Another lost neutral

[StarCat]

Normally the fluctuations in the L-L voltages shouldn’t affect the 240V loads, like you stated earlier.
Depending on what single phase loads are trying to start, the line voltages could shift to 0 on one leg and 240 on the other leg.
still shouldn’t bother the compressor.
I agree with you, looks like a board or some other component would be shot, but not a compressor lockup.
i don’t see how the air handler running or not running will cause a compressor to lock up. I can see it controlling the compressor either off or on, but not lock up.

unless...

the board that controls the compressor start delay was shot, and the compressor didn’t get a delay signal... that I could see.
but I thought the board was 24 VAC...

speculation... I’m not an AC guru.

If this is in reference to a Lennox 2 Speed Condensing Unit they are one of the most complicated systems at large, especially the older generation units and there is more than one time delay device on board the outdoor unit that operate under both startup and speed change scenarios. Those units have a 6 lead compressor. I would like to know the " exact " failure on the compressor in order to get an idea. The older generation units were also typically paired with Gas Furnaces indoor which are 120V primary machines, but they could also be set up on full electric heat FCUs which are 240V primary to the control transformer. Typically if you get into a situation where the control voltage is broken to the Furnace, the TDR on the condensing unit will be forced to recycle.
I have to wonder about a stuck contactor or another missing element as something is not quite adding up. The AC guy is apparently casting blame on the failed neutral because it was conincident with the failure. I'd like to hear his explanation based on the mode of failure and how he proposed it happened. The Engineers would have to comment, but if there is a way for the applied voltage to have gone low for an extended period of time when the contactor was engaged, this could do it.

 

[growler]

+1

I don't know what OP has for "speed control".

Only true VFD units I have ever seen were three phase input and was a complex system with multiple indoor units - the compressor unit was the master unit on that system.

I have seen "two stage" units in dwellings, but compressor motor still runs at full voltage and frequency, in those the second stage just kicks on some solenoid valve to change refrigerant flow in that stage which results in less overall output in that mode.

If this damage was caused by lost neutral on the system it had to happen via the controls that originate from the 120 volt master unit. On/Off signal not so much a problem, rapid cycling can be, though most units have anti-short cycle timers anymore and that makes that possibility less likely also.

The idea of the speed control was some BS that the first AC tech that showed up was saying. I had never herd of such a thing but then I don't work on AC units and have no idea what may be the latest thing on the market. They have a new ceiling fan with VFD speed control built in so it can run a any speed you want and not just three speeds ( I was surprised to see it ).

Once my friend checked on the problem he said that it's not even a two stage unit but a single stage which is a lot cheaper unit.

The AC company was called before I was but showed up a little latter, They didn't even know anything about the lost neutral until I got there and saw lights going form bright to dim. I was the back up plan in case the AC problem turned out to be electrical ( plus they wanted and estimate on installing some more can lights).

 

[kwired]

If this is in reference to a Lennox 2 Speed Condensing Unit they are one of the most complicated systems at large, especially the older generation units and there is more than one time delay device on board the outdoor unit that operate under both startup and speed change scenarios. Those units have a 6 lead compressor. I would like to know the " exact " failure on the compressor in order to get an idea. The older generation units were also typically paired with Gas Furnaces indoor which are 120V primary machines, but they could also be set up on full electric heat FCUs which are 240V primary to the control transformer. Typically if you get into a situation where the control voltage is broken to the Furnace, the TDR on the condensing unit will be forced to recycle.
I have to wonder about a stuck contactor or another missing element as something is not quite adding up. The AC guy is apparently casting blame on the failed neutral because it was conincident with the failure. I'd like to hear his explanation based on the mode of failure and how he proposed it happened. The Engineers would have to comment, but if there is a way for the applied voltage to have gone low for an extended period of time when the contactor was engaged, this could do it.

One possibility that I have seen before with trees rubbing through overhead service drops is it rubbed through an ungrounded conductor and the neutral conductor and the ungrounded lead into the house became grounded leaving you with 120 volts between conductors that should have been 240 volts. The indoor unit of course had to be connected to the good line to get any control voltage, but that would then put 120 volts to the outdoor unit instead of 240. Overload in the compressor should protect it, but if left that way for too long will still take it out eventually.

