If I have two 400 watt RMS subwoofers what size amp do I need? How can you tell?
Probably 800 watts at half the rated ohms of one speaker. Your amp must be able to push 800 watts at, say, 2 ohms if these are 4 ohm speakers or 800 watts at 4 ohms if these are 8 ohm speakers.It will say on the speakers what the impedance in ohms is.Get an amp that is specified with 800 watts or over, at an ohm rating that is SAME or UNDER…half the ohm rating of one of those speakers.That's the answer here's a short ramble:Almost inevitably you will be running the two speakers in parallel. Whether you have two outputs on your power amplifier; or if you go into one speaker then go out from that speaker to the next; the connection you don't see inside the speaker with in and out or inside the amp is a parallel connection. Parallel connections divide the impedance (ohms.) It takes a bigger amp to handle lower impedance, because lower impedance is lower AC resistance. Lower resistance means more amps flowing for the same output voltage. More amps requires a beefier output section. The actual equation for total speaker ohms of speakers wired in parallel is:Total ohms = 1/((1/ohms#1)+(1/ohms#2))…If you put in a third speaker there would be another similar term in the denominator, (1/ohms of speaker #3). More speakers, lower ohms…The amp will say what the watts and ohms are on the back, or at least in the manual it will tell you what it can handle. Be at or below the ohms, and above the watts.
If single phase power is 220 volts, why is 3 phase 440 volts and not 660?
For some reason.. (and its a good reason) electrical quantities have stuck with a circle. A complete circle implies 360 degrees. So for a 3 phase power, to be balanced equally, divide 360/3 = 120 degrees. If I name them A, B and C phases, they would look something like in the image here, 120 degreees apart.The power which is delivered all over the place is sinusoidal in nature (looks like a sine wave). And if I plot them on a paper, they will look as in the image below.From the above image, consider all zero crossings.. whenever a phase is at zero crossing, rest of the phases are at 0.866 values. The vector difference between them is [math]sqrt(0.866^2–0^2) = 0.866[/math]. Now consider whenever two waves are intersecting, they intersect at 0.5 (or -0.5), the other wave is at -1 (or +1). The vector difference between them is [math]sqrt(1^2 - 0.5^2) = 0.866[/math]. Now the line voltage is measured across two phases. Hence the second waveform is actually Vphase1-Vphase3. It can be observed from the graph that the peak of this waveform is more than that of the phase voltages (1, 2 & 3). the rms values of phase and line voltage will have a ratio of 1.732.For calculation, if we consider all the phase voltages to be equal, we could mathematically derive as below:A balanced 3 phase power is like rotating that first image round n round without disturbing those 3 arrows, you get the same result as I explained with the sine wave drawings :)So..that’s how you get 400V = [math]sqrt3 *230[/math] V. I hope the doubts are cleared :)