Help understanding meters vs electric heaters

[weressl]

If you have an accurate odometer and a clock, you can figure out speed.



As commonly used, watthour meters can reasonably be used to figure power consumption as well as total energy used. Most modern digital 'watthour' meters directly report power as well as energy.

-Jon

You're missing the point.

No, watthour meters do NOT record the watts. You can't call up the WHm and ask for the magnitude of watts at a given time. You have so called demand registers that measure the (k)watt-hours within the programmed period and retain the highest value. The demand 'window' is 5, 10, 15 or 30 minutes and it is still not capable to determine the highest and lowest instantaneous (k)watt reached within that time. It is possible that the highest instantaneous (k)watt was reached in another time period. We are talking here about wattchour meters used by the Utilities for billing purposes. Certainly there are devices that are meters as well as registers that are capable to provid data on both instantaneous and quantitive measurements of electrical parameters and retain such data.

 

[rattus]

Sure:

Sure:

081001-1556 EST

rattus:

Add some harmonic content to either or both signals and the PF is still the ratio of real power to apparent power. Calcultate phi from arc cos of PF and phi is not very meaningful in terms of any waveform phase shift.

.

Of course, but we have been assuming ideal sinusoids. I would venture that in most cases, that assumption is reasonable. Introduce significant harmonics and we have multiple voltages, currents, and phase angles. Analysis is much more difficult.

 

[Besoeker]

Of course, but we have been assuming ideal sinusoids. I would venture that in most cases, that assumption is reasonable.

Possibly so. But I'm not sure.

A few years ago, we were awarded a contract for the electrical content of a (water) pumping station. This included a number of variable speed drives. A typical contractual requirement is to demonstrate that the total harmonic voltage distortion does not exceed certain limits. You have to do before and after measurements over a 24-hour period to demonstrate compliance.
Well, the before measurements showed that the voltage distortion was already outside the limit with none of our kit powered up. It was particularly bad from about 6pm to 11pm.
There is an explanation for the timing. The station is in a mainly residential area. A lot of residential equipment takes non-sinusoidal current. Televisions, computers, light dimmers, etc. No single item takes a lot of power. It's just the sheer number of them, the aggregate, that distorts the supply.

I have seen many such instances including the supply into our own works.
And today, we did the final commissioning on a high current rectifier - around 1600 kVA total. At that sort of level, you have to take account of its impact on the supply and design accordingly. As a matter of course, I looked at supply voltage. It was visibly distorted and that was with the plant off load so it had to be present on the incoming HV supply.

My point, in brief, is that I'm not sure that your assumption is entirely valid.
My experience suggests otherwise.

 

[rattus]

Depends:

Depends:

Depends on one's definition of "most".

And too, my experience is admittedly about 40 years out of date!

 

[Besoeker]

Depends on one's definition of "most".

And too, my experience is admittedly about 40 years out of date!

Ah, then maybe you are nearly as old as I am.
I'm mostly involved in the field of variable speed drives and drive systems.
They are, of course, harmonic generating and, in that context, I have had to be concerned with their affect on the supply.
Over the years, I have seen a gradual deterioration in supply quality, often as a result of the increasing use of single-phase non-linear loads. As I said in a previous post, it isn't the power level of any individual unit that is significant - it is the magnitude of the aggregate load. One extreme case I had to look at was a hotel in the middle east which had about 3,000 kW of controlled (i.e. dimmable) lighting. Apart from voltage distortion, there was a huge problem with overloaded neutrals because of third harmonic currents.

 

[rattus]

I do mean OLD:

I do mean OLD:

Ah, then maybe you are nearly as old as I am.

Around 1950 I encountered a Thyratron controlling a universal motor. An older fellow who had been to tech school told me how it worked. That is how old I am!

 

[Besoeker]

Ah, then maybe you are nearly as old as I am.

Around 1950 I encountered a Thyratron controlling a universal motor. An older fellow who had been to tech school told me how it worked. That is how old I am!

OK, OK.
You maybe have me on the age stakes.......lol
In my time I have had to deal with thyratrons, mercury arc rectifiers, carbon pile regulators, and Ward Leonard sets.
That's history I guess but I think it helps in understanding basic principles.

 

[steelersman]

I believe that the watt-meter is based on the equation,

p(t) = v(t)*i(t)

I believe gar's statement about the in-phase component is not literally true, but it is true in effect.

In a reactive circuit, p(t) is negative when energy is flowing out of the load. The watt-hour meter integrates p(t) with respect to time which results in real energy used in KWH. Does the disk turn backward? No, but it would stop with a PF of zero.

BTW, instantaneous values can be represented by rotating phasors of the form,

v(t) = Vpk[cos(wt) + jsin(wt)]

the disk does and can spin backwards....I've seen it.

 

[steelersman]

It wasn't a trick question. It was a question to determine your understanding of the relationship between voltage, current, resistance, and power.

The examination you will be taking will have similar questions that won't look exactly the same, but that will require your understanding of those principles. Unless you are able to examine the questions and frame the problem in the proper way to apply the principles you may not pass the exam.

excellent point Bob. It definitely isn't a trick question. And it should be understood why the answer is that they are the same.

 

[weressl]

Of course, but we have been assuming ideal sinusoids. I would venture that in most cases, that assumption is reasonable. Introduce significant harmonics and we have multiple voltages, currents, and phase angles. Analysis is much more difficult.

Even if we stretch the OP we are only talking about PRACTICAL metering. Both rotary and electronic meters are measuring actual power - both real and phantom - and are harmonic insenitive or corrected. Harmocnic content is an issue mainly for the user for reduced electrical efficiency and malfunctioning of some circuits.

Theoretical computation is rather complex of any resulting parameters, be it flux, power, phase angles at different harmonics, etc.