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Electrical problem

4K views 52 replies 7 participants last post by  autx790 
#1 ·
I have a thread going in the HVAC section regarding an issue I had with my blower motor going out. The reason I'm posting here is because I'm thinking I may have an electrical issue that caused my HVAC issue. My breaker was making a loud buzz sound intermittently when the AC unit kicked on. This sound started happening every time the AC kicked on and after about 3-4 weeks of that, the blower motor stopped working. I replaced the blower motor yesterday but the problem with the breaker making noise is still there. I have replaced the breaker, and moved it to the top of the panel but still not luck. The voltage at the AC unit is normal. This unit is a 3.5 ton Goodman that was installed in 2007. It's not a split unit so it runs off of one double pole 50A breaker. I've had two blower motors go bad in the last 2 years. Does anyone have any insight as to what may be causing me this trouble?
 
#4 ·
thanks, i'll take a look at that. The unit was installed in 2007 but I bought the house in 2009. It sat idle for about 6 months before I bought it and then I really didn't use it until 2011. I was doing a complete renovation on the house so I never turned it on. It never made the buzzing sound until about 4-5 months ago.

The breaker i'm using is HACR rated.
 
#6 · (Edited)
Is the loud buzzing just during start up?

If so this may be related to the magnetic portion of the Over Current Protection. Residential circuit are molded case circuit breakers with thermal and magnetic operation. The thermal operation is for overloads the magnetic operation is for shorts and protection from instantaneous high currents. When we test circuit breakers it is not uncommon to hear buzzing during testing.

There are ways to check the circuit breaker while on line (serving the load) but generally this is not a DIY test as one should wear proper PPE during performing the test.

Basically you measure the voltage drop across the circuit breaker (line bus to load conductor) under load utilizing a multimeter set to millivolts.
 
#9 ·
ritelec i could swap out the breaker with an expensive one but i'm thinking there's something causing the breaker to make the noise and not the breaker being the problem.

Wirenut, i replaced the capacitor since it was reading 16uF instead of 35uF but I still have that noise. i checked the contactor and it's reading the same voltage on the output as the input and i dont hear any "chattering". The noise still isn't as loud as it was just before the blower went out but you can definitely hear it when you're in the same room as the panel.
 
#12 ·
Sorry, i meant to attach the picture of the nameplate last post. It says 30A is max breaker size but i dont think that accounts for having heat strips, which i do.

The wire from the breaker to the service disconnect is 8 gauge and the same from the disconnect to the contactor. Which i thought was low but i saw some calculation done on another page where it showed 8 gauge was sufficient. I'm thinking it should probably be 6 though and something i'll probably change.

RoundedRooster, the breaker only serves the HVAC but it's not a split unit so the 50A breaker protects the whole unit (compressor, heat strips, blower, etc.) Is this what you were asking?
 

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#35 ·
Well, welcome to the money pit club! :) Actually, I think a 175 y/o home is worth this much fixing up compared to a 40 y/o home.
Yeah, no doubt I've learned a whole lot in the process of doing this work.

Where's the contactor or time-delay relay? Are you hearing a Buzzzzzz or a Chatt-att-att-atter? Are we talking the typical one on in the HVAC or do you have something extra near your panel? If the contactor/relay are electrromechanical, chatter is most likely and you cannot miss that. I have worked with electrician's in the past on timer and lighting installs where the old contactor coil would hum loudly before actuating, but this is not so common. Those contactors are very expensive, so, unless it was an annoyance to the customer, were left in. A solid state contactor/relay would typically just permanently open or close, failure of the p/n junction or gate of the thyristors, but typically no noise.
The Buzz is only right when the unit kicks on. No more than 1 second. It was pretty loud a few weeks ago just before the motor went out. It's definitely quieter now, but it started off quiet originally and progressively got worse. So I'm thinking there's still a problem somewhere which caused my compressor cap to go bad and a blower motor. I haven't sat outside to listen for sounds at the unit when it kicks on but after it's already running I go out and listen and don't hear any noises. The contactor is in the unit. The time delay relay is in the unit as well but installed on a circuit board.
 
#40 ·
Yeah, no doubt I've learned a whole lot in the process of doing this work.

The Buzz is only right when the unit kicks on. No more than 1 second. It was pretty loud a few weeks ago just before the motor went out. It's definitely quieter now, but it started off quiet originally and progressively got worse. So I'm thinking there's still a problem somewhere which caused my compressor cap to go bad and a blower motor. I haven't sat outside to listen for sounds at the unit when it kicks on but after it's already running I go out and listen and don't hear any noises. The contactor is in the unit. The time delay relay is in the unit as well but installed on a circuit board.
It sort of makes sense that an older motor would cause more noise, since it may have increased friction from wear, breakdown of lubricants, contamination, etc... But, I would've assumed a new motor and breaker would've corrected the problem. Have you ensured all wire connections, where current is pulled from (including mains), are tightened properly? Another long shot... what about moisture getting into the panel (breaker especially)?

It really sounds like you're near the trip point of the breaker. Kind of hard to believe on a 50A breaker. Have you disconnected your heat strips and tested this system? I realize the heat strips shouldn't be pulling current during warm weather, but I think it's good to subtract possibilities (such as miswired). I'm also assuming this breaker buzzing is only a problem running your AC? Are all the motors turning on at the same time or is there a confirmed delay between each? The total sum FLA of your motors is about 21A FLA (1/4hp = 2.6A, 1/3hp = 3.9A) and an approximate 119A LRA (using the LRA/FLA ratio established from the compressor for worst case).

