Total Motor Currents

[arniepj]

Taking a quick look in the old basic electric circuits book seems to verify what you are seeing.On the subject of parallel ac circuits involving impedance,the calculated branch currents are higher than the total calculated generated current.The basic reason is the phase angle of each branch.Ouch,this is starting to hurt thinking about this stuff.

[Darrel Taylor]

Ouch,this is starting to hurt thinking about this stuff.

Yeah, tell me about it. OUCH!

What you guys are saying does seem to make a lot of sense.
And I do remember seeing different waveforms from the current sensors on each motor, so perhaps the larger motor, which is driving a Positive Displacement pump at 1000psi, is putting enough force back into the circuit to partially drive the 1/2hp motor (some of the time), effectively reducing the overall current reading by an Amp.

I guess the problem now is ... Is the difference something that's calculatable.

I only installed current sensors on each motor. I didn't put one on the main feed. If I put one there, then it reads the number the customer wants. But I didn't.

Can you just multiply by the "Power Factor" of the motors?
Naahhh, that would be too easy.

Thanks again,

[mackrackit]

Can you just multiply by the "Power Factor" of the motors.
Naahhh, that would be too easy.

If you knew:

The motor's efficiency
How different loads effect the motor
Monitor the line voltage
Monitor the ground (earth) potential
and maybe some other stuff, you might be able to run a calc. The formula... I have not a clue.

[Darrel Taylor]

If you knew: ...

Well, the system also collects the Pressure and Flow readings from each pump.

I suppose I could calc a "Load" value from that, and try to match it to some curve I've created from observed data.

But, I'd rather not go there.

And just to add fuel to the fire ....

Since these systems are part of a larger system that has it's own compressors and motors, and whatever. Isn't any "True" reading I get, going to combine with whatever bigger motors that might be running. Making that "True" reading useless?

Ugh!

[Melanie]

In large industrial installations, you are supposed to switch-in banks of Capacitors in the plant room to keep the Power Factor within limits set by the electric company if it is distorted by large amounts of (inductive load) machinery. If you don't, you are liable to fines and/or surcharges by the electric company on your energy usage. If you know how, and throw your current and voltage sufficiently out of phase, you can actually run your entire house for 'free'. If you are really clever, by the correct phase difference, you can not only slow-down your electricity meter, you can stop it completely - and with some older mechanical types have the meter go BACKWARDS, so the electric company ends up paying you for the electricity you use. You may find the 'small print' on your electricity supply contract threatens you with dire consequences if you extend your experiments in self-sufficiency in this direction.

[skimask]

In large industrial installations, you are supposed to switch-in banks of Capacitors in the plant room to keep the Power Factor within limits set by the electric company if it is distorted by large amounts of (inductive load) machinery.

What kind of cap's are we talking about here? Are we talking about those huge oil filled jobs (not the transformers, I know what those are), switched in and out with a huge set of contactors at will by a 'Power Factor Controller' or something to that effect?

[Acetronics2]

Hi, Darrel

When adding the current for the 2 motors, you should calculate a VECTORIAL addition ...

but the sum modulus of two vectors is not compulsory equal to the sum of the modulii ... generally, It's lower ...


Your results mean current for both motors have a different de-phasing ( ? ) ,called Cos Phi in Europe, from voltage.

Alain

[dhouston]

Darrel, you need to read up on Power Factor. It is an issue with all non-linear (i.e. reactive) loads, even very small loads. Voltage and current are out of phase so measurements can be misleading. Wikipedia is as good a place to start as any - it explains the fundamentals.

• http://en.wikipedia.org/wiki/Power_factor

You wrote that the PIC measures currents. How are you doing this?

[Melanie]

What kind of cap's are we talking about here? Are we talking about those huge oil filled jobs (not the transformers, I know what those are), switched in and out with a huge set of contactors at will by a 'Power Factor Controller' or something to that effect?

Yes, absolutely correct.

Don't you all recall having to remember the word CIVIL at school? (referring to the fact that in a Capacitive C load, the current I leads the voltage V. In an inductive L load the voltage V leads the current I). Measure time difference between both (zero cross is easiest), simple math determines the rest.

[mackrackit]

Since these systems are part of a larger system that has it's own compressors and motors, and whatever. Isn't any "True" reading I get, going to combine with whatever bigger motors that might be running. Making that "True" reading useless?

I think your best bet would be to pick a time when the load on the entire system is running normal or average. Then play with moving things around on different branch circuits if possible until the reading near the utility meter is close to what you know it should be. Then just leave it at that. From there it should average out.

Like Melanie said, you can make the utility meter slow or stop or run backwards. In some rare occasions the utility meter will run faster than it should. That is why the system needs to be "balanced". Good electrical contractors on new construction or revamped systems try to do this.
That is why you see them running around with the clamp meters at the end of the job doing pretty much what you are doing.

[dhouston]

Can you just multiply by the "Power Factor" of the motors?
Naahhh, that would be too easy.

It depends on exactly what you want to know. The measured currents are real (and determine wire size, etc.) - it's just that not all of the current is doing work. Average Volts x Average Current gives VA (VoltAmps) and multiplying that by PF gives WATTS. Most residential meters measure watts but most commercial and industrial sites have demand meters which measure peak current.

For small loads there is an inexpensive meter available that measures all of these things. However, it's only for 120VAC and a maximum of 15A.

• http://www.newegg.com/Product/Produc...82E16882715001

[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.

[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

[Archangel]

. . . Most residential meters measure watts but most commercial and industrial sites have demand meters which measure peak current. . . .

• http://www.newegg.com/Product/Produc...82E16882715001

And as I understand it, the leg of the circuit which is consuming the greatest power is the one which you pay for, which is to say if you have 3 phase comming into the building and you are using 120 1 phase @ 2000 watts and 3 phase @ 2000 watts you will be billed @ 4000 watts.

[dhouston]

Just charge them extra for the 1 amp you're magically saving.

[Darrel Taylor]

Just charge them extra for the 1 amp you're magically saving.

ROFL.

I Like that idea!