Why did KVA increase while KW decreased during furnace melting process?

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Discussion Overview

The discussion revolves around the observed increase in KVA and decrease in KW during the operation of an aluminium melting furnace, particularly after adjustments to the PID control settings. Participants explore the implications of these changes on the furnace's performance and the underlying electrical principles involved.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes a drop in KW while KVA increased after changing the PID control to a longer sampling time, suggesting a potential relationship between control settings and power metrics.
  • Another participant proposes that the observed KVA increase may be due to the drive remaining constantly on, leading to continuous switching to maintain power levels, which could affect system capacitance.
  • A participant questions the PID sampling time and suggests that the observed graph may indicate closed-loop oscillation, recommending adjustments to control system parameters.
  • Another participant inquires about the firing method of the thyristors, suggesting that the metering method may influence the readings of KVA and KW.
  • One participant confirms that the thyristors are phase angle controlled and mentions the specific meter being used for measurements.

Areas of Agreement / Disagreement

Participants express differing views on the reasons behind the changes in KVA and KW, with no consensus reached on the underlying causes or the implications of the PID control adjustments.

Contextual Notes

Participants acknowledge the complexity of the system, including factors such as time delays in temperature adjustments and the specifics of the metering technology used, which may affect the interpretation of the KVA and KW readings.

roro36
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Hi Guys,

So we have a melting furnace that is melting Aluminium. After 2 days of holding a melt to see how the furnace performs in a holding situation, I changed the PID control to have a longer sampling time and hence smoother control. What I noticed was the KW seemed to drop, although the average usage was probably the same. The KVA value had a definite increas in the average value. I can post a graph of the results if anyone is interested?

Can this be explained? The elements a SiC and are controlled by a thyristor card by current setting.

Thanks for any help.
 
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Do post your results.
 
Graph

This is using an Elster 3 phase meter, integrating over 5 minutes. The KVAh term is just me looking at what the KVA has been doing over the previous hour.
 

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You can see the kW has stabalised, but using the same average. The KVA value however at the exact time I changed the PID sampling time shot up.

I can only think that because the drive is now staying on constantly and never turning off there is constant switching to keep the power at 20%. Before it could turn on to 100% where it uses the power more efficiently and then off again, meaning the capacitance introcuded into the system was not as great? I'm not really sure though.
 
What is "PID sampling time?"

Your graph looks like a classic case of closed loop oscillation from way too much gain or time delay...

If oscillation is around 2/3 of integral time try lengthening that setting
if it's faster try less proportional gain
try switching derivative all the way out and add back as required to reduce overshoot in response to step change of setpoint

Do some reading on control system tuning. Old line companies like Fisher Controls and Bailey Meter pretty much wrote the books.
 
Hi Jim. Thanks for the reponse. Unfortunately, that was not the answer I was looking for. As it's an aluminium melting furnace, with 2 tons of Aluminium inside, I believe the time delay factor is out of my hands, as it honestly takes ages for even full power to adjust the temperature ever so slightly.
My question was actually on the reasoning behind the KVA jump when the system had changed to a larger sampling time and more stable?
 
Are the thyrisistors fired at zero crossing, ie on and off for interger numbers of cycles, or are they phase angle controlled?

I suspect it's a matter of metering - how does that Elster meter work?

reason i ask is

The harmonics shall be filtered out while measuring Wh, Var
and VArh, and only fundamental frequency quantities
shall be measured/computed.
from http://nrldc.in/Commercial/calculations/SEM_Hand_book.pdf
which i got from a search on Elster

is this it?
http://www.elstersolutions.com/assets/products/products_elster_files/A1700_31052012.pdf
 
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Hi Jim,

Yes it's Phase Angle Controlled and I am using the A1140 meter.