| The phase shift is what gives a good AC side waveform. Take for | example, a rectifier with a "infinite" DC inductance. The input will be | a step function. A 6 pulse rectifier will draw over one cycle 0, 1, | 0, -1 and back to zero, with 1 equal to the positive peak value. Run | this through a Delta-Wye or Wye-Delta and this becomes a waveform of | 1/2, 1, 1/2, -1/2, 1, -1/2. Combine another rectifier without a phase | shift, and now the input is a step function of 1/3, 2/3, 1, 2/3, | 1/3, -1/3, -2/3, -1, -2/3, -1/3. With the greater number of steps the | input now better approximates a sinewave. Of course a real rectifier | would have a real value of DC inductance that will introduce a ripple to | each step, but the input is still far better with 12 pulse than 6. | Another way to look at this is to look in the frequency domain. The | largest (AC side) harmonic of a 6 pulse rectifier is the 5th. The 30? | phase shift reverses the phase of the 5th harmonic. When combined with | the non-phase shifted rectifier, the 5th harmonic is canceled out. | Harmonics from the 7th up are not affected. | To combine to equal 6 pulse rectifiers with no phase shift will result | in no harmonic cancellation.
OK, I see what you are saying. You are combining waveforms from each side of the delta-wye so you have more phases. But that would be 12.
| The point of the 18 pulse is getting a good AC side waveform. | Particularly in AC drives, the savings in the filter is of negligible | benefit because the filtering cap bank required for the inverter side | dwarfs the current from the rectifier. For really large rectifiers or | high voltage rectifiers, the savings in the DC side filter becomes | significant.
Any schematics online?
What about 24 pulse?
| If you are looking at an arc lamp, I am assuming that the voltage is low | (>200V?). In this application, you definetly are looking at a parallel | rectifier.
I was figuring it would be somewhere between 200 and 600 at no load. Of course, while operating, the voltage across the arc drops way low and you have lots of current. I really don't know what voltage I'd need because I don't know that much about it, yet. I just want to do in with DC because I don't want it to be AC modulated.
| Can you get the generator wound for the desired output voltage? This | can be done for less than you might think if you can actually get in | contact with the right person at one of the generator manufactureres. | | It will cost more, but less than the cost of a transformer for the full | output of the generator. You are pretty much on your own for a | regulator, but you want to have the regulator regulating on the DC side | not the AC side anyways.
I'll probably be getting some used generator and going from there. As the rectifier circuits need 3 isolated secondaries, I either need a transformer, or have to break out the winding leads from the generator.
Do you think maybe driving the exciter level from the DC voltage would be the best way to regulate the generator?
| You may want to look at getting the generator wound with either 6 phase | star, or wound with two stators, phase shifted by 30?. | If the 6 phase star is used, balancing transformers or inductors will be | required in both the positive and negative legs to ballance the current | between the two sets of rectifiers because of the common neutral. | With two isolated windings, only one ballancing transformer or one set | of ballancing inductors is/are required. As a side note, locomitives | that still use DC motors use two isolated windings on the AC generator | and switch between series and parallel rectifiers using SCRs to "shift | gears" between low speed and high speed operation.
Interesting.
| I have been involved in a design where about 30kW at 48V DC was desired | from a backup generator. The cost of a custom generator, custom | controls and a custom regulator was cheaper than a stock generator and | controlled rectifier. The customer avoided the cost of a backup | controlled rectifier because the generator now became the backup if the | rectifier died. The application was used at a bunch of cell sites. The | only downside was the long lead time for new or replacement generators. | I believe local repair shops have rewound generators by reverse | engineering the windings. Even though we included a automatic monthly | "exercise" in the controls moisture still took its toll on the generator | windings, particularly in mountain top sites.
So I take it these were not epoxied windings.