 

[Hv&Lv]

One possibility that I have seen before with trees rubbing through overhead service drops is it rubbed through an ungrounded conductor and the neutral conductor and the ungrounded lead into the house became grounded leaving you with 120 volts between conductors that should have been 240 volts. The indoor unit of course had to be connected to the good line to get any control voltage, but that would then put 120 volts to the outdoor unit instead of 240. Overload in the compressor should protect it, but if left that way for too long will still take it out eventually.[/B]

If you have a neutral that rubs a grounded conductor and reduces the voltage to 0, the other leg won’t have 120 but a cycle or two...It will have 0 also.
the CSP transformer will trip out, or if it is a conventional XF, the fault current will either blow the primary fuse or melt the conductor into. If it blows the other conductor into, you only have 120, no 240. The compressor can’t attempt to start because there isn’t a return path.

 

[kwired]

If you have a neutral that rubs a grounded conductor and reduces the voltage to 0, the other leg won’t have 120 but a cycle or two...It will have 0 also.
the CSP transformer will trip out, or if it is a conventional XF, the fault current will either blow the primary fuse or melt the conductor into. If it blows the other conductor into, you only have 120, no 240. The compressor can’t attempt to start because there isn’t a return path.

I was talking about situation where the tree rubbed through the ungrounded conductor and the load side of the break becomes grounded or at least same potential as incoming grounded conductor if it were also compromised. Grounding electrode(s) would have to be low enough resistance to carry the loads in that case or you still end up with well under 120 volts on what is partially working.

 

[growler]

If this is in reference to a Lennox 2 Speed Condensing Unit they are one of the most complicated systems at large, especially the older generation units and there is more than one time delay device on board the outdoor unit that operate under both startup and speed change scenarios. Those units have a 6 lead compressor. I would like to know the " exact " failure on the compressor in order to get an idea.
I have to wonder about a stuck contactor or another missing element as something is not quite adding up. The AC guy is apparently casting blame on the failed neutral because it was conincident with the failure. .

Yes the AC compressor failure and the bad neutral could have been coincidence . The AC tech never said that the bad neutral caused the compressor to fail but did admit that it was possible. Heck , I'n still trying to figure out if it's possible.

This all started as a simple AC problem with a breaker that wouldn't reset and an AC unit that had been making strange noise.The AC company was called early in the morning but could get there until afternoon. By the time they get there I already have the power company replacing cable from transformer to weather-head. I explain that he probably would not have been able to troubleshoot anyway as he would have gotten some strange voltage reading on the control side. With-in 15-20 minutes power was restored and the games begin.

It was me who asked if the missing neutral and power surge could have also caused the AC problems and I don't think the AC Tech really knew the answer. It's a two year old compressor and that sounds a little new for failure unless the unit is just a piece of crap. My friend who has been an HVAC contractor for over 20 years doesn't think to highly of those units even if they are very high priced.

 

[growler]

If you have a neutral that rubs a grounded conductor and reduces the voltage to 0, the other leg won’t have 120 but a cycle or two...It will have 0 also.
the CSP transformer will trip out, or if it is a conventional XF, the fault current will either blow the primary fuse or melt the conductor into. If it blows the other conductor into, you only have 120, no 240. The compressor can’t attempt to start because there isn’t a return path.

We had 246 V volts measured on the line side of the main breaker. I measured as high as 145 V on one phase to neutral/ground and about 100 V on the other phase and then turned of the power. Once you know the neutral is bad it's just safer to turn power off.

It's kind of funny because when you call these problems in to the power company they want to know what percentage of power is available for use by the homeowner. I tell them I don't consider any of it safe to use as the voltages are all over the place and it may burn up equipment from over voltage applied (power surge ).

 

[Hv&Lv]

We had 246 V volts measured on the line side of the main breaker. I measured as high as 145 V on one phase to neutral/ground and about 100 V on the other phase and then turned of the power. Once you know the neutral is bad it's just safer to turn power off.

