Finding the power usage of my project

[goofeedad]

Hey All, I admit its been a long time since I've used alot of this stuff but the formula originally given by birdman0 (Prms = V(in AC) *0.707 * I), that gives Volt Amps, Right?

Is that the same as Watts?

Layman definition is fine, it's for my own use for now.

I ask because when I do the math, Prms = 115VAC * 0.707 * 1.15A, I get P = 93.51VA. 93.51 Watts seems very small. Now I know this is comparing apples to oranges but my wifes hair dryer uses like (I think) 1100Watts. Is this right? Yes, I realize the formula has a lot of debate but I just want something close.

 

[bountyhunter]

Well, it's a mist machine that drives 4 piezos and their driver circuits as-well-as 3 - 4" pc fans, a microprocessor, a 24VAC solenoid, internally it has 2 - 36VDC transformers to drive the piezo drivers and a 24VDC transformer for the solenoid, a 5VDC cell phone power supply for a relay it has. I don't have a schematic, I assembled it (obviously) from off the shelf parts. The 4 piezo driver circuit boards all have a bridge rectifier.

Sorry you asked now I bet. I appreciate everyones help, I really do. At this stage I'm looking for something close would be nice but I don't really expect miracles.

Thanks.

No way to estimate because all of those loads have a different effect. Motors do indeed cause a phase shift in power factor.

 

[goofeedad]

No way to estimate because all of those loads have a different effect. Motors do indeed cause a phase shift in power factor.

Thaught so. For now I'll just have to be satisfied with P=V*.707*I.

 

[bountyhunter]

Hey All, I admit its been a long time since I've used alot of this stuff but the formula originally given by birdman0 (Prms = V(in AC) *0.707 * I), that gives Volt Amps, Right?

Is that the same as Watts?

NO. Volt-Amps is apparrent power which is composed of real power (Watts) and reactive power (VARs). VA is a vector made up of Watts and VARs and the power factor defines the angle which determines how much of each is present.

The references I posted cover this pretty well.

I ask because when I do the math, Prms = 115VAC * 0.707 * 1.15A, I get P = 93.51VA. 93.51 Watts seems very small.

It does seem low, because when you calculate apparrent power, real power (watts) must be equal to (only if it is zero PF) or smaller in magnitude than apparrent power.

What are you using to measure the current? That is almost certainly where the error is.

 

[goofeedad]

It does seem low, because when you calculate apparrent power, real power (watts) must be equal to (only if it is zero PF) or smaller in magnitude than apparrent power.

What are you using to measure the current? That is almost certainly where the error is.[/QUOTE]

A "MICRONTA" digital multimeter set on 10A. The only other settings are in the milliamp range. I put directly in series in a cut up extension cord.

 

[bountyhunter]

It does seem low, because when you calculate apparrent power, real power (watts) must be equal to (only if it is zero PF) or smaller in magnitude than apparrent power.

What are you using to measure the current? That is almost certainly where the error is.

A "MICRONTA" digital multimeter set on 10A. The only other settings are in the milliamp range. I put directly in series in a cut up extension cord.

Those meters give sensible readings ONLY when waveforms are sinusoidal. With three fans chopping away, your current waveforms could look like anything.

 

[goofeedad]

In your opinion, what would be your best guesstimation on what kind of formula or factor I could use. I can't stress the fact enough that I can't get my hands on expensive equipment.

I do realize I'm asking for guesses, but right now I'd be satisfied.

 

[AllVol]

Some fool at national Semi wrote a document with some good info on the subject. Check page 52:

**broken link removed**

Don't know who that fool is, but he did a pretty bang-up job. The document answered a lot of questions I've had about SMPS. Thanks for the link.

 

[The Electrician]

A "MICRONTA" digital multimeter set on 10A. The only other settings are in the milliamp range. I put directly in series in a cut up extension cord.

What is the model no. of your Micronta meter? Is it a "true RMS" meter?

 

[The Electrician]

Devices which have a bridge rectifier and transformer can have bizarre current waveforms which don't read accurately on digital RMS voltmeters anyway.

