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PostPosted: Mon Oct 03, 2011 8:09 am 
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Joined: Fri Jul 01, 2011 11:26 am
Posts: 43
Hi, this forum does not really have an area to discuss the observed behaviour of specific electrical appliances i.e the results of monitoring.

With the ability to monitor individual appliances with an IAM and to chart this, we start to see the patterns of use of everyday electrical appliances, and maybe try strategies to reduce their consumption

At the moment I'm charting a humble fridge. It follows a regular saw-tooth chart with a 50 watt spike every 20 minutes. However a couple of times a day I see a huge pike which goes up to 750 or sometimes 1500 watts. This fridge doesn't have any anti-frost features, so it's not some hidden heating function. The stat is set to the lowest value (i.e highest temperature, it's function is to keep my beer cold. The spike only last a few minutes.

I have seen this behaviour before, when I had the envi hooked up to a laptop using the USB cable and running Dale Lanes monitoring software, I'd see momentary huge spikes when the compressor kicked in on the fridge.


My question is why does this happen only a couple of times a day. Is it a function of the fridge OR is this happening every time the compressor kicks in and the sampling rate of the envi is missing it ?

Perhaps this is connected with Power Factor, which I'm not entirely sure I understand , but I have read that fridges and freezers do not always use electricity in the most efficient manner.


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PostPosted: Mon Oct 03, 2011 12:34 pm 
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Joined: Mon Apr 18, 2011 5:26 pm
Posts: 138
Location: Melbourne, Australia
jives11 wrote:
The spike only last a few minutes.
I assume you mean the 50w spike lasts a few minutes. Or is the 1500w spike as long as that? If the former, it could be inrush current you are seeing, but that only lasts a few AC cycles. See http://en.wikipedia.org/wiki/Inrush_current.
Quote:
My question is why does this happen only a couple of times a day. Is it a function of the fridge OR is this happening every time the compressor kicks in and the sampling rate of the envi is missing it ?
Let's assume it is inrush and that you can detect it for 10 AC cycles. At 50cps it would be detectable for 1/5 seconds after each start. with a 6 second sampling rate there is a one in 30 chance of detecting the inrush. With 3 starts/hour= 72 starts per day you might detect it 72/30 times, say 2 times a day.

Okay, I chose the figures to explain the observations, but it is a possible explanation.

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PostPosted: Mon Oct 03, 2011 3:31 pm 
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Joined: Sat Apr 16, 2011 1:10 pm
Posts: 328
Location: East Sussex
Do the large spike occur at the moment the fridge starts to draw power or at random?

Most fridges (except those with inverter controlled compressors) draw a large spike when the motor starts.

Most defrost heaters (when fitted) are about 15 watts and this load would normally show up when the compressor is not running.

My fridge freezer looks like this...

Image
Today, with one random spike which I think is a CC issue.

Image
Yesterday, the graph was as I would expect...

Automan.

jives11 wrote:
Hi, this forum does not really have an area to discuss the observed behaviour of specific electrical appliances i.e the results of monitoring.

With the ability to monitor individual appliances with an IAM and to chart this, we start to see the patterns of use of everyday electrical appliances, and maybe try strategies to reduce their consumption

At the moment I'm charting a humble fridge. It follows a regular saw-tooth chart with a 50 watt spike every 20 minutes. However a couple of times a day I see a huge pike which goes up to 750 or sometimes 1500 watts. This fridge doesn't have any anti-frost features, so it's not some hidden heating function. The stat is set to the lowest value (i.e highest temperature, it's function is to keep my beer cold. The spike only last a few minutes.

I have seen this behaviour before, when I had the envi hooked up to a laptop using the USB cable and running Dale Lanes monitoring software, I'd see momentary huge spikes when the compressor kicked in on the fridge.


My question is why does this happen only a couple of times a day. Is it a function of the fridge OR is this happening every time the compressor kicks in and the sampling rate of the envi is missing it ?

Perhaps this is connected with Power Factor, which I'm not entirely sure I understand , but I have read that fridges and freezers do not always use electricity in the most efficient manner.


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PostPosted: Mon Oct 03, 2011 6:59 pm 
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Joined: Fri Jul 01, 2011 11:26 am
Posts: 43
SeekerAfterTruth wrote:
jives11 wrote:
The spike only last a few minutes.
I assume you mean the 50w spike lasts a few minutes. Or is the 1500w spike as long as that? If the former, it could be inrush current you are seeing, but that only lasts a few AC cycles. See http://en.wikipedia.org/wiki/Inrush_current.
Quote:
My question is why does this happen only a couple of times a day. Is it a function of the fridge OR is this happening every time the compressor kicks in and the sampling rate of the envi is missing it ?
Let's assume it is inrush and that you can detect it for 10 AC cycles. At 50cps it would be detectable for 1/5 seconds after each start. with a 6 second sampling rate there is a one in 30 chance of detecting the inrush. With 3 starts/hour= 72 starts per day you might detect it 72/30 times, say 2 times a day.

Okay, I chose the figures to explain the observations, but it is a possible explanation.



thanks seeker and automan.

yes - seeker - I think your answer fits perfectly with what I'm seeing. i.e spikes with an initial very short high inrush current, but I'm only seeing charted the few that coincide with the sample frequency. the firdge is rather old (16 years) so perhaps does have a higher inrush demand than automans more modern/efficient fridge


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PostPosted: Mon Oct 03, 2011 10:06 pm 
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Joined: Sat Apr 16, 2011 1:10 pm
Posts: 328
Location: East Sussex
If the spike does not occur when the fridge starts I would suspect another issue...

My one for today has just registered a peak over 3kw which of course is not accurate and has also pushed up the daily consumption from the .6kwh which would be correct.

Image

Automan.

jives11 wrote:
SeekerAfterTruth wrote:
jives11 wrote:
The spike only last a few minutes.
I assume you mean the 50w spike lasts a few minutes. Or is the 1500w spike as long as that? If the former, it could be inrush current you are seeing, but that only lasts a few AC cycles. See http://en.wikipedia.org/wiki/Inrush_current.
Quote:
My question is why does this happen only a couple of times a day. Is it a function of the fridge OR is this happening every time the compressor kicks in and the sampling rate of the envi is missing it ?
Let's assume it is inrush and that you can detect it for 10 AC cycles. At 50cps it would be detectable for 1/5 seconds after each start. with a 6 second sampling rate there is a one in 30 chance of detecting the inrush. With 3 starts/hour= 72 starts per day you might detect it 72/30 times, say 2 times a day.

Okay, I chose the figures to explain the observations, but it is a possible explanation.



thanks seeker and automan.

yes - seeker - I think your answer fits perfectly with what I'm seeing. i.e spikes with an initial very short high inrush current, but I'm only seeing charted the few that coincide with the sample frequency. the firdge is rather old (16 years) so perhaps does have a higher inrush demand than automans more modern/efficient fridge


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