It's kind of funny because when you call these problems in to the power company they want to know what percentage of power is available for use by the homeowner. I tell them I don't consider any of it safe to use as the voltages are all over the place and it may burn up equipment from over voltage applied (power surge ).

246V isn’t unheard of. We like to keep it high... keeps our revenue up.
And that coincides with the 145V and 100V on leg to ground.
The problem when you call in to about any POCO is your talking to someone that knows absolutely nothing about electricity, ours included.
They think they know, and show their ignorance when they say these things to people that really know what’s going on, like yourself.

I just thought of a true story and had to share.. I had a problem with my cable box blowing out about every time we had a thunderstorm. Cable company had installed a signal amplifier and it wasn’t grounded properly.( found out later).
On one of my trips to have yet another box replaced (free) I asked the lady why the cable boxes continually go out like that.
She asked me how many devices I had plugged in where the box was at. Confused, I told her the cable box, a lamp, and a television. She told me that was probably the problem, there was too much plugged into one outlet and the signal getting to the cable box, lamp, and television was getting weak. She thought she had me because I could’nt help but look at her with this confused and probably stupid looking expression...:huh:

I just let let it go, took my new cable box home and laughed it off....:lol:

 

[mbrooke]

One more thing I should mention about this problem. The AC unit did make some strange noise ( home owner thought it was the fan bearings or something) and it tripped the 50 Amp breaker. He said that he tried to reset the breaker but he may have reset it until it would no longer function.

I didn't get there until 11 AM the next morning and the power was still on. When I checked and confirmed a bad neutral I shut the power off so as to do as little damage as possible. But after power was restored (around 1 PM) and the AC guy showed up we tried to turn on the AC and the breaker was bad and would not reset.

Normally my job would be over as I don't get involved with AC problems. The HVAC tech that showed up gave and estimate of over $4 grand in labor to repair the problem ( parts still under warranty). I suggest that the home owner call a friend of mine to get a second opinion on cost as this seemed a little high. My friend said the job should only take about a half day (actual work time not counting time to get warranty parts).

Since I have never seen an AC unit damaged like this due to a lost neutral it has kind of got me interested. Even we old dogs have to learn a new trick now and then.

I have been reading a bit and have learned that Intermatic even has a surge protector for just an AC compressor that may be worth looking into
( the AG3000). If it's going to cost that kind of money just to replace a unit compressor you would want to protect it as best you can.

I have also learned that I wouldn't buy that particular unit as it's known to have bad condenser coils that need to be replace often. People really need to do their research before making any major purchase these days.

A surge protector will not prevent damage for two reasons:

1) It will not prevent the compressor from short cycling- the contractor will still pull in and out.

2) MOVs will are for brief transients not sustained sine wave over voltages. Depending on how the surge protector is built the MOVs may even catch fire. Better units have thermal fuses that will open cutting power to the MOVs or better yet the load itself.

You say the breaker would not reset- sounds like the compressor has shorted internally.

BTW- what brand are we talking about

 

[growler]

You say the breaker would not reset- sounds like the compressor has shorted internally.

BTW- what brand are we talking about

The brand of AC is Lennox. I'm not sure of the model number.

I'm quite sure the compressor is shorted internally. What I don't get is that it was only two years old. You would kind of expect a top quality (high end unit) to last longer than that.

When I mentioned the surge protection I wasn't suggesting that it would have prevented this problem but considering the cost of some of the compressors it could be worth the investment to try and protect the unit from other forms of power surge (lightning).

The sad thing is that if the unit failure was caused by lightning it would be covered by his homeowners insurance.

 

[mbrooke]

The brand of AC is Lennox. I'm not sure of the model number.

I'm quite sure the compressor is shorted internally. What I don't get is that it was only two years old. You would kind of expect a top quality (high end unit) to last longer than that.

When I mentioned the surge protection I wasn't suggesting that it would have prevented this problem but considering the cost of some of the compressors it could be worth the investment to try and protect the unit from other forms of power surge (lightning).

The sad thing is that if the unit failure was caused by lightning it would be covered by his homeowners insurance.

