Home AC Compressor Motor Electrical Questions

[DC73]

In the near future, I'm going to be helping a friend with some power quality issues. I'm trying to resurrect some brain cells and get my head around home AC compressor motor operation.

1) Is a home AC compressor motor a constant load motor? A motor that serves a constant load will see increased current when the voltage drops and if the drop is enough, the motor could see too much current and overheat. I want to determine if that is a possibility with a home AC compressor. As an example, a garage door opener motor is a constant load motor. A box fan motor is not as if it sees lower voltage the fan speed drops accordingly.

2) Does the answer change if the AC compressor is a scroll type variable speed compressor?

3) What is the purpose of the contactor?

4) If the contactor is failing (burned contacts), what are the likely causes?

5) If the wiring specs on a home AC compressor say to use a "30-35" amp breaker, is there any reason to favor a 30 amp breaker over a 35 or vice versa?

6) If a home AC compressor motor is NOT a constant load motor, are there any consequences of low voltage other than the unit just won't cool as quickly?

Thanks much,

DC

 

[wyliesdiesels]

Are you asking about a hermetic compressor for air conditioning?

No the current draw will vary depending on head pressure and refrigerant level.

How low of a voltage are we talking about? Because ac compressors have voltage ranges- min and max

What is the problem you're experiencing? Inefficient cooling? Could be a leak in the system.

Have an ac tech check the pressures

 

[DC73]

I don't know exactly what all is going on yet as I haven't been fully briefed.

What I can say is there is a small group of new homes in a rural subdivision that are having various power quality issues. Some of the issues involve the HVAC systems. I know there are AC breakers that may be nuisance tripping. One electrician told one homeowner the POCO was experiencing low voltage which causes the compressor to draw more current than rated which tripped the breaker, hence the first question. There also appears to be an issue with contactors burning up. Trying to learn what might cause some of the breakers to trip while other units are frying contactors instead. There is other weirdness going on as well.

Do you have the answer to any of my questions? Just trying to get a handle on generalities until I have a chance to find out more.

DC

 

[brewchief]

Most newer A/C condensers are rated 208/240v, there is also a range above and below that it will operate at, I believe plus or minus 5%. As the voltage drops amp draw will increase.

If the voltage drops enough the control voltage may drop low enough that it could chatter the contactor, that's not good for contactor or compressor life. Typical control voltage is 24v but will normally be a touch higher, say 26 or 27.

Contactors aren't built that great any more, most are coming from overseas and quality control isn't exactly that great.

A delay on break time delay could be added if the contactor is droping out.


A typical nameplate will list a minimum circuit ampacity and a max fuse or breaker size, it could be something like min circuit ampacity 18.1 and max breaker of 30, this means you could have a 12awg wire protected by a 30 amp breaker, breaker should be near the max allowed, some guys will see the 18.1 and put it on a 20 when it really should be a 30.

Sent from my SM-G965U using Tapatalk

 

[Norcal]

Most newer A/C condensers are rated 208/240v, there is also a range above and below that it will operate at, I believe plus or minus 5%. As the voltage drops amp draw will increase.

If the voltage drops enough the control voltage may drop low enough that it could chatter the contactor, that's not good for contactor or compressor life. Typical control voltage is 24v but will normally be a touch higher, say 26 or 27.

Contactors aren't built that great any more, most are coming from overseas and quality control isn't exactly that great.

A delay on break time delay could be added if the contactor is droping out.


A typical nameplate will list a minimum circuit ampacity and a max fuse or breaker size, it could be something like min circuit ampacity 18.1 and max breaker of 30, this means you could have a 12awg wire protected by a 30 amp breaker, breaker should be near the max allowed, some guys will see the 18.1 and put it on a 20 when it really should be a 30.

Sent from my SM-G965U using Tapatalk

If it's run in pipe, it could be 14 AWG, in that application it's 20A wire.

 

[SGKent]

different compressor types have different behaviors these days. There isn't a blanket answer. If the POCO is having voltage issues then other things will be going bad too, not just HVAC. The POCO can put a device on the home that records the voltage 24/7 for a week or two and if the incoming voltage falls the device will see it. Call the POCO.

The contactor would be chosen by the manufacturer for whatever compressor they use. I doubt if it is a problem with the HVAC system itself unless it was charged improperly. Since these units are new and likely under warranty I would stay out of it unless you are a HVAC person with an EPA 608 cert. If there is a problem with a bad run of contactors the manufacturer will know what is happening.

Just my 2 cents worth.

 

[wyliesdiesels]

I don't know exactly what all is going on yet as I haven't been fully briefed.

What I can say is there is a small group of new homes in a rural subdivision that are having various power quality issues. Some of the issues involve the HVAC systems. I know there are AC breakers that may be nuisance tripping. One electrician told one homeowner the POCO was experiencing low voltage which causes the compressor to draw more current than rated which tripped the breaker, hence the first question. There also appears to be an issue with contactors burning up. Trying to learn what might cause some of the breakers to trip while other units are frying contactors instead. There is other weirdness going on as well.

