




#83168  01/17/03 12:32 PM
Re: Super Neutral Type MC Cable

Member
Registered: 10/10/01
Posts: 615
Loc: Minneapolis, MN USA

Folks,
This is an interesting one. I have found the key to understanding nonlinear loads centers on how one describes the nonsine wave current. It helps to start by looking at each halfcycle of voltage that the power company provides, that is put across the load that is connected to one hot conductor of a branch circuit. The voltage can be thought of, essentially, as a clean sine wave with a frequency of 60 Hz. 60 Hz is the base frequency. . .the fundamental, or the "first harmonic".
Now, if the load on my branch circuit conductor is an incandescent light bulb, or a resistance heat element, as the voltage rises and falls through the sine wave, the current in the bulb or the element will rise and fall exactly the same way, in the shape of a clean sine wave. The current, at any instant in time, will be exactly that calculated by Ohms Law. The resistance of the load stays constant, and, as the voltage rises, the current rises in direct proportion. I=V÷R.
Set inductive and capacitive load to the side for the moment.
Let's jump straight to loads that receive their voltage, not directly from the branch circuit voltage, but rather from the output side of a silicon controlled power supply (SCPS). The SCPS, during any one half cycle, turns on and off (in its simplest form), and generally turns on and off a bunch of times in a half cycle. When the SCPS is off, no current flows, when it is on, the current that flows is a function of the logic chip(s) of the SCPS. The half cycle wave form of the current going through the branch circuit to the SCPS is a bunch of pulses more like square waves than any thing else.
This half cycle of branch circuit load current is now very complex, compared to the simple sine wave of the resistive load current. With the help of good math, electrical engineers learned to think of this half cycle of pulses of current as the sum of a lot of currents in the shape of pure sine waves that are multiples (harmonics) of the fundamental frequency (the Fourier Series, so named for Joseph Fourier near the turn of the 18th century, a French mathematician and friend of Napoleon).
In fact, the pulses of current that a silicon controlled power supply draws during a half cycle of voltage are made up of an infinite number of current sine waves of every multiple of the fundamental frequencies of the voltage.
When the neutral current of the branch circuit is then combined with the current of other branch circuits in a multiwire branch circuit neutral, some of the currents add to each other.
OK. Enough. I've probably really confused it.
The math really helps, the words are hard.
Al
_________________________
Al Hildenbrand

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