Motors and W versus VA

[Simon]

I see, some assholes they have at this university I am studying with! The module for this question is about motors, not how power is billed and how on earth am I to know how power companies think when it is totally irrelevant to the subject I have been taught.

So you say that despite the fact that they meter the power instantly rather than simply clock up a total use (or how else do they know about the peaks) they bill the whole period at a price that assumes both opposing worse case scenarios simultaneously for the whole consumption?

If that is the case then it makes sense and I'd hate to be the customer and the question is rather irrelevant to the subject material. Last I checked i could not find a textbook on the inner workings of power companies and the course only talks about the bare bones of motors.

In the real world, the revenue meter accumulates both kWh and kVARh - two separate indicators.  At the end of the month these values are used to compute average power factor for the billing month.

Independent of all that, the meter also accumulates kWh and kVARh in 30 minute buckets called demand intervals.  The billing demand is the bucket with the highest accumulated kWh.  The billing demand interval may change across industries.  Maybe some company has a really huge demand for just 15 minutes.  The utility may decide that a 15 minute demand interval is more appropriate to capture the peak and change to a different rate schedule.  One can imagine some benefit in turning on a large short duration load 1/2 way before the end of one interval and running it half way into another interval to cut the apparent demand.  The utility won't give you a signal that shows the beginning and end of the interval for that very reason.

But despite the average being calculated this problem counts any instantaneous occurrence as the averaged norm.

[Simon]

@Simon: Maybe the university is trying to teach you about dealing with incompetence and the usual mix of those who talk a lot and don’t say anything useful. 
Wait until you try to sell real of energy savings to management.  Instead management will spend more by hiring a consulting group with pretty power point presentations of pure guesswork in regard to power savings and claim to the stockholders that they are saving at least the advertised amount and get a huge bonus for nothing as they never follow up with real data. 
What rstofer is saying about facilities projects is absolutely spot on.  The only projects that are implemented are those that will provide some manager(s) with bigger bonuses and/or promotion possibilities.

The university is incompetent. At work I am one of 4 that are studying distance learning with this university and all say the same, the materials are rubbish, i am the furthest ahead as i was the first to start and I am further into my modules of choice and the further in you go the worse you get. The material has some strange and unorthodox ways of explaining stuff and questions like this one are totally irrelevant to the material supplied or so strangely worded that you have to study the convoluted material.

[rstofer]

The material has some strange and unorthodox ways of explaining stuff and questions like this one are totally irrelevant to the material supplied or so strangely worded that you have to study the convoluted material.

But I like this problem; I even delayed breakfast for the first part.  It helps that I used to do this stuff for a living and have spent literally decades dealing with electric bills.

I will be very interested to find out if I got the right answers to the second part.  Don't forget to post back!  It seems correct, the "Law of Neat Numbers" seems to apply to the pf correction, so I suspect it is right.

Oh, wait!  That's a made up law!

[rstofer]

I see, some assholes they have at this university I am studying with! The module for this question is about motors, not how power is billed and how on earth am I to know how power companies think when it is totally irrelevant to the subject I have been taught.

Since the subject is motors, maybe they are trying to show an application for a synchronous motor.  If the load is essentially constant and the motor can provide leading VARs then it might make sense to replace an induction motor (always lagging pf) with the synchronous motor and reduce any penalty.  Notice that they didn't mention a reward if you got above 0.8.

Economically, it depends on having a relatively constant load over the billing interval.  That's why the problem stated a continuous 2 MW motor.  If the motor only ran one hour per week, it wouldn't produce enough VARs to be useful.

At one company I worked for, aerospace as it turns out, there were two large air compressors serving the entire plant and they were both powered by 500 HP synchronous motors (I think...  It's been 45 years).  Aircraft manufacturing is a 3 shift operation and uses a LOT of compressed air.

The stationary engineer that worked in the area would tweak the DC excitation as necessary for power factor correction.

[Simon]

I find anything interesting but I judge questions that do not pertain to the material taught to be in bad taste when I am to be marked on them. I can learn, but I can't do clairvoyance.

I would love to do the module on EMC next but I am literally shitting myself about how bad it may turn out to be and this is becoming an excise in getting the piece of paper with no expectation of learning much. Meanwhile in the real world I have plenty of learning to do of the sort you do not get qualifications for.

[rstofer]

I have been pretty cavalier about the calculation of demand.  It is not really instantaneous but rather the amount of energy consumed during the demand interval multiplied by some factor to account for units.

Suppose the maximum consumption during any demand interval in the billing cycle is 100 kWh and the demand interval is 1 hour (by tariff).  In that case, we divide 100 kWhr by 1 hr and we get 100 kW of demand.

The length of the demand intervals may vary so it might be necessary to multiple by 2 if the demand interval is 1/2 hr.

I'm not sure this helps but I did want to clear up the idea of instantaneous demand.  No, we don't measure motor inrush.  We measure consumption over some demand interval.

[Simon]

Yes I understand but the answer is rather odd as the interval is 1 week.