Have you been able to measure operating current? If you measure it using an ammeter with a current clamp, make sure it's an RMS meter with it's maximum range just over that of the current drawn. "AmpClamps" are generally not very accurate anyway, but a 300A clamp will be worse since the significant digits (rounding effect) will be greater near the lower end of a range.

Another issue I'm going to throw out is CEMF (aka... Back EMF or Back Voltage). CEMF, perhaps from another motor requiring much more current (such as a compressor motor) can create high enough negative voltage swings to breakdown transformer wire insulation and reduce the effectiveness of breakers (even destroy electronics). The insulation breakdown is not noticeable right away, but eventually a motor will become less efficient and draw more current, eventually leading to operational failure. You might be able to see this by placing a good analog meter (600V rated) across your 240VAC. You would need to watch the meter as the blower kicks in and see how high the meter goes or if it pegs the needle. It will be very fast, such that a <$500 digital meter will not catch (they'd need a fast transient integrating RMS IC). CEMF is present as soon as current passes through a winding and opposes EMF, giving a voltage drop (impedance) across the winding. V=L*di/dt, if you're into calculus. When current suddenly drops to zero, because the delta in di/dt is so rapid, huge voltage swings into thousands of volts can be generated. Such a concept is used on ignition coils, where 12V In = 30kV out.

There are MOV suppressors you can by that are designed for these applications. These suppressors run about $15.00 + S&H. The instructions will normally tell you to connect into your panel, but you would be better placing these as close to the potential source or sources as possible. The reason is you have other things on the same circuit, so they'll see the elevated voltage levels before the MOV does (yes, there really is a delay). Any surge suppressors you buy should be in compliance to ANSI/UL 1449 preferably Third Edition. This is a newer update and much more rigorous than previous editions. But, even an old edition will most likely serve you long enough.

Here's a link for an ICM MOV. If you get these, you must make sure the ground wire is connected to ground for full protection. Even without ground, you will still have clamping from L1 to L2. Again, I am just throwing the CEMF out there and it may not be the cause, but low-cost protection doesn't hurt.

http://www.supplyhouse.com/ICM-Cont...qdsE8jBAfD17gn2rfA_e1LabG0hah0HdPXkTNdl_D_BwE

A motor shouldn't simply die in 2 years. These motors are designed for their environment and should last until the mechanics fail, probably about 10 or so years.
 
#37 ·
Yes. I have one of those.

I'm asking where it is is all.


Personally, I could put the 12's on the 20 and 10's on the 30. or 8's on a 40.

Will it break the bank? NO. Does it upset me not knowing where or when I can put 14's on a 20 or 12's on a 35. NO

Just asking is all.


Thanks anyway.
 
#41 ·
Thanks roundedrooster, that's a lot of good info to think about.

The noise did seem to start as winter was over and we switched over to AC. intermittent at first then occurring every time it switched on. I have tightened all connection I could. Charleston is a humid environment so there is a chance for moisture problems, but I don't see any indications of that in the panel. The upstairs unit has a 40A breaker right next to this one and there are no problems with that circuit. I think today i'll go outside and watch the unit when it kicks on and see if the motors have a delay between turn on or if there are any noises out there at turn on.

I can't measure current since I don't have a meter that goes high enough. I have a pretty nice Agilent meter at work I can probably bring home to watch for fast transients on the voltage, but that one can't do high enough current measurements.

I measured the transformer in the unit and it's putting out 27V instead of 24V...not sure if that is out of tolerance. Seems ok to me, but just thought i'd mention it.

Yeah, something is not right. The first blower motor went out 5 years ago, and the next one lasted a little under 2 years. Hopefully I get this figured out soon.

Thanks for the info!
 
#52 ·
Is that the Agilent 34401A? I read your Bio, what type of engineer are you? Do you have any current probes/xfmrs at work that can connect to your meter? Like the Pearson current xfmrs?

I'm not really leaning towards moisture, I just thought I'd throw it out there, since this does happen on blue moons. You mentioned a 40A breaker on a second unit. Why 40A?

Was the 27V open circuit measurement? 27 should be ok.
 
#43 · (Edited)
Well i'm stumped. I haven't done anything since my last plan, but today i am working from home and sitting in front of the panel and I have yet to hear the sound once. The weather hasn't changed any except it is a bit hotter today. This morning i was thinking that because it was staying off longer in between cycles that maybe that was helping it out, but now it's close to 3pm and i still haven't heard it. Im stumped

EDIT: Spoke too soon. Right after typing that i heard thunder. It's now 3pm, the AC kicked on and i heard the buzz.
 
#46 ·
So i rechecked all the connections and they seem fine. I looked at the heat strips area and there was some surface rust on the heat strip terminals that i scraped off. It also appears the installer put it in backwards, as the air pointing out the direction of air flow is currently pointing back into the blower.

I didn't hear any noises in the unit when it kicked on. Single solid click on the contactor and a short while later (2-3 seconds) the blower motor kicked on. I didn't have the meter to check the voltage.

Another thing i noticed is they use the contactor terminals as a junction point to run the wire down to the heat strips area. I'm thinking this probably isn't ideal. I attached a photo of that too.

So far i've really only heard the noise once or twice today. Not sure why that is.
 

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