Momentary Power Loss or Drop (Long post)

[SG-1]

Have you monitored the N-G voltage during the speed change ?

If I understand correctly, it is normal for the blower to run all the time ?

 

[MikeS]

Re: flickering lights. On the initial service call there was a complaint of flickering lights in one room that was controlled by a dimmer. I never observed a flicker. During the experiment of killing all 240V breakers HO observed a flicker the first time but not the second, I haven't been back yet to see first hand.

Re: N-G Measurement during slowdown. I haven't yet...didn't occur to me grrr.

Re: Voltage consistency. Consistent at all times at all points during the slowdown. And yes, normal for blower to run full on all the time during a call for heat.

Re: Air flow obstruction I thought that too but all duct work is new both supply & return. Is it correct? Don't know not an HVAC Tech, looks consistent with other units i've seen.

 

[hillbilly1]

Is the transformer feeding the house shared with other houses? Real common up north, but not so common here in the south. My cousin had a 5hp air compressor in his garage, and while it was running, his neighbors lights would dim. The poco was using a 10 KVA transformer to feed four houses.

 

[MikeS]

Normally with a loose Pico neutral you will notice some lights going bright as others dim. Any mwbc will exhibit the same behavior. What you describe now could be a poor connection of any kind.

I have a variable speed blower on one of my furnaces. It speed is determined somewhat by the back pressure in the duct work and also varies along with the stage of heating. This is a zoned two stage unit. Is your unit switching backz forth between the two stages? Did they put in new thermostats?

Would a MWBC elsewhere in the house (not sharing with furnace) affect the furnace or vice versa?
Yes, new thermostats, no not switching between stages.

Re: Transformer, Yes, at least 2 houses being fed from same trans. It seems that problems are contained to this house, neighbors are either unconscious or have no issues.

 

[mivey]

Would a MWBC elsewhere in the house (not sharing with furnace) affect the furnace or vice versa?

MWBC elsewhere is not a concern.

You need to bring a good reference plane to the unit for taking measurements at the units. Bring an extension cord from an outlet near the panel. Put in a temp breaker if you have to. readydave8's suggestion of a dedicated temp circuit would also be a good idea.

But you need to make sure everything up to the panel is good first. Take L, N, & G readings from a known stable point under various load conditions, like bringing in a wire from the ground rod outside if it is a good ground. I would not use loads in the house as if you have a loose neutral, you are risking burning them up (cut the single pole breakers off). Bring some 1500W space heaters or heat guns or hair dryers, etc so you can throw some significant load unbalance on the panel. Light bulbs just don't have enough hind-end to do what you need to do (well, enough of them would).

 

[gar]

100208-0929 EST

mivey and others:

What is confusing to me is what I interpret that MikeS has said is that the voltage at the furnace to the furnace is essentially constant when the variations in the motor speed and current occur. If this is correct, then at least part of the problem is in the furnace.

As others and I have said there is a real need for a recording volt, amp, and watt meter on this house to prove what is happening. Probably first used at the furnace and then moved to the main panel.

Lacking that just a Fluke voltmeter in MIN-MAX mode at the furnace input would help to verify if there are major voltage variations at the furnace.

.

 

[mivey]

gar,

I'm not sure what "essentially constant" means. I wonder what he would see if he read the voltages with an analog meter (all trouble-shooters should have one, BTW)?

If the voltage is really steady, and he is getting the amp readings indicated:

I would use a manual thermostat (i.e., connect the wires with appropriate jumpers or use a good thermostat) to eliminate a thermostat issue. It may have been wired wrong. Then, verify the thermostat voltage (could be a bad transformer in the unit), and then verify the correct signals are reaching the unit from the thermostats wire.

If the thermostat and LV wire checks out, it is a unit problem (again, if the voltage are really good). But I am suspect of the voltages because changing other loads in the house seemed to aggravate the issue.

 

[SG-1]

The thing I cannot wrap my head around is how does the voltage remain constant while the current nearly triples ?? Impossible, unless there is a controller ??

I think a scope meter at the motor to compare the voltage & current wave forms would be useful.

Mike S. would you mind telling us what brand and model of ammeter & voltmeter you are using ? Or if you know if they are RMS or averaging ?

I have an old clamp on Amprobe, RED LED Display, and one day I measured about 80 watts of CFL lighting ( 120V ). It read about 3.2 amps. Should have been less than one amp. replaced the bulbs with incandesent and the ampere reading was correct. I found the instructions for the meter and it reads the peak current then converts it to an RMS reading. Since the CFLs are non-linear the reading was way-off.

I am not doubting your meters accuracy, just trying to understand the readings better.

 

[mivey]

The thing I cannot wrap my head around is how does the voltage remain constant while the current nearly triples ??

Taking "constant" in context: The change in voltage he posted would be consistant with a run of #12 about 35 ft long.

 

[Dennis Alwon]

Firstly, I would like to welcome Mike to the forum. On a personal note Mike and I know each other thru our local chapter of the NCAEC. Mike is our secretary and a well rounded electrician.

Mike I am also somewhat confused by this thread. I am sorry for not responding earlier but I have had a bad few days. Give me a call and we can talk about it more easily.

My first thought was that air flow was the problem but the flickering of lights on other circuits seems a bit odd so I am not sure of that.
Is the furnace on the same circuit as the problem flickering?
Is the flickering contained to one circuit?
Is the unit wired to a MWBC?
Have you pulled the meter to check load side of the meter with meter in place and current on?

One last thing-- is it a house I wired? :grin: