Instantaneous 3-phase power:

[physis]

Re: Instantaneous 3-phase power:

Sam, your calculator apparently is taking the sine of 1.57 degrees. You should convert back to degrees for the calculator.

And wt is an angle in radians.

BTW, back in 1971 I worked with the hp guys in Palo Alto on the development of the hp-35 scientific calculator.

Let me do the math for sin (wt)

sin (2Pif)(t) =

sin (2 x Pi x f x t) =

sin (2 x 3.141 x 60 x .004167) =

sin 1.5709

You see. I'm giving the calculator 1.5709. Because that's the way I'm reading the equation. That's why I ask why is the sine of a number of radians being taken. A number of radians isn't an angle.

Palo Alto, HP. That's my "hood". I knew a guy with an HP calculator eons ago. That's before I knew what reverse Polish notation was, and it used it. I don't know what model it was but it had program modules you plugged in. HP would produce new programs all the time and sell the modules. He loved that thing.

[ January 29, 2005, 02:27 PM: Message edited by: physis ]

 

[rattus]

Re: Instantaneous 3-phase power:

Sam,

pi/2 radians = 90 degrees

both are angles.

You need to take the sin of 90 degrees because your calculator does not work with radians.

The calculator you mention may have been the hp-67. I designed the program memory for it. It had a little card reader built into it.

 

[rattus]

Re: Instantaneous 3-phase power:

deleted duplicate

[ January 29, 2005, 02:59 PM: Message edited by: rattus ]

 

[rattus]

Re: Instantaneous 3-phase power:

Deleted duplicate post

[ January 29, 2005, 02:40 PM: Message edited by: rattus ]

 

[physis]

Re: Instantaneous 3-phase power:

If I don't say the word "radian", and nobody else tells the equation that there are such a thing as radians. How does that equation know what to do other than what I did with it?

Come to think of it, I learned to use reverse Polish notation on that calculator.

[ January 29, 2005, 02:52 PM: Message edited by: physis ]

 

[rattus]

Re: Instantaneous 3-phase power:

Good point Sam,

You have to keep up with the units yourself. You have to know that your angle is in radians, but your calculator works on degrees.

Some calculators may work on either angles or degrees.

Likewise, you need to keep up with your voltages, currents, etc. It is good practice to always write the units, Volts, Amps, Ohms, etc.

 

[physis]

Re: Instantaneous 3-phase power:

Hey, You did do IC and digital. I was under the impession you worked with things alien to me.

 

[physis]

Re: Instantaneous 3-phase power:

But everything in that equation reduces to an exact number. If my thinking of it in a different way changes it's outcome then it isn't math. It's philosophy.

 

[rattus]

Re: Instantaneous 3-phase power:

Sam, don't think of it in a different way. Radians are radians, degrees are degrees--just like inches and centimeters. Just keep all your units straight in your head.

The Hubble telescope problem was the result of some engineers using the wrong units.

 

[physis]

Re: Instantaneous 3-phase power:

Rattus, The units I am using are labeled

0, 1, 2, 3, 4, 5, 6, 7, 8 and 9

Can you show me an error in the last calculation I did or not?

 

[physis]

Re: Instantaneous 3-phase power:

I remember the Hubble thing. I did something similiar working on a controller design for a cable feeder. The engineer's and I were scratching our heads for a couple of days because we knew this thing should work. It tuned out I was using frequency instead of RPM.

 

[rattus]

Re: Instantaneous 3-phase power:

Sam, I don't see any labels, but it is this simple,

sin(1.57 radians) = sin(90 degrees) = 1.00000000

480*sin(90 degrees) = 480*1.00000 = 480

[ January 29, 2005, 07:21 PM: Message edited by: rattus ]

 

[physis]

Re: Instantaneous 3-phase power:

So the answer is either that it is implied that the number of radians will be converted to the coresponding angle or there is more than one way that sine works.

Ok. I don't like, but ok.

So now I'll need to how to make the decision whether to convert to an angle or use sine the other way.

Is it the presence of Pi.

They must cover in this in calculus.

 

[steve066]

Re: Instantaneous 3-phase power:

Sam:

If you take the equation sin(wt), wt is always in radians. Likewise, 2*pi*f*t is also always in radians. Your calculator will have a way to set it for radians or degrees. When you punch in sin (1.57), it needs to know if you mean 1.57 radians or degrees.

Now consider the equation sin(wt+u), where u is some phase angle. When we add two numbers, they must have the same units. So you must either convert wt to degrees, or convert u to radians before you add them.

Unless you are really going to work with these equations a lot, I would suggest converting everything to degrees, and leaving your calculator set to degrees. After all, most people are more comfortable with degrees. We know at once that 90 degrees is a quarter of a cycle.

To convert radians to degrees, multiply radians by 180/pi. To convert degrees to radians, just multiply by the inverse - pi/180.

I hope you try your example again, I think you'll have better luck.

Edit: Sam, since I'm suggesting you work this in degrees, I should have just given you the same equation in degrees. the "2*pi" is actually what makes the units radians (there are 2*pi radians in one revolution.) So we just substitute 360 degrees for 2*pi radians.

sin (360*f*t + u) in degrees is the same as sin (2*pi*f*t +u) in radians.

And in degrees, w=360*f instead of 2*pi*f.

Steve

[ January 29, 2005, 11:34 PM: Message edited by: steve066 ]

 

[physis]

Re: Instantaneous 3-phase power:

Is that all? That's easy. Thank you Steve.

You see Rattus, Steve wasn't all difficult about it.

 

[rattus]

Re: Instantaneous 3-phase power:

I agree with everything Steve says with one nit picking exception:

Lower case omega, which we type as w, is the standard symbol for radian frequency, so one must be careful in converting to degree frequency.

It is far easier though to tell your calculator to expect radians. On my hp 15C, you press g, then 8, that is, the blue button, then the 8.

A little reminder appears on the LCD screen to tell you that you are in radian mode.

There is another button which converts radians to degrees and vice versa. Don't use that one.

 

[physis]

Re: Instantaneous 3-phase power:

I hope you try your example again, I think you'll have better luck

I don't even have to try it again. It's obvious it'll work.

Sin (radians).