Then what would you use to measure the RMS value of the curent?

 

[bountyhunter]

Then what would you use to measure the RMS value of the curent?

If you want an accurate reading, you need a meter that can read TRUE RMS for arbitrary waveform shapes. It is specified in the meter specs.

here is one example:

**broken link removed**

There are "true RMS" electronic meters that have circuitry that approximates the integration to get true RMS, but they are generally limited to a crest factor of three. That's the ratio of the peak current value to the average value. One of these may read close or it may not. Unless you know what the current waveform is, you can't really predict if they are good enough.

 

[bountyhunter]

Don't know who that fool is, but he did a pretty bang-up job. The document answered a lot of questions I've had about SMPS. Thanks for the link.

I still have the original manuscript. The total document I wrote is a lot longer than that.

 

[bountyhunter]

In your opinion, what would be your best guesstimation on what kind of formula or factor I could use. I can't stress the fact enough that I can't get my hands on expensive equipment.

I do realize I'm asking for guesses, but right now I'd be satisfied.

Your best guess would be to piece it apart. The fans should have a rating on them. The loads that are fed DC can be viewed as single loads: measure the DC current into a board and the voltage and you have the power. You can estimate power losses in rectifiers and transformers by temp rise and physical size. Add up all the watts you can measure directly and the ones you estimate based on temp and you have an estimate of total power.

 

[goofeedad]

What is the model no. of your Micronta meter? Is it a "true RMS" meter?

MICRONTA 22 - 194. Doubt if it's a true RMS meter, it's a hand held unit. Used it in college but they supplied all the big equipment.

 

[goofeedad]

I still have the original manuscript. The total document I wrote is a lot longer than that.

I would appreciate having a copy of the whole document, if you don't mind. I'm about 1/3 of the way through and find it very informative. Not to mention it's bringing back a lot of my college stuff.

 

[goofeedad]

Your best guess would be to piece it apart. The fans should have a rating on them. The loads that are fed DC can be viewed as single loads: measure the DC current into a board and the voltage and you have the power. You can estimate power losses in rectifiers and transformers by temp rise and physical size. Add up all the watts you can measure directly and the ones you estimate based on temp and you have an estimate of total power.

OK. I can do that, I can't do it right now though.

How far off can the reading I got be? Surely not VERY far off.

 

[bountyhunter]

I would appreciate having a copy of the whole document, if you don't mind. I'm about 1/3 of the way through and find it very informative. Not to mention it's bringing back a lot of my college stuff.

I will try but not sure it's feasible. It is on my MAC and the only way to transport documents to my PC is on floppy disk, and that limits capacity to about 2 pages per trip. I have to write the pages to postscript format then save on floppy, take them to my PC, open and convert to PDF using a PDF distiller. Takes about ten minutes to do a disc which is about 2 pages at most material.

I'll see if there is a way around it, but it's pretty laborious to try to haul 100 page manuscript.

 

[bountyhunter]

I would appreciate having a copy of the whole document, if you don't mind. I'm about 1/3 of the way through and find it very informative. Not to mention it's bringing back a lot of my college stuff.

Ask and ye shall receive. I found the files and figured out a way to transport and convert them to PDF.

 

[Mr RB]

You wrote that bountyhunter? Just drawing up the schematics and charts alone would have been weeks of work... I can only guess the hours you put in including the text, research and error checking etc. Very nice!

 

[bountyhunter]

You wrote that bountyhunter? Just drawing up the schematics and charts alone would have been weeks of work... I can only guess the hours you put in including the text, research and error checking etc. Very nice!

Thanks, it was a lot of work but at least I did get paid for it. I wrote tons of technical material for customer seminars and also internal training for our field engineers. The 20 years I worked there, I was always writing, updating, and reformulating info to give to customers and engineers.

The drawings take the most time, the text is faster if you know the material.

I always took great pride in the finished writings, and really went for top notch graphics which takes a lot of time. Most of the writing done at that place was schlocked out, but I never wanted my name on anything that was junk.