[perieanuo]

I worked in a megawatt consuming factory.when you have low cosphi you just buy a compensatory intelligent unit.do the calculations yourself or speak to dedicated tech, conclusions are the same.it's ok to buy one.


Envoyé de mon iPad en utilisant Tapatalk

[nsrmagazin]

And you have to pay for that imaginary power too!

[rstofer]

Yes I understand but the answer is rather odd as the interval is 1 week.

Yes, that is odd.  It makes the problem easier to approach, I think, but it would be an odd rate schedule that did something like that.  Then too, billing intervals are seldom as short as one week yet the problem revolves around a weekly cost.

Around here we have for kWh of energy, kW of demand (largest 30 minute window) and a penalty for power factor below 0.8.  We do not pay directly for kVARh, it is buried in the power factor penalty which is a month long average.

[Simon]

It made the answer no easier as this calculation has nothing to do with the theory of operation of synchronous motors which this a module on.

30 minute windows I think are common. My own home smart meter reports at 30 minute intervals.

[rstofer]

It made the answer no easier as this calculation has nothing to do with the theory of operation of synchronous motors which this a module on.

30 minute windows I think are common. My own home smart meter reports at 30 minute intervals.

Well, it kind of did apply in that the synchronous motor provided a leading 1.5 MVARs and replaced an induction motor with lagging 1.5 MVARs for a net change to the plant of 3.0 MVARs which improved the overall power factor and so on.

I agree, it was the long way around to doing a simple pf calculation.  Having to do all the cost calculation seemed a little outside the scope but, who knows, it might be important someday.  Without the original calculation, it wouldn't be possible to compute the savings because we first needed the penalty.  Yes, the problem could have been simplified.

I do know this:  I was never able to make pf correction pay.  Whether I did it on the incoming 12 kV or at the 480V motor control centers, I could never pay it back for the $1k monthly penalty I was seeing.  Remember the 2 years thing?  Well, $24k doesn't buy a lot of electrical work!  Now, if they would stretch the payback to 10 years, I could do some work.  Funny thing is, the plant was still there 10 years later and still paying $1k/month penalty.  Of course we should have done the project.

[Simon]

The problem I have with this question apart from it's lack of information is that the module I am studying is the bare basics of electric machines. The concept of power factor correction with a synchronous motor has been expressed but this question is over thought as there is not much of a question here.

[rstofer]

The problem I have with this question apart from it's lack of information is that the module I am studying is the bare basics of electric machines. The concept of power factor correction with a synchronous motor has been expressed but this question is over thought as there is not much of a question here.

I had an entire semester course on Motors, Rotors and Dynamos called "Electric Machines".  It was not my finest class and I don't remember anything about the material.  I still have to book should I ever have a problem sleeping.

[Simon]

Let's try the last half - I'm a lot less certain of this arithmetic.

First, the plant : 11 MVA at 0.6 pf => 6.6 MW and 8.8 MVAR
The existing motor: 2.0 MW at 0.8 pf => 2.5 MVA and 1.5 MVAR

The new motor, instead of lagging by 1.5 MVAR, leads by 1.5 MVAR for a net gain of 3 MVAR to the plant.

The new plant: 6.6 MW and 5.8 MVAR => 0.75 pf, 6.6 MW (no change expected) and 8.8 MVA
Now we only have 1 increment of power factor penalty ((0.8 - 0.75) / 0.05) instead so 0.03 * $17,160 or $514.80.

We save $1544.40 per week.

$250,000/$1544.40 = 162 weeks

Again they have made a mess of this one. If the 2MW or 2.5MVA motor replaces an existing one then that 2.5MVA needs subtracting from the 11MVA before calculating the new power factor, so you have 8.5MVA with 0.6PF lagging and 2.5MVA with 0.8PF leading. This actually adds up to a new power factor of 0.8 lagging so there will be no penalties.

[beesbees]

Looks like they're still trotting out this question, I cannot for the life of me get the £17 644.50 in part iv. Working backwards, sideways and all the other ways.

I'm pretty sure I have the new power factor which reduces the reactive penalty charge to 1*3% surcharge, £514.80. And the question states "assume the max demand penalty does not change"... These numbers just will not fit to the one provided in the questions.

My method so far:
take the existing factory, calculate the load without the induction motor by subtracting the real and reactive components,
calculate a new power factor based on the new real and reactive values,
calculate new MVAR and MW figures for an apparent power of 11 MVA,
add in the synchronous motor to these new figures by subtracting 1.5 MVAR and adding 2MW to real,
then calculating the new PF (0.71).

But if we "assuming the max demand penalty does not change" then we need a reactive penalty of, erm, £484.50, to get the figure in IV...

[Simon]

They told me they were taking it out when I complained that it had nothing to do with the materials provided and was not something I could simply look up online. I was very fortunate to find someone on here that know how this works.

I would suggest contacting them and explaining that the question is not relevant to the module unless they have updated the module which I doubt.