Do you have the answer to any of my questions? Just trying to get a handle on generalities until I have a chance to find out more.

DC

An inductive load will draw more current as voltage decreases.

If the breakers match the size called for on the nameplate then the breakers are tripping due to low voltage.

Need to get a meter on the power and figure out what the voltage is dipping to.

If this was caused by PoCo issues then they should be paying for the failed equipment. file a claim with them

 

[DC73]

Can someone answer question number 5 please?

different compressor types have different behaviors these days.

That's why I posed question number 2. Can someone expound on how scroll type compressors differ from standard compressors with regards to voltage fluctuations?


brewchief - thanks for your input on contactors. Very helpful.

Thanks much to everyone for all the information. It should prove to be very helpful. As of now, this is not going to be an adversarial situation. There are several individuals and companies working together to resolve various power issues. The problems are not restricted to HVAC.

I've been asked to participate as an observer and advisor to one of the parties.

At this point, I suspect we are going to find various issues. It won't surprise me to first find out that some of the AC units have oversized breakers installed.

This location is smack dab in the middle of the Permian Basin oilfield. There are numerous pump jacks and other oilfield motors operating in the vicinity, all with VFDs. There is some speculation that the VFDs are causing harmonics issues. There may also be issues with power factor.

I also think it's possible there may be poor grounds involved. Due to soil conditions, some of the oil field installations in the Permian Basin have difficulty using traditional grounding methods and so they must install chemical grounds. Not sure if the housing development will have grounding issues but I'll recommend the grounds be tested.

The area has been having tremendous heat of late and at least some of the problems occurred on hot days when the power system would have it's greatest load. But, there have been other issues when the temperatures were much cooler.

Just trying to get my head back in the game before the various parties get together.

Appreciate all the good information. Keep it coming if you have more to offer.

DC

 

[SGKent]

Google single stage vs multiple stage, or two stage compressor, hvac etc.

I had a compressor at this house which would burn up one wire right at the contactor, and the contactor would melt on really hot days. It was random. About 10 years into it, I replaced the siding on the house and found that a nail had been shot thru the romex when the house was built about a foot into the wall from where the line and contactor were burning up. On hot days things would expand enough that the nail would ground out one leg, causing the wire to crisp at the contactor. Really lucky the house did not burn down. Doubt if that type error would happen on multiple homes. The POCO really needs to put a monitor device on the power coming into the house and look at it for a couple weeks. Another example - we had a transformer here on the street causing random voltage dips. The POCO could not find the cause but one extremely cold morning it blew out that leg on the street. Another time the underground splices were leaking to ground after 20 years. Eventually they would eat away the copper until the line failed. Once the bad connections were fixed, and that transformer replaced, even the street lights were brighter and didn't ever flicker. The POCO needs to solve it, that is what they get paid to do.

 

[wyliesdiesels]

Yeah this is a PoCo issue most likely.

Why hasnt someone gotten the PoCo involved from the beginning?

A bad grounding electrode would have nothing to do with voltage sags as grounding electrodes are for shunting lightening strikes to earth, primary line surges, and limiting voltage to earth.

 

[ard]

Yeah this is a PoCo issue most likely.

Why hasnt someone gotten the PoCo involved from the beginning?

A bad grounding electrode would have nothing to do with voltage sags as grounding electrodes are for shunting lightening strikes to earth, primary line surges, and limiting voltage to earth.

I was going to post this too.

First thing is attaching a logging voltmeter to the main and see if it is delivering rated voltages over some time period- a day, a week.

There is a hard line between "homes" and "utility". If the problem is on the utility side, why have homeowners get spun up over grounds and whatnot?

 

[wyliesdiesels]

Need to get power quality meters logging right away.

 

[wyliesdiesels]

I will sumarize a similar situation i came across on a service call one time.

Call was in a mobile home park.

Complaint was tripping breakers.

Microwave breaker would trip, AC breaker would trip.

The microwave current was not at or above the breaker rating and there was nothing else on the circuit.

So i measured voltage at the panel with no loads on- 228v. Thats pretty low.

Turned on microwave and it dipped quite a bit lower.

Turning the ac on caused it to dip close 100v.

I explained to the customer that its a utility issue.

She then began to tell me that Pacific Gasp and extortion has had major issues with the power in this park. At one point they had to put the park on generator power while they fixed all the switchgear and equipment.

I told her looks like they still havent fixed it.

 

[walta]

Question 1 Yes. Low voltage is bad for an AC compressor if it often is less than 90% of the voltage on the AC name plate. If so call the electric Co and they are generally very responsive if you commonly see voltage below 228 volts across the main breaker.