A compressor is a hardy device, I would not be worried about lightning. If lightning is enough to kill a compressor you will most certainly have bigger problems.

I am tempted to say the overload wasn't working well causing the wingdings to overheat and short circuit but honestly I'm just guessing.

 

[oldsparky52]

I quit doing residential decades ago, so can someone inform me, do y'all take 3-wire circuits to the air handler (w/heat strips) and/or to the condensing unit?

The reason I ask is when I did residential work, they were 2-wire 240-volt circuits. Never a neutral was used. I see this talk about how a neutral problem could effect the 240-volt units, but I don't see how.

For those that don't use electric heat strips in the air handler, is the circuit a 120-volt circuit?

 

[LarryFine]

For those that don't use electric heat strips in the air handler, is the circuit a 120-volt circuit?

I've seen them come set up for both 120v and 240v. You need to know the specs of the unit you're installing.

I've never seen a 240v HVAC unit, in or out, that required a neutral, too.

 

[Eddie702]

Went on a call yesterday where the problem was an open neutral. It was a power company problem as their cable had the open.

Here is the question. Open neutral may have caused the AC compressor to burn/short. May have been a coincidence.

It was a Lenox unit and only a couple of years old. High efficiency unit with speed controlled compressor. An HVAC tech was there and he seem to think that a lost neutral would effect the control module for the compressor.

Anyone ever seen or herd of a lost neutral causing an AC compressor to lock up and short out. I would have figured that the controller board in the air handler would have burned and shut the unit off during a power surge like that but would like other opinion or comments.

As a former HVAC mechanic I agree with the post by mbrooke. I can't see an open neutral specifically causing compressor damage. But and open neutral to the AHU (if 120 volt) could cause problems with the 24 volt control transformer in the AHU causing the compressor contactor to chatter. Also, since the compressor is variable speed it may have cause havoc with the speed signal and caused compressor damage.

 

[Borderland Sparky]

Honestly, I can’t see the neutral issue causing a compressor failure, even with a 120V Air Handler and screwy control signals.
I do have to say that it has been my experience that most catastrophic and premature failures can be traced to poor installation practices.

 

[hillbilly1]

Under voltage on the controls can cause the contactor to chatter, which can blow the start capacitor, motor current goes up, trips the breaker.

 

[ptonsparky]

I get a lot of work by being able to quickly and accurately diagnose lost neutrals. It really is not that complicated. Blaming compressor failure on it might be a stretch for me.

 

[garbo]

Went on a call yesterday where the problem was an open neutral. It was a power company problem as their cable had the open.

Here is the question. Open neutral may have caused the AC compressor to burn/short. May have been a coincidence.

It was a Lenox unit and only a couple of years old. High efficiency unit with speed controlled compressor. An HVAC tech was there and he seem to think that a lost neutral would effect the control module for the compressor.

Anyone ever seen or herd of a lost neutral causing an AC compressor to lock up and short out. I would have figured that the controller board in the air handler would have burned and shut the unit off during a power surge like that but would like other opinion or comments.

No such thing as a neutral anymore in NEC. Have to call it grounded conductor. If AC compressor was running on 240 volts an open grounded conductor should not have any effect. In my area a open grounded conductor usually has little affect as long as you have a copper water service and a metal gas line. Electricity will always take every available return path. Can not count the # of times that I did a 100 or 200 amp residential 3 wire 120/240 volt along with the 8 foot ground rod and had between 0.4 to 0.6 amps on both the copper water service ans steel gas line. Not aware of any control board that will shut down a unit for a surge. They usually install the cheapest control that will do the job. Had a cheap customer who refused to install loss of phase/under voltage relays on four 25 ton 480 volt AC units. After he paid over $6,000 twice for a 100 foot plus crane to replace compressors did he install the relays.

 

[Eddie702]

If it is a split system you usually have only 240 + an equipment ground and no neutral. Did it even have a neutral run to it?

If it was a packaged unit with 120 volt controls that would be a different story

 

[ptonsparky]

No such thing as a neutral anymore in NEC. Have to call it grounded conductor. ...

The ‘17 has enough of one column in the index referencing ‘neutral’ to indicate otherwise.