Page BottomI have never been fond of AC power, and less fond of single phase power systems. The concept of a power supply that's delivering useable power only in pulses is just wierd. Power flow should be continuous, even if it's not DC. That's the basis of multiphase power systems. Many people have heard of 3phase, and most industry and commercial businesses use some 3 phase somewhere. This page is
You may be thinking 2 phase is esoteric, not found anywhere any more. But it is found nearly everywhere, because no single phase ac motor can start under load, or start in a specified direction reliably, without a method to induce a rotational drag. That's roughly 90° 2 phase power, altho it's usually used in the single phase motor only to start it, called "capacitor start" motors. There are heavy duty motors that use a tougher capacitor to continuously shift the voltage, these are called "capacitor run" motors, they make their own 2 phase for high running torque. There are motors that use both methods, the capacitor start for high start torque, and capacitor run for high continuous running torque. There is also the shaded pole motor, which has a copper bar or wire around a portion of the stator iron. This causes a magnetic lag, due to the time it takes to charge the single coil of wire, and the time that magnetism decays. It's a small difference, but
This is a plot of true 90 degree 2phase voltage: The 2nd phase is 90 degrees lagging the 1st phase. I did the plot points with a calculator. The three pics on this page are all phase-aligned, altho not exactly the same size, because i photographed them, i didn't scan |
This is the fullwave rectified 120Vrms, 90 degree, 2phase sinewaves : |
As a power source for a device needing stable dc that's always there, you can see this beats a single phase. If you trace the phase1, you can see it drops to zero volts, and at that point it has 170volts of ripple. If all you have is single phase, it's up to you to fill in that power demand, and most people do so with capacitors. Problem with capacitors is they tend to be really low impedance devices and people connect them right to the rectified powerline, and when they recharge off the peak of the sinewave, the capacitor sucks so much current that the sinewave peak is cut off, and sometimes the rectifier diodes are zapped by the surge current. Soft start circuits help significantly when first powering up. As do current sinking supplies that follow the sinewave, emulating a resistive load (these are now mandatory in the European Union, and the power companies in the usa near data centers would like to see them mandatory
The 60hz sinewave gives you a total of 8.33ms of dc pulse each 1/2 of the rectified sinewave. Of that, you generally get roughly 1.5 to 2 millisecs (3ms maximum) to charge the capacitor before the line voltage drops below the stored voltage in the capacitor and stops charging it. Then the capacitor supplies power for ~6ms as the sinewave drops below the capacitor voltage, goes to zero, and then comes back up to recharge the capacitor again. Rule of thumb with great capacitors is 1000uf per amp, and lets say you need 30amps... i'd use 70,000uf, in several capacitors. Why several?, because capacitors are not ideal devices, nothing is ideal. They have some internal resistance, among other things. That resistance is passing 30amps for 6ms while discharging, and then recharging at 60amps for 3ms, flat-top, clipping the sinewave. Even at 0.1 ohms resistance, that's some wattage dissipation inside the capacitor, and they don't dissipate power well, they tend to get hotter and hotter and then explode to some degree or another. Or leak out all their electrolyte and the capacitance goes to zero. So i use several capacitors to spread the heat around. Besides, several are cheaper than one 70,000u cap. The capacitor's ESR factors into the
As a study aid, if you resize the browser width for this page so the full cycle shown in the pics here is 16 centimeters on your monitor, you can read out the timing at a scale of 1millisec per centimeter.
So examine the plot with 2phase. When the phase1 goes to zero volts, the phase2 is up at 170v. The peak ripple is 50 volts at 45, 135, 225, and 310 degrees. If you don't want to use capacitors, and can tolerate 50v ripple on your dc, this will deliver that power. It beats the 170v ripple on single phase, or using huge capacitor values in heavy current applications. In fact, the usa used this 2phase system to charge batteries in their diesel-electric submarines starting in World War 2, right up till they got rid of the last WW2 sub. It's more cost effective than 3phase or dc, and far easier on the batteries than single phase. Car alternators are fullwave rectified 3phase, mostly so the rms of the resulting dc is higher, with less peak ripple to damage devices using the car's electrical system (like radios, etc). For more info on 2 phase power,
This is what would happen if you connected a device across the two phases. You get some really odd voltage. It doesn't really do anything for you, except for one thing. Remember i said the capacitor input power supply will clip the tops off the sinewave? This crossphase sinewave is not in phase with either of the two generated phases, so it's peaks do not occur at the same time as any peak in the rectified plot above. It's peaks occur on the rising and falling edges of the two generated phases. For a heavily capacitive load, in a pinch, you could use this as an odd 3rd phase, 45 degrees behind the phase1, 45 degrees ahead of the phase2. You could trash it's peaks and not be affecting the peaks of
This is a pic of an oscilloscope display of 2 phase power being generated. As far as sine waves go, this is pretty bad, but one of those traces is the line voltage here in my house. The other is from the "generator". I say it's a generator, only because it is actually making some power, and lighting a incandescent light bulb. It's really a 1 phase electric motor, with identical capacitor start and run windings. This configuration is common in motors meant for easy reversing: you simply power up the other winding. What's less known is that winding can deliver 90°
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