Question 2 A Scroll compressors should handle low voltages better because its load is more constant.

Question 3 The contactor allows the low voltage low current from the thermostat to turn on and off the high voltage high current the compressor needs.

Question 4 Old age, excessive current or environmental contamination like water or insects.

Question 5 35 amp is likely to be a high cost special order item.

Question 6 is not applicable.

Walta

 

[MattT]

5) If the wiring specs on a home AC compressor say to use a "30-35" amp breaker, is there any reason to favor a 30 amp breaker over a 35 or vice versa?

Depends on what kind of motor control the condenser uses. If it's conventional, contactor controlled, folks usually put the larger breaker on those to prevent the starting inrush current from nuisance tripping the breaker.

On variable speed inverter, AKA VFD, units it's best to breaker, or preferably fuse, those as tight as possible to try to protect the electronics. Starting inrush isn't an issue with these units so there's no need for a heavier breaker.

And if the subdivision has a mix of conventional and VS equipment that could be why they're seeing a mix of burned contactors and tripped breakers.

But it's all academic. Just tell the light company to fix it. And if they don't do it have a lawyer tell them to fix it.

 

[TRWham]

...

That's why I posed question number 2. Can someone expound on how scroll type compressors differ from standard compressors with regards to voltage fluctuations?

...

There is nothing unique about a scroll compressor that makes it more vulnerable to voltage issues. If it is VFD driven, then the inverter may object to bad power, but there are other types of modulating scrolls (2 step and Digital come to mind) that do not use inverters. If anything, a scroll is easier to start at low voltage because it always starts unloaded, then builds enough speed to seal the flanks.

I will add, that in my experience with Copeland (about 13 years), the most common issue with warranty return compressors was...


NOTHING.


That's right, parts changers in the field simply swapped out perfectly good compressors and other parts until they stumbled across the real problem.

 

[American Locomotive]

1) Traditional split air conditioning systems have fixed-speed compressors. They're
designed to run a certain speed, but will experience varying load as the indoor/outdoor conditions change. Low voltage will cause them to draw more current in an attempt to maintain that speed.

2) More modern and high end split systems may have variable speed compressors, in which case the inverter drive would likely throw an alarm for low voltage and shut the system down.

3) Contactor is just a switch. The thermostat isn't capable of handling the high currents of the compressor, so the thermostat turns the contactor on, which turns the compressor on.

 

[DC73]

Why hasnt someone gotten the PoCo involved from the beginning?

Why would you assume the PoCo is not already involved? As I said, there are multiple power quality issues, and multiple parties involved in working together to find solutions.

 

[DC73]

Question 1 . . .

Walta

Thank you Walta for taking the time to answer each of my questions. Your answers were very helpful.

Depends . . .

There . . .

1) Traditional . . .

Thanks to you guys as well for your recent posts. Very helpful too.

DC

 

[DC73]

Ok, I thought of one more related question.

Around here, the PoCo is required to maintain voltage in the +/- 5% of nominal range during system normal conditions. So, 240 could range from 228 to 252.

They are allowed to reduced the tolerance to +/- 10% of nominal during temporary system abnormal conditions. That means 240 could range from 216 to 264.

Will voltages at the low end of these ranges cause problems for a typical home HVAC system? I'm especially interested in how low voltages in the system abnormal condition range could impact the systems (i.e. 216 to 228). I get it that time is a factor when system conditions are abnormal. But, if abnormal conditions persist for an hour or more, would that be detrimental?

Thanks again for all the input.

DC

 

[Norcal]

Most residential A/C compressors are 200-230V so there is a pretty wide voltage range possible.

 

[ard]

Why would you assume the PoCo is not already involved? As I said, there are multiple power quality issues, and multiple parties involved in working together to find solutions.

Well, he assumed- and I assume- that if you didnt tell us what the poco was delivering you didnt have them out and they havent tested.



But it sounds like you did

best of luck

 

[wyliesdiesels]

Ok, I thought of one more related question.

Around here, the PoCo is required to maintain voltage in the +/- 5% of nominal range during system normal conditions. So, 240 could range from 228 to 252.

They are allowed to reduced the tolerance to +/- 10% of nominal during temporary system abnormal conditions. That means 240 could range from 216 to 264.

Will voltages at the low end of these ranges cause problems for a typical home HVAC system? I'm especially interested in how low voltages in the system abnormal condition range could impact the systems (i.e. 216 to 228). I get it that time is a factor when system conditions are abnormal. But, if abnormal conditions persist for an hour or more, would that be detrimental?

Thanks again for all the input.

DC

Check the nameplate on the AC. Almost all nameplates Ive seen have min and max acceptable voltages listed.

The PoCo voltages may be within spec for the listed voltages on the nameplate.

Has the PoCo been out and done a power quality test?

