Buzzing from dishwasher breaker and weird current draw?

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cerfsud

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I had a power quality meter hooked up to my house's panelboard to get an idea of how much power I was using at certain times of the day (yes, it was overkill). As I was getting ready to disconnect it and return it to my work, out of curiosity, I checked the scope function. I noticed the dishwasher appeared to be dragging the voltage down for a millisecond or so near the drop of each peak. Is this to be expected or does this indicate a problem with my dishwasher?

Here are pictures of the voltage and current waveforms: http://imgur.com/a/tDxqC (note that the color coding on the amp reading is reversed from voltage - I must've had the CTs swapped).

Also, since I was close to the panelboard when disconnected this, I noticed that there was a faint buzzing sound coming from my expansion subpanel. It went away whenever the dishwasher momentarily shut off between its cycles, so I assumed it was from the dishwasher's breaker. I tried gently pressing on the breaker and didn't notice a difference. My ammeter read about 7.5A for that circuit (it's a 15A breaker). After taking my clamp off the wire and the wire snapped back, I did notice a slight change in volume/pitch (probably volume). Any ideas?

Thanks in advance!
 
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cerfsud

Member
It's a Square D QO115. I bought a new breaker on my way home from work today, and plan on changing it out.
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
111209-2025 EST

You need a voltage plot with the dishwasher off.

What actually are you displaying. You mentioned CTs. That would imply current. But your display seems to refer to two voltages.

One thing is clear. Internal or external to the home there is possibly a very high rectifier with capacitor input filter load on the system.

There are 15 increments per 1/2 cycle. There should be 8.33 ms per half cycle for the frequency shown. This does not correlate with 15. So the dots are not 1/2 millisecond markers.

At dot number 5 is where the heavy loading starts, and stops about 8 or 9 where energy seems to be returning to the system.

I would assume the dishwasher is 120 V. Get voltage and current to the dishwasher while not operating and at different points in the washer cycle. The washer cycle likely has heat only and pump only portions to the cycle as well as the two combined.

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cerfsud

Member
There are two images in the link I included. You have to click on the second thumbnail in the top to see the second plot.

Voltage: http://i.imgur.com/wnVpf.jpg
Current: http://i.imgur.com/JRJZE.jpg

This is for my whole house. Hence the two different waveforms. This wasn't part of my diagnosis of the breaker noise, it was just a curiosity I noticed before I disconnected the meter to take it back.

The sag on the voltage waveform (and spike on the current waveform) goes away when the dishwasher cycles off. So, it's definitely from the dishwasher.

As for the breaker noise - I replaced the breaker and still get the noise. The bus bar looked unscathed. I didn't check the neutral bar for tightness, so I'll do that once the dishwasher is done (it's currently running).
 
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cerfsud

Member
kwired said:
Increase in current and decrease in voltage sounds like normal activity to me when a load is added.
Click to expand...

Right. I just didn't know what the load should actually look like. The spike on the downward slopes of the peaks seemed odd to me, and whenever this "odd" load from the dishwasher would present itself, that's when the breaker started making noise.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Why is current waveform stair stepped? Must be some solid state device involved, probably also has something to do with the spike.

Possibly a ECM motor on the dishwasher?
 
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the blur

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Location
cyberspace
I replace buzzing breakers. Just had one recently, and it caused the entire panel to buzz. It was sending a virbration up the buss bars. This one was for an A/C unit. it would buzz on compressor startup real bad, and then the buzzing would slowly decrease. Until the next A/C cycle.

I opened the breaker, but there was nothing physcially wrong that I could see.
 
kwired

kwired

Electron manager
Location
NE Nebraska
the blur said:
I replace buzzing breakers. Just had one recently, and it caused the entire panel to buzz. It was sending a virbration up the buss bars. This one was for an A/C unit. it would buzz on compressor startup real bad, and then the buzzing would slowly decrease. Until the next A/C cycle.

I opened the breaker, but there was nothing physcially wrong that I could see.
Click to expand...

All it needs is a component that is loose enough to move when an alternating magnetic field influences it to move, at motor starting this field can briefly be up to 10 times running level.

The instantaneous trip mechanism may be a place that will move enough to make the buzz and yet not quite move enough to trip.

If replacing breaker did not stop it maybe it is in the main breaker if a main breaker panel.
 
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wirenut1980

wirenut1980

Senior Member
Location
Plainfield, IN
kwired said:
Why is current waveform stair stepped? Must be some solid state device involved, probably also has something to do with the spike.

Possibly a ECM motor on the dishwasher?
Click to expand...

I agree. Some solid state device controlling the dishwasher motor is firing too early or too late. Seems strange for a 120 V device. I'm not sure what all is going on inside new dishwashers, but I read that the newer ones have some kind of drive for the motor for lower power consumption.
 
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