18th February 2012 04:29 PM
Good man, glad we didn't lose you :-)
Originally Posted by KeepItSimpleStupid
18th February 2012 06:09 PM
A circuit consisting of a 12 V voltage source and two 6 ohm resistors can be analyzed as follows.
12= 6I+6I ---> I=1 or 0=-12 + 6I +6I which is the generic form of the equation
This also means the the voltage across each resistor is 6V
Now Power = VI
If we did the same thing we would have (12)(1)W = 6(1) W+6(1) W
In the generic form of the equation
0=-12 W + 6W + 6W
Power is a negative quantity.
Power generated, by convention has a negative sign. Power dissipated, a positive sign.
Yea, it might make sense to have it the other way, but just like conventional current that's the way it is.
The analysis is generic and is generally known as a "loop equation". The algebra gets messy, but it is appropriate for multiple adjacent loops.
We don't talk about -10 MW generators, but when we analyze, they are negative. Your 100 W light bulb dissipates say 100 W of energy as heat.
Last edited by KeepItSimpleStupid; 18th February 2012 at 06:11 PM.
18th February 2012 06:43 PM
Alright!!. Now where were we...
All very true. However:
Originally Posted by KeepItSimpleStupid
Any motor, for instance, by its very nature, while turning, is also a generator. Electric Motive Force (EMF) is said to be the "power" that causes the motor to turn. All well and good. But as it turns, it's also generating what's called a Counter-EMF (CEMF) that flows in opposition to the EMF (this can set up so called "Standing Waves", [which are really bad for causing energy losses] but that really is down the road).
Anyway, as in KISS's example, yes, there is power generated that does no useful work due to losses that are abundant. But keep in mind that a 10MW plant does not always produce at full capacity. The main power producing apparatus is throttled to the load. This does not eliminate losses, but it does reduce unnecessary loses. And, in essence, CEMF can be a power saving phenomenon.
So, for DC, since by way of a bridge rectifier system (and other ways) we can move the negative element of the sine wave to the positive side of ground we are, thereby, now making it part of the "power" side of the equation. This is as you saw in the excepts you introduced in a previous post.
It's good to see that that your curiosity is leading you to search out the answers to some of the questions you have.
Sometimes it's necessary to explain a fundamental concept in such a way is isn't exactly complete or in all cases true. Certainly in a number of fields thee are ambiguities and nuances that are difficult to grasp until a certain underlying concept is understood.
That's when you know better what questions to ask.
And you are rapidly getting there.
Last edited by cowboybob; 18th February 2012 at 06:46 PM.
Semper ubi sub ubi.
18th February 2012 06:54 PM
Another example with your trying to say that a negative wave does nothing. Take a three phase motor which as 2 windings energized with 3 sine waves 360/3 degrees apart. When the sine wave is negative it could be pulling rather than pushing or vice versa, hence it is doing work.
18th February 2012 09:03 PM
A DC motor can act as a generator as well as a motor. In fact the voltage produced by an unloaded generator is proportional to speed so V =k(rpm) or some constant k times the rpm. The motor also has a DC resistance, so there is an IR drop across the windings.
So the Voltage V across the motor is V = Vm - IR; where Va is the Voltage across the motor. If you can compensate that IR drop, you can gain speed control of the motor.
A recent application note from Linear Technology illustrates the earlier fact. The motor is pulsed for nearly the entire time. When it's coasting the voltage is measured and fed back to the speed control.
At the same time torque is proportional to current.
18th February 2012 11:08 PM
Sorry guys, baled & had one of those rare visits to pub
Back on it tomorrow
CBB, tried to self help when site was down but I know you know the way forward, never could there be amore eager student :-)
20th February 2012 09:31 AM
20th February 2012 09:36 PM
20th February 2012 09:39 PM
Now you have time to work on your triac stuff. You shouldn't be complaining. <G>.
20th February 2012 09:55 PM
See, Graham? Another addiction to deal with...
Your right, KISS. Prob has been that our DSL link to the world has gotten very bad, lately. Keep checking the stupid thing and wasting time doing it.
On to Triac play, yeah!!
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