If they have, you didnt list that here so thats why i made my previous comment about the PoCo. Quite frankly if there were power quality issues, the PoCo should be the first entity involved.

 

[DC73]

Most residential A/C compressors are 200-230V so there is a pretty wide voltage range possible.

That's helpful to know that they can handle lower voltage than the PoCo is required to provide. Thanks

Well, he assumed- and I assume- that if you didnt tell us what the poco was delivering you didnt have them out and they havent tested.

I don't have all the information yet. As I said, I haven't been fully briefed. Just helping one of the parties and I haven't made a site inspection yet. There are others working on the issues, which are numerous, and the PoCo is involved. I assume they've either already monitored the voltage or are in the process of doing so. I'm just gathering information to resurrect my brain cells so I can be an effective participant.

Check the nameplate on the AC. Almost all nameplates Ive seen have min and max acceptable voltages listed.

The PoCo voltages may be within spec for the listed voltages on the nameplate.

Has the PoCo been out and done a power quality test?

If they have, you didnt list that here so thats why i made my previous comment about the PoCo. Quite frankly if there were power quality issues, the PoCo should be the first entity involved.

I'll check the nameplates as soon as I make a site inspection. I'm in information gathering mode at this time to get my brain around what to look for and what to ask for and to resurrect some brain cells that have been dormant for a long time.

Not sure what all the PoCo has done at this point. I just learned the PoCo has directed the oilfield operation to mitigate harmonics from their operation and both of those groups are working with a power quality specialist. Not sure at this point how harmonics are impacting the homes nearby.

Thanks much.

DC

 

[wyliesdiesels]

That's helpful to know that they can handle lower voltage than the PoCo is required to provide. Thanks

I don't have all the information yet. As I said, I haven't been fully briefed. Just helping one of the parties and I haven't made a site inspection yet. There are others working on the issues, which are numerous, and the PoCo is involved. I assume they've either already monitored the voltage or are in the process of doing so. I'm just gathering information to resurrect my brain cells so I can be an effective participant.

I'll check the nameplates as soon as I make a site inspection. I'm in information gathering mode at this time to get my brain around what to look for and what to ask for and to resurrect some brain cells that have been dormant for a long time.

Not sure what all the PoCo has done at this point. I just learned the PoCo has directed the oilfield operation to mitigate harmonics from their operation and both of those groups are working with a power quality specialist. Not sure at this point how harmonics are impacting the homes nearby.

Thanks much.

DC

Harmonics would cause the transformers to overheat, causing voltage sag and power quality issues.

heres an article for you:

https://www.fluke.com/en-us/learn/blog/power-quality/rural-transformer-failure

 

[DC73]

Harmonics would cause the transformers to overheat, causing voltage sag and power quality issues.

heres an article for you:

https://www.fluke.com/en-us/learn/blog/power-quality/rural-transformer-failure

Thanks for the info and the article. It was a good read. Very helpful.

DC

 

[MattT]

Harmonics would cause the transformers to overheat, causing voltage sag and power quality issues.

They can also cause other problems which don't show up as voltage sag/drop which makes them harder, and more expensive, to track down. Motors overheating, breakers tripping and electronics acting squirrelly are some possible effects. LED lighting flickering falls under the electonics category.

Sounds to me like the light company has found the problem and is working to resolve it

 

[wyliesdiesels]

They can also cause other problems which don't show up as voltage sag/drop which makes them harder, and more expensive, to track down. Motors overheating, breakers tripping and electronics acting squirrelly are some possible effects. LED lighting flickering falls under the electonics category.

Sounds to me like the light company has found the problem and is working to resolve it

thanks. i was too lazy to add all that

 

[DC73]

Motors overheating, breakers tripping and electronics acting squirrelly are some possible effects. LED lighting flickering falls under the electronics category.

Sounds to me like the light company has found the problem and is working to resolve it

Thanks for this. Based on the incomplete picture that has been painted for me so far, I think you've just described most of the problems. And I think you may be right that the main problem has been discovered. By the time we get around to having the site meeting, we may learn that the problem has been resolved or at least that there is a resolution in the works. My part will be to explain all of this to average folks who aren't in the business of electricity.

I appreciate all the good info and help with this. If y'all were all local to me, I'd buy everyone a beer. Well, maybe not wyliesdiesels. I might be too lazy to buy him a beer. Just kidding, I owe you one too.

DC

 

[wyliesdiesels]

if it turns out to be PoCo and oil field customer caused, everyone definitely needs to file claims.

The PoCo shouldve been monitoring their grid to alert them to this issue.

 

[DC73]

The PoCo shouldve been monitoring their grid to alert them to this issue.

PoCos don't routinely monitor for harmonics issues (at least around here they don't). They do investigate power quality issues and will test for harmonics when the more common issues are first ruled out.

I might find out a bit more about the situation this coming Thursday.

DC