digits maybe one night per year, and below zero one night every few years - but not any longer.
It will always get colder than design temp but you don’t design for the yearly low temps or record lows. Just grab an extra blanket and a sweater;)
I don’t see the extra cost of getting a cold climate heatpump down here. Maybe if you were just putting in a single mini split. The two stage equipment comes in 3,4or 5 ton with first stage being 70% of rated output. So second stage at 20 degrees is a little less than first stage at 45. Defrost would be cold but doable, I wonder if they could set fan to use low speed setting for defrost with no heat strips.

The Mr cool and units alike are single speed fan. I can’t recommend them as they just won’t dehumidify well unless you undersized a bit.

I have messed with my equipment so it rarely goes into second stage and the house is much more comfortable moving lesser amounts of air more continuously rather than cycling 2nd stage and shutting off.
 
I don’t see the extra cost of getting a cold climate heatpump down here.
So you're saying you don't think I don't need propane backup ?
 
So you're saying you don't think I don't need propane backup ?
I’m saying you have options. If I had propane on site that seems like the obvious choice. If I didn’t I’m not sure that’s what I would choose.

A service upgrade would be on my list if I wanted to stay there long term.

The incentives for solar, heatpumps, and efficiency upgrades are enticing. Done thoughtfully a service upgrade could be part of the qualifying upgrades. But the cheapest option without credits is probably cheaper than the qualifying options.

Duke really wants to increase our rates. I don’t have the link to their proposed increases but it’s enough it changes the ROI on efficiency upgrades considerably. They won’t let you install a solar system larger than your last 12 month usage. Which for those electrify means you have to electrify first then build solar a year later. Drifting.

Two stage compressor and gas pack sized appropriately would the best option for comfort. Very happy with my trane/American standard unit.
 
A service upgrade would be on my list if I wanted to stay there long term.
I have 200 amps now. I think 225 amps is as high as I can go, due to the buried cable from our transformer, 200ft or so, maybe even the transformer itself. (So more than 225 would be quite expensive). But I guess that extra 25 amps would be enough for some heat strips.

Would also need a panel upgrade of course. Which I could use anyhow, current panel being chock'a'block full of space-saver breakers (the ones that fit two breakers in one space). Yes, it's legal, my panel being a 20-space/40-circuit model. Shouldn't be too expensive, as I can DIY, but house would probably be out of commission a few days.
 
I have 200 amps now. I think 225 amps is as high as I can go, due to the buried cable from our transformer...
This is something that has interested me, as our old house is on a 200 amp service, but all of the other (newer) houses in our neighborhood are on at least 400 amp services. I always figured that, with all of our lighting, air conditioning, and sheer square footage, we'd be bumping up against the service threshold, but it doesn't appear to be the case at all. My Emporia Vue shows we very rarely use much more than 55 amps, relatively well-balanced.

One significant factor is that our primary heating is oil and wood, whereas all others are heat pump. But we have two traditional HVAC units, two minisplit systems (three heads), and even a heat pump for the pool, with no trouble at all on the 200 amp system.
 
200 amps is plenty (dare I say, overkill) for most single family homes. Rarely does anyone actually need a 400A service.

Installing a heat pump does not necessitate heat strips. They are one option. But if you have propane you can continue to use the propane as your backup heat.
 
I have 200 amps now. I think 225 amps is as high as I can go, due to the buried cable from our transformer, 200ft or so, maybe even the transformer itself. (So more than 225 would be quite expensive). But I guess that extra 25 amps would be enough for some heat strips.

Would also need a panel upgrade of course. Which I could use anyhow, current panel being chock'a'block full of space-saver breakers (the ones that fit two breakers in one space). Yes, it's legal, my panel being a 20-space/40-circuit model. Shouldn't be too expensive, as I can DIY, but house would probably be out of commission a few days.
I have 200 amp service. 3 ton heatpump. 10kw strips. 9kw steam generator 3kw car charger.

If you have 200 amp service heat strips won’t require service upgrade. New panel. Or panel reorg sure. I’d just add a 100 amp sub panel for strips and heatpump and a car charger.