Total Motor Currents

[Darrel Taylor]

Well, Thanks for all the information.
At least now I know where I screwed up.

The problem is that these things are spread all around the world. And many are on ships at sea, or sitting on oil rigs in the Gulf of Mexico. There's no way for me to go retro-fit anything. A software update was the only possibility.

The current is sensed with current transformers, and processed with an RMS converter which by definition, averages the waveforms. The result is measured with a 16F676. The voltage is rectified, filtered, and read with a 12F675. So I can't see anything like phase angles or zero-crossing.

I was hoping there would be an easy way to take the 2 current readings that I have, and figure out the actual total. But apparently, I'm out of luck. At least the individual motor currents are correct.

I guess they'll just have to buy the latest "System Upgrade", a third current sensor.
If they absolutely have to have the exact combined reading. $$$

[Darrel Taylor]

And just an afterthought.
The Total current reading was never part of the original requirements.

Since I had the current reading for both motors, I thought I'd just add them together and display the total current, just because I could.

Wasn't till some technician decided to verify it, that it became a problem.
Didn't even need to be there, and now it's a problem.

Be careful what you add to your designs.

[sougata]

Hi Darrel,

Great to know that at least your barber wouldn't be out of business. We all try to enhance our designs cause the customer may not know the possibilities of the system, and for me every project has some or other emotional attachment too. More $$$$= more emotions...

The current is sensed with current transformers, and processed with an RMS converter which by definition, averages the waveforms. The result is measured with a 16F676.

The voltage is rectified, filtered, and read with a 12F675. So I can't see anything like phase angles or zero-crossing.

BTW if you remove the rectification, filtration and still read the voltage (half cycle) you can determine the zero cross programatically. Also in most cases if your input is not AN3 you already have the option of determining zero-cross in some way or the other. Cin+, Cin-, INT. Not very precise though.

[Acetronics2]

Hi, Darrel

May be you could find a turnaround ...

if you found an info, on motor id. plates ; a number like "cos xx = .85"

Generally it's written there. May be also in the motors datas ...


your intensity will be

SQR ( ( I1_meas. * sin xx1 + I2_meas. * sin xx2 )^2 + ( I1_meas. * cos xx1 + I2_meas. * cos xx2 )^2 )

with I1_meas, xx1 related to motor 1 and I2, xx2 to motor 2.

this could seriously simplify things ... don't you think ???

Alain

[Darrel Taylor]

your intensity will be

SQR ( ( I1_meas. * sin xx1 + I2_meas. * sin xx2 )^2 + ( I1_meas. * cos xx1 + I2_meas. * cos xx2 )^2 )

Got me all excited with that one Alain.

Made a spreadsheet, was playing with the numbers, kinda almost could see how it might work.
Went to find the cos = .xx ...
Oh no! Don't have that info.

[dhouston]

Went to find the cos = .xx ...
Oh no! Don't have that info.

For an inductive circuit like this, it's the Power Factor. However, for more complex non-linear circuits it gets more complicated.

• http://findarticles.com/p/articles/m..._/ai_n17364837

[Acetronics2]

Got me all excited with that one Alain.

Went to find the cos = .xx ...
Oh no! Don't have that info.

Hi, Darrel

You've one left chance ... on the motor itself, there's a metal little sheet where it MIGHT be written. ( a summary of the main characteristics ... including this COSxx )

Alain

HEYYYYYYYYYYYYYYYYYYYYYY ..

POWER FACTOR = 66 ...% on 17th line !!! .... means Cos xx = .66 and Sin xx = .75

You had it under the eyes !!!

[amgen]

the only way to make a utility meter to run backwords is to supply power back out to them.
a pure capacative load will draw amps at 90' but not turn meter.
don

[Luciano]

Hi,

Distributed Power factor correction (PFC):
http://www.drasa.co.za/news_articles_detail2.html

(From the above link)

If a customer only applies PFC at the utility metering point then a load reduction is only
experienced on the utility’s network and not on the customer’s internal reticulation network.
If however, the PFC is applied at the points where the inductive reactive energy is consumed,
then a loading reduction and consequently also an associated Capex reduction will result on the
customer’s own network. It then follows that the customer will enjoy the same benefit for the
reduction in loading due to the application of PFC as the utility enjoys.

* * *

PFC: (ABB - Asea Brown Boveri )
http://library.abb.com/global/scot/s...with%20PFC.pdf

* * *

Capacitors for Power Factor Correction:
http://www.epcos.com/web/generator/W...locale=en.html

Applications & Cases
http://www.epcos.com/web/generator/W...product=247926


Best regards,

Luciano